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I just bought a lower-mid priced 120V/20A welder (Forney Easy Weld 140MP), and the manual details that the welder be plugged into a 20A outlet (with a 30A circuit breaker), or you can use a provided adapter to plug into a 15A outlet (with a 20A circuit breaker) for welds requiring lower power draw.

I’ve researched a bunch on breaker/receptacle amperage and building code, and thus am now aware that multiple 15A receptacles can be on a 20A circuit.

That aside, is the suggestion from the manual common practice? I would assume that a 20A outlet be on a 20A breaker, not a 30A breaker. I would assume that a 15A outlet would be on a 15A breaker (unless I was specifically informed otherwise).

I’d figured that a 20A welder on a 20A circuit would be a-ok. I definitely don’t have 30A circuits to plug into in my humble garage.

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  • From the product label what is the I(ieff) or primary current?
    – NoSparksPlease
    Jul 12, 2021 at 3:32
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    So a 20 amp outlet with a 20 amp breaker is not an option? Weird.
    – MonkeyZeus
    Jul 12, 2021 at 14:43
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    If you are using a 15A outlet and your breaker panel is not in your garage, you may want to invest into a sturdy power strip or some other power distro unit that has a built-in circuit breaker to plug your welder into. Otherwise you will be making frequent trips to the breaker panel if you are welding on the hotter end of your settings.
    – Christopher Hunter
    Jul 12, 2021 at 19:28
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    More background info: The purpose of a 30A breaker is to prevent more than 30A from flowing through the wire in the wall. The wire has to be sized for 30A (probably AWG 10). The purpose of the 20A outlet is to keep you from plugging in a device that draws more than 20A. So, code compliance aside, having a 20A outlet on a 30A wire with a 30A breaker should be safe. You can never exceed the current of any of the parts of the circuit.
    – Kevin Keane
    Jul 13, 2021 at 5:18
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    The reason for using a 30A breaker (and wire) instead of a 20A breaker is that welders (and a few other types of equipment) can draw much higher currents for brief periods of time. Because the times are brief, it's still safe despite exceeding the outlet's nominal amperage, but may falsely trip a 20A breaker (a "nuisance trip").
    – Kevin Keane
    Jul 13, 2021 at 5:20

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You're correct that 20A receptacles are not allowed on 30A circuits. 30A receptacles only.

The manufacturer is competent and the item is UL-listed. What the UL-approved instructions really mean is you would be allowed to wire a dedicated 20A welder circuit.

Special rules for a dedicated welder circuit: since it is dedicated to the welder, certain NEC rules allow a breaker size "bump" to avoid nuisance trips. The rules essentially allow the welder to surge above 20A for short time periods (see "Duty cycle") since the short overload times and long cool down times will prevent wire overheat. To avoid the surges tripping breakers, a breaker bump is authorized.

But that bumped circuit must be dedicated to that one appliance. Preferably the appliance should be hard-wired; your AHJ might allow you to use a single socket (not the usual duplex) which is labeled "Welder only" or some such.

You are NOT allowed to bump the breaker for a general-purpose circuit that is used for other stuff.

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    To add clarity: the breaker bump is permitted above the usual required gauge of the wire, e.g. 20A on AWG14 wire to the dedicated socket.
    – P2000
    Jul 12, 2021 at 17:07
  • @P2000 is the welder itself then required to have a breaker/fuse for its powercord so that in the event of a failure state that had it drawing eg 25A sustained it won't overheat or ignite the wiring and receptacle? Even if it's code legal, I think I'd have more peace of mind running heavier gauge wire because then only the receptacle would be at risk. Being nearby while welding I'd have a reasonable chance of noticing, shutting off the welder, and if needed using a fire extinguisher before the situation could get out of control.
    – Dan Is Fiddling By Firelight
    Jul 12, 2021 at 19:12
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    @DanIsFiddlingByFirelight The welder applies these exceptions as approved under UL. So the welder's cord would be included. Remember, there will be current peaks but no overheating of the chord or electrical wiring, as warranted by the manufacturer and validated by their UL certification. It would have some sort of internal protection circuit to not exceed 30A, or use 15A-30A more than a certain amount of time.
    – P2000
    Jul 12, 2021 at 19:21
  • @P2000: Allowing a breaker bump seems dangerous. Far safer would be to allow breakers with trip curves that allow higher peak currents for short durations.
    – supercat
    Jul 13, 2021 at 22:08
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    It is my understanding that arc welders, in general, regulate the current they deliver -- typically with a 'big knob' on the front. The reason the bump is allowed is that, when initially pulling an arc, there may be an instantaneous current spike, which is well understood and characterized. That, being no threat to the wiring overheating on the given circuit, is therefore allowed by the NEC, as long as the circuit is properly rated for the equipment otherwise.
    – MrWonderful
    Jul 14, 2021 at 23:49
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Generally the NEC prohibits 20A receptacles on 30A breakers, but Instructions are part of the NRTL Listing (UL/CSA/ETL), and override general NEC provisions. So if the product is listed it's code legal.

If you have a 20A receptacle on a 20A breaker you will need to operate at a less than max setting.

2017 NEC 110.3(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.

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    Caution to OP: to get 30A you cannot just replace the breaker, you also must verify that the wiring is AWG10. It likely isn't. 15A circuits are usually wired AWG14, and 20A are AWG12. So if you have a 20A receptacle, it will be AWG12 wiring and a 20A breaker.
    – P2000
    Jul 12, 2021 at 2:16
  • @P2000 i don't think we have enough information to determine minimum wire size. Wire size should be based on Art. 630 using I(ieff) or primary input current times the duty cycle multiplier.
    – NoSparksPlease
    Jul 12, 2021 at 3:36
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    Thanks P2000 - and yes, I would I no way be considering simply swapping out breakers/receptacles to create a 30A pathway that the wiring does not support - I’d have an electrician run a meeting 30A circuit if need be. I was more just confused by the recommended mismatch of breaker and receptacle.
    – b. insler
    Jul 12, 2021 at 3:49
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    Yes, NoSparksPlease, and my comment was for a general purpose circuit, as I assume this is OP's case since nothing about a dedicated (industrial) circuit was mentioned. From his reply it now seems he's well aware of the context.
    – P2000
    Jul 12, 2021 at 5:16

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