Improper MWBC

Here’s what I found today, and I thought it would be a good exercise for the newer inspectors. So to the veterans and electrical experts, please give the other a chance to answer first.

This panel was installed as a 120 volt panel. Forget the multiple conductors under the single lug for a moment.

The top left breakers are wired as a multi-wire branch circuit (MWBC) with a shared neutral.

  1. In a proper MWBC, the handles should be tied together, but in this case, a MWBC should not be installed. Can you tell us why?

In the bottom right, there is a tandem breaker - two separate circuits also with a shared neutral.

  1. Is the tandem breaker allowed to utilize a shared neutral? Please explain your answer.




I think the MWBC can’t be there because it’s a 120 volt panel. Each breaker on an MWBC must be on a different phase but i don’t think that panel has alternating phases.

This could allow 40 amp on the neutral because they are on the same phase and the conductor is not rated for that.

Am I close? :slight_smile:

Judt a note: the requirement for a handle tie or a two pole breaker is a recent change. Before the change you only needed a simultaenous disconnect is both hots landed on the same yoke.

Actually, you’re spot on and so is Juan.

MWBC’s must be on separate legs of 120, otherwise the neutral can become overloaded.

The tandem breaker requires two separate neutrals (although I believe there’s an exception) otherwise the neutral may become overloaded.


I am guessing by the type of enclosures that this if for an inside installation. Is this correct?

Also I see in the second picture conductors that raises questions. What is this panel set up to do?
Are those enclosures beside the panel timers?

The panel and timers were all exterior mounted, but enclosed in an exterior rated utility panel. One breaker box and four timers.

They served exterior lighting, a barbeque area (exterior kitchen) and the pool equipment.

The reason I asked was for outside lighting circuits Article 225 states
225.7**(B)** Common Neutral. The ampacity of the neutral conductor shall not be less than the maximum net calculated load current between the neutral conductor and all ungrounded conductors connected to any one phase of the circuit.

I don’t see any conductor that seems to be sized larger than a conductor on the breakers so those who have answered are correct but I wanted to be sure to post this.

I have on many occasions installed a #8 neutral for Christmas Tree lots for the festoon lightning.

So theoretically, if you had a size 8 neutral the tandem breakers would be ok?

Yes, that was the exception that I was referring to.

In this case the majority of it was wired with MC (Greenfield) 12/4, with relatively light loads whereby the neutrals may never have become overloaded. However, without an “up-sized” neutral I decided it was best to call it out.

In my professional opinion you made the correct call and I stand behind you all the way.

If I stand behind you and they start swinging they will get you first lol

I was pretty confident with my call, but confirmation is always a good thing. What’s your phone number? :smiley:

For Article 225 to apply the wiring would have to be on the outside of the building.

Jeff how did you know it was an MWBC. Sometimes I try to follow the wires if I suspect an MWBC but its usually to messy and i don’t move my hands into the panel to move stuff around.

I didn’t know that, but this was indeed exterior wiring. None of it was within the structure. Even the boxes were surface mounted.

Then you could use the common neutral provision of Article 225. Here’s the scope of that Article:

Thanks Robert.

I guess based on 225.7(B) I may have made the wrong call with regard to the lighting circuits, but without knowing the net load, I can’t say for sure. However, there were MWBC’s other than lighting-circuits in the panel, such as a garbage disposal, small outdoor refrigerator, barbeque-spit, etcetera. Plus several receptacle outlets, all on a single phase of 120V.

These were pretty apparent with the MC 12/4. The MC has one black, one red, one green and one white conductor.
All of the other colored wires in the panel (orange, grey, blue, yellow, etc.) were part of the timers.

Each red and black wire terminated at a breaker (even though they were difficult to follow) and there was only one neutral in each conduit, so it couldn’t have been anything else but an attempt at MWBC’s.

Jeff you made the right call. The white conductor would have to have been twice the size of the black and red conductor of each branch circuit as well as for the feeder supplying the panel. Or to be perfect it would have to be sized to twice the load on any one hot conductor.

I stand behind you all the way.
Good call.

Gotcha. Kinda seems like a stupid question now.