TIL: You can share a neutral and use a single black/red/white/green cable for two circuits

[SwampNut]

Well, I learned it yesterday.  This is useful, would have saved me some time and money when I added several things, particularly the wood shop.  I knew this was technically ok, but didn't know it was LEGAL and fully supported.  Plus it's legal to use it as both 120v and 240v at the same time.  As long as the neutral can carry the load of one circuit, you can run both to max (and if you do, the neutral sees 0 current of course).  You use dual breakers, so you stay on different phase legs, and so the entire circuit is de-energized at once.

 

[CALCXX]

Yes, the neutral will carry the imbalance of current. 

 

This link is an FYI,  if you ever get into J-boxes and need to seperate any neutral connections.

Shock potential risk is higher. 

 

https://circuitiq.ai/blogs/blog/the-dangers-of-shared-neutrals-how-to-avoid-being-shocked#:~:text=A shared neutral wire%2C often,neutral can still carry current.

 

another fyi, if the home run neutral is disconnected, the 120V devices will experiance 240v usually only once.

Pay close attention to the neutral. Especially the home run neutral.

[SwampNut]

1 hour ago, CALCXX said:

usually only once.

 

[CALCXX]

True haha

[superhawk996]

Seems like it would be perfectly safe, if those wires are good for the amperage, but WTF did they do that vs. single pole breakers?

[SwampNut]

If you have a circuit that includes two hots, whether the flow is intended to be hot-hot or hot-neutral, you have to have both hots turn off at once.  I'm not sure if that answers the question.  

[superhawk996]

7 hours ago, SwampNut said:

If you have a circuit that includes two hots, whether the flow is intended to be hot-hot or hot-neutral, you have to have both hots turn off at once.  I'm not sure if that answers the question.  

It appears that you have several 120v items connected to double pole breakers like you'd use for 240v stuff instead of individual breakers, kinda odd.

[superhawk996]

On 7/25/2025 at 8:44 AM, SwampNut said:

Plus it's legal to use it as both 120v and 240v at the same time.  As long as the neutral can carry the load of one circuit, you can run both to max (and if you do, the neutral sees 0 current of course).

I'm still trying to figure this out.  What 240v stuff uses a neutral?

[SwampNut]

51 minutes ago, superhawk996 said:

It appears that you have several 120v items connected to double pole breakers like you'd use for 240v stuff instead of individual breakers, kinda odd.

 

Not my panel, and that's the whole thing with the MWBC (Multi-Wire Branch Circuit).  The point of this post is a shared neutral, with a single cable carrying 2x 120v and 240v on a shared neutral and ground.  That has to be on a ganged breaker.  The part that's new (ish, since about ten years ago I think) is running actual 120v outlets and singular devices from such a circuit.

 

3 minutes ago, superhawk996 said:

I'm still trying to figure this out.  What 240v stuff uses a neutral?

 

Semantics are critical.  Nothing actually using the 240v split phase uses neutral, but a device can contain both 120v with neutral and 240 in the same chassis.  Ovens, dryers, many things use both 120 and 240 at the same time.  EVSEs technically should never run from your old-ass 6-50 outlet in the garage because they are supposed to have a neutral, don't really know why, but there's a document about it (which I didn't read but gets posted all the time).