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I have a 200amp service, thats loaded with twins, my son just bought house, anyway I want to put a 100amp sub panel inside basement off the main panel, the run is nomore than 20', I have about 50' of #6 that Id like to use to see 100amp sub, now thing is off this sub panel I have to running 3-double pole 20's for 3 8' baseboard heaters, the Main 20amp arch fault breaker for room & a homerun 20amp breaker dedicated to a window AC unit (Im pulling lights of that line, 7 high hats.........is my #6 (4 wire) enough to carry that load and to add to down road or should I splurge to play safe and use #4 or #2?

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    Why are you using three double-pole 20's for three 8' baseboard heaters? They are typically only 2000W@250V, or 8A. You can land two per 20A/240V breaker. Is the wire to them #12 or #10? – Harper Mar 13 '18 at 0:35
  • What size basement is this panel providing circuits to? – ThreePhaseEel Mar 13 '18 at 1:27
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You can feed as many 100A subpanels as you want with #6 wire, as long as the wire is supplied from 60A breakers. Breakers protect wires. You need a 60A breaker to protect #6 wire. (or #4 Al).

In fact you can use any size of subpanel whose amp rating is >= the breaker that is feeding it. Upsizing the panel is a very good idea if it means getting more spaces. Always get so many spaces that you never need twins/double-stuff breakers -- you can never have enough spaces! 42 spaces is not too many!

The required size for subpanel feeder is exactly the same as the required size for branch circuits, and comes out of NEC table 310.15(b)(16), formerly 310.16. Do not use the service drop/lateral table, those are outside and can't burn your house down obviously.

You need 4-wire cable to feed a subpanel. It is both illegal and unsafe to supply hot-hot-neutral (ungrounded) to a subpanel. However, 3-wire cable can be used to a subpanel that is 240V-only and supports no 120V or 120/240V loads whatsoever -- for example water heater, A/C unit or any of those 8' baseboard heaters. If you are stuck with useless 3-wire cable, sell it on Craigslist and let someone else use it.

If you have individual wires (THHN) also keep in mind your neutral must be natively white and your ground must be green or bare. Different deal for wire #4 or larger, as it can be "re-defined" using black, red, white or green tape.

If you want to buy the correct wire for serving the panel from a 100A breaker, buy #3 Cu or preferably #1 Al.

  • I picked up a double pole 50 to use from Main, are you saying to get a double pole 60 to feed sub panel? Then im guessing another double pole 60 in sub panel? – Ken K Mar 14 '18 at 12:39
  • Yeah he bought this house n didnt look at panel, the Main is a 200amp service but only a 20 space panel & in that panel they squeezed at least 6-7 twin 20's......wth & project he wants done I need 8 spaces, 6 of which are dedicated double pole 20's for his baseboard heaaters, noway I can put in Main, so Im putting a 40 space sub panel in, he'll never use or need that much but I like to do things once – Ken K Mar 14 '18 at 12:47
  • @KenK You have the right idea using a double pole 50 in the main panel, and 60 is also legal there, given #6 wire. The subpanel's main breaker can be anything you please - 20, 60, 100, 225 (obviously 20 would limit you lol) since it's used for nought but a shutoff switch. Or you can use a main lug panel which has no main breaker at all, if the subpanel is in the same building. However in your case I would aim for a 200-amp, so someday the service feed could be rereouted to this new panel, making it the main. – Harper Mar 14 '18 at 13:15
  • I thought I could do that, running the double pole 50 from Main to double in sub with #6 wire........I mean I ran sub at my house with double pole 50 out n in for hot tub plus a few outlets........he will never draw that much amperage in one room, even with baseboard heat – Ken K Mar 17 '18 at 15:00

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