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I have a detached garage. I would like to run power to it and have a 400 amp service on my house. I already have a bunch of 2-2-2-4 aluminum ser wire, which I know is only good for 90 amps.

I have installed a 125 amp 12 space subpanel and intend on running wire from main service (where first 90 amp breaker will be) to garage with another disconnect on the outside.

The inspector is saying that since I have a 125 amp panel installed, I need to run wire that is rated for 125 amps. I'm saying that since I am protecting the wire with the 90 amp breaker at the main disconnect then it is acceptable, the subpanel is rated for 125 max, it's kind of like using a 70 amp AC disconnect for a 30 amp AC unit that has 10/2 feeding it, right?

I just need a code reference and cannot find one. I'm even fine with using a smaller breaker than 90 amps, I just want to use this wire.

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    You can use a 200A panel, so long as the feed breaker on the main supply size fits the wire you are using (90A or less) - your inspector is ignorant. 215.2(A)(1)
    – Ecnerwal
    Feb 27, 2023 at 16:07

3 Answers 3

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I'm going to venture to guess that your inspector is conflating a service with your feeder. A service is the connection from the house to the utility supply. Normally there is no over-current protection (fuse or breaker) at the source end of service conductors -- the utility assumes that the service conductors are safe from physical damage and it's adequate to protect against overcurrent by having the conductor size matched to the size of your main circuit breaker.

A feeder is different. You're building a feeder. Feeders are basically just a branch circuit that feeds other sub-branch circuits (via a subpanel). As you know, feeders do have overcurrent protection at the source end of the circuit, and in fact there's no requirement for a main breaker at all at your detached garage. A disconnect, yes, but no main breaker or fuse is necessary. As with any branch circuit, a person must size the circuit by doing a load calculation, then use conductors and other equipment that have ampacity sufficient for the use.

I think you might be able to reset and guide the inspector's view by bringing written calculations. Call it the "Detached Garage Feeder Calculations Sheet."

First, a load calculation. (See online calculator at Ask the Electrician, as helpfully pointed out in a semi-related answer by manassehkatz there) The process for this defined in article 220 of the NEC. Include items like general lighting and receptacles, any special equipment like an air compressor or welder, etc. Maybe even pad it out by including allowance for equipment you don't have any may never get -- a vehicle lift or an EVSE charger, for example. At the end of the load calculation section, highlight the fact that the minimum subpanel and feeder ampacity are X amps.

Second, show feeder conductor sizing calculations. This is really just as simple as "minimum required conductor ampacity -> chosen conductors are 2-2-2-4 aluminum -> all terminals are rated 75 C or greater -> OK, 2-2-2-4 is adequate and protecting it with a 90 amp breaker is appropriate."

Third, include some conduit sizing calcs for the mast and weather head (SER can't be buried!).

Finally, some commentary to wrap up your calculations. Something like the following:

  • feeder's minimum ampacity is X amps; I choose to oversize and provision for 90 amps instead.
  • will use 90 amp breaker model Y in main panel
  • will attach 2-2-2-4 conductors to main panel
  • will use conduit size Z
  • will position disconnect model A with ampacity B amps on the exterior of the garage
  • will use more 2-2-2-4 from outdoor disconnect to indoor subpanel
  • will use subpanel model C, which could be lug-only or main-breaker style, with ampacity D amps because that's what's available and satisfies the calculated minimum of X amps.

And.. hopefully, when presented that way, the inspector will say to himself "ahh yes, this is right. What was I thinking?? Oh, I was thinking about a service.."

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    Except for bold headings and snark, I thought this was a Harper answer at first!
    – FreeMan
    Feb 28, 2023 at 19:28
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Maybe 408.36 is what you need: (This is from 2023 edition, but I don't think it's changed significantly in a long time.)

408.36 Overcurrent Protection. In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard.

[Emphasis added.]

You meet the requirements with your configuration. The 90A feeder protection is not greater than the 125A panel rating, and the upstream location of the feeder protection is on the supply side.

A practical point: Does the panelboard have a main circuit breaker, or lugs only?

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Oh man, this question keeps coming up over and over again. The inspector is wrong. Just because you have a 125 AMP panel does not mean you need to supply it with 125 amps. The breaker is for protecting installed wiring. Often people oversize a panel to make it future-proof and have plenty of breaker space. No problem with that.

If the breaker is rated for the size of the wire, you're ok. Heck, you could supply it with 10/3 as long as it would be protected by a 30 amp breaker....not that I'd do that or even remotely suggesting that, just mentioning it to give some perspective.

I found this NPFA ampacity worksheet. Hopefully it will help.

The inspector is wrong.

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    The OP knows the inspector is wrong, he's looking for a code reference to prove it. Unfortunately, this answer doesn't provide said reference.
    – FreeMan
    Feb 27, 2023 at 13:40
  • Thanks for the response! I cannot find the code and I know the inspector is going to ask, but I also don't know of a part of the code that states the panel would have to be fed by a wire that is good for 125 amps. You know how some inspectors are and they stick to their guns on certain things . . . Her argument is that someone could come later and increase the breaker size and i would argue that, that could be done with any feeder or branch circuit for that matter
    – Will
    Feb 27, 2023 at 14:10
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    "someone could install a bigger, code-violating breaker" is true of any and every breaker in your panel, @Will, and seems to me to be a reasonable counter-argument. However, my experience with inspectors is limited, but the stories I've heard indicate that "reasonable" isn't always a word in their vocabularies...
    – FreeMan
    Feb 27, 2023 at 14:34

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