UNITED STATES DEPARTMENT OF AGRICULTURE

Rural Utilities Service 

RUS BULLETIN 1730B-121 

RD-GD-2013-71

SUBJECT: Wood Pole Inspection and Maintenance

TO: All Electric Borrowers

EFFECTIVE DATE: Date of Approval. 

OFFICE OF PRIMARY INTEREST: Transmission Branch, Electric Staff Division. 

FILING INSTRUCTIONS: This bulletin replaces RUS Bulletin 1730B-121, "Pole Inspection 

and Maintenance," issued April 15, 1996. 

PURPOSE: To furnish information and guidance in establishing a continuing program of pole 

maintenance. 

August 13, 2013_

 Date

Assistant Administrator 

Electric Program

Disclaimer: The contents of this guidance document does not have the force and effect of law and is not 

meant to bind the public in any way. This document is intended only to provide clarity to the public 

regarding existing requirements under the law or agency policies.
RUS Bulletin 1730B-121

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TABLE OF CONTENTS

Page

1 PURPOSE................................................................................................................................3

2 GENERAL DISCUSSION OF POLE DECAY ......................................................................3

3 PLANNED INSPECTION AND MAINTENANCE PROGRAM..........................................6

4 INSPECTION METHODS......................................................................................................8

5 ADDITIONAL INSPECTION TOOLS AND METHODS ....................................................9

6 RESULTS OF WOOD POLE INSPECTION .......................................................................10

7 REMEDIAL TREATMENT..................................................................................................11

8 DETERMINING THE SERVICEABILITY OF DECAYED POLES..................................14

Exhibits

Exhibit A: Metric Conversion Factors

ABBREVIATIONS

ACA - Ammoniacal copper arsenate

ACZA - Ammoniacal copper zinc arsenate

ANSI - American National Standards Institute

AWPA - American Wood Protection Association

CCA - Chromated copper arsenate

EPA - Environmental Protection Agency

NaMDC - N-methyldithiocarbamate

NESC - National Electrical Safety Code

MITC - Methylisothiocyanate

pcf - pounds per cubic foot

RUS - Rural Utilities Service
RUS Bulletin 1730B-121

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1 PURPOSE 

To furnish information and guidance to electric cooperatives in establishing a continuing 

program of pole maintenance and to operating personnel in performing inspection and 

maintenance of standing poles. Included in this bulletin are methods and procedures for 

determining the minimum permissible groundline circumferences of distribution and 

transmission poles.

2 GENERAL DISCUSSION OF POLE DECAY 

Pole Decay. Decay of treated poles is usually a gradual deterioration caused by fungi and 

other low forms of plant life. Damage by insect attack (termites, ants and wood borers) is 

usually considered jointly with decay because preservative treatment of wood protects 

against both fungi and insects. In most cases, the decay of creosote and penta-treated 

poles will be just below the groundline where the conditions of moisture, temperature and 

air are most favorable for growth of the fungi. Factors affecting pole life, such as species 

of wood, type and thoroughness of treatment, geographical location, and soil conditions 

are discussed below.

a Pole Species. Of the millions of poles on RUS financed systems, about 85 

percent are the thick sapwood southern pines. Untreated, the sapwood is 

especially vulnerable to attack by wood destroying fungi, termites, and carpenter 

ants. In the Gulf States, where temperature and moisture are most favorable for 

growth of these major wood-destroying organisms, the time to pole failure of an 

untreated pole would be 2 to 3 years. In areas of lower rainfall and few frost-free 

days, the time to pole failure would increase to 5 to 10 years.

The bulk of the remaining pole population is classified as the western species, 

comprised of Douglas-fir, western red cedar, red pine, lodgepole pine, ponderosa 

pine and a small amount of jack pine.

Adequate preservative treatment protects the pole sapwood and the underlying 

heartwood. Heartwood of these pole species varies not only in decay resistance, 

but is difficult to treat with preservatives. The heartwood decay resistance for the 

major pole species is as follows:

Durable - Western red cedar heartwood.

Moderately Durable - Douglas-fir and most of the pines.

