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NATL INST OF STANDARDS & TECH R.I.C.

A1 11 00992825

Meese, William J/Analysis of current tec TA435 .U58 V63;197S C.I NBS-PUB-C 1975 NBS BUILDING SCIENCE SERIES 63

U.S. DEPARTMENT OF COMMERCE / National Bureau of Standards
The Building Science Series

The Building Science Series disseminates technical information developed at the National Bureau of Standards on

building materials, components, systems, and whole structures. The Series presents research results, test methods, and pt^rformance criteria related to the structural and environmental functions and the durability and safety characteristics of building elements and systems. These publications, similar in style and content to the NBS Building Materials and Structures Reports (1938-59),

are directed toward the manufacturing, design, construction, and research segments of the building industry, standards organizations, and officials responsible for building codes. The material for this Series originates principally in the Center for Building Technology of the NBS Institute for Applied Technology. The publications are divided into three general groups: Building Systems and Processes;

Health, Safety and Comfort; and Structures and Materials. For further information regarding these publications

please contact the Scientific and Professional Liaison Section, Center for Building Technology, Institute for Applied Technology, National Bureau of Standards, Washington, D.C. 20234.

[See mailing list announcement on last |)agej
"^^^ sis of Current Technology on Electrical

' /.V

U Si

Connections in Residential Branch Circuit Wiring

William J. Meese and

Ramon L. Cilimberg

Center for Building Technology

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

Sponsored by

Office of Policy Development and Research The Department of Housing and Urban Development

Washington, D.C. 20410

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W. Roberts, Director

Issued March 1975
Library of Congress Cataloging in Publication Data

Meese, William J. An Analysis of Current Technology on Electrical Connections

in Residential Branch Circuit Wiring.

(National Bureau of Standards Building Science Series; 63) Includes bibliographical references.

1. Electric Connectors. 2. Electric Wiring, Interior. I. Cilimberg, Ramon L., joint author. II. Title. III. Series: United States. National Bureau of Standards. Building Science Series; 63. TA435.U58 No. 63 [TK3521] 690'.08s [621.319'24] 74-31122

National Bureau of Standards Building Science Series 63

Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 63, 23 pages (Mar. 1975) CODEN: BSSNBV

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1975 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 (Order by SD Catalog No. C13.29:2/63>.

Price 70 cents
Contents

Ptt/c 1. Introduction 1

1.1. Scope 1

1.2. Background 2

a. Fire-hazard considerations 2

b. Electric shock considerations 2

c. Industrialized housing needs 2

1.3. Definitions of special terms 2

a. Electrical connection component 2

b. Continuous current path element 2

c. Dielectric element 3

d. Enclosure element 3

e. Other building components 3

2. Traditional approach 3

3. . Current evaluation system 3

3.1. Local authorities 3

3.2. National Electrical Code 3

a. NEC requirements for electrical connections 4

b. NEC requirements for boxes 4

c. NEC accessibility requirements 4

3.3. Underwriters' Laboratories 4

4. Other electrical connection specifications, standards and tests 5

4.1. American National Standards 5

4.2. Federal Specifications 5

4.3. Military Specifications 5

4.4. National Electrical Manufacturers' Association 5

4.5. American Society for Testing and Materials 5

5. Field performance of electrical connections 6

6. Electrical connection research and testing 6

6.1. U.L. accelerated tests 6

6.2. Seven year tests in the United Kingdom 6

a. High current tests ^ 6

b. Cyclic loading tests 7

6.3. Battelle Memorial Institute research 7

7. Failure mechanisms 7

7.1. Resistance heating 8

7.2. Loose connections—arcing 8

7.3. Wire breakage 8

8. Properties and behavior of materials used in electrical connections 8

8.1. Contact resistance 8

8.2. Surface fibns 8

8.3. Effect of contact pressure 9

8.4. Creep and stress relaxation 9

8.5. Thermal expansion and contraction ... 10

8.6. Diffusion across material boundaries 10

8.7. Strength, ductility, and hardness 10

8.8. Electrical conductivity 10

9. Materials properties, considerations in electrical connector design 10

9.1 Surface layers on wire 10

9.2. Creep and stress relaxation 10

a. Connector design approach 10

b. Metallurgical bonding 11

c. Wire alloy development 11

d. Copper cladding 11

e. Nickel plating 11

iii
Page

10. Recent innovations }}

10.1. Electrical harness 1|

10.2. Split-beam receptacles 11

10.3. Quick splicing connectors 12

10.4. Baseboard receptacle system 12

11. Research considerations of innovative electrical connections , 12

12. Summary • 1^

13. Recommendations 1^

14. References 1^

Appendix NEC requirements which directly affect innovative electrical connections 14

ADDENDUM TO NATIONAL BUREAU OF STANDARDS BSS 63 (MAR. 75) by Meese and Cilimberg

In late 1973, the Consumer Product Safety Comtnission requested the National Bureau of

Standards to investigate the safety of aluminum branch circuit wiring in homes. This

work included the evaluation of reports on the hazards, study of technical work of other

organizations, and a technical study of the problem. The work at the National Bureau

of Standards continued actively through 1974.

The NBS Programmatic Center for Consumer Product Safety issued a special report to the Com- mission on this subject, NBSIR 75-723 -- Aluminum Branch Circuit Wiring in Residences; Sum- mary Report for the Consumer Product Safety Commission, January-September, 1974. This report

is available for purchase from the National Technical Information Service, Springfield, Va., 22151, as is another NBS report, NBSIR 75-677 — Hazard Assessment of Aluminum Electrical Wiring in Residential Use, published under date of March 27, 1975.

The Consumer Product Safety Commission held Public Hearings on the Safety of Aluminum Wire in Washington, D.C. in March 1974, and in Los Angeles in April 1974. An abstract of these hearings is included in NBSIR 75-723.

ERRATA TO BSS 63

Replace Figure 6 (p. 9) with the following:

lO' 10^ 10' 10* 10" PRESSURE (Grams)*

Figure 6. Contact Resistance versus Pressure (Two Conductors Crossed Under Load).

*Force (grams) — one gram-force = .009806650 newton

iv
An Analysis of Current Technology on

Electrical Connections in Residential

Branch Circuit Wiring

William J. Meese and Ramon L. Cilimberg

In the Operation BREAKTHROUGH research and demonstration program the U.S. Department

of Housing and Urban Development became concerned with the inability to properly evaluate innovative eletcrical connections. Long life requirements, fire safety considerations, the lack of adequate

technical information, and long established conventional practices and evaluation procedures have

led to slow-changing regulations concerning electrical connections used in branch circuit wiring in housing. This report discusses the present methods of evaluating electrical connections, the technical parameters involved, and innovative electrical connection developments. Innovations involving elec- trical connections may lead to significant advancements in housing construction if it can be demonstrated that functional and safety requirements over the expected life of the electrical con- nections are adequately satisfied. Research is needed to enable prediction of long-term performance

of electrical connections based on the results of accelerated performance tests.

Key words: Contact resistance; electrical codes; electrical connections; fire safety; house wiring;

materials properties; performance testing.

1. Introduction

In 1969 the U.S. Department of Housing and

Urban Development initiated the Operation BREAKTHROUGH program; one of its purposes was to en- courage and demonstrate innovations in housing [1, 2, 3, 4].^ In the Operation BREAKTHROUGH program

several innovations involving electrical connections were

proposed, including connections which would be inaccessible after completion of building construction. Conservative reviews were required due to the fire hazard potential and the lack of durability and other

technical information available. Approvals for innovations involving electrical systems were sparingly given.

However, the possibilities of almost immediate signifi- cant breakthroughs were recognized.

The U.S. Department of Housing and Urban Devel- opment agreed to sponsor at the National Bureau of Standards a long-range research project on electrical connections in residential branch circuit wiring. The

long-range goal of this project is to develop criteria and

test methods for evaluating innovative electrical connections. This report is the first publication for this project.

1.1. Scope

This report covers the state of the art of electrical connections used in branch circuit wiring in housing.

"Branch circuit" is defined by the National Electrical Code [5] as:

A branch circuit is that portion of the wiring system between

the final overcurrent device protecting the circuit and the

outlet (s). A device not approved for branch circuit protection such as thermal cutout or motor overload protective device is not considered as the overcurrent device protecting the circuit.

1 Figures in brackets refer to references at end of paper.

Branch circuits in housing are generally limited to nominal 115 or 230 volts with rated ampacity (current

carrying capacity in amperes) of not less than 15 or more than 50. Generally, wires involved are not larger

than #6 AWG nor smaller than #14 AWG for copper

conductors, and #12 AWG for aluminum or copper

clad aluminum conductors. Tap conductors for lighting

fixtures (fixture wiring) and other equipment con- nected to branch circuits may be as small as #18 AWG

for copper conductors. Both wire-to-wire and wire-to-terminal (such as wire- to-terminal parts on receptacles, switches, lighting fix- tures or permanent appliances, etc.) connections which

are made in the field or in a housing or mobile home

factory are included in this study. Connections which

are normally made as a part of the manufacturing

process of electrical equipment are not included. This research project was not created in response to reported problems with aluminum wire electrical con- nections [6] and it is not the intent of this state-of-the- art report to evaluate the use of aluminum wire in branch circuits. However, nearly all recent research concerning branch circuit electrical connections has

been addressed to aluminum wiring problems. Pertinent

information based on this research is included in this report. All electrical cables with aluminum conductors in

sizes used in residential branch circuits which were

listed by Underwriters' Laboratories (U.L.) prior to 1973 have been withdrawn from U.L. listings [7]. U.L.

has subsequently listed a number of other electrical cables with specific aluminum alloys [8]. Also, U.L. has

revised its requirements for connections listed for use with aluminum conductors [9]. This has out-dated all pre-1973 connectors (such as terminals on receptacles)

listed for use with aluminum conductors in residential branch circuits. Information concerning what could or should be done with pre-1973 aluminum wiring in- stallations is outside of the scope of this report and this project.

