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enter image description hereenter image description hereI have empty space at the bottom. Will just adding a 100 amp breaker and running connecting it to a 100 amp sub panel 50' away work?

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Terry Myers
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  • Can you provide us with photos of the panel directory labeling, as well as the square footage of your house please? – ThreePhaseEel Jun 25 '23 at 18:23
  • What will you be running on the sub? If it's a "grow farm" or bitcoin mining operation, those are considered continuous loads and the feed needs to be derated by 20% and yes, in this case you should do measurements and a load calc. . If for tools, like working., Unless you're running a large planer (IE: 5 HP or more) and dust collector at the same time, you should be fine. Next: What other large loads on the main panel: IE electric range, electric clothes dry, electric water heater , electric heat or heat pump, EV charger? etc. – George Anderson Jun 25 '23 at 19:52
  • Also that bundling is a code violation (unless appropriate de-rating was performed, and I’d bet money it wasn’t). – nobody Jun 25 '23 at 22:53
  • One more question: can you post photos of the nameplates of your air conditioning units please? Also, we still need the square footage of your house – ThreePhaseEel Jun 27 '23 at 03:24
  • I'll try to a pic of the AC unit plates, this is actually my in-laws house and I will have to get the pic for them tomorrow. But they are standard central air units. The one labeled upstairs is actually the attic air handler unit. There is no upstairs. – Terry Myers Jun 27 '23 at 14:48
  • @TerryMyers -- yeah, we'll need those AC nameplate pics and the square footage to continue (you can't do a load calculation *at all* without the square footage of the space since NEC lighting loads are on a VA/square foot basis) – ThreePhaseEel Jun 28 '23 at 03:57
  • Ok added the AC plates, hopefully that will help. – Terry Myers Jul 05 '23 at 17:30
  • @TerryMyers -- I still need the house's square footage as that's what the NEC uses to compute the lighting & general receptacle loads when doing a load calculation – ThreePhaseEel Jul 07 '23 at 00:22

2 Answers2

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There's plenty of room for a 100A breaker here.

You will need #1 aluminum or #3 copper feeder wire, and it must be 4 wires with separate neutral and ground. We advise aluminum - copper is costly and buys you nothing useful, unless you need to fit into small pre-existing conduit.

If you would prefer to use the much more popular #2 aluminum or #4 copper, then use a 90A breaker instead of 100A. If your subpanel is rated 100A, look at your car tires - I bet they have a rating like 112 MPH. You don't need to drive 112 MPH nor feed a full 100A, it's just a "redline" :)

The loads on the subpanel need a NEC Article 220 Load Calculation against the feeder size. All loads (main panel and subpanel) need another Load Calc based on service size. If this is for EV charging, ask us advice before buying stuff - 99% of first-time EVers spend a lot of money they don't have to. And Harper's Law: Buy the wire last :)

Harper - Reinstate Monica
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You have plenty of space for the breaker. What you'll need to do is do an NEC load calculation on your main panel to determine your load and on your sub panel. You've got a 200 amp main which is a fairly normal residential size. If the subpanel is for a EV charger or hot tub that will normally be used late in the evening, you should be OK. If the sub panel will be heavily loaded during times when your main panel will be loaded, then you're going to have some problems. I've seen both sides of the coin where some people were tripping their main breaker and others never had any problem.

JACK
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    No, the proper way to assess load vs. service size is to perform an NEC load calculation - not to freelance amp readings at arbitrary times. – nobody Jun 25 '23 at 22:57
  • I'll get photos of the panels labels. The house is 1800 sq feet with all the normal appliances. Gas range and oven. I intend to run a fridge and small power tools in the shop and install a 50 amp service for a RV. – Terry Myers Jun 26 '23 at 11:39
  • @TerryMyers Gas heating and water heater? – JACK Jun 26 '23 at 12:21
  • Added the panel labeling. – Terry Myers Jun 27 '23 at 02:18