Power provisions for the GF

Basically sharing the neutral wire?..not a good idea.

Wouldnā€™t the white and green wires need to be sized for 40 amps - 20 amps for each circuit? That would mean 10 AWG for those wires whereas the two hot legs would only need 12 AWG. No reason you couldnā€™t use 10 AWG for all of them, of course.

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Probably. But I expect itā€™s only a shortcut and ought not be used that way.

Although not sure why sharing that wire at the outlet is any different than sharing it at the bus bar which is what happens in your breaker box normally. All white wires are connected together on a shared bar in the box just as all green/ground wires are (green & white do get different bars).

I think the difference is that the white wire from an outlet or connection only carries the current that was delivered to that connection by the hot wire. In a standard 20-amp duplex outlet the 20 amps are split between both receptacles. In the split duplex outlet each receptacle is delivered with 20 amps and the combined current (40 amps) must be returned to the neutral bus bar by a single white wire. That white wire needs to be rated for the maximum current that it can see, which is 40 amps in this case. All of the neutrals for the circuit breaker panel do get connected to a single (neutral) bus bar but that bar is rated for much higher current than any of the circuits it supplies. Sorry, Iā€™m probably not explaining this very well Maybe @rpegg or someone else more knowledgeable will step in here.

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There is a major difference between an Electrical Engineer (me) and an Electrician. Just means when I electrocute myself Iā€™m able to understand why. Yeah, sharing a neutral could potentially put too much current on the neutral wire. But I think there are a couple other code violations. One that comes to mind is that if you have two circuits using a single neutral and you turn one breaker off with anything powered on the other circuit the neutral becomes energized at a receptacle you thought was dead.

edit: A little too tired to read all of the posts. I think others have said the same thing. Either way itā€™s really bad to get electrical advice from someone who just understands theory. Wired half of my house but kept pretty much to standard practice, nothing fancy.

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Why? Theyā€™re all sharing the same neutral at the bus. Without the load being live (black), thereā€™s no circuit from just the white back to the box and the black to the disconnected breaker. Any live black attached to the same neutral still is separated from making a circuit at the disconnected outlet.

I do agree the neutral would need to be sized for the full potential load of the shared circuitry but am not buying the argument that the shared neutral is in and of itself dangerous absent an overload. (I had similar issues in college - I still donā€™t believe in energy quanta, I just know thatā€™s the right answer to solve many problems. An electron that is here and then there but is never in between strains my credulity but I acknowledge that explanation is necessary at this point in our knowledge.)

My last thoughtā€¦ In theory you are correct. The code has to do with the fact that connections are not always perfect and that there is resistance in normal copper wire. One bad connection to get back to that ground and you could have energy flowing on the neutral seeking a better ground than the one you have wired. In a single circuit with the breaker off there is no way to get power on the circuit no matter how bad the connections on the neutral wire or ground.

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@MikeH: Yep, if you share a neutral between two circuits that can supply 20A each then the neutral has to be large enough to carry 40A (unless itā€™s a balanced three phase system with no neutral current such as you would find on an industrial site)

And as you say, while the bus bar is rated for much more than 40A, the neutral conductor running through the house may not be.

@jamesdhatch: a shared neutral definitely is dangerous. Unshared neutrals are also dangerous, for the same reason, but itā€™s much easier to be safe when you are working on neutrals fed from only one point. If you feed neutrals from multiple points, itā€™s very easy for an unwary electrician to accidentally shock or kill themselves.

If you have a single phase conductor and a single neutral conductor (as in any normal circuit), and you break that neutral either by removing and separating the looped wires going in and out of an outlet or by disconnecting it from the neutral bus, then that neutral will become live. You are holding a wire that goes back to your outlets, through some appliances, and back to a live bus bar. It isnā€™t held at neutral potential anymore because you have disconnected it from the neutral.

If you use a single neutral for more than one circuit, you could turn off the isolator for the circuit you are working on, disconnect the neutral, and then find that it was still carrying current supplied by the other isolator and receive a shock.

If you wanted to share a neutral between circuits, you would need to mechanically interlock the overloads so that if one circuit was isolated, they all must be isolated. Then you are effectively back to the same scenario as earlier with only a single circuit

In theory, you can work safely no matter how many circuits share a neutral. The problem is that it needs to be designed in a way that makes it impossible to not realise that the neutral is shared, or in practice somebody is going to end up dead.

Sorry, that ended up being a long response

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Weā€™ve run them with true sine wave UPSs but nothing else.

Yes, peak is just that, and will generally not be seen for a sustained period.

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My take away on this thread has been an invitation to understand electric power in all its variations. I had a rudimentary understanding of AC and DC and current and power, but getting in to the complexity of power supplies had me do some study. The Glowforge will make this all seem trivial, but some understanding is necessary when you are preparing shop circuits and power backup units. Hereā€™s to electrical engineers: the magicians of modernity. All the math whizzes of the world can come up with the Spells, but it takes the EEs to make the magic wands!

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The only piece of equipment that I ever used with the 20A plug was a Amp for a shaker table.
It was always fun using it, but would annoy some people in the office. I was usually running it in the upper end of the audible range (15-20kHz).
Fun fact, you can tell how old people are by slowly cycling down from 25kHz to 10kHz and seeing when they start to complain. :grin:

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Never noticed but will check when Im back in the office. For example, what devices would have the plugs on the right??

Large photocopiers sometimes do. (The Xerox Color C70 I use at work has a 5-20P plug.) You donā€™t often see NEMA 5-20 in the home, but where I work all of our outlets are 5-20R. But Iā€™ve only ever seen a 5-20P plug a handful of times. Some of the UPSes in my server rack have 5-20P plugs.

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Ooooā€¦thatā€™s cold :smile:

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can you say that again a bit louder sonny? I canā€™t quite hear you. I have a banana in my ear.

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I resemble that last sentence. Hearing is really not bad for the life Iā€™ve lived.

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ā€œItā€™s not the years, itā€™s the mileage.ā€

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Right? 40 years of hearing hazard career fields and just some High freq. losses.