1
1.2. Background

In the United States approximately 28,000 electrical contractors employ 237,000 electrical workers and

60,000 overhead workers [10]. (Electrical Contractor Magazine.) They have approximately $12 billion an- nually in sales. Approximately 35.2 percent of this total

is spent on electrical labor, 38.4 percent on materials,

and 26.4 percent on indirect expenses, overhead, profit

and taxes. Of the $12 billion in sales, approximately 25.8 per- cent is for one- and two-family residential, 6.3 percent

for apartment residential, 24.6 percent for commercial,

22.3 percent for industrial, 12.0 percent for institutional, and 9.0 percent for other types of occupancies. The

$12 billion in sales primarily represents electrical work

by contractors in conventional construction and does

not include electrical work in mobile homes or factorybuilt housing. Also, it includes only a small portion of the electrical work done by utilities and only part of

the electrical work done by industrial establishments.

The major portion of the electrical work by contractors in conventional residential construction is in branch circuit wiring. The major part of the work of branch circuit wiring is the making of electrical connections, including the installations of outlet, junction

or other boxes and the mechanical fastening of electrical cable to boxes. Electrical connections in branch circuit wiring appear to be the primary area in electrical construction where considerable savings in cost and

time are almost immediately possible.

a. Fire Hazard Considerations

Electrical wiring and equipment are among the leading causes of accidental fires. In 1971 they ac- counted for 160,900 of 996,600 building fires. Motors

and power-consuming appliances accounted for 62,100

of these fires; wiring and general equipment accounted

for the other 98,800 [11]. (NFPA Statistics.) While

available statistics are not in sufficient detail to deter- mine how many of these fires involve connections or

terminals, it is believed that a large number of them

which do not involve motors or other power-consuming

equipment are started at connections.

By 1968 field reports indicated that aluminum wiring was sometimes overheating at screw terminals on re- ceptacles in branch circuit residential applications [7].

Further indications of problems with aluminum wiring

appeared in 1970 when 25 incidents of fire in 2000

California homes wired with aluminum were believed

to have been caused by faulty electrical terminations

[6]. Subsequently a study by Underwriters' Laboratories estimated that 80 percent of the failures in branch circuit wiring involved connections and showed

that failures involved both aluminum and copper wire

installations [9].

b. Electric Shock Considerations

Protection against electric shock and electrocution is a major consideration in determining requirements for

electrical installations. Approximately 1,000 deaths an- nually are attributable to electrocution. Of these ap- proximately 300 are associated with residential occu- pancies [12] (National Safety Council Statistics).

It is usual practice to require that those parts of

electrical equipment operating at 50 volts or more

(Sec. 250-3 of NEC) be guarded against accidental

contact. It is also usual practice to ground non-current- carrying conductive parts of electrical equipment. Large

metal objects, such as metal siding, are often grounded

for protection in the event they may come in contact with live electrical conductors. Section 250—44 of the National Code [5] recommends grounding of large

metal objects.

c. Industrialized Housing Needs

Nearly all major parts of houses, such as exterior and interior walls, ceiling, and "floor-ceiling" assemblies, incorporate electric wire and equipment (switches,

receptacles, lighting fixtures, etc.). Installing the type

of electrical systems used in conventional housing

becomes an impediment to industrialized or mass- produced housing since it is required that all electrical connections be in junction, outlet, switch or other

boxes. The advantages of not requiring that electrical connections be accessible after completion of building

construction became apparent with advancements in the construction processes.

1.3. Definitions of Special Terms

Several special terms used throughout this report are

defined as follows:

a. Electrical Connection Component

Electrical connection component is defined as all parts of the hardware directly concerned with the proper functioning and safety of the electrical connection. As described below, this would include the continuous current path elements, the dielectric element,

and the enclosure element.

b. Continuous Current Path Element

Continuous current path element is defined as the

electric connection point or area and electric wire and

other parts in the vicinity of the connection point

through which current is intended to flow and which

are directly involved in the proper functioning and

safety of the connection. In conventional construction

this would include metal portions of "wire-nuts" in wire-to-wire connections and wire binding screw ter- minals on wire-to-terminal connections. Also included

are the grounding connection point or area; and the wire and metal parts in the vicinity directly involved

in the proper functioning and safety of the grounding

connection. In this report "continuous current path ele- ments" are generally referred to in the plural because an electric connection usually involves more than one

joint or connection.

2
c. Dielectric Element 3.1. Local Authoritiee

Dielectric element is defined as insulating material

in the vicinity of the electric connection point or area. Insulating material must completely surround the con- tinuous current path elements. The dielectric element

may be "air." This is the case where receptacles or switches are rigidly held in place within an outlet box

and electrical connections are made by means of wire

binding screws.

d. Enclosure Element

Enclosure element is defined as rigid metallic or nonmetallic material which surrounds an electrical connection and prevents persons, other building components

or other objects from coming in contact with the con- tinuous current path elements. In conventional construction a switch, outlet or junction box is the enclosure. The same piece of hardware may perform both

the function of the dielectric element, which is an

electrical function, and the function of the enclosure,

which is a mechanical function.

e. Other Building Components

Other building components is defined as parts of the building other than electrical connection components.

This would include studs and other structural parts,

nails, gypsum board, pipes, ducts, and wiring other than that included in Definition 1.3.b.

2. Traditional Approach

The traditional approach to branch circuit wiring

connections is the on-site: (1) fastening of outlet, junction or other boxes to building components, (2) fasten- ing of electrical cable to boxes, and (3) making of

electrical connections in boxes. Wire-connectors (wire

nuts) are normally used for "wire-to-wire" connections

and wire binding screws or studs and nuts having up- turned lugs are normally used for "wire-to-terminal"

connections. In some cases connections are soldered wifh a fusible metal alloy. Pressure cable connectors, pressure terminal connectors and soldering lugs are sometimes used. There have been improvements in electrical cable

insulation. New materials have been introduced, such

as aluminum wire and non-metallic sheathed cable and

non-metallic outlet boxes. However, the traditional ap- proach to branch circuit wiring has changed very little over the past 25 or more years.

3. Current Evaluation System

In the United States, the evaluation and approval of

electrical connections and other electrical construction

in housing primarily involves three entities, which are;

(1) local authorities, (2) the National Electrical Code

(NEC) [5] and (3) Underwriters' Laboratories, Inc. (U.L.).-

Local authorities usually determine legal require- ments which apply to electrical construction in their areas. In practice, however, local authorities usually

follow the National Electrical Code for installation re- quirements and approve only manufactured electrical components which have been listed by Underwriters'

Laboratories, Inc. In some cases local authorities adopt the National

Electrical Code with modifications. Such modifications

usually make legal requirements more restrictive than

corresponding NEC requirements. For example, some

jurisdictions prohibit the installation of non-metallic

sheathed cable [13] or electrical cable with aluminum

conductors [7].

The inspection of electrical construction is usually

done by local authorities. The National Electrical Code

and any local amendments to the NEC are referred to in the event of any controversy concerning wiring

methods or requirements. Otherwise, the safety and

adequacy of electrical installations are evaluated by

the judgment of electrical inspectors.

3.2. National Electrical Code

The National Electrical Code [5] is promulgated

under procedures of the National Fire Protection Association (NFPA) and the American National Standards

Institute (ANSI). The Rules of Procedure for amending the NEC are detailed in an appendix to the NEC.

The National Electrical Code, as published, is a voluntary standard. However, it affects construction

throughout the United States because of its adoption

by state, local, or other enforcing authorities. In areas where there are very limited or no legal requirements,

or where legal requirements are not rigidly enforced, some contractors still conform to the National Electrical Code. In the event of fire or accident, courts generally

accept the National Electrical Code as acceptable good

practice. Eight hundred thousand copies of the 1968

NEC, which was superseded in 1971, were published.

The National Electrical Code is an "installation" code. It states in generic terms what may be installed for different types of construction and for given conditions, and sets forth installation requirements. It is a combination of performance (stating what is to be

accomplished) and specification (detailing specific

methods, materials or components) requirements. An

Electrical Code for One and Two Family Dwellings,

which is excerpted from the National Electrical Code

[14] also has been published.

Responsibility for revision of the NEC is divided

among approximately 22 panels (subcommittees) with

a correlating committee overseeing and coordinating

the work of the panels. The panels consist of repre- sentatives from varied interests such as electrical manufacturers, electrical contractors, electrical utilities, elec- tricians, insurance, electrical and building inspectors,

and other governmental authorities. The NEC is revised

3
on a three-year schedule and there are provisions for interim amendments. All properly submitted revisions are considered by the appropriate NEC panel. Approximately 1800 revisions to the 1974 NEC edition have

been proposed.

The NEC covers requirements for electrical construction except for the suppliers' or utilities part of elec- trical systems. Requirements for electrical and communication utilities' facilities are generally covered by the National Electrical Safety Code [15, 16, 17, 18].

It is virtually impossible for new products to enter

the market place in any significant quantity unless there is conformity with the National Electrical Code. Because of potential fire and shock hazards of electricity, it is difficult for innovations to gain approval

without in-use experience. National Electrical Code

requirements are principally the result of long experi- ence with electrical construction.

a. NEC Requirements for Electrical Connections

Section 110-14 Electrical Connections, of the National Electrical Code, which is listed in Item A of the Appendix to this report, requires that connecting de- vices be "suitable for the purpose." This performance

language permits the use of innovative connecting

devices which have been properly tested and evaluated

by organizations such as Underwriters' Laboratories.

