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I'm wiring a steam generator (Thermasol PROIII-240) for my shower (it's a residential unit) and the specs call for a NEMA 6-50p outlet. As the unit draws 46 amps, I will be using a 60amp breaker with #6 NMD90 cable.

Stupid question, but is it OK to have a 50amp outlet on a 60amp breaker?

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    Why are you using a 60 Amp breaker and #6AWG?
    – JACK
    Jan 23, 2023 at 20:27
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    @JACK Depends on the cable. NM (which is almost certainly the case here) can only go up to 55A. USE can go to 65A, as can individual wires in conduit. Jan 23, 2023 at 20:33
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    @manassehkatz-Moving2Codidact Good point on the NM. I still think everything has to be in conduit.. raised in Chicago. :-)
    – JACK
    Jan 23, 2023 at 20:40
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    Aha! NMD90 is the part that we didn't know. Normal assumption (and 95% of the time it is correct) is NM-B. Jan 23, 2023 at 20:51
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    @manassehkatz-Moving2Codidact The one we had was installed in am enclosed shower. we'd turn it on and in 15 minutes take a steam bath, then turn it off until next time.
    – JACK
    Jan 23, 2023 at 22:03

2 Answers 2

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The manual does not specify a breaker size. However, it does list power requirements for each model:

MODEL       INPUT VOLTAGE KW RATING AMPS PHASE ELECTRICAL MAX. ROOM SIZE
PROIII-84   208-240VAC    6.0       28   1     NEMA 6-30P 84 CU.FT.
PROIII-140  208-240VAC    8.0       37   1     NEMA 6-50P 140 CU.FT.
PROIII-240  208-240VAC    10        46   1     NEMA 6-50P 240 CU.FT.
PROIII-395  208-240VAC    11        48   1     Hardwire   395 CU.FT.
PROIII-575  208-240VAC    15        69   1     Hardwire   575 CU.FT.
PROIII-750  208-240VAC    20        92   1     Hardwire   750 CU.FT.
PROIII-1200 208-240VAC    24        110  1     Hardwire   1200 CU.FT.

In addition to the PROIII-240 listing 46A and NEMA 6-50P (OP's question), the PROIII-84 is listed as 28A and NEMA 6-30P. If the breaker was to be sized based on 125% of listed Amps, then the PROIII-84 would need 28 x 1.25 = 35A and a 40A breaker (assuming there are no 35A breakers) and a 50A receptacle (since there are no 40A receptacles). Which tells me that through some combination of:

  • The rated Amps already including the derate (not so clear based on kW rating, but that is a possibility)
  • A decision, hopefully in consultation with UL, ETL or similar, that this device based on duty cycle or other factors doesn't need a full (or possibly any) derate

Based on that, and based on the next size up requiring a hardwired connection, I think it is reasonable to say that the PROIII-240 is designed for use with a 50A circuit and matching wire. Note that the manual specifies 90C rated THHN wire, not cable, though your receiving local approval of NMD90 seems perfectly reasonable as the issue is really "90C" and not "wire instead of cable". An interesting side effect is that it is likely that the PROIII-140 at 37A and 6-50P receptacle is, I suspect, really a 40A (but not 40A receptacles so specifies 6-50P) device that would match with a 40A breaker and appropriately sized 90C-rated wire.

It is not OK normally to pair a 50A receptacle with a 60A breaker, but in certain circumstances it is OK, subject to inspector approval. That being said, if the manufacturer says it is OK to hardwire, I would absolutely do that. Eliminates one common point of failure.

I did not find any reference to UL or ETL or similar safety rating. It could be I missed it, or it could be that they do not have official UL or ETL listing due to the costs involved relative to a small-volume product. The company appears reputable, so the concern there is that your AHJ needs to approve the specific product in order to avoid any problems in the future as the normal situation is "only connect UL or ETL (or similar) listed appliances".

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  • Really appreciate you taking the time to dig through the manual. In the interest of safety, passing inspection and time (I need to get this done ASAP), I'm thinking of hard-wiring it with the 60amp breaker and #6 NMD Jan 23, 2023 at 21:30
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    @AfterWorkGuinness since the manual says hardwire, do that. It gives your AHJ one less thing to take issue with. Edit: nevermind, it's the next size up that needs hardwire. I would still hardwire if the until supports it, it's one less thing to go wrong.
    – KMJ
    Jan 23, 2023 at 21:32
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    As I understand it, the primary match that has to be made is between breaker and wire, and your 6 AWG NMD90 with 60A breaker is perfectly fine. The question is whether the device is OK with that. It probably is, but I can't say for sure. Have you tried a simple question of the company: "What size breaker to you recommend for the PROIII-240?" Jan 23, 2023 at 21:32
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    @KMJ I absolutely prefer hardwiring. And I suspect that would be fine here. But again, the manufacturer would have that answer. Jan 23, 2023 at 21:34
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    It is not OK normally to pair a 50A receptacle with a 60A breaker, but in certain circumstances it is OK, subject to inspector approval. That being said, if manufacturer says it is OK to hardwire, I would absolutely do that. Eliminates one common point of failure. Jan 23, 2023 at 21:42
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50A sockets are not legal on 60A breakers. They do make 60A sockets, but they're industrial items with an industrial price.

Since it is a 46A device it needs a 57.5A circuit. You must round up to the next available breaker size.

The cable must also be appropriate for a 57.5A circuit.

6 AWG copper with a 60°C thermal rating is only good to 55A, so it won't cut the mustard.

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