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The heat pump requires a 15A fuse/circuit breaker. I was told that it should work as long as there are 4 wires coming into the panel because that would make it 240V... 2 hot, 1 neutral, 1 ground. Mine seems to have this.

But since the total breakers already installed = 115A and the panel is only rated for 125A, this would put me at 130A. Do I need a bigger sub panel? I'm uncomfortable with the total amount of fuses/breakers on the panel exceeding the maximum capacity of the panel itself, when it doesn't have a main breaker on the panel.

I've read that the "rated current" is typically a lot less than the fuses/circuit breakers sizes, so in this case - I wouldn't be coming close to that 125A maximum. Can someone please suggest me a solution, or am I good to go as it is already? Thanks!

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    Breakers adding up to more than the panel rating is common and unproblematic, as long as the upstream breaker is sized to protect the panel. Commented Apr 4, 2023 at 5:49
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    @AbeKarplus I agree that over-subscribing a panel is very normal but the upstream breaker is for protecting the installed wiring, not the sub-panel. Clearly it shouldn't be more than the sub-panel is rated for, but technically the upstream breakers purpose is to protect the installed wiring. Commented Apr 4, 2023 at 10:33
  • What kind of heat pump do you want to install? Where is this? Commented Apr 4, 2023 at 11:58
  • You may have the power capacity for adding another circuit, but from your picture, it looks like you don't have room in your circuit breaker panel to add any additional breakers. If you need to add an additional beaker for your heat pump, you'll need to expand your breaker panel to provide the space.
    – Milwrdfan
    Commented Apr 4, 2023 at 16:21
  • Is this a heat pump water heater or a heat pump minisplit for HVAC? Commented Apr 4, 2023 at 21:15

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Actually the max draw on each phase is 20+15+30 = 65A and 15+20+15 = 50A because of how the phases are laid out in the panel (one phase feeds the top and third row, while the other feeds the second and fourth row) and the neutral will only ever see the max of the phases. So adding 15A on both is perfectly fine.

As long as the breaker feeding the subpanel back in your main panel is less than 125A then it will be fine.

You are looking at what the load calculation says you can have vs. what the maximum potential draw is.

Because most loads aren't drawing their max most of the time if ever you can exceed the max draw of a panel in breaker total, as long as there is a main overcurrent protection that will trip when the maximum rating is actually exceeded.

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But since the total breakers already installed = 115A and the panel is only rated for 125A, this would put me at 130A. I'm uncomfortable with the total amount of fuses/breakers on the panel exceeding the maximum capacity of the panel itself...

Not how it works. It's totally normal to oversubscribe panels. Circuits are unlikely to all be maxed out at the same time, so a different method is used to determine panel capacity: a NEC Article 220 Load Calculation. It generally uses VA, which are comparable to Watts. It starts with the square footage of living space served out of that panel (3 VA per square foot is a catch-all for lighting and general plug-in loads)... gives a 1500 watt allocation for kitchen and laundry room general-purpose receptacle circuits (if powered from this panel), and adds in certain fixed-in-place loads (like that heat pump).

As far as the amps, because of the oddity of North American 120/240V power, you have twice the amps you think. Here's probably the funniest video to learn about this, watch at least the first bit when it gets to panel phasing.

Now let's talk about that 30A breaker. Does this outlet serve a small Travel Trailer using a TT30 120V/30A socket? That is the only legit use of a 30A breaker I see regularly. The travel trailer outlet is allocated 3600 watts in the Load Calculation because of the RV rules (NEC 551.73).

when it doesn't have a main breaker on the panel.

On the panel? ... OK but let me ask you this question. See those 4 big wires? Where do those go?

Dollars to donuts, they land on a circuit breaker in another panel somewhere else. And that circuit breaker protects both the cable you just followed, and also this micro-panel from your photo. It is, functionally, the "main breaker" for this panel.

So yes, you have one after all.

Do I need a bigger sub panel?

Yes, but only because this panel doesn't have nearly enough breaker spaces. I mean you've already filled it and it's not even at capacity. This is why we always encourage people to "think BIG" - panel spaces are cheap. When you're buying the panel. When you've run out, spaces are expensive.

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  • Are those "spaces" on the upper right availble for breakers? Commented Apr 4, 2023 at 21:42
  • @Jim I mean it'll be full after they install the heat pump. Commented Apr 4, 2023 at 21:49

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