Developers of innovative connections do not consider

rules strictly relating to electrical connections as being

restrictive.

b. NEC Requirements for Boxes

Section 300-15 (b) of the National Electrical Code

states that a box shall be installed at each conductor

splice connection point, outlet, switch point, junction

point or pull point for the connection of metal clad

cable, mineral insulated metal sheathed cable, aluminum

sheathed cable, non-metallic sheathed cable, or other

cables, and at each outlet and switch point for concealed

knob and tube wiring. (See item B of appendix.) For

concealed knob and tube work, splices may be made

outside of boxes; however, boxes are required for terminal connections (see item F of appendix). Section

370-6 requires a certain volume of free space in each

box depending on the number and size of the con- ductors and the fittings or devices such as switches

or receptacles which are in the box (see item C of appendix). For example, each number 14 conductor

requires 2 in^ of free space; each number 12 requires 2.25 in^. These requirements concerning boxes appear

to constrain innovations.

The requirements concerning boxes appear to constitute an immediate issue which this research project

must face. Reference [19] (National Electrical Handbook) states that the "purpose of an outlet box is to provide an enclosure for circuit wire where they are brought out for connection to a fixture or other device. This enclosure is particularly necessary on account of the splices which the box may contain. The enclosure

should be made complete by means of a cover of some

kind." Boxes are necessary for fire and electrical safety

purposes for conventional electrical systems. However,

whether boxes are necessary in innovative electrical systems is a very important question. It appears that very significant cost and time saving advancements in branch circuit wiring systems could be effected, almost

immediately, if the requirements for boxes were elimi- nated, provided, of course, that fire and electrical safety

aspects were provided for in other ways. There are exceptions to the requirements for boxes

for (1) exposed cable wiring, (2) concealed work in rewiring existing buildings, and (3) wiring in mobile

homes. A technical reason for these exceptions is not

obvious.

Section 331-11, Devices of Insulating Material,

permits the use of switch, outlet, and tap devices of insulating material without the use of boxes in exposed

cable wiring and for concealed work for rewiring in existing buildings where the cable is concealed and

fished (see item E of appendix)

Section 550-8 ( j

) of the National Electrical Code

permits outlet boxes of dimensions less than those required in Tables 370-6(a) (1) and 370-6(a) (2)

in mobile homes (see item D of appendix). There is essentially "zero space" for some innovative connections

which are described in this paper and are listed by

Underwriters' Laboratories (see subsection 10.2 of this report )

Section 550-8(j), however, only applies to mobile

homes. A technical reason for less stringent require- ments for mobile homes is not known. Actually, mobile

homes may need more stringent requirements because

they may be subject to more severe vibrations than

conventionally built homes.

Requirements for boxes in conventionally-built and

factory-built homes need intensive evaluation because

of the very significant and very immediate effect they

have on housing technology.

c. NEC Accessibility Requirements

There has been a long established requirement that

all electrical connections be accessible after building

construction has been completed. Section 370-19 of the National Electrical Code states that "junction, pull, and

outlet boxes shall be so installed that the wiring con- tained in them be rendered accessible without removing

any part of the building, sidewalk or paving." (See item

G of appendix. ) In conventional electrical systems there

is need for repairing or replacing connections or con- necting devices. Because of this it appears certain that inaccessible connections will not be permitted unless there is convincing proof of their durability and safety.

However, with the advent of industrialized housing,

there is a growing desire to make use of inaccessible connections. In Operation BREAKTHROUGH, two

housing producers proposed using inaccessible "quick

make-up" connectors. Neither was used because of the

lack of technical knowledge and tests to evaluate the

durability of these connectors.

3.3. Underwriters' Laboratories

r^early all testing for safety of manufactured elec4
trical products is done by Underwriters' Laboratories

(U.L.), a non-profit, private corporation. Underwriters'

Laboratories lists manufactured products which they

hare tested and found satisfactory. U.L. develops its own test methods and standards. Non-metallic sheathed

cable, armored cable, wire connectors and solderjng

lujjs, and outlet boxes and fittings are some of the

eh'ctrical products covered by Underwriters' standards

which interface with branch circuit electrical connectit.ns [20, 21, 22, and 23]. Other U.L. standards which

include specifications and tests for wire leads and wire

terminals intended for connections to branch circuits,

include equipment such as snap switches [24], attach- ment plugs and receptacles [25], and electric light fixtures [26]. In the case of innovative or other products for which

a standard does not exist, the product is evaluated in accordance with the judgment of Underwriters' Laboratories, using established tests or procedures to the extent applicable or possible.

"Testing for Safety" [27], published by Underwriters' Laboratories, describes its purpose, procedures,

facilities and equipment, services available, and policymaking councils. Underwriters' Laboratories has no

legal authority to "approve" products for installation or sale. U.L. uses the word "list" and not "approve"

in referring to products it has found satisfactory. Approval of products is the function of the enforcing

authority having jurisdiction. The procedures of most

enforcing authorities do not call for listing by Underwriters' Laboratories as the sole criterion for approving

electrical products. However, in practice, listing by

U.L. usually becomes tantamount to local approval

provided local electrical regulations are not more

stringent than those of the National Electrical Code, in which case the use of certain types of electrical products

may be prohibited [13].

Electrical products are listed in two volumes; Electrical Construction Materials List [28], and Electric Appliance and Utilization Equipment List [29]. Thousands of specific manufactured electrical products are

listed in each of these volumes. Additions and deletions

to these lists and to other products listed by Underwriters' Laboratories are published quarterly [30].

A long-standing policy of Underwriters' Laboratories

is to list only products which may be installed in accordance with the National Electrical Code. Special

conditions and instructions governing the use of a product category or a specific listed product are published in their lists [28, 29]. For example, a part of the text for "wire connectors"

states "wire connectors are listed for No. 18 AWG or

larger copper conductors and/or No. 12 AWG or larger

aluminum conductors. They are for use in accordance

with the National Electrical Code."

The text, listing a type of receptacle described in subsection 10.2. of this report, states: "Receptacles for factory assembly on type NM or NMC cable in mobile

homes without a separate outlet box, 15a, 125v, grounding type Type HW-1."

4. Other Electrical Connection

Specifications, Standards and Tests

The state of the art concerning the requirements for and the evaluation of branch circuit electrical connections in the United Slates is represented by the National

Electrical Code and appropriate standards of Underwriters' Laboratories as discussed in subsection 3.2., 3.3. and 6.1. of this report. Other standards, specifications, and test methods which may have some application are listed below.

4.1. American National Standards

The C73 series of American National Standards specifies dimensions and configurations of caps, plugs, and

receptacles [31]. The other American National Standard on electrical connections concerns highly reliable soldered connections in Electronic and Electrical Applications [32].

4.2. Federal SpecilBcations

The Index of Federal Specifications [33] lists two

electrical connector specifications [34 and 35]. In the context of these Federal Specifications, attachment

plugs and receptacles are connectors and the connection

between these devices is the primary concern. The

permanent connection between a receptacle and the branch circuit wiring is not specifically addressed.

4.3. Military Specifications

There are over 900 military specifications for electrical connectors [36]. Most of these are for very

special or very limited applications. References 37

through 41 list some of these specifications. None of these specifications is concerned with electrical connections in branch circuit wiring. A number of test methods to determine various properties or characteristics are described. These tests appear to have little application to this project.

4.4. National Electrical Manufacturers'

Association

Standards of the National Electrical Manufacturers'

Association concerning electrical connectors generally

involve connections using larger wires and applications other than branch circuit wiring. Reference 42 on

Electric Power Connectors defines and describes various

types of electrical conductors. Test methods are not

described.

4.5. American Society for Testing and

Materials

Reference 43 describes standard methods for measuring contact resistance of electrical connections (static contacts). (See subsection 8.1. of this report.) Contact

resistance is not measured by Underwriters' Laboratories in their test procedures.

5
5. Field Performance of Electrical

Connections

An attempt to determine the field performance of

electrical connections is represented by Reference 9. U.L. responded to the aluminum wiring problem [6, 7]

by sending 1000 questionnaires to industrial users of

electrical equipment and 11,500 questionnaires to elec- trical contractors and electrical inspectors. The response from industry was 17 percent and from contractors and

inspectors was 13.5 percent. The survey is briefly summarized as follows:

(a) There is a usage of copper to aluminum of 4 to 1. (b) Estimated causes of failure and percent occur- rence : Loose connections 54%

Design 16%

Environment 14%

Improper Metals 9%

Overload Conditions 6%

(c) 71 percent of the failure problems were corrected by replacing damaged parts and 22 percent were cor- rected by replacing wire. A second list of more detailed questions was sent by U.L. to 194 electrical inspectors who provided the following information:

(a) Loose connections are caused by failure to tighten screws (workmanship) and/or small bolts, small screwdriver slots and weak connector metals (design).

(b) Wire, particularly aluminum wire, often breaks

when being bent to fit into available wiring spaces.

(c) Corrosion due to improper use of dissimilar metals or to wet or damp environments contributed to connector failures. (d) Consideration of the properties of thermoplastic

insulation is necessary to assure that involvement of the insulation in an overheating problem will not be a contributing cause to more rapid deterioration of the connection itself.

test results. Therefore, predictions of durability involve

judgmental factors which are based on changes in properties of materials relative to how these materials

are installed in systems and the subsequent service conditions of the systems.

6.1. U.L. Accelerated Tests

In the United States acceptance testing of conventional electrical connections in branch circuit wiring is principally the accelerated tests performed by Underwriters' Laboratories [44]. Specific tests include (1) heat cycling with wire disturbance, (2) heat cycling

with vibration, (3) environmental and (4) stripping

torque.

6.2. Seven-Year Tests in the United Kingdom

British Insulated Callender's Cables Ltd. (BICC) per- formed tests (high currents, cyclic loading) on con- nectors not commonly used in the United States or Canada, over a period of seven years [45]. These tests compared the results of using solid copper, copper-clad

aluminum and solid aluminum wire.

a. High Current Tests The effects of high currents on standard connectors and wire were determined by measuring the time to cause failure in typical simulations. Connector temperatures were measured with themocouples and failure was defined as the time taken to reach 175 °C. Figure 1 shows current (in amperes) plotted against

the time to failure (in hours) for a circuit. The dotted

portions of the curves are extrapolated. Figure 2 pro- vides data for a lighting circuit. The significance of these curves is that they show

the relative durability of copper, copper-clad aluminum,

and solid aluminum wire as a function of wire gage, current rating, and failure. Thirty thousand hours are considered to be equivalent to a 20-year life for wiring.

