I'm installing a sub-panel in a metal building. I want to put in a 220V 100 amp service. I have 500 feet of 3 AWG THHN wire. The distance is 90 ft and it will be run in 1-1/2" PVC Conduit.

Is the wire sufficient for 100 Amps?

  • Copper or aluminum? And what temperature are the lugs at each end rated for? Commented Jan 19, 2022 at 19:43
  • This Cable is 3 awg Copper. I have not purchased the box yet. What temperature do you recommend? The lugs in the Garage on the house I do not know. Commented Jan 19, 2022 at 20:15
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    Sell it, buy aluminum wire, laugh all the way to the bank. <Seriously.>
    – Ecnerwal
    Commented Jan 19, 2022 at 20:17
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    You can sell it at a discount from current pricing (so it will sell), use a small fraction of the money to buy suitable aluminum wire, and have money left over, or you can "use it because you have it" and not have money left over - your call.
    – Ecnerwal
    Commented Jan 19, 2022 at 20:25
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    Beware "fallacy of sunk costs". That spool retails for $789 with lead times into February and you could probably get that on Craigslist or FB marketplace because you have it today. The aluminum you need is under $300 for 100A or ~$200 for 90A. Hmm, yeah, OK I can see where that's a wobbler given the annoyances of F2F trading in the middle of Omicron. Your call. Commented Jan 19, 2022 at 20:54

1 Answer 1


Yes, 3 AWG copper is legal for 100A. We don't worry about voltage drop til at least 180 feet.

Make sure to use the No-alox, since you'll be landing on aluminum lugs. Those are the "universal donor", they play well with Al and Cu wire.

However it would be a crying shame to use wire that nice for a ground wire that only needs to be #8 copper or #6 aluminum. There is nothing wrong with aluminum wire at these large sizes.

Copper wire is going for top dollar these days because of the shortages, and the many DIY projects being done by novices. (what gets novices tense about aluminum is a fiasco in the 70s relating to small branch circuit wiring, but those causes don't affect heavy feeder.) So if it were me, I would "take their money" and sell that copper spool for top dollar and get Al. 1 AWG Al is good to 100A, but 2 AWG sits in a pricing/availability "sweet spot" and is good to 90A.

Now don't make the same mistakes we see from many. First, 100A of feeder can power a world more stuff than most people realize, so get a nice big panel - 30 space is not excessive. We have a 24-space on 65A feeder in our shop, and it's nearly full. Second, "copper purists" invariably go for HOM or other "el cheapo" sub panels, and don't pay attention to the metal the bus bars are made of. Copper bus bars can be had, and I for one care about that when the bus bars are outdoors or in non-climate-control space. Maybe I'm being superstitious! But I have >50 year old CH panels that live in miserable conditions, still going strong.

  • I understand everyone's concern about copper pricing, but that being said. My last house main lines had Aluminum buried underground to the the house and it gave me nothing but problems. 3 times FPL had to come out and find the wire break spot and I was told by the employees it was because of the aluminum corroding. I don't know? next to Silver, Copper is the best conductor in my opinion. Thank you all for the advise. Good fortune to all! O and just to mention I am using #6 copper for the ground. Commented Jan 20, 2022 at 12:15

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