I'm wiring a heater that uses 7.5kW and 240V. It therefore uses 7500/240 = 31.25 amps. Can I use a 35 amp breaker? The instructions suggest to use a 40 amp breaker.

The reason I am asking is I have a 6 wire that I will be splitting, and I'd like to use it for two items: a dedicated 20 amp circuit for a hot tub and have the remaining 35 amps for the heater. The tub technically only requires a dedicated 15 amp circuit, but I am afraid that a whirlpool hot tub should have more.

If it matters, I will not be using the whirlpool tub and heater at the same time.

Here's the description of the heater. On page 6 it lists the amperage and breaker size of the 7.5kW heater.

Here's the description of the whirlpool heater. It clearly says the motor only requires a dedicated 15 amp circuit, but that, to me, is very surprising. I wanted to install a 20 amp circuit just to be safe.

That way, I'd have 35 amp breaker for the heater and 20 amp breaker for the whirlpool motor, so 55 amps for the 6 wire that is there. Or, I could do what both instruction manuals recommend and do 40 amps for the heater and 15 amps for the whirlpool motor. What do you think? Thanks!

  • 3
    Not following the instructions is an automatic inspection fail. Breaker size as two duties, the first is to protect the wires inside of wall from burning. Second and less important is for device protection, an extra 5 amps on a motor might be dangerous.
    – crip659
    Commented Jan 25 at 21:01
  • 2
    @PeteBecker But go to 20 for a 15 amp motor. I have never noticed(or looked) for a 35 amp breaker. Do not think a 35 will work for the 125% derate either, OP needs the 40 for 31.25
    – crip659
    Commented Jan 25 at 21:18
  • @crip659 -- yup; deleted my misguided comment. Commented Jan 25 at 21:21
  • What is a 6 wire? A 6 gauge cable feeding a subpanel?
    – jay613
    Commented Jan 25 at 21:31
  • 1
    A 35 amp breaker will have a derate of 28 amps, which is less than your needed 31.25 amps, so you need the 40 amp breaker.
    – crip659
    Commented Jan 25 at 21:37

1 Answer 1


The heater uses 31.25A when running. Since a heater can run for long periods of time, it has to be derated. Meaning the circuit (breaker and wire) have to be rated to 1.25 times as much current, 39A, which means a 40A breaker. In fact, you'll find a lot of things have some pretty strange sounding current requirements until you add 25% and see that they get close to, or often exactly match, a standard breaker size.

So 40A is an absolute for the heater.

As far as the hot tub, if it says 15A then that's what you need. That being said, it is quite normal and safe to use 12 AWG wire and a 20A breaker where you only strictly speaking require 14 AWG wire and a 15A breaker.

And now the fun part:

a 6 wire that I will be splitting

You can't just "split" a wire to two separate loads that happen to add up to the total capacity of the wire. Well, in some cases you can, but I certainly wouldn't recommend that here (40A + 15A). The correct thing to do is to either run two separate circuits with separate breakers or to install a subpanel, and install the two separate circuits with separate breakers in the subpanel.

You seem to have a fixation with 6 AWG wire. Remember Harper's Law - Buy the wire last. In addition, once you get to larger sizes there is no one capacity. 6 AWG is a good example. See this chart for details:

  • 6 AWG NM-B copper cable (a.k.a., Romex) = 55A
  • 6 AWG copper wires in conduit = 65A
  • 6 AWG aluminum wires in conduit = 50A

In addition, once you are in a subpanel feed situation, you should seriously consider large aluminum wire. For example, 2 AWG aluminum is good for up to 90A and will cost a lot less than 6 AWG copper. Aluminum wire does require compatible devices on each end, but all modern breakers and subpanels can handle it without any problem.

But before you do anything, figure out whether you actually have enough capacity for the heater and hot tub. That is done using a Load Calculation on your main panel. If it is good, then you can look at:

  • Wire size
  • Subpanel type/size - bigger generally better for future expansion
  • Ground rods if this is in a separate building

Note that anything relating to the hot tub will require GFCI protection. I don't think the hardwired heater will require GFCI protection, but the code keeps changing so anything is possible.

If it turns out you don't have the capacity to properly add this huge heater, consider a heat pump instead. Higher up-front cost but much lower running cost and much lower power requirement.

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