US Electrical Supply Arrangemrnt - Page 1
Forum section: electronics / beginners

author: Brianf
author_rank: Regular Contributor
timestamp: March 09, 2023, 02:55:53 pm
content: I'm trying to get me head around electrical supplies in the US. I'm happy about what goes on inside the property (I have a copy of the NEC) but I am struggling to find out what happens outside. This is what I believe...1) The supply company has a big transformer somewhere which supplies many properties.2) This has a star output with the star point locally grounded, this is the neutral output.3) The other three outputs are the phases.4) This outputs around 12kV....so now we have a four-wire three-phase distribution cable at 12kV.Nearer to a group of properties there will be another transformer. For normal domestic properties this will have a single phase input, connected across the distribution neutral and one live conductor.The output from this is a centre tapped 120-0-120 winding with the centre tap being locally grounded and forming the neutral. One or both of the 120V output go to a property. So the property receives either a two-wire or three -wire feeder cable.Now, here's what I haven't been able to find out...1) Does the supply company then provide a ground rod somewhere at the property?2) If so, where is it located? Inside or outside?3) Where does the incoming supply terminate? Inside or Outside?4) I assume (dangerous) that if the supply company doesn't provide a local rod then the consumer must?5) Is any local rod connected to neutral or left separate?Thanks.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2023, 03:13:41 pm
content: A couple of details about residential electrical connections for the Chicago electrical code:I had my house re-wired a few years ago.Three wires come from the local pole-mounted transformer, which I share with a few neighboring houses, providing 120-0-120 (240 VCT).The electrician grounded the service at the circuit-breaker box to two locations: outdoors, near the inlet, to a ground rod; indoors, to the city water service inlet (copper pipe).Both ground connections use a heavy (6 AWG?) copper wire inside an aluminum conduit: both wire and conduit connect to the rod or pipe.The neutral bus inside the circuit-breaker box is grounded to the ground bus (the two connections mentioned above).All wires inside the house are within metal conduit: other jurisdictions do not require metal conduit.This is all single-phase: there is one high-voltage wire driving the primary of the pole transformer, I think at 3 kV, with the other side of the primary to a grounded low-voltage connection.Edit: to clarify, the three wires enter my house through a meter box mounted on the outside of the wall, and the ground wire connects to that box.A short pipe connects the meter box to the circuit-breaker box inside the house.

author: Nusa
author_rank: Super Contributor
timestamp: March 09, 2023, 03:45:08 pm
content: Quote from: Brianf on March 09, 2023, 02:55:53 pmThe US is BIG, and transmission voltages aren't necessarily 12kV in any particular place. Doesn't matter to the layman of course, it's stuff you don't touch. Skim this if you want a high level view of how the US grids (plural) work.https://www.energy.gov/sites/prod/files/2015/12/f28/united-states-electricity-industry-primer.pdfNormal domestic residences get bi-phase input (two phases 180 degrees out of phase), unless it's a VERY old unimproved install in which case it may only have a single phase input. Three phase power is generally unavailable outside of industrial areas. Which means 240V is available to most main panels, but in most instances is only wired to a few points (electric dryers, ovens, water heaters, car chargers, etc.) while everything else works on 120V.The NEC will tell you that neutral grounding (often to a long ground rod literally pounded into the ground, or an approved equivalent for local conditions) is required at the main panel of the residence. It's also literally grounded at the distribution transformer (typically on a pole in older/rural neighborhoods, or a surface or underground box in newer ones).

author: themadhippy
author_rank: Super Contributor
timestamp: March 09, 2023, 03:58:12 pm
content: QuoteHere in the uk the connection to the incoming water pipe (and gas if its metal) isn't part of the incoming earthing arrangement,in fact using them as your earth rod is against our regs,but part of the requirements for equipotential bonding,