6. Electrical Connection Research

and Testing

Branch circuit electrical ^connection research and

testing consists principally of U.L. accelerated tests, seven years of testing by the United Kingdom, and

unpublished work at the Battelle Memorial Institute. The goals of accelerated testing are (1) to induce, in a much shorter period of time, changes in properties

representative of those caused by long-term service,

and (2) to relate the time required to induce a property change to that required by long-term service. The second goal of accelerated testing is seldom fulfilled and the lack of an accurate time relationship

between the results of accelerated testing and long-term

service makes difficult the interpretation of accelerated

AMPS ' 1 1 1 III RING (MAIN CIRCUIT 1

1 — -^^Amm^

1 (11 1/4 AWG) COPPER CLAD ALUMINUM |

, 1

1 --^ COPPER RATING 1 ALLOWABLE ^ ] ~* - -{30 AMPS

4fn mZ 1 1 1 1/4 AWG) ALUMINUM^""^——_

"~ "^22 AMPS

116 AMPS

30.000h

1 1 iiiiiil 1 1 IiimJ 1 1 iiiiiil 1 I illllJ 1 1 llliill 1 ilJUllL 0.1 1.0 K) WO 1.000 10.000 100.000 TIME TO FAILURE: HOURS

Figure 1. Twenty-year circuit ampacity—ring main circuit (branch circuit).

6
AMPS

LIGHTING CIRCUIT

30,000 h 1.5mm'= ( 151/2 AWG) COPPER

1.5mm2 COPPER CLAD ALUMINUM

I Smm'^ ( 151/2 AWG) ALUMINUM ^t.-

-—J,

lis AMPS

I I IIMIll I I NjiiiI I I I I II

|10 AMP»

1 1 mill I I L iiiiil ill I 10 10 100 1.000 TIME TO FAILURE: HOURS

10.000 100.000

Figure 2. Twenty-year circuit ampacity—lighting circuit.

35

30

PATTERN OF FAILURE RING MAIN CURRENT 30 AMPS

I r.

\ \ FAILURE RATE

IN EACH PERIOD

~-4mm2 ( 1 1 1/4 AWG)

ALUMINUM

4mm2 (11 1/4 AWG)

COPPER CLAD ALUMINUM

.2 5mm2(13 1/4 AWG) _ 3%,_j/^°''''"

mJ I 1 1 mill n^i imil i 1 1 iiml i i 1 1 mi

1,000 10,000 LOADING HOURS

100,000

b. Cyclic Loading Tests

Cyclic loading tests were performed on groups of connectors wired as a ring main circuit with a terminal

box between connectors so they could be isolated and

replaced. A daily check was made on temperature rise, and a connector was considered to fail when it reached

a temperature of 175 °C. Figure 3 shows the relative durability of connectors wired with copper, copper-clad aluminum and solid aluminum as a function of cyclic loading at 30 amperes. The failure rate of aluminum appears to be clearly

unacceptable under these conditions, and the failure rates of copper as well as copper-clad aluminum are questionable.

Figure 4 represents a more practical service condition

with cyclic loading at 22 amperes representing typical use of a receptacle in the home. The failure rate of aluminum appears to remain unacceptable under these

test conditions while copper-clad aluminum as well as copper exhibited no failures. The BICC work points out the need for long-term

testing of electrical connections with potential con- ductor materials to establish the validity of acceleration

factors. Figure 4 probably represents 6- to 7-year per- formance for the designs tested. Data is needed for long-term performance of different designs to prove the durability of those designs. Long-term performance

testing is especially needed where replacement provi- sions are not made.

6.3. Battelle Memorial Institute Research

(BMI)

Research began in 1970 at the Battelle Memorial

Institute in response to field service problems arising

from the use of aluminum wiring with conventional

connectors. The results of this work have not been

published, because it has been funded by private in- dustry sponsors who wish the information to remain

proprietary at the present time. This research represents

the most significant research effort in the United States on electrical connectors and includes stress relaxation,

thermal, and environmental performance testing.

Figure 3. Service reliability test (the number of aluminum

connections failed versus the hours energized—10 percent failed

after 100 hours, 27 additional failed after 1000 hours, and 23

percent additional failed after 10,000 hours). The number of copper connections failing is 3 percent after 1000

hours.

PATTERN OF FAILURE RING MAIN CURRENT 22 AMPS

3S

O 25 -

10

5

NIL

FAILURE RATE

IN EACH PERIOD _ 24%

8%

I'll

4mm' (11 1/4 AWG)

ALUMINUM

4mm2 (11 1/4 AWG)

'COPPER CLAD ALUMINUM

>mm2(13 1/4 AWG) COPPER"

NIL FAILURES

III II I lli'il I I ' lliiJ III liiii 10 100 1,000 10.000 100.000 LOADING HOURS

Figure 4. Service reliability test (the number of aluminum

connections failed versus the hours energized—8 percent failed

after 100 hours and 24 percent after 10,000 hours). Copper and copper-clad connections did not fail after 10,000 hours

energized at 22 amperes.

7. Failure Mechanisms

There does not appear to be any published research information which substantiates causes of failures in

electric connections. An evaluation of available infor- mation (unpublished and published) indicates that

failure might occur as a result of one of the three postulated mechanisms listed below.

7
7.1. Resistance Heating

Fire at electrical connections might be started by

resistance heating in the wire-to-connector joints. Resistance heating may be caused by the presence of in- sulating layers in the joints. The insulating layers may

form during power operation by a combination of low

contact pressure in the joint and exposure of the joint

to elements in the atmosphere which form insulating

layers. Overheating accelerates the rate of formation

of insulating layers and hence the rate of increase in the resistance of the joint, leading to additional over- heating. This is a runaway phenomenon. Overheating

might occur to such an extent that combustible material

could ignite.

7.2. Loose Connections—^Arcing

A loose connection may result in disconnection of a

circuit but may also result in overheating. A potential

fire hazard is created by arcing in a loose connection.

Temperatures sufficiently high to melt metals in the

joint may be reached. Accident investigations indicate

that molten metal has been ejected explosively from

high-voltage motor control centers as electricians opened

the door to examine the cause of noises in a panel box.

Two theories can be expressed which might account

for such an incident, as follows:

a) Hydrogen

Excessive temperatures in an electrical connection

could generate hydrogen chloride by the decomposition of plastic insulation containing chlorine,

with subsequent liberation of hydrogen by reaction

of hydrogen cholride with metals. Hydrogen might

also be produced by the electrolytic decomposition

of water. Hydrogen gas might be ignited explosively by an arc in the electrical box.

b) Metal/ Oxygen Reaction

Metal might react explosively with a sudden flow

of oxygen into an electrical box after arcing has

burned all of the oxygen in a box and removed

oxide films from the conductor metals. A metal

stripped of its oxide film might behave like a pyrophoric material. The tendency for such a reaction to occur increases as the metal changes

state from solid to liquid vapor.

7.3. " Wire Breakage

In the process of making electrical connections, the wire in the vicinity of the connection may receive

stresses which may be a factor in subsequent wire

breakage. Subjecting an electrical connection to vibration might increase the wire stresses sufficiently to cause wire breakage or reduce the wire cross section enough to result in overheating. Bending and pulling the wire during installation of connections could result in wire breakage or overheating.

8. Properties and Behavior of Materials

Used in Electrical Connections

In conventional systems used in the United States, the current path at the interface in an electrical con- nection is established mechanically and maintained by

a "wire-nut", binding head screw, or other means.

Properties discussed in this section assume mechanically established connections. (See 9.2. b. for a discussion

of metallurgical bonding.)

8.1. Contact Resistance

Establishing and maintaining a low contact resistemce

is the principal property necessary for a successful

electrical connection. Reference 43 states

"Contact resistance is a measure of the functional extent of true metallic contact area that has been achieved in making

an electrical connection. High contact resistance indicates poor utilization of the apparent contact interface area with

relatively small areas of true metallic contact. Zero contact

resistance is achieved vfith full utilization of the clean contact

interface. Acceptably low values of contact resistance are

attained with less than complete metallurgical bonding of the

contact interface. The practical evaluation and comparison of

electrical connections depends in large part on their contact

resistance characteristics." As contact resistance increases, attributes which are

directly necessary for the satisfactory performance of

electrical connections, particularly "fire safety" (temperature rise), are greatly affected. Much of the other

information discussed in this section and in section 9, Materials Property Problems in Electrical Connector

Design, concern the problem of maintaining low contact

resistance throughout the life of an electrical connection.

8.2. Surface Films

As indicated by Fan [45], surface films which are always present on exposed surfaces of the wire can have

much higher resistivities than their base metals, so con- duction paths must be created through these surface

films or layers when making a connection.

The mechanism which permits current conduction

through the oxide layer on copper is designated as

"fritting" by Holm [46]. Fritting is defined as a mild

electrical breakdown of surface film during current

flow. Reference 45 emphasizes that the fritting voltage

for a 100 angstrom layer of copper oxide is less than

0.0001 volt, while the fritting voltage for the same

thickness of aluminum oxide is 40 volts. Because of its high fritting voltage, aluminum oxide

may interrupt the current path. This potential interruption has resulted in special wire/connector designs,

termination techniques, and performance testing for

electrical connections terminating aluminum wire. Virgin metal-to-metal contact should be established

and maintained between aluminum wire and other conducting components in the continuous current path

of an electrical system if efficient electrical function

and safety are to be achieved under required service

conditions.

