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I have a plasma cutter which can have 35 amps of 220 draw. The factory plug is NEMA rated for 50 amps, 220V single phase. My circuit is rated for 30 amps. I never intend to go above 30 amps when cutting.

What kind of receptacle with metal conduit should I use? (I am in the United States.) What size junction box does this require? Should I use an adapter/splicer/pigtail - not sure of the term, but it is a short length of wiring with a different receptacle and plug on each end.

I've searched various sites and none directly address this issue. Thanks.

Paul C
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2 Answers2

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This is a common mistake that I admit I have answered incorrectly in the past.

There is a table in the 2020 NEC (See note below):

old version of table 210.23(B)(3)

It's easy to quickly say "that's the rule, we're done here", but hold on a minute, when does the Code say that table applies?

NEC 210.21(B)(3) Receptacle Ratings. Where connected to a branch circuit supplying two or more receptacles or outlets, receptacle ratings shall conform to the values listed in Table 210.21(B)(3), or, where rated at higher than 50 amperes, the receptacle rating shall not be less than the branch circuit rating.

So that table applies to multioutlet circuits. What about a single outlet? If you look back a couple of paragraph you find a different rule for a different situation that does not reference any table.

210.21(B)(1) Single Receptacle on a Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.

So since 50 is not less than 30, then single 50A receptacle on a 30A circuit complies with the rule.

Note: I would have arranged the answer differently if I had noticed before completing answer that the image of the Table represents the table as shown in the 2020 and earlier versions of the NEC. The text of 210.21(B)(3) hasn't changed, but I think the 2023 NEC has tried to clarify that the table only applies to multioutlet circuits by re-titling the table Receptacle Ratings for Circuits Serving More Than One Receptacle or Receptacle Outlet.

NoSparksPlease
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If you are putting in a 50 amp outlet, you should put in a 40 or 50 amp circuit. 'Why is 40 a possibility?' you may ask. It's possible to use a 40 amp circuit on a 50 amp outlet because there is no common 40 amp outlet. If you are putting in a 30 amp circuit, put a 30 amp outlet on it and wire it for 30 amp, i.e. usually with 10 gauge wire. Then make yourself a 30 to 50 amp adapter pigtail. My suggestion is to use a 14-30 as it has neutral and ground, so it's useful for all sorts of equipment in the future.

You also may ask 'why can't I just put a smaller circuit on a bigger outlet?' It's because you have to assume whatever you install will be safely used by someone who has no idea what strange choices you have made in the past.

KMJ
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