Least Durable - Lodgepole pine. (The use of this species has been limited 

primarily to the Mountain States areas.)

b Preservative Treatments. There are two general classes of preservative treatment, 

oilborne (creosote, penta in petroleum, and Copper Naphthenate) and waterborne 

(arsenates of copper). Creosote was the only preservative for poles on rural 
RUS Bulletin 1730B-121

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systems until 1947, when post-war shortages prompted the introduction of 

pentachlorophenol (penta) and Copper Naphthenate. Both of these preservatives 

were dissolved in fuel oils from petroleum or mixtures with creosote.

For many years, penta has been the most widely used preservative for poles. With 

the increase in cost of petroleum-based products, penta-in-oil treated pole costs 

have also increased. Presently, both penta and waterborne preservatives are 

widely used with both preservatives having performed satisfactorily. Where 

problems have occurred with penta treated poles, the decay can be tied to poor 

conditioning of the poles, to the loss of solvent carrier due to migration and 

bleeding or to loss of dissolved penta to retentions below the effective 

preservative threshold. To overcome these losses, treatments and quality control 

have been improved.

Standard wood preservatives used in waterborne solutions include ammoniacal 

copper zinc arsenate (ACZA), and chromated copper arsenate (CCA) (types A, B, 

and C). These preservatives are often employed when cleanliness and paintability 

of the treated wood are required. Several formulations involving combinations of 

copper, chromium, and arsenic have shown high resistance to leaching and very 

good performance in service. Both ACZA and CCA are included in many 

product specifications for materials such as building foundations, building poles, 

utility poles, marine piles, and piles for land and fresh water use. Treatment 

usually takes place at ambient temperature. Care needs to be taken during 

treatment to ensure heat sterilization of the pole when treating Douglas-fir with 

ACZA.

c Decay Zones. The following Decay Severity Zones on a map of the United States 

was originally based on summer humidity and temperature information and later 

on a pole performance study conducted by RUS. Decay severity ranges from 

least severe in Zone 1 to most severe in Zone 5. Service life records and 

individual experience or a planned sample inspection will indicate if the decay 

hazard for a system is typical of the zone in which the system is located.
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Decay severity zones for wood utility poles as defined by the USDA Rural Utilities 

Service. Decay is least severe in zone 1, most severe in zone 5.

d Types of Decay. Internal decay may be found in southern pine poles that were 

not properly conditioned or in which penetration or the amount (retention) of 

preservative is inadequate. Internal decay of the western species usually involves 

the heartwood in butt-treated western red cedar, lodgepole pine, and Douglas-fir 

which have been improperly seasoned prior to treatment. After installation, decay 

organisms invade the heartwood through the poorly treated sapwood zone checks, 

or woodpecker holes. Internal decay may also occur in field framed poles when 

supplementary treatment is neglected.

Insufficient treatment or migration of oil-type preservatives is the principal cause 

of external decay in southern pine poles. This decay is generally the result of 

improper seasoning or treatment. Inspection methods should be directed toward 

discovery of this type of defect and maintenance efforts to supplement the 

treatment with additional chemicals.

External decay above ground, or better known as "shellrot", may occur in butttreated western red cedars after 12-15 years of service.
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3 PLANNED INSPECTION AND MAINTENANCE PROGRAM 

Purpose. The purpose of a planned inspection program is to reveal danger poles and 

poles which are in early stages of decay so that corrective action can be taken to prolong 

the service life of the pole. The end result of the inspection program is the establishment 

of a continuing maintenance program for extending the average service life of all poles 

on the system. The steps in developing a planned pole inspection and maintenance 

program are outlined below.

a Spot Checking. Spot checking is the initial step in developing a planned pole 

inspection and maintenance program. Spot checking is a method of sampling 

representative groups of poles on a system to determine the extent of pole decay 

and to establish the priority for a pole maintenance program. A general 

recommendation is to inspect a 1,000 pole sample made up of continuous pole 

line groupings of 50 or 100 poles in several areas of the system. The sample 

should be representative of the poles in place. For instance, all the poles on a line 

or a map section should be inspected as a unit and not just the poles of a certain 

age group. The inspection of the sample should be complete, consisting of 

hammer sounding, boring, and excavation as described in Section 4. Field data 

should be collected on the sample as to age, supplier, extent of decay, etc.