8
Virgin metal-to-metal contact is reported to be not as important for the fire safety of copper wiring systems

because of the low fritting voltage of copper oxide. However, other surface films caused by corrosion might

cause overheating in copper wire connections [47].

The significance of an insulating surface layer to the performance of an electrical connection is repre- sented by the curve in Figure 5 [48]. Williamson ex- plains that as long as the resistance of the joint remains

small the temperature-dependent processes occur slowly, but when the constriction resistance increases by

slow reoxidation at the interface, the temperature rises and this accelerates the reoxidation. The data for Figure 5 were derived by measuring the overall re- sistance, the current (I), and the temperature of an

electrical joint of two aluminum conductors. Williamson

then estimates the constriction resistance (Ro) as the

difference between the observed resistance and the

resistance of an equivalent length of conductor of the same diameter as the joint. Molten aluminum and high

temperatures in the electrical joint present the potential for unacceptable fire safety. Williamson concludes:

(1) that the interfacial temperature in a well-made joint

under normal operating conditions is only a few degrees

Celsius above the bulk temperature, and (2) uncertainties about the constriction resistance are due to a lack

of knowledge of the way contact spots are distributed over the joint interface. Figure 5 shows the temperature in a compression

joint between two aluminum conductors in an ambient

temperature of 20 °C. The abscissa has been plotted in terms of the product of the constriction resistance,

measured with a low current at 20 °C and the current through the joint.

Ol I 1 I I I 0 20 40 60 80 100

iRo . 'Millivolts

Figure 5. Relation between the temperature created at the

interface of a joint and the potential difference across the

restriction resistance [48].

8.3. Effect of Contact Pressure

Reference 45 describes a continuous current path

(metal-to-metal contact) through insulating surface

layers in wire-to-connector interfaces. Low contact re- sistance was achieved by the application of high conRESISTANCE (Ohms)

Figure 6. Contact resistance versus contact pressure for electrical connections wired with the indicated conductors.

tact pressures as shown in Figure 6. Figure 6 shows

that electrical connections wired with copper and cop- per clad aluminum exhibit low contact resistance at much lower contact pressures than electrical connections wired with aluminum.

8.4. Creep and Stress Relaxation

Once a continuous low resistance current path has

been established, it must be maintained under various

service conditions. Creep and stress relaxation present a special problem

relative to maintaining a continuous current path over an extended period of time. "Creep" and "stress relaxa- tion" are names for the same property of metals under

different conditions. Creep is an increase of strain at a

constant stress. Relaxation is a decrease of stress at constant strain. The high contact pressure which is needed to establish metal-to-metal contact applies stress to the wire. Certain metals when under stress in a

connector, can flow to reduce the stress. This flow

results in a decrease in contact pressure and a possible

increase in the oxide surface layer, the contact resist- ance, and the temperature of the interface. This

mechanism is referred to as cold flow, creep, or stress relaxation. Creep occurs at stresses below the )ield

strength of the metal, and the rate of creep increases

with increasing temperature.

Creep of metals such as copper and steel is reported

to be not significant at temperatures commonly exhibi- ted by electrical connections.

Some aluminum alloys which were used in electrical systems prior to 1973 exhibit a significant amount of creep or stress relaxation. In conventional electrical connections aluminum wire might exihibt creep to such an extent that the increase in interface temperature is accelerated by the cumulative mechanism of contact pressure decrease, oxide layer increase, and

9
contact resistance increase. The result is overheating

and eventual failure of the connection [45].

8.5. Thermal Expansion and Contraction

Thermal expansion is important in electrical connections when there is a significant difference between the properties of the wire and the connector to an over- heating mechanism.

Thermal expansion is also important when wire is improperly installed with a heat-producing gap between

the wire and the connector. The ratio of the coefficients of thermal expansion of the steel connector and the wire is about 2 to 1. If the temperature of the connection increases by some

heat-producing mechanism, then differential thermal

expansion will lead to increased contact resistance and

a further increase in temperature. The wire will tend

to expand at a greater rate than the connector while current is flowing, but because of its being confined

by the connector it will deform plastically in such a way that on cooling it will have a reduced diameter.

During periods of no current flow, the connection cools and the contact pressure at the interface between the wire and the connector decreases as the metals contract. This mechanism may lead to excessive overheating and

an accelerated destruction of the connection if the connection is current cycled.

8.6. Diffusion Across Material Boundaries

Diffusion is the spontaneous movement of atoms or molecules within a material or across material boundaries. The importance of diffusion in the long-term per- formance of electrical connections involving dissimilar metals is the change in properties that rhay result over an extended period of time. The rate of diffusion in- creases with the temperature.

Consider a connection with zinc-plated steel connector

plates and aluminum wire. Such a connector is common

because zinc is applied to steel to impede corrosion of the steel. If overheating has increased the temperature of the connection, then zinc may diffuse more rapidly into the aluminum wire. The newly-formed alloy of alumi- num and zinc would have an increased electrical re- sistance which might contribute to an accelerated

failure through overheating.

8.7. Strength, Ductility, and Hardness

The mechanical properties of copper and aluminum

wire are a function of composition and work hardening.

Composition and processing variables establish the

strength, hardness, and ductility of the wire. Strength

and hardness increase and ductility decreases with

increasing applied stress. The significance of these properties is that a wire may be so weak that it will

fail with a small amount of applied stress or it may be

strong and "brittle" so that it will break in bending

during installation. The optimum, as-fabricated, properties for aluminum wire as specified by U.L, are a tensile

strength of 15,000 to 22,000 psi and a minimum

elongation of 10 percent [8].

These properties are also important after termination

in a connector because contact pressure can increase

the hardness and decrease the ductility of a wire to such an extent that th^- wire can break on bending

after installation.

8.8. Electrical Conductivity

Electrical conductivity measured in percent lACS

(International Annealed Copper Standard) is the property of a conductor which indicates the electrical effi- ciency of the flow of current. Electrical conductivity is the reciprocal of electrical resistivity. The electrical conductivity of aluminum and copper wires decrease

as alloys are modified so as to improve properties such

as creep, stress relaxation, and ductility, which are

related to durability.

9. Materials Properties Considerations

in Electrical Connector Design

The purpose of this section is to show how problems

with the performance of electrical connections may be

minimized by taking materials properties into account

in connector design.

9.1. Surface Layers on Wire

One approach to the establishment of a continuous

current path through surface layers in wire to connector

interfaces requires a special geometry in connector

design. This approach involves the use of indiumplated connector plates which contain small indentations on their surfaces which contact the wire. When

pressure is applied to the binding head screw of this design the surface layer on the wire is broken as the wire metal is extruded into the indentations making

metal-to-metal contact with the connector plates. It is not known what contact pressure is required to achieve

metal-to-metal contact by this method, but an ad hoc

committee sponsored by U.L. specifies a torque of 12 pound-inches [7].

The purpose of the indium is twofold, first to flow through the breakage of the surface layer and then to inhibit environmental reaction at the connector inter- face by acting as a barrier to elements in the atmos- phere during subsequent operation of the electrical system.

9.2. Creep and Stress Relaxation

There are essentially three approaches to the problem

of creep and stress relaxation, which are in the areas of connector design, metallurgical bonding, and materials

development.

a. Connector Design Approach

A connector might be designed with spring tension

or the equivalent to maintain contact pressure between

the connector and the wire.

10
b. Metallurgical Bonding

The detrimental interfacial surface layer which can form in a mechanical electrical connection could be

eliminated by metallurgically bonding the conductor

wire to the connectors. Soldering is the only process used in the United States. Possible improvements in branch circuit electrical connections lies in the fields of welding, cold forming,

and explosive bonding. It appears that practical appli- cation of these processes could only be done in a factory

environment because of equipment requirements.

c. Wire Alloy Development

Nine wiring manufacturers have developed aluminum

alloys for electrical wiring which have been accepted

by Underwriters' Laboratories as indicated by Refer- ence 8. It should be noted that as of March 29, 1973,

this acceptance excludes many of the alloys which are

presently installed in homes.

d. Copper Cladding

Aluminum wire can be clad with a metal such as copper to overcome the property changes which result in a thermal failure mechanism. Reference 45 provides

the information on copper cladding. Essentially, copper

cladding of aluminum wire provides an electrical bond

at terminations and prevents the increase in contact

resistance initiated by aluminum oxide which triggers

creep, overheating, and eventual destruction of an

electrical connection. Copper-clad aluminum appears to perform nearly as well as solid copper. A failure mechanism can be postulated for clad aluminum if the coating is perforated and exposed to moisture. Under

these conditions the wire would tend to corrode at an

accelerated rate and promote overheating and wire

breakage.

e. Nickel Plating

The same performance comments that apply to copper-clad aluminum also apply to nickel-plated alumi- num. The advantage of nickel-plated aluminum wire is

its reported lower cost compared to copper, copper-clad,

and the new U.L. listed aluminum alloys.

10. Re6ent Innovations

The principal developments of innovative electrical connections for branch circuit wiring in housing are described below. Information concerning innovative

electrical connections is based on information published

by the organizations developing the products described,

in engineering or test reports concerning these products, on examination of the products and on discussions

with people concerned with their development.

10.1. Electrical Harness

For the Operation BREAKTHROUGH program, an

electrical harness for use in branch circuit wiring was

developed. The basic idea was the adaptation of the

principles of automobile electrical harnesses (12-volt

systems) to electrical systems in housing (nominal 115- and 230-volt systems )

.

Each harness consisted of a central junction box or

circuit breaker panel and a network of legs of pre- determined length using outlets and switches specifically

designed for attachment in controlled, high-volume,

factory assembly lines. The junction boxes were lo- cated at ceiling fixtures or other convenient access points such as closets, utility or service areas. A housing module electric power distribution system consisted of one or more spider harnesses installed at the housing producer's plant.

