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I was reading the post from the link below. I had a question with regard to that line of discussion. I can post there but it was old. I have a GE main service box, 24 positions, that has a double pole 125A main breaker.

Do the breaker amp readings sum up to dictate my current panels total / possible amperage?

There is currently a 40 Amp double taking up 4 slots on the right side for an apartment size electric stove. There are 4 more circuits for plugs, and a regrigerator is on one of those. I will add a disposer onto the kitchen circuit but that only has the overhead light plugs. The main service is for a 450sf tiny apartment of one bedroom, a bath and kitchen. Its being renovated but budget is tight.

I want to add breaks for a 24Kw on demand water heater and a small split unit AC. Water heater at 240V, requires two 60 amp breakers, and the AC, MRCool, 18K BTU, will require a 15A to 20A, double pole.

Can the main service that is currently in operation accomodate the additional load. I know the breakers will fit fine. Looks doable to me.

Breaker main Service Panel

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  • Welcome to diy.SE! Just a note, that is a 12-space panel, not 24. Some panels allow some (or all, or none) of the spaces to have those thin double-circuit breakers. So, it may very well be a 24-circuit panel, but it is only 12 spaces. Can't tell for sure without the model number or a view of the bus bars.
    – mmathis
    Nov 12, 2016 at 20:00
  • Why the tankless heater btw? A well-insulated tank type unit pulls much less instantaneous power, and is actually better from an energy conservation standpoint than a tankless unit.
    – ThreePhaseEel
    Nov 12, 2016 at 22:09
  • The current 15gal heater is in the kitchen in the corner under the end of the sink base. I feel it's woefully inadequate for a nightly rental with kitchenette but I agree if I could do away with the tankless I would. Wish there was gas .
    – John Rost
    Nov 13, 2016 at 2:17

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The breakers will physically fit into the panel, but you'd probably start tripping things whenever the water heater turned on.

At 24 kW @ 240 V, it's pulling 100 A* on its own. Add in lights, receptacles, fridge, or especially the stove (pulling maybe 30 A*), and you're well over the limit of the 125 A main breaker.

If you want to install that water heater, you'll need to upgrade the main service - which could mean upgrading the service entrance wires from the utility, the meter, the panel, the wires from the meter to the panel, or any combination of the above.

Installing the AC unit should be doable, though, since it's such a small unit.

*the water heater won't actually pull 100 A (nor the stove 30 A) all the time. Depending on your incoming water temperature, hot water set temperature, etc it may not need that much power. The stove only pulls 30 A when all burners and the oven are on. That said, you don't want to have things work ok in the summer but not the winter. Or only if you're using 2 of the burners rather than all 4.

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