After the data has been collected, it should be analyzed to determine the areas 

having the most severe decay conditions and to establish priorities of a pole-bypole inspection of the entire system. It may be desirable to take additional 

samples on other portions or areas of the system to determine if the severity of 

decay is significantly different to warrant the establishment of an accelerated pole 

inspection and maintenance program for that portion of the system. The results of 

the spot check will aid in scheduling a continuous pole inspection and 

maintenance program at a rate commensurate with the incidence of decay.

b Scheduling the Inspection and Maintenance Program. The results of the spot 

check will aid in determining when the planned program should be started. The 

suggested timing for initial pole-by-pole inspection and subsequent re-inspection, 

when supplementary treatment is applied after each inspection, is as follows:

Decay Zone Initial Inspection Subsequent

Re-inspection

Percent of

Total Poles

Inspected Each Year

1 12 – 15 Years 12 Years 8.3

2 and 3 10 – 12 Years 10 Years 10

4 and 5 8 - 10 Years 8 Years 12.5

The vulnerability of poles to decay is generally proportionate to the decay zone in 

which they are located. As a general recommendation, the initial pole-by-pole 

inspection program should be inaugurated at a yearly rate of 10 percent of the 

poles on the entire system when the average age of the poles reaches 10 years. If 
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a spot check indicates that decay is advanced in 1 percent of the pole sample, the 

inspection and maintenance program should be accelerated so that a higher 

percentage of poles are inspected and treated sooner than the figures shown 

above. If the decay rate is low for a particular decay zone or area of the system, 

the pole-by-pole inspection can be postponed accordingly. Historical inspection 

data indicates that the ratio between the decaying/serviceable poles to reject poles 

in the 10-15 year age group is about six or more to one. In a 30-year age group, it 

is about one to one or less. In the latter group, the survivors have more than 

sufficient residual preservative to protect them indefinitely. The poorly treated 

poles in the 30-year old group have already decayed and been replaced.

The greatest economic benefit from regular inspection is in locating the 

decaying/serviceable group. Treatment of poles in this group can extend pole life, 

thereby saving the cost of emergency replacement. Inspection and proper 

maintenance can more than pay dividends by extending the serviceable life of the 

poles. With the costs of replacing poles rising, the economics of extending the 

service life are more favorable.

c Establishing the Program. The pole-by-pole inspection and maintenance work 

may be done by system employees or by contracting with an organization 

specializing in this type of work. The choice should be made on the basis of the 

amount of work to be done, the trained employees available, and a comparison of 

the costs. Developing the necessary skills in the system's own crews may require 

considerable time and be contingent upon the availability of an experienced 

inspector to train system employees. Therefore, qualified contract crews may be 

preferable for this work in many instances. An inspector to be qualified should 

have inspected, as a minimum, 5,000 poles in conjunction with a qualified 

inspector and another 5,000 on his own, but under close supervision. The 

inspector's work should be checked every week or two by the system's 

representative and the inspector's supervisor. To check an inspector’s work 

select at random about 10 poles, inspected in the previous few weeks, re-excavate, 

take off paper and treatment, and re-inspect. Check for hollow sounds, take a 

boring, check soft surface wood, especially adjacent to shaved areas or along 

checks, re-measure the pole, recheck the calculations, then retreat and backfill. If 

any serious errors are discovered, all the work between these spot checks should 

be re-inspected.

The pole inspection and maintenance program may result in a large number of 

replacements. If the reject rate is high, the system's crews may not be able to 

replace rejected poles in a reasonable time because of other work. The temporary 

addition of skilled personnel for inspection or pole replacement may be required. 

It is generally necessary to use at least one crew full time to keep up with the pole 

inspector. An average pole inspector can check 150-200 poles per week or 800 

poles per month. It is desirable to have one person responsible for supervision 

and coordination. 
RUS Bulletin 1730B-121

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d Re-inspections. Information obtained during the first pole-by-pole inspection can 

serve as the basis for scheduling subsequent inspections. As a guide, it is 

recommended that a re-inspection be made every 8 to 12 years as shown in 

paragraph 3c of this Section, according to the decay zone and severity of decay. 