The system used conventional materials and components. Conventional non-metallic sheathed cable was

used. The switches and outlets were of conventional

design insofar as switching actions and face configurations of receptacles are concerned. The unique characteristics of the switches and outlets were the wire was factory terminated and an integral junction box

was used. The switch and outlet designs included a back case cover, made of electrical insulation material,

which functioned as an integral part of the junction box. The principal waivers of the National Electrical Code provisions which were necessary to permit the use of this harness were those requiring a specific

volume of "free space" within the box. (See sec. 3.2.b. of this report.) In this system all electrical connections were made

in the factory producing the electrical harness. In the housing producers' factory, only mechanical connections were necessary. Portions of U.L. Standards 20

(Snap Switches), 498 (Attachment Plugs and Receptacles) and 514 (Outlet Boxes and Fittings) were used

as a guide in the acceptance testing of the harness

system [24, 25, 23]. The test results, which included

sixteen different types of tests, appeared to be satis- factory when compared to criteria established by

Underwriters' Laboratories for similar conventional

components. The tests were actually of the harness and

not specifically of the electrical connections.

The economic justification for this innovative elec- trical harness was the saving of labor in a housing

producer's factory. Estimates were made that the costs of materials, if there were volume production, would

be about one-third more than materials in a conventional electrical system. However, it was estimated that

installation labor would be cut, possibly by as much

as 70 or 80 percent.

The question of replaceability in the event of failure of one of the components such as a switch or receptacle was considered. It was determined that components

could be replaced with reasonable effort with corres- ponding conventional devices as long as sufficient slack was left in the cable. A minimum of 12 inches for each

device or for each leg was recommended.

10.2. Split-beam Receptacles

The most successful innovation concerning electrical connections in branch circuit wiring up to the present

time has been split-beam receptacles. Non-metallic

sheathed cable is prepared by a special tool which cuts the insulation but not the wire, and positions the wire

in the proper position for quick placement into one

11
piece of a two-piece receptacle. The special tool (cost- ing about $50.00) is then used to clamp the receptacle

together making a non-visible electrical connection

within the receptacle.

These receptacles have, within the last year, received

widespread use in mobile homes and limited use in factory-built and conventionally built homes. National

Electrical Code provisions concerning the box (see

3.2.b.) have been the principal reasons for their nonuse in housing. Special NEC exceptions for mobile

homes and for re-wiring in existing buildings have

been taken advantage of for its use in limited appli- cations. (See appendix.)

These receptacles, which do not employ a separate

"box," cost more than conventional receptacle assemblies. The economic justification for this device, however, is the saving on installation labor, either in a housing factory or in conventionally-built housing.

Underwriters' Laboratories has issued a fact-finding

report on this device. Underwriters' subjected this device to the tests which they believed relevant. Tests used for conventional devices, such as U.L. 498 on

Attachment Plugs and Receptacles [25] were, in general, used as a guide. U.L. also subjected these devices

to the vibration test outlined in [27] to simulate the

vibrations encountered from movement of land trans- portation vehicles. Tests to simulate time were not made. As a result of the tests, U.L. has listed these devices for applications permitted by the National

Electrical Code [5].

The question of replaceability was examined by

Underwriters' and it was determined that these devices

could be replaced by a conventional receptacle with

reasonable effort as long as there was sufficient slack

left in the cable during installation. It would be difficult to get less than 18 in of slack if the installation were

done in accordance with recommended procedures

because of the amount of cable needed to operate the

special tool. Development work is being done on a replacement

receptacle which would not require a special tool for

its installation. Development of split-beam switches,

lighting fixtures, and appliance mountings on the same

general principles used in the innovative receptacle is underway. Completion of this work with a satisfactory

evaluation by proper authorities would permit the use

of this system of electrical connections for nearly all normal applications in branch circuit wiring.

10.3. Quick Splicing Connectors

Quick splicing connectors were developed by at least two manufacturers in response to Operation BREAKTHROUGH opportunities.

The intent was to use these connectors without boxes. Otherwise it was contended that the innovations would

not be feasible or competitive. However, for lack of

technical knowledge these connectors were permitted

to be installed only in accessible enclosures which

contained no combustible materials. Because of this and construction schedules, the Operation BREAKTHROUGH housing producers did not use these connectors' and installed conventional wiring systems.

10.4. Baseboard Receptacle System

An innovative wiring system was developed on an

Operation BREAKTHROUGH Type B contract [49].

This development was concerned primarily with an

economical way of providing more electrical receptacles

in each room of a building. The principal features of

this development concerned the connection of attach- ment plugs to receptacles and not permanent connections in branch circuit wiring. However, this system

would eliminate some connections which would or- dinarily be made in branch circuits. Only prototypes

of this system have been developed.

11. Research Considerations of

Innovative Electrical Connections

Tests have been run (see subsection 10.2. of this report) on innovative concepts which appear to meet

or exceed the acceptance requirements of Underwriters'

Laboratories.

The principal reason preventing the application of these innovative systems in the field is the potential use of wiring materials other than copper and the lack

of durability data. The technical factors of greatest importance to the durability performance of innovative electrical connections are temperature-increasing mechanisms and

materials properties which will result in failures of the continuous current path.

Adequate metal-to-metal contact should be established during installation and maintained during service to minimize temperature increases. Materials for a mechanical-type electrical connection

should be selected with the required strength and

ductility to prevent breakage during installation and

service. These materials should also exhibit properties which

will not increase the temperature of the electrical con- nection above an acceptable operating level. Metal-to-metal contact can be measured directly by

destructive examination. However, destructive examination can be useful when related to non-destructive in- direct performance measurements.

Contact resistance can be measured electrically and

can be related to joint temperatures. More information

is needed concerning contact resistance, contact temperture, contact pressure, and metal-to-metal contact with time. A method is needed to measure the contact temperature at the interface between the wire and the connector. The development of the above measurement tech- niques would permit the indirect determination of metal-to-metal contact and contact temperatures during

the performance testing of electrical connections under

cyclic load. Measurement techniques should be first developed

and then applied to the performance testing of innovative electrical connections. The following performance

testing is needed:

1. Accelerated tests should be performed to provide

data which can be related to the performance of

electrical connections under real life conditions. The

12
basis for acceleration factors, such as cycles, cycle

times, current overload, ambient temperature, and

aggressive environment, is not presently known.

2. Long-term (10,000 hours) cyclic loading tests at currents typical of household use should be performed

to establish data for the performance of electrical con- nections under real life conditions.

3. Accelerated environmental tests should be per- formed in aggressive laboratory environments to pro- vide data which can be related to the performance of

electrical connections exposed to environments in the

field (various geographic locations) such as marine,

industrial, et al. The relationship between accelerated environmental results and field environmental results

is not known.

4. Long-term (10,000 hours) exposure of electrical connections to field environments should be performed

to provide data which can be related to accelerated environmental test data. This research is needed to establish the validity of acceleration factors and to enable the prediction of the long-term thermal performance of innovative electrical connections based on the results of accelerated per- formance tests. Additional research is needed to develop perform- ance tests which will ensure the structural integrity of

electrical connections relative to strength and ductility

requirements.

12. Summary

1. In the United States the principal basis for the evaluation and approval of electrical connections in branch circuit wiring is the judgment of local electrical inspectors; these are men with practical experience in conventional electrical construction.

2. The National Electrical Code [5] is the principal

guide covering the installation of electrical wire and

equipment, including the requirements for and the methods of making field electrical connections; unless an innovation can be installed in accordance with the NEC, it will be unable to attain widespread use in the market place.

3. Underwriters' Laboratories requirements and tests are the principal criteria applying to the components

interfacing with branch circuit electrical connections.

4. As a result of the use of aluminum wire, research and proposed performance-test programs concerning

the failure and hazard mechanisms of branch circuit

electrical connections, including some durability as- pects, are now under way at Underwriters' Laboratories

and Battelle Memorial Institute (see subsections 6.1. and 6.3. of this report)

.

5. In the construction of both conventional and fac- tory-built housing the traditional system of installing

electrical boxes at switch, outlet and junction points, the mechanical fastening of electrical cable to boxes

and the making of electrical connections in boxes, are required by the National Electrical Code and most

local jurisdictions. It is the requirements for "boxes"

and not specifically the requirements for electrical con- nections which appear to constrain innovation in branch

circuit wiring systems (see subsection 3.2.b.).

6. There are some exceptions in the National Electrical Code to the requirements for "boxes." These

exceptions apply to mobile homes, to exposed cable wiring and to concealed work in rewiring existing

buildings.

7. Electrical connections have failed under field service conditions and some of the overheating-type failures are believed to have been the caus«j of fires. Wire

breakage failures have occurred as a result of wire

being bent during installation.

8. Some failures are believed to have been caused

by abnormal temperature increases of wire-connector

interfaces. The temperature increases are caused ljy a variety of short and long-term mechanisms which

increase the contact resistance at the wire-connector

interfaces.

9. Performance tests are needed which measure the properties of electrical connections as they relate to durability and overheating aspects.

10. Measurement techniques are needed to establish a quantitative relationship between contact resistance, contact temperature, contact pressure, and metal-to- metal contact with time under cyclic loading conditions.

11. Data are needed to establish the validity of acceleration factors and to enable the prediction of the long-term thermal performance of innovative electrical connections based on the results of accelerated per- formance tests (see section 6 and 11 of this report).

12. Metallurgical bonding represents an innovative

approach which may avoid the thermal failure potential

problem inherent in mechanical-type electrical connectors.

13. Considerable uncertainty is associated with an

evaluation of the durability of electrical connectors,

because very few applicable data are available and

much of the available information is proprietary.

13. Recommendations

1. Initiate a research program to develop information and tests on the performance of innovative elec- trical connectors. This program would consist of the following parts:

(a) Establish an interface with organizations con- ducting on-going electrical connector testing to obtain information and perform routine testing.

(b) Develop techniques for measuring the properties of electrical connectors with time under cyclic

loading conditions.

