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I have 100 amp main panel that is feeding a subpanel (125 amps). The amperage usage in the subpanel is more like 40 amps or so. I am adding an addition where I plan to add another 100 amp subpanel (with load of around 60 amps). What is the recommended wiring to the new subpanel?

I have heard that

  1. you can wire the new subpanel to the old subpanel.
  2. You can somehow connect the new subpanel to the house main panel.

Edit: The main panel has only one breaker. Load calculation- taking in everything comes to around 90 Amps. What works and what is recommended?

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    Need pictures showing all existing panels - breakers and (if present, usually on the door) label showing allowable breaker types, etc. What is your utility service feed? Most important, you need to do a Load Calculation. Sep 21, 2023 at 5:16
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    40+60=100 means there could not be any loads fed from the main panel other than these two sub-panels, which seems highly unlikely, so you probably need a service upgrade. The load calculation will be informative.
    – Ecnerwal
    Sep 21, 2023 at 11:43
  • So the one breaker in the main (a 100A breaker?) is feeding the 125A panel? Or there's a main 100A breaker and some other breaker and the 125A panel is being fed by feed-through lugs off the main panel?
    – Ecnerwal
    Sep 21, 2023 at 17:38
  • So the one breaker in the main (a 100A breaker?) is feeding the 125A panel? <yes>
    – FuzzyWuzzy
    Sep 22, 2023 at 6:13

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First, if your NEC 220.82 Load Calculation is currently 90 amps on a 100A service, you will not be adding 60A of additional load... except using a load management technique. This is easier or harder, depending on whether the load is a dumb load like a hot tub, or a smart load like an EV. (guess what we get a lot of requests for... speaking of EVs, 50/60A is crazy overkill if you're not a hotel, see Technology Connections' primer on the subject particularly 28:15 and 32:55.)

Now you're saying your main "panel" only has one breaker. It sounds like a "Meter-Main. It's possible to tee off that, but it's annoying, and will involve at least $100 worth of Polaris connectors. It's fine to treat your "main subpanel" as if it's a main panel for this discussion. For the Load Management stuff, it doesn't matter that much whether you go to the meter-main or the "one with all your breakers", however it's going to get pretty crowded in that meter-main with both 3 Polaris splices and also the load management box.

I recommend that conduit be built legally (built empty and the wires pulled in) despite the added expense. Because #1 you're going to need that load management system, and it'll need data wires. And #2 if it's EV, Vehicle 2 Home is right around the corner, and we have no earthly idea which wires will be required for that (HVDC cables?) I would recommend 1" conduit, or preferably, 1-1/4" conduit.

Once that's in, decide which wire suits your budget - for 60A either #6 copper or #2 aluminum individual wires in the conduit. If you go with cable, just go 2-2-2-4 aluminum, though #6 copper SER (not NM) would suffice.

If this is for basic at-home EV charging, you could side-step the Load Management equipment by simply turning charge rate down to sensible levels for home charging. Adopt an ABC "Always Be Charging" strategy (within favorable energy tariffs if possible). That simply requires 12/2 cable from a 15A/240V breaker to a suitable location in the garage for a "wall unit" (the classy kind you see in the commercials). Configure the wall unit for 15A breaker/12A actual charge, and done. This is more than twice the speed of level 1, plenty for most people.

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