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2023, 05:36:16 pm
content: Quote from: Brianf on March 09, 2023, 02:55:53 pm1) Typically the electric utility's responsibilities end at the service meter. The ground rod will be installed 'after' this point ('after' being a somewhat uncertain term in some cases) and is typically installed by the customer's electrician when the service is installed.2) I've always seen them outside only and there can be more than one. Typically it would not be practical to install a grounding rod indoors. If the service panel is indoors, the rod is outdoors with a wire going through the wall.3) I'm not sure what you mean by 'terminate', but the meter and the service panel can be inside or outside.4) Again, for a typical USA installation I'm familiar with, you need a grounding rod. In a dense urban environment the grouding may be provided a bit differently.5) The ground rod is typically bonded to the metal plumbing and to the neutral bus barin the service panel only.

author: jmelson
author_rank: Super Contributor
timestamp: March 09, 2023, 06:59:40 pm
content: Locally (St Louis, MO area) we usually have nominal 7200 V feed on the poles. The substation, a few blocks from our house feeds 7200 V, 14.4KV and 45 KV to other substations. We happen to have our OWN pole-mount transformer, but in many cases several houses share a transformer. I'm pretty sure the HV feed has a return wire strung on the poles, but there is also a ground rod right against the pole. There is also a ground rod driven near our service entry. In our case, we have a buried feed running from the pole to the service entry. On many homes, the service "drop" is strung in the air between pole and house. On our house, there is a conduit that goes up to a meter box, then a conduit goes through the wall to a master breaker, and then feeds to the main panel.Jon

author: Jwillis
author_rank: Super Contributor
timestamp: March 09, 2023, 09:39:13 pm
content: Because Canada and the United States share power distribution, the standards are the same .Transmission Substationsrange of 155,000 to 765,000 volts.The 10kV to 12Kv comes from the 3 phase distributionGridwhich goes to the 3 phase distributionBuswhich is reduced to 7200V. This in turn goes to aRegulator Bankwhich maintains voltage regulation. From the Regulation Bank there is 3 phase (4 wire )Distribution lines. These can be Tapped further to 2 phase (3 wire) or Single Phase (2 wire) 7200V . This depends on where you are on the distribution path and the service provided by the power distributor. From the 3 phase or 2 phase distribution lines there will be a transformer that is tapped from one of the individual phases reducing one of those phases into 240V single phase "Split" into (120V - 0V -120V) to your home. Thus called Split Phase. Each Split phase is a single phase 120V . The transformer has the same circuit as a transformer with 2 Primary wires (Live 7200V and Neutral 0V ) and 3 Secondary wires (Live 120V - Neutral 0V - Live 120V) . On Split phase each 120V tap (relative to 0V common) are 180 degrees out of phase to each other. The 240V is a direct link to both 120V lines excluding the 0V Common.The 0V Neutral or Common is always connected to Earth or Ground to keep it at 0V potential. If it wasn't the potential voltage can change relative to earth because of static build up. This could destroy any component or seriously hurt or kill if you were to come in contact with it.

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2023, 09:49:10 pm
content: Attached is the form used in Chicago to order a new installation from Commonwealth Edison, the local utility.The standard choices are what the Edison transformer will supply to the customer's metering connection.Of course, most of the options are for commercial or industrial applications: the normal option for a residence is "120/240V 1-phase, 3-wire".https://600westchicago.info/pdf/comedservice.pdf

author: bdunham7
author_rank: Super Contributor
timestamp: March 09, 2023, 10:00:41 pm
content: Quote from: TimFox on March 09, 2023, 09:49:10 pmFrom the form: 120/240V 3-phase, 4-wireI didn't know you could still get that setup. Do they mandate one and only one service entrance per building?

author: TimFox
author_rank: Super Contributor
timestamp: March 09, 2023, 10:19:43 pm
content: Quote from: bdunham7 on March 09, 2023, 10:00:41 pmIIRC, from another version of the form, that configuration was not available in all jurisdictions.A co-worker who was doing volunteer electrical work at his church (an old building) was surprised to find that connection.Essentially, a delta winding with one leg center-tapped, CT to neutral.His surprise was finding the third node at a higher voltage (wrt neutral) than 120 V.I don't believe there is any requirement for only one service entrance per building, especially since large commercial buildings may well have multiple customers, each with its own meter.At my pre-retirement employer, there were two Edison transformers, one supplying the original building and the other supplying a later addition (continuation of the original).Each transformer had its own switchbox indoors: the first was Square-D and the second was Siemens.The two transformers were mounted near each other, where the utility easement was on the property for the underground feed cable.

