I just wrapped up adding new overhead 100 amp service for my parent's garage outside Chicago. My Mom is a retired ceramics teacher and wanted a 30A two phase kiln and/or welder in there and the original install was terribly inadequate, with buried #10 a long distance from the house, but the building is right near the power pole. So I put in a new POCO approved meter can, ground rod, panel, etc. I used #4 copper for the main service conductors, according to NEC 310.15(B)(7). But according to other charts I need bigger wire. It's only about 12' from the weatherhead through the meter to the panel. One thing nagging me about understanding the NEC is: is a residential garage / outbuilding allowed the same derating as a 'Dwelling?' The inspector isn't likely to be too strict based on neighbor's experience, but thought it was an interesting code question.

  • I'm not an electrician, so I won't even attempt to answer this. But if there is some ambiguity as to the size of the wire, then why not just get the bigger size to be on the safe side? If it's only 12 feet, then the difference in cost can't be that much. Commented May 14, 2018 at 13:46
  • If you contact the utility ahead of time they will have a planning guide book/pdf that defines engineering standards and expectations.
    – Tyson
    Commented May 14, 2018 at 14:07
  • The power company has documents but defers to the local municipality's code requirements, which is NEC 1999 and what I'm trying to dissect. Residential Install Guide and Meter Requirements. As for buying more wire, it's a $60 difference, far harder to work with, and I'd never expect this building to draw more than 50amps but put in a 100a meter box so it's a question of the main breaker protecting that wire.
    – Neil
    Commented May 14, 2018 at 14:17

3 Answers 3


The idea of using #4 copper (85A @ 75C) or #2 aluminum (90A @ 75C) is relying on 310.15(B)(7) (310.12 in the NEC 2020 renumbering) - a weird exception for dwellings.

Because dwellings have a "diversity of loads" which is well understood by engineers, they are permitted to use slightly smaller wires, rated for 83% of the service size, for the service wires (from main breaker to the power company's demarcation point, i.e. the weatherhead on a pole line.) So 83A for 100A service, or 166A for 200A service. (2/0 Cu or 4/0 Al).

To avoid dumbness, 310.15(B)(7) aka 310.12 also says any feeders run off that service, do not need to be bigger than the service wires have to be. So if you have a farm or trailer home where the main breaker is on a pole outside and you bring "main FEEDER" into the home, that feeder doesn't need to be larger merely because it's a feeder.

E.G. if you have 200A service and 100A feeder to a subpanel, it's not saying you can apply 83% favorable derate to every feeder on the property LOL! It's saying no feeder ever needs larger than 4/0Al or 2/0Cu.


The jurisdiction of NEC normally ends at a specific demarcation point, e.g. the meter. Beyond that, the installation is under power company Codes -- I believe IEEE's National Electric Safety Code (NESC). That Code is built around different assumptions applicable to power companies:

  • The wires will be outdoors or underground
  • Given the outdoor-scale distances, cost of wire matters a lot
  • The wires will be out of contact of humans or structures, so they can run hotter
  • Whole neighborhoods don't peak at once, so wires can be significantly oversubscribed
  • System monitoring equipment will warn when those assumptions are wrong

As such, they are allowed to use thinner wire. However sometimes situations arise where outdoor wiring looks, walks and quacks like power company wiring that would be subject to NESC, but has landed in NEC jurisdiction because of a technicality. An example is rural Texas where the PoCo slaps a meter at the edge of your property and leaves it to you to provide your own service drop/lateral. It would be prohibitive to make, say, a 500' run using wire out of the usual 310.15(B)(16) (formerly known as 310.16) table. So NEC 310.15(B)(7) and accompanying Code allows you to use wire sizes more like what NESC would recommend in that instance.

  • The OP has 12' to the service drop not a sub panel.
    – Ed Beal
    Commented May 14, 2018 at 17:51
  • @EdBeal Confusing section removed. Commented May 14, 2018 at 17:53
  • It looks like 310.15 was rewritten and cut off quite heavily in the 2020 edition. This article does not even exist in the 2020 edition of the NEC (at least not in section 310.15) and 310.15(B)(16) is back to being 310.16 ...
    – Marian
    Commented Sep 30, 2019 at 2:02
  • 1
    @Marian Nothing was deleted from Article 310. They just did a massive renumbering of the subsections. It's now at 310.12. I still use NEC 2017 numbering because while most cows and cornstalks are on NEC 2020, most humans are still on NEC 2017. Commented Mar 6, 2023 at 20:55

According to NEC Article 110.28 The code definition of a "Dwelling Unit is "A single unit, providing complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, cooking, and sanitation." So unless your stand alone garage meets all of these requirements, it cannot be classified as a Dwelling Unit. That does not mean you couldn't classify it a something else, for example the NEC does have a definition for just a "Building" which actually describes your situation better. And no NEC 310.15 (B)(7) does not allow derating for anything else other than dwelling units. You could reduce the service size per 230.79 (D) to an 80A service or minimum 60A.

I would suggest a separate meter and service, but I am not familiar with Chicago's requirements. In this state you can have that, but it must meet certain zoning and code requirements (probably what your neighbor went through). This could be relatively easy or a logistical nightmare. I would suggest you look at @Tyson's comments on where to start.

  • Caught in the act of answering while Ed Beal was answering but you get the picture. Commented May 14, 2018 at 15:43
  • Thanks for lending your experience and great details here. This is for a separate overhead service and meter install and was approved by the POCO before we got too far into plans.
    – Neil
    Commented May 14, 2018 at 16:13

This garage is residential per the NEC, code is Basicly broken down into residential, industrial and commercial. I am not sure where your table is from because the ampacities haven't changed. For residential ampacities under 100 amp are required to use the 60 deg collum not the 90 deg that I believe you used. I did check current 2017 310.15.B.16 and the 2 prior (14 & 11) just to make sure and all 3 have the same ampacitys. As a detached garage your AHJ may allow a 70 amp main or possibly 80 amp with the wire you have as a residential service can be 83% of the service disconnect. But for single family the code minimum is 100A (I would expect them to allow a smaller service for a garage) The best way to find out would be to call your inspector (I live on the west coast and haven't worked in your area since the 80' S) in my state the wire would need to be #2 copper for your 2 hot legs and neutral with #6 to a driven ground rod or #4 to a concrete encased ground (this is 240v single phase not 2 phase).

  • Thanks for lending your experience and great details here.
    – Neil
    Commented May 14, 2018 at 16:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.