(c) Study the validity of acceleration factors to en- able the prediction of the long-term thermal perform- ance based on short- and long-term data.

2. Obtain information on metallurgical bonding to permit an evaluation of this innovative approach.

3. Develop interim performance criteria and tests. 4. Make a study of the technical criteria and para- meters concerning the necessity of "box" requirements

for branch circuit wiring.

5. Make a study concerning the parameters, the per- formance criteria and the feasibility of non-accessible

electrical connections in housing.

13
14. References

[1] Guide Criteria for the Design and Evaluation of Operation BREAKTHROUGH Housing Systems—Volume 1 —Multifamily High Rise, December 1, 1970.

[2] Guide Criteria for the Design and Evaluation of Operation BREAKTHROUGH Housing Systems—Volume 2 —Multifamily Low Rise, December 1, 1970.

[3] Guide Criteria for the Design and Evaluation of Operation BREAKTHROUGH Housing Systems—Volume 3 Single Family Attached, December 1, 1970.

[4] Guide Criteria for the Design and Evaluation of Operation BREAKTHROUGH Housing Systems—Volume 4 —Single Family Detached, December 1, 1970.

[5] National Electrical Code—1971 ANSI CI-1971, NFPA

#70-71.

[6] Quinn, G. C, Aluminum Wire for Branch Circuits: Past Problems^ But Bright Future, Electrical Construction and Maintenance, April 1973.

[7] The Use of Aluminum Conductors with Wiring Devices in

Electrical Wiring Systems—A statement prepared under

the auspices of an Ad Hoc Committee sponsored by

Underwriters' Laboratories, Inc., dated March 1973.

[8] List of Recognized Aluminum Conductor Material (ACM)

for use in Insulated Wire and Cable Employing Solid Aluminum Conductors, No's 12-8 AWG, U.L. memo

dated 3-29-73 on subjects 4, 44, 83, 468, 493, 719, and

854.

[9] Summary of U.L. Fact Finding Study, Terminals for Aluminum Conductors in Electrical Distribution Equipment, 3-31-70 memo to J. Kark of NEMA from G.

Schall of U.L.

[10] 1973 Guide to the Electrical Contracting Market and Profile of the Electrical Contractor. Special Publication of Electrical Contractors (National Electrical Contractors

Association )

[11] Fires and Fire Losses Classified, 1971 Fire Journal (NFPA) September 1973.

[12] Accident Facts, National Safety Council, 1972.

[13] Why Non-Metallic Sheathed Cable Should Be Prohibited,

Orange County Chapter, National Electrical Contractors

Association 1971. [14] Electrical Code for One and Two-Family Dwellings

NFPA No. 70A-1972, ANSI C.l. 1-1971.

[15] National Bureau of Standards Handbook H30, National

Electrical Safety Code (Parts 1-5) March 1948. [16] National Bureau of Standards Handbook 81—Rules for the Installation and Maintenance of Electric Supply and Communication Lines—Comprising Part 2, the defini- tions and Grounding Rules of the sixth edition of the National Electrical Safety Code, November 1, 1961, (ANSI C 2.2-1961).

[17] NBS Handbook 110-1—National Electrical .Safety Code,

Part I, Rules for the Installation and Maintenance of Electrical Supply Stations and Equipment, June 1972 (ANSI C 2.4-1971).

[18] National Electrical Safety Code, Part 4, Rules for the Operation of Electric Supply and Communication Lines and Equipment (ANSI C 2.4^1972). (Being published

by the Institute of Electrical and Electronic Engineers.)

[19] National Electrical Code Handbook, Abbot and Steka, 1962 (McGraw-Hill Publishing Company).

[20] U.L. Standard 719—Non-Metallic Sheathed Cables (De- cember 1970).

[21] U.L. Standard 4—Armored Cable (November 1970).

[22] U.L. Standard 486 (June 1969), Wire Connectors and

Soldering Lugs.

[23] U.L. Standard 514 (May 1971), Outlet Boxes and Fittings. [24] U.L. Standard 20 (March 1971), Snap Switches.

[25] U.L. Standard 498 (November 1970), Attachment Plugs and Receptacles.

[26] U.L. Standard 57 (August 1972), Electric Lighting Fixtures. [27] Vibration Encountered from Movement of Land Transportation Vehicles (MIL STD 202D, Method 201A).

[28] Electrical Construction Materials List, Underwriters' Laboratories, June 1972.

[29] Electrical Appliance & Utilization Equipment List, Underwriters' Laboratories, June 1972. [30] Quarterly Supplement to Annual Product Directories, Underwriters' Laboratories, November 1972.

[31] Dimensions of Caps, Plugs, and Receptacles (various de- vices of different types and ratings—different dates), American National Standards Institute. [32] Highly Reliable Soldered Connections in Electronic and

Electrical Applications, Criteria for Testing and Inspection, ANSI C 99.1-1966.

[33] Index of Federal Specifications and Standards, January 1, 1973, (FPMR 101-29.1, 41-CFR 101-29.1) General

Services Administration. Federal Supply Service, Interim Federal Specification. [34] Federal Specification—Connector, Plug, Electrical, Connector, Receptacle, Electrical W-C 00596C, October 21, 1970.

[35] Federal Specification, Connector, Plug, Electrical, Connector, Receptacle, Electrical W-C 596A, July 7, 1964.

[36] Index of Military Specifications and Standards, Depart- ment of Defense, July 1, 1972.

[37] Connectors, Electric, AN TYPE, General Specifications

for Military Spec, MIL-C-008015 (NAVY), October 8, 1971.

[38] Connectors, Electrical, Printed Wiring Board, General

Purpose, General Specifications for, MIL-C-21097L, July

14, 1970.

[39] Connectors, Electrical, Circular, Multi-contact, High-Environment, Quantitative Reliability, General Require- ments for, MIL-C-38300A (USAF), November 19, 1964,

(later amendments).

[40] Connectors, Plugs and Receptacles, Electrical, Waterproof, Quick Disconnect, Heavy Duty Type, General

Specifications for, MIL-C-22992D, February 2, 1972. L41] Connectors, Electrical, Environment Resistant, for Use

with Flexible Flat Conductor Cable and Round Wire, General Specification for MIL-C-55544A, June 25, 1971. [42] Electric Power Connectors, Standard Publication CC-1-

1972, National Electrical Manufacturers Association.

[43] Standard Method for Measuring Contact Resistance of Electrical Connections (Static Contacts), ASTM B539-

70. [44] Tentative Program for the Invesigation of Wiring Devices Intended for Use with Aluminum Wire—Appendix A,

enclosed with U.L. memos dated 1-23-73, 11-24-72, and 9-29-72 on subjects 22, 44, 83, 468, 496, 719, and

873.

[45] Fan, J. C, Mechanisms of Building Wire Connection

Failure, A Texas Instruments Engineering, Inc., Report,

Bulletin No. 516-TB93-1070.

[46] Holm, R., Electric Contacts—Theory and Application,

Springer-Verlag, New York, 1967, 4th Edition.

[47] Evans, Ulick, R., Metal Industry (London) 73:10-13

(1946).

[48] Williamson, J. B. P., Significance of Non-Destructive Tests

of Compression Joints, IEEE Proceedings, June 1962.

[49] Wiring systems—Operation BREAKTHROUGH Type B

Contract—Final Report (Electronic Modules Corp.).

Appendix. NEC Requirements Which

Directly Affect Innovative Electrical

Connections

A. NEC Requirements for Electrical

Connections (see subsection 3.2.a.)

Article 110 General 70-15

110—14. Electrical Connections. Because of dif- ferent characteristics of copper and aluminum, devices

such as pressure terminal or pressure splicing con14
nectors and soldering lugs shall be suitable for the material of the conductor and shall be properly in- stalled and used. Conductors of dissimilar metals shall not be intermixed in a terminal or splicing connector where physical contact occurs between dissimilar con- ductors (such as copper and aluminum, copper and

copper-clad aluminum, or aluminum and copper-clad

aluminum), unless the device is suitable for the purpose

and conditions of use. Materials such as solder, fluxes, inhibitors, and compounds, where employed, shall be

suitable for the use and shall be of a type which will not adversely affect the conductors, installation, or equipment.

(a) Terminals. Connection of conductors to ter- minal parts shall insure a thoroughly good connection

without damaging the conductors and shall be made by means of pressure connectors (including set-screw type)

,

solder lugs or splices to flexible leads except that No. 8

or smaller solid conductors and No. 10 or smaller stranded conductors may be connected by means of wire-binding screws or studs and nuts having upturned

lugs, or the equivalent. Terminals for more than one

conductor and terminals used to connect aluminum

shall be of a type suitable for the purpose. (b) Splices. Conductors shall be spliced or joined

with splicing devices suitable for the use or by brazing,

welding, or soldering with a fusible metal or alloy.

Soldered splices shall first be so spliced or joined as to be mechanically and electrically secure without solder and then soldered. All splices and joints and the free ends of conductors shall be covered with an insulation equivalent to that of the conductors or with an insulat- ing device suitable for the purpose.

B. NEC Requirements for Boxes (see subsection 3.2.b.) Note particularly

300-15 (b) below.

Article 300 Wiring Methods—General 70-93

300—15. Boxes or Fittings Where Required.

(a) Box or Fitting. A box or fitting shall be

installed at each conductor splice connection point, outlet, switch point, junction point or pull point for the connection of conduit, electrical metallic tubing, surface raceways or other raceways. Exception No. 1 : A box or fitting is not required for a conductor splice connection in surface raceways, wireways, header ducts, multi-outlet assemblies and

auxiliary gutters having a removable cover which is accessible after installation. Exception No. 2: As permitted in Section 410-26.

(b) Box Only. A box shall be installed at each

conductor splice connection point, outlet, switch point,

junction point, or pull point for the connection of metal-clad cable, mineral-insulated metal-sheathed cable.

aluminum-sheathed cable, nonmetallic-shcatln-d cable, or other cables and at each outlet and switcli point for concealed knob-and-tube wiring.

