2 same phase circuits sharing a neutral

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Twoskinsoneman

Twoskinsoneman

Senior Member
Location
West Virginia, USA NEC: 2020
Occupation
Facility Senior Electrician
Can you have 2 circuits on the same leg share a neutral back to the panel.
Given that this neutral is sized to handle the full load of both circuits. Would it make any difference if it was a feeder to a subpanel? I know this is a weird one. An example would be two 30a circuits (two 30a breakers each having a #10 wire) with a single #6awg wire for a shared neutral. All single conductors run in the same conduit. Is it forbidden?
Thanks
 
In my opinion you can as there is nothing in the NEC that says you can not.

You can find in 225.7(B) that it is specifically referenced.

The 2002 Hand book even has examples of doing it.

That said I can't think of any way it would be a helpful.
 
Twoskinsoneman said:
I know this is a weird one.
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I agree with Bob: allowed, but not a great idea.

Just out of curiosity, can you share with us the reason for your asking this question? :-?
 
charlie b said:

I agree with Bob: allowed, but not a great idea.

Just out of curiosity, can you share with us the reason for your asking this question? :-?
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Sure. I'm trying to decide the best way to wire a vehicle's generator to a load panel. You may have seen a thread I started a few days ago about generator wiring. Basically we have a vehicle with an onboard generator. The generator is a single phase 120vac output. You wire into the generator by means of a 2 pole circuit breaker built into the unit. There is no potential between the two poles, meaning it is a single leg output, protected by a 2-pole breaker. From there it will go to two separate tranfer switches. The reason for two transfer switches is the vehicle has two external power inlets.
 
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Are you using this one leg output of the generator to power the two poles of your loadcenter? If so be careful of MWBC within the dwelling because of the extra current on the neutrals of those particular circuits. I dont know how it would effect your 240v loads being there would be no potential.
 
Cody K said:
Are you using this one leg output of the generator to power the two poles of your loadcenter? If so be careful of MWBC within the dwelling because of the extra current on the neutrals of those particular circuits. I dont know how it would effect your 240v loads being there would be no potential.
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This generator will only produce 120vac. No 240v loads.
 
Assuming this generator, and the inlets are only serving equipment on or temporarily utilized from or around the VEHICAL? NEC doesn't even apply! (well unless you live in it.) That said - after the generator is isolated from the load by the transfer switch it would be fine because it would just be a regular multi-wire circuit if you powered it by 120/240 or 120/208. I am also assuming 120 loads only?

30a you say???? GFI protection? "first make - last break" on the ground and neutral of the inlets? (Say pin and sleeve?) And why two inlets - are they isolated from each other?

Care to draw a diagram?
 
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e57 said:
Assuming this generator, and the inlets are only serving equipment on or temporarily utilized from or around the VEHICAL? NEC doesn't even apply! (well unless you live in it.) That said - after the generator is isolated from the load by the transfer switch it would be fine because it would just be a regular multi-wire circuit if you powered it by 120/240 or 120/208. I am also assuming 120 loads only?

30a you say???? GFI protection? "first make - last break" on the ground and neutral of the inlets? (Say pin and sleeve?) And why two inlets - are they isolated from each other?

Care to draw a diagram?
Click to expand...

Two inlets because of the nature of the use of the vehicle. Normally the generator is the primary source of power. If that source fails, the inlets are there for a backup. In that situation the best they will be able to hope for is to be able to draw off of some utility circuit somewhere. Or possible the shared output of another truck. These are almost always a 15 or 20 amp circuit. So we provide two inlets with shore cables that have 15amp ends.

In a situation where the operators of the vehicle know where they will be and how long they will be there, they can make arrangements to have as much power as they need through shore cables with 30amp twist locks on the end.

As far as a drawing. The drawings I do for my company belong to them but I've got a copy of AutoCAD at the house and I'll whip up a quick drawing of what I'm taking about.
 
I do understand that we don't fall under the NEC. There is no agency that is going to approve or disapprove my designs. Having said that, there are going to be good men working in the vehicles for extended times. Sometimes men risking their lives to save people in danger. I want to be diligent in finding the best way to provide these guys with a working, safe vehicle.
I try to treat the design like it is a house my kid is going to live in.
 
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