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I'm wiring an electric kiln that consumes 43 amps. Since it's a continuous load device, I understand that the wiring needs to be capable of handling 125% of that load, which is 53.75 amps.

I have a 25ft length of 6/3 NM-B (https://www.amazon.com/Wirenco-Non-Metallic-Sheathed-Residential-Equivalent/dp/B08R6LQGGR/) whose allowable ampacity is 55 amps.

There are no 55 amp breakers, but I've read that for applications under 800 amps, one can use the next largest standard size breaker - in this case 60 amps.

I'm clearly not an electrician (and am trying to learn so please don't ream me out for asking).

Is my understanding correct?

If so, why is an upsized breaker safe in this scenario? It seems like if there were a continuous load of 59 amps it would not trip the breaker but would exceed the capacity of the wiring. How is that okay?

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  • 1
    Good question. My instincts say go up a size on the NMC, especially if you can convince Amazon to take the roll you bought back and issue credit for it.
    – keshlam
    Commented Apr 16, 2023 at 3:26
  • If returning the cable, use conduit and THHN so the 75°C rating applies which makes 6 AWG copper 65A wire. Or the 4 AWG aluminum you spend a lot less on is.
    – Ecnerwal
    Commented Apr 16, 2023 at 11:27
  • I actually didn't but the wire yet, but I've used the exact same wire for a less amperage kiln in the past, so that was my plan. Any 4/3 I've found is much more expensive, and I'd rather not have to use conduit. Commented Apr 16, 2023 at 14:24
  • * Buy the wire. Commented Apr 16, 2023 at 15:19

2 Answers 2

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That is correct.

It is deemed safe because of the fineness of "standard" breaker sizes.

  • every 5 amps from 10A to 50A
  • every 10 amps from 50A to 110A
  • every 25 amps from 125A to 225A

So you're not likely to be more than 10% over, and that's about the manufacturing tolerance of the breakers anyway.

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  • Regarding "manufacturing tolerance of the breakers", are you saying they'll break at 90% of their stated amperage (so somewhere around 54 amps for a 60amp breaker)? Commented Apr 16, 2023 at 15:22
  • Otherwise, I still don't understand why it's safe to use wiring rated at 55 amps and a breaker rated at 60 amps... it seems like pulling a constant 59 amps from that circuit will eventually cause a fire or melt wiring. Commented Apr 16, 2023 at 15:24
  • Perhaps my entire understanding is faulty... I assume that the wiring should be rated for higher amperage than the breaker, such that the breaker would shut off before the wiring hits its limit. Commented Apr 16, 2023 at 15:26
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    @AaronCicali a 60 amp breaker doesn't trip at 60 amps in any case, it is a curve vs. time. The wire ratings have enough safety margin to deal with this, unless you stuff them inside insulation or similar.
    – jpa
    Commented Apr 16, 2023 at 16:29
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    @AaronCicali We'll have to tell you the secret. All this stuff is built with a fairly wide safety margin. NM and UF cable are limited to 55A because the insulation will discolor and degrade over years if run higher than 60C thermal. (but it's cheap). But #6 wire inherently is good to 65A at 75C or 75A at 90C. Wires with better insulation can exploit that. However breaker lugs are only rated 75C. Also, slight overloads are very unusual failures. So NFPA determined that it was an unusual enough edge condition with low enough consequences that allowing a "round up" was alright. Commented Apr 16, 2023 at 22:48
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I see two reasons why this is allowed:

  • Hardwired Devices

This is not, generally, for plug/cord connected devices. It is for hardwired devices. With receptacles there is concern about overdoing it, and in fact there are real problems with people putting multiple 15A-rated (12A continuous usage) space heaters on a 15A or 20A circuit. But with a 43A kiln, you are not putting other stuff on the same circuit. You can't put receptacles on the circuit because this one device uses more than 50% of the circuit capacity. And you can't add more hardwired equipment because you would exceed the 55A rating of the wire.

  • Failure Modes

I would expect an electric kiln to be basically a giant toaster - i.e., resistance heat. You are unlikely to see a 40% increase in current/power usage. That actually can happen (and more, but usually for a very short amount of time) with motors, but not with resistance heat. The more likely problem is some sort of catastrophic failure, which is more towards short-circuit territory and the breaker will do its job just fine.

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