The recommended re-inspection intervals are based on treating poles during the 

previous inspection cycle. Shorter re-inspection intervals are recommended if 

poles were not treated. These recommendations may be modified by experience, 

but the intervals should not be extended by more than 3 years. It is advisable to 

recheck some poles which have been groundline treated. At the completion of the 

inspection of a pole, a small, weatherproof tag should be attached to the pole 

indicating the organization that performed the inspection and the date of the 

inspection.

4 INSPECTION METHODS 

Inspection Types. There are varying types of inspection, each with a different level of 

accuracy and cost. Inspection methods with low accuracy require more frequent reinspection than methods which are detailed and more accurate.

a Visual Inspection. Visual inspection should be considered the first step to 

inspecting poles but has the lowest accuracy. Since most decay is underground or 

internal, this method will not detect the majority of defective poles. Obvious data 

can be collected on each specific structure, such as the condition of the pole above 

ground, crossarm, and hardware. This method is not recommended for detecting 

decay.

b Sound and Bore. This method involves striking a pole with a hammer from 

groundline to as high as the inspector can reach and detecting voids by the hollow 

sound. An experienced inspector can obtain significant information about a pole 

by listening to the sounds and noticing the feel of the hammer. The hammer 

rebounds more from a solid pole than when hitting a section that has an internal 

decay pocket. The internal pocket also causes a sound that is dull compared to the 

crisp sound of a solid pole section.

Some contracts require all poles to be bored, while others require boring only 

when decay is suspected. Boring is usually done with either an incremental borer 

or power drill with a 3/8" bit. An experienced inspector will notice a change in 

resistance against the drill when it contacts decayed wood. The shavings or the 

borings can be examined to determine the condition of the wood, and the borings 

can be analyzed for preservative penetration and retention.

When voids are discovered a shell thickness indicator can be used to measure 

them. This information can be used to evaluate the reduction strength by the void.

The effectiveness of the sound and bore method varies by species. For southern 

yellow pine poles, which represent a majority of the poles in North America, 
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decay normally is established first on the outside shell below ground. The decay 

moves inward and then upward to sections above ground. By the time sound and 

bore inspection can detect internal decay pockets above ground, the pole is likely 

to have extensive deterioration below ground.

Sound and bore method is more effective with Douglas-fir and western red cedar. 

Decay on these poles is likely to begin internally near the groundline, or in the 

case of Douglas-fir, above the groundline. Therefore, sounding and boring can 

identify at least some decay at a stage before the groundline section is severely 

damaged. All borings should be plugged with a treated wood plug which is 

properly sized for the respective hole.

c Excavation. The effectiveness of the sound and bore inspection is greatly 

increased when excavation is added to the process. Excavation exposes the most 

susceptible section of the pole for inspection. For southern yellow pine this is 

particularly true, since decay begins externally and below ground.

Poles should be excavated to a depth of 18 inches in most locations. Deep 

excavation may be required in dry climates. After excavation the exposed pole 

surface should be scraped clean to detect early surface decay.

Shell rot and external decay pockets should be removed from the pole using a 

specially designed chipper. Axes or hatchets should not be used for this 

application. The remaining pole should be measured to determine if the pole has 

sufficient strength with the reduced circumference. Tables 2, 3, and 4 assist in 

adjusting circumferences for various size voids.

After complete inspection and application of preservative treatment, the pole is 

backfilled. The dirt should be tamped firm every 6 to 8 inches. The backfill 

should mound up around the pole to allow for future settling and drainage away 

from the pole.

5 ADDITIONAL INSPECTION TOOLS AND METHODS

Additional Methods. Over the past several years there has been a considerable amount of 

research work done by companies to develop additional products that can be incorporated 

into the in-line pole inspection process. These products, involving many diverse 

technologies, are intended to improve both the accuracy and reliability of the in-line 

inspection programs used by utilities, as well as decrease the time necessary to carry out 

the inspection process. In developing comprehensive inspection programs, cooperatives 

are encouraged to examine all potential inspection tools and processes to determine the 

best system for their particular needs. The data from these testing devices does not 

always correlate exactly with the actual bending strength determined by full scale testing 

and, as a result, should be used to establish trends showing changes in strength.
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6 RESULTS OF WOOD POLE INSPECTION

a Inspection Results. Inspection results should be used to update pole plant records, 

evaluate pole condition, plan future inspection and maintenance programs, and 

provide information for map revisions. The inspection process will result in 

identifying the condition of each individual distribution or transmission pole. 

