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I'm having a new power line run out to a detached garage that is about 50 feet from the main house and another 25 feet to the box inside to the main panel, so a total run of about 75 feet. Currently I have 1-20 amp line out there and I want to future proof so my electrician suggested either a 60 amp setup, or 100 amp sub panel.

Future plans may only be lighting as it currently is now, or, electric car chargers (1 or 2), compressor, light welding, or anything a true DIY may get into. Like I said this is more about future proofing while we are already spending the money. A new conduit line was already run so that is all set and sized properly.

My electrician first said a 60 amp sub panel should be sufficient. Then when I mentioned potential of charging 2 cars he said we may as well go 100amp, or said something about 100amp, but with a 90 amp breaker in the panel. Not sure if that has to do with certain codes or what not, or is using a cable that's capable of 100, but better rated at 90amp. Almost sure he said a 1 or 2 gauge wire, but I could have mistaken and I'm not sure how many wires/conductors that wire contains.

He said he will be putting in a sub-panel, grounding rods, etc.

Just looking for some feedback. I told him to proceed with the higher 100amp/90amp setup since the cost was negligible.

We are upgrading the main house service from 100amp to 200amp, so the garage is being done at the same time so the main panel will have plenty of space.

Is this 100/90amp setup more than sufficient? Anything I should be asking him specifically about this setup? Any reason to go 60amp if cost is negligible?

Any info appreciated.

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    As many potential duplicates mention, 90 is a magical figure because 2-2-2-4 cable (or 2 awg wire) is cheap, comparatively, and 2 AWG aluminum is rated for 90A. The size of the subpanel merely needs to be "anything larger than that" in Amps, and mostly needs plenty of spaces.
    – Ecnerwal
    Oct 31, 2023 at 20:55

1 Answer 1

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Basically two separate issues:

  • Feed Circuit Size

This is determined by wire size. 90A is a common value because 2 AWG aluminum has a maximum capacity of 90A when used as a subpanel feeder. If you have sufficiently large conduit then you can install small wire (cheaper) and a smaller feed breaker and upgrade later by replacing the wire and breaker.

  • Subpanel Size

The subpanel can be any size equal to or larger than the feed circuit size. 100A is a typical size. But often a 200A main panel is more cost-effective than a 100A sub panel. Why? You need a disconnect for service to a separate building, and with a "main" panel you get that included as the main breaker. It doesn't matter if it is larger than the feed breaker because the purpose is a disconnect and not overcurrent protection. In addition, you can often find bundles that include "bonus breakers" with larger panels, saving you even more.

There is nothing wrong with a 30-space panel that only uses 8 spaces. But if you get an 8 space panel and now need space # 9, you have a big problem.

So get a big panel, feed it based on what you need now, and you can always upgrade later. It is quite possible to fill up a 20 or 30 space panel while using only 90A, as many circuits (e.g., LED lighting) add very little load.

1 AWG aluminum is good for 100A and 2 AWG aluminum is good for 90A. Either one makes a reasonable subpanel feed. The feed is actually four wires - 3 (hot, hot, neutral) of the main size, plus a ground wire which is normally a little smaller - the exact size depends on various code issues.

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  • Thanks @manassehkatz-Moving 2 Codidact. If you can share some extra insight that would be great. Feed Circuit Size So maybe he's planning to run 2 AWG to save a little bit on cost and thinks I may never actually need the whole 100amp, and will stick me with 90amp? I may ask him to go with the 1 AWG to give me a true 100amp. Do I need the whole 100amp out there if I were charging 2 electric cars simultaneously? It may never happen, but just thinking of the possibility. I have 1 conduit run that is about 1-1/2" wide inner diameter, so almost sure code allows 1 AWG or 2 AWG to be pulled.
    – RocketManZ
    Nov 1, 2023 at 16:55
  • Subpanel Size: The main in house is going to be a large 200amp setup. I forget how many circuits but I think he said 36 or 40 space? I will not be tapped out based on current usage so I should still have a little space left, even after all these upgrades in the main box. In the garage, almost sure he said a 16 or 20 space panel, but I could be wrong.I don't have major plans out there, so I'm assuming this should be fine. 90amp vs 100amp, if he runs 2AWG and I'm stuck with 90 amp, is that an issue or should I ask him to just pull 1AWG for the true 100amp?
    – RocketManZ
    Nov 1, 2023 at 17:01
  • 90A is almost certainly enough for most uses. Consider that the biggie for most people is EV charging. You could allocate, for example, 40A for EV charging shared between two cars allows 20A each (enough to charge most cars most of the time overnight) and also allows one at a time to get 40A (32 actual) or a dynamic split (EVSE can do that very well.) That would leave 50A for lights, tools, etc. True 100A is only if you really need it, because the panel (and its main shutoff breaker) can be larger. There is no "magic" to 100A, it is just a nice round number. 2 AWG is fine. Nov 1, 2023 at 17:53

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