I will be installing a 125A subpanel using a #2 Cu THHN via 2" x 3"(L) conduit secured with four 2-in conduit locknuts (inside and outside of panels). The sub-panel shut-off will be a Q2125 125-Amp Double Pole Type QP.

The first photo shows the current configuration with a small, 8 circuit subpanel that will be replaced with a 20/20 125A box. I will further be adding 40A 240V (oven/micro combo), and 50A 240V (induction cooktop), as well as two 20A circuits, one for a dishwasher and another perhaps for an island outlet with beverage fridge (no sink or stove on the island).

I am trying to calculate the maximum capacity I can put on this subpanel, and whether I can just add the new circuits I mentioned above, or whether I need to rebalance with the main panel.

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  • Can you provide us information about what the GFCI circuit serves, as well as the wattage of the cooktop and the oven/microwave combo? Also, why are you using copper for a 125A feeder? – ThreePhaseEel Sep 3 at 22:53
  • @ThreePhaseEel, yes, the GFCI is for kitchen counter but after posting I realized that I already have plenty of GFCI circuits in there that I can use (4x). I added a diagram in the original question of what the two panels look like now (except sub is currently is small 8 circuit). I will be adding a 40A 240V (oven), and 50A 240V (induction cooktop), as well as 1 or two 20A circuits one for a dishwasher and another perhaps for an island (no sink or stove on the island). I have a new 125A 2020/panel that will be replacing the small one. – David Sep 4 at 18:10
  • @ThreePhaseEel Edited the original question to better reflect what I am trying to do. – David Sep 4 at 18:48
  • We don’t worry about double pole breakers when balancing by load, on the single pole you try to distribute the total on the odd breakers with the total on the even breakers for example we know 20’s can Cary more if they are all on even breakers L2 would normally be loaded heavier. Without the connected loads that would be your best guess at balancing getting as close to even numbers of 15 or 20 on the odd breakers as the even breakers. Pros use amp clamp meters and Measure with different things turned on to balance the loads. – Ed Beal Sep 4 at 19:37
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    That is reasonable and probably what I would do small loads are easier to to move around. – Ed Beal Sep 4 at 20:39

The trick is adding up the loads not the breakers you might have close to 200 double pole amps on a 125 amp panel and it could be ok but the size of the loads and the diversity of loads like heating / cooling all come into play but if you are putting in a new panel go big they don’t cost that much more.

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  • That makes sense, however I really don’t want to put another large panel there and already bought the hardware. I am thinking, the only toke I will ever be drawing full amperage is if I have washer+dryer+microwave+oven+all induction cooktop spaces on full power which will never ever happen. Worse case scenario it will trip the 125A breaker, isn’t it? If that starts happening, I could move a 59A breaker to main one and move a couple of 15A/20A ones over to sub. Does that sound okay? Also what do tou think of my wiring question? Thank you! – David Sep 3 at 20:35
  • Ok that makes sense but the actual loads need to be calculated if your hot tub actually draws 30 amps and your dryer 20 both are 240 v loads and right there you are at your max, washer and microwave probably around 12 amp max, ovens are usually 30 and up. Based on those loads I would say no it would not meet code if you have room in the main panel it’s possible to draw from there if it is large enough. – Ed Beal Sep 3 at 20:44
  • Ed, Thanks for the tip. I have added a current diagram. Currently, I have a small 8-circuit subpanel that I am replacing with this 20/20 one. I added a photo that shows the current distribution. To this, I will be adding 40A 240V for the oven, 50A 240V for the induction cooktop, and maybe two 20A c120V circuits. Could you help me balance it? Thanks.David – David Sep 4 at 18:02
  • Edited the original question to better reflect what I am trying to do. – David Sep 4 at 18:48

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