In general the National Electric Safety Code (NESC) requires that if structure 

strength deteriorates to the level of the strength reduction factor required at 

replacement, the structure shall be replaced or rehabilitated. The inspection 

results should indicate if a pole is "serviceable" or a "reject". 

b Serviceable. The characteristics of "serviceable" are based on the following 

conditions:

(1) Large portion of completely sound wood.

(2) Early stages of decay which have not reduced the pole strength below 

code requirements.

(3) Pole condition as stated above but a defect in equipment may exist, such 

as a broken ground or loose guy wire. Equipment defects should be 

subsequently repaired.

c Rejects. Any poles that do not meet the above conditions should be classified as 

"rejects". Their characteristics are:

(1) Decay, insect or mechanical damage has reduced pole strength at the 

groundline below code requirement.

(2) Severe woodpecker hole damage has weakened the pole below safety 

standards.

(3) Hazardous conditions exist above ground, such as split top.

d Reinforce or Replace. Rejected poles may be classified further depending on the 

severity of the deterioration and whether they are reinforceable.

(1) A “reinforceable reject” is any reject which is suitable for restoration of 

the groundline bending capacity with a method of reinforcement.

(2) A “replacement” candidate would be a rejected pole which is not suitable 

for the necessary rehabilitation.
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7 REMEDIAL TREATMENT

Purpose. The purpose of remedial treatment of standing poles is to interrupt the 

degradation by the addition of chemicals, such as pesticides, insecticides and fungicides, 

thereby extending the useful life of the structure. Treatment may be external groundline 

treatment or internal treatment.

a Regulations and Licensing. The majority of states require that applicators or the 

job supervisor obtain a pesticide applicator license. Testing for this license 

includes a "basic skills test" to show knowledge of the rules and regulations 

governing pesticides. Some states also give a "category test" which is specific to 

wood poles and wood preservation. The uses of every pesticide are classified by 

the U.S. Environmental Protection Agency (EPA) as either "general" or 

"restricted". 

(1) A "general use" pesticide is not likely to harm humans or the environment 

when used as directed on the label. These preservatives may be purchased 

and applied without a pesticide applicator license. However, a 

manufacturer may choose not to make a product available for purchase by 

the general public.

(2) A "restricted use" pesticide could cause human injury or environmental 

damage unless it is applied by competent personnel (certified applicators) 

who have shown their ability to use these pesticides safely and effectively. 

These wood preservatives can only be purchased and applied by someone 

who has a pesticide applicator license or whose immediate supervisor has 

a pesticide applicator license.

b Groundline Treatment. All treated poles eventually lose resistance to decay. 

Groundline treatment with effective preservatives provides an economical 

extension of their physical life. Experience has shown that a well designed and 

implemented groundline inspection and maintenance program can significantly 

increase the service life of many poles. Groundline external treatment is 

recommended under the following conditions:

(1) Whenever a pole is excavated during an inspection, and the pole is sound 

or decay is not so far advanced that the pole must be replaced or 

rehabilitated.

(2) Whenever a pole over 5 years old is set.

(3) Whenever a used pole is installed as a replacement.

External preservatives used for groundline treatment typically contain active 

ingredients that are either water soluble, oil soluble or practically insoluble. 
RUS Bulletin 1730B-121

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Before application of external preservatives, decayed wood should be stripped 

from the pole and removed from the excavation. The preservative paste is most 

commonly brushed onto the pole following label directions. A polyethylene 

backed paper is then wrapped around the treatment and stapled to the pole. The 

paper aids the migration of the preservative into the critical outer shell.

c Internal Treatment. The three basic types of preservatives used for internal 

treatment are liquids, fumigants, and solids. 

(1) Liquid Internal Preservative: Liquid internal preservatives should be 

applied by pressurized injection through a series of borings that lead to 

internal decay pockets or voids. Adequately saturating the pocket and 

surrounding wood should arrest existing decay or insect attack and prevent 

further degradation for an extended time.

Liquid internal preservatives contain water soluble or oil soluble active 

ingredients. Sodium fluoride, boron and various forms of copper solutions 

are the principle active ingredients used today. Moisture that is present in 

the pole will help facilitate diffusion of the active ingredients into the 

wood beyond a decay pocket.

