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I’ve got a 100 amp main breaker in my center fed panel, and I see that the bus bars are rated for 125 amps. I’d like to add a 50 amp circuit for EV charging, but my current load calculation is about 65 amps (all gas appliances).

I called my power company to ask if they would have to upgrade the service drop - I eyeball it to be 4 AWG - and if they would have to sign of on me upgrading the main breaker. They came out to look and gave me a hesitant answer of “no” to both questions.

Does that seem right? If it were 4 AWG copper coming in from overhead, I think the feeder has an ampacity rating of 100 amp. I read there’s a 83% service factor that applies, but even with that I don’t think the service drop would support 125 amps.

Picture of the panel sticker

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  • What is the rating of the meter cabinet?
    – NoSparksPlease
    Commented Jun 30, 2021 at 5:52
  • @NoSparksPlease I believe this panel is what you call a meter main. Would the meter half of this panel be rated differently than the bus bar half? I recall a sticker on the meter half stating the rating is 125 amps.
    – aerospark
    Commented Jun 30, 2021 at 5:58
  • This is mostly a local ordinance issue. The 2020 NEC would require #2 awg copper riser for 125A. Issue that need to be resolved is if the NEC is adopted locally without exception and modifications. You can get tripped up by added minimum size requirements, their hesitation may have related how much modification is allowed by local policy before requiring completely meeting the new requirements.
    – NoSparksPlease
    Commented Jun 30, 2021 at 6:28
  • Your house is using 65 amps at all times? There's no reason you couldn't install the EV charging station and just make sure to reduce your power consumption while the vehicle is charging.
    – MonkeyZeus
    Commented Jun 30, 2021 at 16:23

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To run 125A, you need your service entrance cabling, meter pan, and main breaker panel to be ready for 125A.

Your meter+main+panel assembly is suitable for use as a 125 amp rated unit when a 125 amp THQL21125 circuit breaker is field installed, replacing the factory installed 100 amp circuit breaker. Per the large text on the panel label. "Suitable" refers to the whole product, so that ticks the boxes for "meter pan" and "main breaker panel".

Now we have service entrance wiring. Since it's an all-in-one, the only service entrance wire of concern is from the meter-main to the top of the weather-head. That is in NEC jurisdiction, but is subject to the 83% favorable derate from NEC 310.15(B)(7). 125 x .83 = 103.75 amps. So that will require wire large enough for that. Sounds to me like #1/0Al or #2 Cu. It will be attaching to aluminum at both ends.

The size of power company's service drop is a big bag of "not your problem". They follow different rules, called NESC, and they have a smart meter there to tell them when you're overheating those wires, which are in free air obviously. Even NEC grants higher ampacity to wires in free air. (table 310.15(B)(17)).

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  • I forgot I own from the meter to the weatherhead =[ Is there any safe way to measure the gage of the wire from the meter socket to the weatherhead while the wires are installed? Or do I need a pro with pro eyeballs to eyeball it?
    – aerospark
    Commented Jul 3, 2021 at 4:44
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Are you sure you eyeballed the size correctly?

The power company plays by different rules than you have to. Their wires are in free air and they have different ways to determine loading. They have standard sizes for service drops and won't just change one out because you want to upgrade your panel. You'd have to submit an upgrade request, they would do the disconnect, you'd do your work and they would reconnect. They might not reconnect unless you have it inspected and get an "OK" to reconnect. At that point they might make a determination to replace the service drop.

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  • I am not sure I eyeballed it correctly. The service drop conductors have faded markings that says "Southwire Company for SCE" and that's it.
    – aerospark
    Commented Jul 3, 2021 at 4:42

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