author: Bud
author_rank: Super Contributor
timestamp: March 09, 2023, 10:55:47 pm
content: Quote from: TimFox on March 09, 2023, 10:19:43 pmThey had to set it up in compliance with the "Divine power" specification.

author: james_s
author_rank: Super Contributor
timestamp: March 09, 2023, 11:32:09 pm
content: Grounding requirements vary by locale, the NEC has some requirements, and then individual states and cities have additional requirements. When I did a service upgrade on a house around 15 years ago I had to install at least two ground rods a minimum of 6' into the ground and at least 6 feet apart. These had to be connected by a continuous #8 copper conductor to the ground bus in the panel which was then bonded to the neutral bus in the panel. A separate ground wire then bonded the water pipe coming into the house to the ground bus in the panel. In that case the requirement was to NOT ground bond the gas pipe but I'm pretty sure some places do require that.

author: Jwillis
author_rank: Super Contributor
timestamp: March 10, 2023, 01:37:37 am
content: Quote from: james_s on March 09, 2023, 11:32:09 pmGrounding requirements are defined by more than just local regulations. Soil composition, structure, moisture content play a factor as well. For example, more gravely soil may require more grounding than say a clay based soil .Also the size of structure , distance from main distribution line and obstructions can also play a factor in what grounding is required.

author: Nusa
author_rank: Super Contributor
timestamp: March 10, 2023, 03:07:13 am
content: Also note that code requirements are pretty universally grandfathered to the year of installation in the US, so far as the law goes. Any upgrades have to meet the code of the day, however. The 1925 house I grew up in still had active knob-and-tube wiring and no safety ground on the entire 2nd floor and the ceiling fixtures on the 1st floor when we sold it a few years ago. Completely legal as is, but I'm pretty sure the new owners have had most of it replaced now.

author: james_s
author_rank: Super Contributor
timestamp: March 10, 2023, 06:20:30 pm
content: There's nothing wrong with knob & tube wiring if it's in good shape and hasn't been messed with. The problem is when people add onto existing circuits, using improper technique and overloading it, or adding insulation that buries wires and causes them to overheat. I've seen places where someone rewired a house to replace knob & tube with modern romex and did a really poor DIY job that was arguably less safe than the original. I advocate leaving it alone, especially for lighting circuits. Add new circuits for receptacles and appliances using modern wire.

author: TimFox
author_rank: Super Contributor
timestamp: March 10, 2023, 06:30:40 pm
content: My grandparents' house had knob-and-tube wiring.My problem with it was that the wires were exposed between the joists in the basement, and could be damaged by accidental contact (maybe moving a stepladder around).Also, without metal conduit, the grounding was not well-guaranteed.Worse was the actual wire in use, with older insulation that deteriorated with age.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: March 10, 2023, 07:28:28 pm
content: Quote from: bdunham7 on March 09, 2023, 05:36:16 pmFor 2) There is a "ground rod" inside of the house. Metal plumbing is used for this. This different from a "bonding metal" ground. There should be a ground wire to a metal pipe as thick as the wire going to the outside ground. It is supposed to act as an alternate ground if the rod fails. (At least this was why my house failed an inspection.)

author: james_s
author_rank: Super Contributor
timestamp: March 10, 2023, 08:37:02 pm
content: Quote from: TimFox on March 10, 2023, 06:30:40 pmI can see that being an issue. Basements are rare in my part of the world although more common in houses that were originally knob & tube. If I had such an arrangement I think I would be inclined to sheetrock over the areas that had wires run, or something else to protect the wires from damage. Exposed romex is common here in unfinished areas, it too is subject to damage.