Oil based internal preservatives most often incorporate Copper 

Naphthenate as an active ingredient with fuel oil or mineral spirits as the 

solvents. Since oil-based Copper Naphthenate is not soluble in water, it is 

likely to migrate into the surrounding wood only as far as the oil will 

travel.

(2) Fumigants: Most of the fumigants in use for wood poles today were 

originally developed for agricultural purposes. Applying fumigants to soil 

will effectively sterilize the ground. Due to high levels of microorganisms 

and chemical activity in soil, the fumigants will degrade fairly rapidly and 

dissipate so that new crops can be planted in a short time.

These same fumigants do not degrade rapidly in wood and will remain 

affixed to sound wood cell structure for many years. Fumigants have also 

been found to migrate longitudinally in wood, several feet away from the 

point of application. This helps control decay in a large section of the 

pole. When the vapors migrate into a decay void, however, they may 

dissipate through associated checks and cracks. This reduces the long 

term efficacy and requires more frequent application.

Registered pole fumigants include Sodium N-methyldithiocarbamate 

(NaMDC), Methylisothiocyanate (MITC),tetrahydro-3,5-dimethyl-2H1,3,5-thiadiazine-2-thione (Dazomet) and Chloropicrin. Chloropicrin is a 

very effective wood fumigant. However, the liquid must be applied from 
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pressurized cylinders, and the applicator must wear a full-face air 

respirator.

MITC, NaMDC and Dazomet are the most widely used wood pole 

fumigants. Pure MITC is a solid below 94°F and contains 97 percent 

active ingredient. Solid MITC sublimes directly into fumigant vapors. 

Avoiding the liquid stage helps to minimize loss of fumigant during 

application through checks and cracks. MITC is packaged in aluminum 

tubes to facilitate installation. Just before placing the tube into a treatment 

hole, the cap is removed. As with any fumigant, application holes should 

be plugged with pressure treated wooden or plastic plugs.

NaMDC is soluble in water to a maximum amount of 32.7 percent. 

Treatment holes drilled in a wood pole are filled with the aqueous solution 

so the appropriate dosage is applied. Recommended dosages vary 

according to pole size. The NaMDC solution decomposes and generates 

MITC as the main fungi-toxic ingredient. The maximum theoretical 

amount of resultant MITC at ideal conditions is 18.5 percent by weight. 

After decomposition the MITC vapors then migrate up and down the pole 

to help control decay.

Dazomet is a very fine granular material that is 98-99% active ingredient. 

Treatment holes drilled in a wood pole are filled with the granular material 

so the appropriate dosage is applied. Like NaMDC, Dazomet decomposes 

and generates MITC as the main fungi-toxic ingredient. The maximum 

theoretical amount of resultant MITC at ideal conditions is 45.0 percent by 

weight. The MITC vapors then migrate up and down the pole to help 

control decay.

(3) Solids: Currently there are several solid diffusible rods available as a 

supplemental preservative treatment for wood poles. Active ingredients 

used in these rods include Sodium Fluoride, Boron and Copper. The 

migration of these ingredients through the wood to control or prevent 

internal fungal decay is aided by the moisture content present. The zone 

of effective treatment is determined by the distance the active ingredients 

move from the point of application at fungi-toxic levels. Studies have 

shown wood moisture content in excess of the fiber saturation point 

(approximately 30%) are necessary for significant migration to occur. 

However, at wood moisture levels typically found at groundline in the 

internal regions of in-service utility poles, adequate diffusion can be 

achieved with an appropriate drill pattern in the treatment zone. Active 

ingredients from rods will tend to move slower than fumigants. 

Preservative rods should be applied according to the label directions. The 

rods are typically applied through a pattern of downward angled holes 

beginning at groundline or below with application rates varying with pole 
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circumference. Diffusible rods can also be used to sterilize inspection 

holes and to control or prevent pole top decay.

d Woodpecker Damage. Woodpecker damage is an on-going issue that must be 

continually addressed. Many preventative methods are available but each 

typically has a varying degree of success.