Exception No. 1 : As permitted by Section 336-11

for insulated outlet devices supplied by nonmetalUcsheathed cable. Exception No. 2: As permitted by Section 410-60

for rosettes. Exception No. 3: Where accessible fittings approved

for the purpose are used for straight-through splices in mineral-insulated metal-sheathed cable.

C. NEC Requirements Concerning Free Space

in Boxes (see subsection 3.2.b.)

Article 370 Boxes and Fittings 70-165

370-6. Number of Conductors in a Box. Boxea

shall be of sufficient size to provide tree space for all conductors enclosed in the box. The provisions of this Section shall not apply to terminal housings supplied with motors. See Section

430-12.- , Sections 370-6 (a) and (b) do not apply to con- ductors used for rewiring existing raceways as referred

to in Table 1, Chapter 9. (a) The maximum number of conductors, not counting fixture wires, permitted in outlet and junction

boxes shall be as in Tables 370-6 (a) (1) and (a) (2)

with the exceptions noted. Tables 370-6(a)(l) and (a)(2) apply where no

fittings or devices such as fixture studs, cable clamps,

hickeys, switches or receptacles, are contained in the box and where no grounding conductors are part of the wiring within the box. Where one or more fixture studs, cable clamps or hickeys are contained in the box,

the number of conductors shall be one less than shown

in the Tables; an additional deduction of one conductor

shall be made for each strap containing one or more

devices; and a further deduction of one conductor shall be made for one or more grounding conductors entering

the box. A conductor running through the box is counted as one conductor, and each conductor originating outside of the box and terminating inside the box

is counted as one conductor. Conductors, no part of which leaves the box, are not to be counted. The volume

of a wiring enclosure (box) shall be the total of the volume of the assembled sections. (b) For combinations or conductor sizes not shown

in Tables 370-6(a)(l) and (a)(2), Table 370-6(b)

shall apply. . . (c) Boxes, other than those described in Tables

370-6(a)(l) and 370-6(a) (2), shall be durably and

legibly marked by the manufacturer with their cubic- inch content. All boxes shall be durably and legibly

marked with the manufacturer's name or trademark.

15
NATIONAL ELECTRICAL CODE

Table 370-6(a)(l). Deep Boxes Box Dimensions, Cubic Maximum No. of Conductors

linches T -cl- inch Trade Size Cap. No. 14 No. 12 No. 10 No. 8 3% X 1% Octagonal 10,9 5 4 4 3 3% X 1% Octagonal 11.9 5 5 5 3 4 X 1% Octagonal 17.1 8 7 6 5 4 X Octagonal 23.6 11 10 9 7 4 X 1% Square 22.6 11 10 9 7 4 X 2% Square 31.9 15 14 12 10 4 11/16 X 11/2 Square . 32.2 16 14 12 10 4 11/16 X 21/8 Square . . . 46.4 23 20 18 15 3 X 2 X 1% Device ... 7.9 3 3 3 2 3x2x2 Device 10.7 5 4 4 3 3x2x2% Device 11.3 5 5 4 3 3 x 2 x 21/2 Device 13 6 5 5 4 3x2x2% Device 14.6 7 6 5 4

3 x 2 X 3% Device .... 18.3 9 8 7 6 4 X 21/8 X 11/2 Device . . 11.1 5 4 4 3 4 X 2% X 1% Device . . 13.9 6 6 5 4

4 X 21/8 X 21/8 Device . . 15.6 7 6 6 5 See Section 370-18 where boxes are used as puU and junction boxes

Table 370-6 (a) (2). Shallow Boxes Maximum Number of Conductors Box Dimensions, In Trade Size No. 14 No. 12 No. 10 3% 4 4 3 4 6 6 4 1% X 4 Square 9 7 6 4 11/16 8 6 6 Any box less than 1%-in deep is considered to be a shallow box.

Table 370-6 (b) . Volume Required Per Conductor

Size of Free Space Within Box

Conductor for Each Conductor

No. 14 2. in3 No. 12 2.25 in3 No. 10 2.5 in3 No. 8 3. in3 No. 6 5. in3

D. Exceptions to Box Requirements in Mobile

Homes (see 550—8 (j) below).

Article 550 Mobile Homes and Parks 70-369

550-8. Wiring Methods and Materials. Except as

specifically limited in this Section the wiring methods

and materials included in this Code shall be used in mobile homes.

(a) Nonmetallic outlet boxes are acceptable only

with nonmetallic cable. (b) Nonmetallic cable located 15 in or less above

the floor, if exposed, shall be protected from physical

damage by covering boards, guard strips, or conduit. Cable likely to be damaged by stowage shall be so protected in all cases. (c) Metal-clad and nonmetallic cables may be

passed through the centers of the wide side of 2-in by

4-in studs. However, they shall be protected where they

pass through 2-in by 2-in studs or at other studs or frames where the cable or armor would be less than

IV2 in from the inside or outside surface. Steel plates on each side of the cable, or a tube, with not less than No. 16 MSG wall thickness, are required to protect the

cable. These plates or tubes shall be securely held in

place.

(d) Where metallic faceplates are used they shall be effectively grounded.

(e) If the range, clothes dryer, or similar appliance

is connected by metal-clad cable or flexible conduit, a

length of free cable or conduit should be provided to permit moving the appliance. The cable or flexible conduit should be adequately secured to the wall. Clear- ance space behind a range may provide the required

protection when a range is connected by Type SE cable. When used. Type SE cable shall have an identified and

insulated neutral plus an equipment grounding conductor. Nonmetallic-sheathed cable (Type NM) shall not be used to connect a range or dryer.

This does not prohibit the use of Type NM cable between the branch-circuit overcurrent protective device

and a range or dryer receptacle.

(f) Rigid metal conduit shall be provided with a locknut inside and outside the box, and a conduit

bushing shall be used on the inside. Inside ends of the conduit shall be reamed.

(g) Switches shall be rated as follows: (1) For lighting circuits, switches shall have a 10-ampere 125-volt rating; or higher, if needed for the connected load. (2) For motors or other loads, switches shall have ampere or horsepower ratings or both adequate

for loads controlled. (An "AC general-use" snap switch

may control a motor 2 horsepower or less with full-load current not over 80 percent of the switch ampere

rating.)

(h) At least 4 in of free conductor shall be left at each outlet box except where conductors are intended

to loop without joints.

(i) Under-Chassis Wiring. (Exposed to Weather) . (1) When outdoor or under-chassis line-voltage

wiring is exposed to moisture or physical damage, it shall be protected by rigid metal conduit. The conductors shall be suitable for wet locations. Exception: Electrical metallic tubing may he used

when closely routed against frames and equipment enclosures. (2) The cables or conductors shall be Type

NMC, TW, or equivalent.

(j) Outlet boxes of dimensions less than those re- quired in Tables 370-6 (a) (1) and 370-6 (a) (2) may

be used provided the box has been tested and approved

for the purpose. (k) Boxes, fittings and cabinets shall be securely fastened in place.

Exception: Snap-in type boxes or boxes provided

with special wall or ceiling brackets that securely fasten boxes in walls or ceilings may be used.

16
E. Exceptions to Box Requirements in Exposed

Cable Wiring and for Concealed Work for

Rewiring in Existing Buildings

Article 336 Nonmetallic-Shcathed Cable 70-135

336-11. Devices of Insulating Material. Switch,

outlet, and tap devices of insulating material may be

used without boxes in exposed cable wiring, and for concealed work for rewiring in existing buildings where

the cable is concealed and fished. Openings in such

devices shall form a close fit around the outer covering

of the cable and the device shall fully enclose that part

of the cable from which any part of the covering has

been removed.

Where connections to conductors are by binding- screw terminals, there shall be available as many ter- minals as conductors, unless cables are clamped within

the structure and terminals are of a type approved for multiple conductors.

F. Splices—Concealed Knoh-and-Tube Work

Article 324 Concealed Knoh-and-Tube Work 70-127

324-11. Splices. Splices shall be made only where

close to knobs or tubes using solder or specially ap- proved splicing devices. In line or strain .splices shall not be u.sed.

G. NEC Requirements for Accessible Electrical

Connections (see subsection 3.2.3).

Article 370 Boxes and Fittings 70-169

370-19. Junction, Pull and Outlet Boxes to Be

Accessible. Junction, pull and outlet boxes shall be so

installed that the wiring contained in them may be rendered accessible without removing any part of the building, sidewalks or paving.

I

17
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM. Bibliographic data

SHEET

1. PUBLICATION OR REPORT NO. NBS BSS-632. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE - „ , . An Analysis of

Current Technology on Electrical Connections in Residential

Branch Circuit Wiring

5. Publication Date March 1975

6. Performing Organization Code

7. AUTHOR(S)

William J. Meese and Ramon L. Cilimberg

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION N^ME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No. 4624892

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Department of Housing and Urban Development

Washington, D.C. 20410

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Library of Congress Catalog Card Number: 74-31122

16. ABSTRACT (A 200-word or less factual summary ot most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

In the Operation BREAKTHROUGH research and demonstration program the U.S. Department

of Housing and Urban Development became concerned with the inability to properly

evaluate innovative electrical connections. Long life requirements, fire safety

considerations, the lack of adequate technical information, and long established

conventional practices and evaluation procedures have led to slow-changing regulations concerning electrical connections used in branch circuit wiring in housing.

This report discusses the present methods of evaluating electrical connections,

the technical parameters involved, and innovative electrical connection developments.

Innovations involving electrical connections may lead to significant advancements

in housing construction if it could be demonstrated that functional and safety

requirements over the expected life of the electrical connections were adequately

satisfied. Research is needed to enable prediction of long term performance of

electrical connections based on the results of accelerated performance tests.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Contact resistance; electrical codes; electrical connections; fire safety;

house wiring; materials properties; performance testing.

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