author: Nusa
author_rank: Super Contributor
timestamp: March 10, 2023, 08:47:49 pm
content: Quote from: james_s on March 10, 2023, 06:20:30 pmI agree, and that's why it was still there. All the exposed stuff in the basement and accessible first floor outlets were replaced in the 1970's by my dad after the main panel was updated to 200 amps. It was also exposed in the unfinished attic, but that was only an issue if someone went up there. And the insulation did fail in places that mattered...there were several fixtures where I had to slide heatshrink up the wires to keep them separate. Otherwise, knob and tube doesn't actually need insulation for safety.

author: james_s
author_rank: Super Contributor
timestamp: March 10, 2023, 09:03:33 pm
content: Quote from: Nusa on March 10, 2023, 08:47:49 pmI would argue that it absolutely needs insulation for safety, but it doesn't need insulation for function.You can use the same self-amalgamating tape that is used to cover splices to cover spots where the insulation fails, although if it's crumbling in multiple spots that's a good argument for replacing it.

author: Nusa
author_rank: Super Contributor
timestamp: March 10, 2023, 09:20:20 pm
content: Quote from: james_s on March 10, 2023, 09:03:33 pmIn exposed locations. In enclosed locations, it's best to leave it alone. There are still ceramic insulators at every point that matters. Replacing the wire in the first floor ceiling was my last option...would have required tearing up plaster and lathe in many spots to run it. Strong possibility we would have abandoned the fixture and plastered over the hole if it came to that.

author: Brianf
author_rank: Regular Contributor
timestamp: March 11, 2023, 01:43:45 pm
content: One last question I hope...at the service panel is the consumer's earth rod connected to the incoming neutral ine?

author: james_s
author_rank: Super Contributor
timestamp: March 11, 2023, 08:15:31 pm
content: Yes, the ground rod(s) will connect to the ground bus in the main panel where every ground wire in the system terminates. The ground bus will be bonded to the neutral bus in the panel. If a subpanel is used, the bonding screw will be removed so that the subpanel neutral is not bonded to ground, it is only done at one place in the main panel.

author: Brianf
author_rank: Regular Contributor
timestamp: March 12, 2023, 09:13:58 am
content: OK, so the supply arrangement is some form of TN-C-S (non-PME).

author: themadhippy
author_rank: Super Contributor
timestamp: March 12, 2023, 02:24:34 pm
content: QuoteThink they've invented a new arrangement TN-C-CS as it sounds like neutral and earth are combined on the consumers side before going there separate ways

author: bdunham7
author_rank: Super Contributor
timestamp: March 12, 2023, 02:55:08 pm
content: Quote from: themadhippy on March 12, 2023, 02:24:34 pmI'm not sure it is worthwhile to try to understand US power systems in terms of those designations. If you want to, in most areas outside of dense urban settings the overall connection for residences and small commercial looks more like TN-S except the utility does not provide any ground at all. So for a 208/120 three-phase entrance, the utility would provide 3 phases and a common connection to be used a neutral. For the residential 240/120 split phase, they provide three wires--two for the 240V output of the transformer and 1 (often uninsulated) for the center tap. The rest of the circuit in a TN-S diagram--the ground itself and the interconnections--are provided by the customer's installation.In rural areas and in light commercial installations you might have one transformer per service entrance. In suburban areas like mine, 3 or 4 houses may share one transformer and each house will have its own local ground, but the utility does not provide a common ground. However, they are all bonded together by the 'neutral' CT conductor from the transformer.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 12, 2023, 03:56:15 pm
content: Ground is always a local reference.I dont know about ground rod requirements in the USA.On most of this planet, ground its bonded to neutral at the main breaker panel.bonding is not applied to sub panels, they will use the ground from the main panel.

author: Brianf
author_rank: Regular Contributor
timestamp: March 13, 2023, 10:41:45 am
content: Quote from: Terry Bites on March 12, 2023, 03:56:15 pmApart from miles and miles of rural properties in the UK fed by TT on overhead lines.