It is difficult to predict which poles woodpeckers may select. Frequently the first 

hole invites further attack by other woodpeckers. For these reasons, it is good 

maintenance practice to seal up the smaller holes. Various materials are available 

for plugging the holes, and a wire mesh can be used to cover the plugged hole as 

well as large areas of a pole. In addition, as a preventative measure, wire mesh 

can be applied to poles in areas where woodpecker activity is expected. Some of 

these repairs restore varying degrees of strength to the pole, while others simply 

plug the hole.

e Pole Reinforcement. Various methods are available to reinforce a deteriorated 

pole at the ground line. A determination must be made as to whether or not a 

sufficient cross section of sound wood remains at the ground line if pole 

reinforcement is to be used. When considering reinforcement options, consult 

with the pole reinforcement system supplier for design and installation support.

Even though reinforcement may not be economically justified, other factors may 

need to be considered such as difficulty of access to the pole for replacement or 

system critical load which would prevent the line from being taken out of service 

at the time when the pole may be in danger of failing.

8 DETERMINING THE SERVICEABILITY OF DECAYED POLES

Serviceability. The decision to treat or replace a decayed pole depends upon the 

remaining strength or serviceability of the pole. The permissible reduced circumference 

of a pole is a good measure of serviceability. The following procedure may be used to 

assist in determining if a pole should be replaced or reinforced.

a Decay Classifications. Decay at the groundline should be classified as:

(1) General external decay

(2) External pocket

(3) Hollow heart, or

(4) Enclosed pocket.

b Permissible Reduced Circumference. The 2012 edition of the National Electric 

Safety Code (NESC) requires that wood structures shall be replaced or 

rehabilitated when deterioration reduces the structure strength to 2/3 of that 

required when installed for NESC District loading. The NESC 2012 also requires 

that wood structures shall be replaced or rehabilitated when deterioration reduces 
RUS Bulletin 1730B-121

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the structure strength to 3/4 of that required when installed for NESC extreme 

wind or extreme ice with concurrent wind loadings.

Computer programs are available that can calculate the remaining capacity of the 

pole by taking the voids into account when determining the effective remaining 

section of the pole.

Tables 1 through 4 will assist in determining when replacement or rehabilitation is 

necessary. If the reduced circumference indicates a pole at or below the minimum 

reduced circumference, the pole should be replaced, splinted, stubbed 

immediately, or otherwise rehabilitated.

c General Procedure for Using Table 1, 2, 3 and 4

(1) General External Decay. After removing all the decayed wood, measure 

the circumference above and below the decayed section to determine the 

original circumference. Then measure the reduced circumference at the 

decayed section. Enter Table 1, Column 1, with the original pole 

circumference. After determining the acceptable level of deterioration as 

described in paragraph 8c, find the minimum acceptable reduced 

circumference from the appropriate column - Column 2 for 2/3 of the 

original circumference, or Column 3 for 3/4 of the original circumference 

- on the same line as the original circumference. If the actual reduced 

circumference is larger than the acceptable calculated minimum, from 

Column 2 or 3, replacement or rehabilitation is not required yet.

(2) External Pockets. Remove decayed wood and take measurements of the 

depth and width of the pocket. Measure the pole for the original 

circumference. Refer to Table 2 to determine the circumference reduction. 

Use the procedure from paragraph 8d(1) to determine the minimum 

acceptable reduced circumference. If the actual reduced circumference, 

i.e. the original circumference minus the circumference reduction from 

Table 2, is larger than the acceptable calculated minimum from Table 1, 

Column 2 or 3, replacement or rehabilitation is not required yet.

(3) Hollow Heart (Heart Rot). If hollow heart is found, determine the shell 

thickness and measure the original circumference of the pole. Refer to 

Table 3 to determine the circumference reduction. Use the procedure from 

paragraph 8d(1) to determine the minimum acceptable reduced 

circumference. If the actual reduced circumference, i.e. the original 

circumference minus the circumference reduction from Table 3, is larger 

than the acceptable calculated minimum from Column 2 or 3, replacement 

or rehabilitation is not required yet.

To determine the shell thickness, bore three holes (preferably of 1/4- or 

3/8-inch diameter), 120° apart; measure the shell thickness at each hole, 
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add the measurements, and divide by 3. Treat and plug holes with tightly 

fitting cylindrical wood plugs that have been treated with preservative 

once shell thickness is determined. A transmission pole with a shell 

thickness less than 3 inches should be removed from service.

