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I've seen this setup in use in other shops and am interested in using the technique myself. However, the mere mention of it seems to incite rabid pandemonium rather than civil discourse.

Drawing

I'd very much like to learn more about any potential safety concerns, especially if its use violates the national electrical code. I've been researching this for some time and have thus far found no issues with its use despite stern warnings and admonition from folks that were unable to articulate the reasons they felt it was a bad idea or cite the NEC code that prohibits the configuration.

If there are indeed issues with its use as many suggest, I would like to understand the reasoning behind those issues.

This is for a 1-man garage workshop where GFCI is required for 120V receptacles. I'd like to put several of these in. Each electrical box will have a dedicated home run back to a 100A subpanel. The 240V outlets will be used for tools with 240V single phase motors like a 3HP table saw, 3HP jointer, 2HP band saw, and 2HP dust collector. 120V outlets will be used for smaller tools like handheld drills, sanders, routers, etc.

Each electrical box:

  • One (1) 240V 20A double pole breaker
  • 12/3 Romex; red (hot), black (hot), white (neutral), bare copper (ground)
  • One (1) two-gang PVC electrical box
  • One (1) commercial spec 6-20R receptacle
  • One (1) commercial spec 5-20R GFCI receptacle

There are commercial dual-voltage outlets available (without GFCI protection), so it would seem this notion is not as outlandish, unacceptable, or dangerous as some would suggest. Here are a couple examples:

  • Leviton 5842-I (20A)
  • Eaton/Cooper 829V (15A)

This article seems to indicate this configuration is acceptable and safe as long as all rules and exceptions are accounted for. As far as I can tell, my drawing above complies with those rules and exceptions.

Is this truly acceptable? Complies with NEC? Will pass inspection? It seems pretty clear to me there is no issue. Perhaps I'm still missing something, hence the question.

Additional reference: NEC 210.4 Multiwire Branch Circuits (MWBC)

Thanks!

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Yeah, I work a lot with MWBC. That looks fine to me... It should limbo under the NEC 2017 guidelines. If you don't get this permitted by the time your state adopts NEC 2020, then your entire plan falls apart. At that point you'll need a 2-pole GFCI breaker, and the GFCI recep will be superfluous.

Your best defense to NEC 2020 is to get a HUGE panel. That way if the inspector nails you on a bunch of 2-pole GFCI requirements, you can just pop em in. For that reason I recommend you plan never to use double-stuff breakers; you can't upgrade them to AFCI/GFCI. Huge panels (like more than twice what you ever envision using) are cheap insurance. We generally try to help people save money, but chintzing out on panels is false economy.

The only exception is more of a suspicion. You generally have 240V loads when the load is too large for 120V. So that suggests a 240V appliance that will be 8A or more. If you try to use that at the same time as a high draw 120V appliance, you could overload that leg. For instance if you have a 11A 240V load and a 12A 120V load, the current on your 3 wires will be 11A - 12A - 23A. Which ain't gonna fly on a 20A breaker.

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    "one man shop" so unlikely to be using a drill at the same time as a saw - however, the dust collector probably should just get a dedicated circuit, period, as it's always used with other things running (and some are 240v with own circuit, but some (sanders, for instance) are 120V so....)
    – Ecnerwal
    Apr 23 '20 at 11:27
  • I'd be fine with 2-pole GFCI breakers honestly. I'm looking to get the biggest 100A (or 125A) subpanel I can. My main panel is an EATON 40/40 with full size breakers, so I was going to go with a smaller EATON that also uses full size breakers. Yep, the dust collector will be on a dedicated 240V 20A circuit. Apr 23 '20 at 15:27
  • If I were to use a 240V load and 120V load in the same box and the amp draw exceeds the 20A, wouldn't the breaker open the circuit? That's no different than plugging too many things into a 20A household receptacle circuit, pulling too many amps, and having the breaker trip right? The breaker is designed to protect the wiring isn't it? Apr 23 '20 at 15:27
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    @PaulFitzgerald That's the theory, yes. Of course Code requires you plan so this is the exception and not the rule; when a household breaker trips you are expected to rearrange loads so it doesn't happen again. If you're simpatico with GFCI breakers, that allows a favorable change at the receptacle end: you can use a plain 120V recep and split it so you have two subcircuits of 20A each (minus the amps in use by the 240V load). Apr 23 '20 at 17:11
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This is fine, provided you obey a few rules

While a MWBC serving a mix of 120V and 240V receptacles certainly may look strange to the uninitiated, or those who interact with electrical systems in a professional capacity yet are not trained to the full panoply of NEC requirements, this setup is Code-legal in the 2017 NEC, provided the breaker protecting it provides common trip functionality, which all full-width two-pole breakers do these days. This is laid out in NEC 210.4(C) Exception 2:

(C) Line-to-Neutral Loads. Multiwire branch circuits shall supply only line-to-neutral loads.

Exception No. 1: A multiwire branch circuit that supplies only one utilization equipment.

Exception No. 2: Where all ungrounded conductors of the multiwire branch circuit are opened simultaneously by the branch-circuit overcurrent device.

With the upgraded GFCI requirements in the 2020 NEC, this setup requires a two-pole GFCI breaker instead; however, that lets you split the 120V receptacle here to balance the load better, and means that all receptacles on the circuit are GFCI protected, which is also a plus.

However, despite the above, there are a few caveats set not by the NEC itself, but by the limitations of available hardware. First and foremost is that two-pole dual function breakers (that provide both arc-fault and ground-fault protection) just aren't made, so if you were doing this in a location where AFCI was required, you'd need to have a two-pole GFCI breaker in the distribution panel for this circuit feeding an auxiliary distribution panel with a two-pole AFCI breaker in it. (Of course, if you're feeding more than one circuit of these, you can use a bigger GFCI breaker to feed a subpanel with multiple AFCI breakers in it.) Also, some more modern AFCIs (GE MOD 3 CAFCI breakers, most notably) are incapable of providing common trip overcurrent protection to a multiwire branch circuit due to design tradeoffs they made in order to ease installation in shared-neutral situations generally. This isn't an issue if a single MWBC is used as the two-pole GFCI breaker provides common trip overcurrent protection for the circuit at the correct breaker rating, but is a showstopper in a multi-MWBC setup, as the GFCI breaker gets enlarged (up to 50 or even 60A) in order to feed the loads from the additional circuits.

Other than that, you'll need to treat the downstream wiring with the same cautions that are required for any other MWBC: namely, the hots and neutral for this circuit need to be grouped in the panel as per NEC 210.4(D) and 200.4(B) (a ziptie will do) and the neutral must be pigtailed to the 120V receptacles so that removing a receptacle doesn't leave the neutral floating downstream, as per NEC 300.13(B). Last but not least, in your situation, while most of the primary tools can be on a single MWBC, you'll want dedicated circuits for stuff that is running at the same time that a tool is (most notably, the dust collector).

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  • Thank you for the additional detail! Apr 23 '20 at 23:05
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    @PaulFitzgerald Note that double-stuff (Quadplex) breakers do not necessarily provide common trip. Check their labeling. Companies that make "non-common-trip" quads also make common-trip quads; it's just a matter of getting the right one. Apr 23 '20 at 23:12
  • @Harper-ReinstateMonica Understood. Thanks for the clarification. Apr 23 '20 at 23:34

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