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I'm installing a 240V outlet in my shop to run a welder and I'm trying to understand when a neutral is needed and when it's not. The welder has a NEMA 6-50P plug, so I don't need to run a neutral line, right? That said, there are plenty of 240V receptacles/plugs that have 4 holes/prongs. I presume these are for L1-L2-N-Gnd. Is this because whatever is being plugged in here has some 120V components?

Here is part of my confusion: my dryer can be wired in either a 3-wire and 4-wire configuration. If the dryer has 120V components (I suspect all of the control hardware runs on 120V and the only things powered by 240V is the heating elements) then what is the return path since there is no neutral? Does current return through the ground in this configuration (seems bad)?

Regardless of whether a neutral is needed or not, I plan on running the neutral to my new outlet just in case I ever need it (bad idea?). FWIW I'm in the US.

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    The old 3-wire dryer cable had a neutral, it was missing a ground. There were 3 insulated conductors. In the 3-wire connection the case of the dryer was bonded to the neutral at the dryer connection block. In the 4-wire connection the case ofvthe dryer is instead connected a separate ground which goes back to the panel.
    – Jim Stewart
    Commented Mar 10, 2021 at 16:45

3 Answers 3

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A nema 6 outlet has two hots and a ground, it is used for pure 240V equipment.

A nema 14 outlet has two hots, a ground and a neutral. It is used for equipment that has both 120V and 240V components.

A nema 10 outlet nominally has two hots and a neutral but no ground. However in practice the neutral pin is used as a combined neutral and ground conductor. Nema 10 outlets are forbidden in new work, but existing range and drier installations that were correctly installed to the code of the time are grandfathered and new appliances can be fitted with 3 wire cords and nema 10 plugs for connection to existing installations.

Combining the neutral and ground functions is problematic because if the combined neutral and ground wire breaks, the appliance case will become live. The system is therefore not "single fault safe".

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A neutral is needed when the device has components, like controls or motors, than run on 120v. Welders typically do not.

The 3 and 4 wire dryer are both 120/240, but a special exception to save copper during wars years allowed grounding via the neutral conductor. 50 years after WWII they finally decided the risk was greater than cost savings and removed that special exception to use 3 wire for certain appliances. The NEMA 10 configurations are specified as two hots and a neutral, that has some code implications for type of cable that was allowed and insulation and color of the grounded neutral conductor.

Not a bad idea to run neutral now, make sure you do your 314.16 box fill calculation for 4 conductors rather than 3. If you run #6 that would require 30 in³ (4 conductors, plus 2 for device, 5 in³ per conductor).

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    I actually think it was a more pervasive argument that there was too much confusion created by allowing NEMA 10's that the extra unique configuration just created unsafe violations.
    – NoSparksPlease
    Commented Mar 10, 2021 at 16:39
  • if that's the case, then they created even more confusion by having a mix of grandfathered NEMA 10 and new NEMA 14 all legal to use. Sometimes people will even switch their NEMA 14 for a NEMA 10 to plug in a three prong dryer, because they don't realize they should just switch the dryer's cord instead.
    – Nate S.
    Commented Mar 10, 2021 at 18:39
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    I really wonder why they use 120V components and add an extra wire requirement, rather than powering the electronics from 240V. The parts needed to do that should be easy to source from the rest of the world!
    – Criticizing Israel not allowed
    Commented Mar 11, 2021 at 13:26
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    @user253751 "The parts needed to do that should be easy to source..." not in the parts of the USA where "the rest of the world" is only a hypothetical concept.
    – alephzero
    Commented Mar 11, 2021 at 17:38
  • Not even from the rest of the world -- we use 240V 60Hz here too! It shouldn't be too hard to find a dryer motor, for example, designed to run on 240V. Control circuits are best implemented digitally these days, and power supplies for that are almost always 240V compatible.
    – Nate S.
    Commented Mar 11, 2021 at 18:27
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When do you need a neutral? When the instructions say you do. You would not need a neutral for a straight 240 volt welder. Your 3-wire dryer would have two hots, a neutral and a cabinet ground wire. You can tap its ground to the neutral but you couldn't tap it's neutral to a grounding conductor in the supply circuit. Running a neutral is always the right move but remember you'll be limited on loading because of the breaker.

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  • Not sure of why the down vote.
    – JACK
    Commented Mar 10, 2021 at 16:25
  • Ya, not sure. I found your answer helpful and actually upvoted it.
    – tnknepp
    Commented Mar 10, 2021 at 16:55
  • Maybe somebody thought "you can tap a ground to the neutral" applied to more than just the narrow NEMA 10 exception?
    – NoSparksPlease
    Commented Mar 10, 2021 at 18:21
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    @NoSparksPlease Good thought. I edited it so it refers to the dryer in question..
    – JACK
    Commented Mar 10, 2021 at 18:36

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