(4) Enclosed Pocket. An enclosed pocket is an off-center void as shown in 

Table 4 and its diameter should be measured by boring holes as described 

in paragraph 8d(3). Using the minimum thickness of the shell, refer to 

Table 4 for the reduction in circumference. Measure the original 

circumference. Use the procedure from paragraph 8d(1) to determine the 

minimum acceptable reduced circumference. If the actual reduced 

circumference, the original circumference minus the circumference 

reduction from Table 4, is larger than the acceptable calculated minimum 

from Column 2 or 3, replacement or rehabilitation is not required yet.
RUS Bulletin 1730B-121

Page 17

Table 1

Reduced circumferences for NESC Rules 250B, 250C, and 250D

Rule 250B –

Combined ice and wind district 

loading

Rule 250C - Extreme wind 

and

Rule 250D - Extreme ice and 

concurrent wind

Original 

Circumference 

(in)

Minimum Reduced Circumference 

(in)

(Based on 2/3 initial strength)

Minimum Reduced Circumference 

(in)

(Based on 3/4 of initial strength)

30 26.2 27.3

31 27.1 28.2

32 28.0 29.1

33 28.8 30.0

34 29.7 30.9

35 30.6 31.8

36 31.5 32.7

37 32.3 33.6

38 33.2 34.5

39 34.1 35.4

40 35.0 36.3

41 35.8 37.3

42 36.7 38.2

43 37.6 39.1

44 38.4 40.0

45 39.3 40.9

46 40.2 41.8

47 41.1 42.7

48 41.9 43.6

49 42.8 44.5

50 43.7 45.4

51 44.5 46.3

52 45.4 47.3

53 46.3 48.2

54 47.2 49.1

55 48.1 50.0

56 48.9 50.9

57 49.8 51.8

58 50.7 52.7

59 51.5 53.6

60 52.4 54.5
RUS Bulletin 1730B-121

Page 18

Table 2

Reduction in Measured Circumferences to Compensate

for External Pockets

Pocket 

Width (in) 1 2 3 4 5 6

Pocket 

Depth (in) 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5

Measured 

Circumferen

ce of Pole 

(in)

Reduction in Circumferences (in)

20 to 30 1 1 2 - - 2 2 3 - - 2 3 4 - - 3 4 5 - - 4 6 8 - - 6 8 - - -

30 to 40 1 1 1 2 - 1 2 2 3 3 2 3 4 4 4 2 4 5 5 6 3 5 6 7 8 5 7 8 9 -

40 to 50 1 1 1 2 2 1 2 2 3 3 2 3 3 4 4 2 3 4 5 6 3 4 5 6 7 3 5 6 7 8

50 to 60 1 1 1 2 2 1 2 2 3 3 2 3 3 4 4 2 3 3 4 5 3 4 4 5 6 3 4 5 6 7

Table 3

Reduction in Measured Circumferences to 

Compensate

For Hollow Heart

Measured Minimum Thickness of Shell (ins) Circumference 

Of Pole (ins) 2 2.5 3 3.5 4 4.5

20 to 25 1 - - - - -

25 to 30 2 1 - - - -

30 to 35 3 2 1 - - -

35 to 40 4 3 2 1 - -

40 to 35 5 4 3 2 1 -

40 to 45 7 5 4 3 2 1
RUS Bulletin 1730B-121

Page 19

Table 4

Reduction in Measured Circumferences to Compensate

For Enclosed Pockets

Diameter of Pocket 

(in) 3 4 5 

Shell Thickness (in) 1 2 3 1 2 3 1 2 3

Measured 

Circumferences Of 

Poles (in) 

Reduction in Circumferences (in) 

20 to 30 2 1 - 3 1 - 4 2 -

30 to 40 2 1 1 3 1 1 4 2 1

40 to 50 2 1 1 3 2 1 4 3 1
RUS Bulletin 1730-B 121

Exhibit A

Page 1

METRIC CONVERSION FACTORS

To Convert From To Multiply by

Foot (ft.) Meter (m) 0.3048

Inch (in) Centimeter (cm) 2.54

Degrees Fahrenheit

(X°F)

Degrees Celsius

(°C)

5/9 (X° - 32)