Multi Wire Branch Circuit Or Not?

[A/A Fuel GTX]

Hey Guys.........This is becoming more and more common. I go to do a panel change out and come across 3 conductor romex where the black and red wires are going to the same phase with the common neutral. The dilemma is, do I wire the new panel the same way or do I land the black and red wires to different phases and take a chance that the hack that did the original install properly wired it as a MWBC?

 

[infinity]

We had a lengthy discussion about this same topic a few months ago. IMO you need to make the correction.

Balanced neutral?

Wondering how you guys handle balancing a neutral on an existing job (residential 3 wire). Either doing a panel change or working in a panel, you come across a multi wire branch circuit with both hots on the same phase. Do you leave it that way? re install it that way after a panel change...

forums.mikeholt.com

 

[Dsg319]

Hey Guys.........This is becoming more and more common. I go to do a panel change out and come across 3 conductor romex where the black and red wires are going to the same phase with the common neutral. The dilemma is, do I wire the new panel the same way or do I land the black and red wires to different phases and take a chance that the hack that did the original install properly wired it as a MWBC?

Personally I would not replace it as it was. Can you investigate at the destination of the circuits, and maybe open junction/device boxes and see what all he has done. At least maybe get an idea.

 

[A/A Fuel GTX]

I read the aforementioned thread and picked up some good points. Rather than spending hours tracing out the affected circuit, splicing the black and red together creating a single circuit seems the safest play. Worse case scenario is the breaker trips from an overload. Chances of that are quite rare and then I don't have to worry about what was done downstream.

 

[James L]

I read the aforementioned thread and picked up some good points. Rather than spending hours tracing out the affected circuit, splicing the black and red together creating a single circuit seems the safest play. Worse case scenario is the breaker trips from an overload. Chances of that are quite rare and then I don't have to worry about what was done downstream.

Actually, worst case scenario would be if the circuit is double fed and nobody knows about it until they go to work on one of those circuits.

 

[A/A Fuel GTX]

Actually, worst case scenario would be if the circuit is double fed and nobody knows about it until they go to work on one of those circuits.

The two home runs are spliced together in the panel, going to one breaker so it's not a double fed circuit.

 

[infinity]

The two home runs are spliced together in the panel, going to one breaker so it's not a double fed circuit.

Electrically that is not a problem but if the original MWBC was the two circuits required to service the kitchen counter then there may be a problem.

 

[James L]

The two home runs are spliced together in the panel, going to one breaker so it's not a double fed circuit.

That's not what double fed means.

In your OP you said the black and red were on the same leg.
Then in post #4 you said you were planning to pigtail them, creating a single circuit.

What I'm saying is, and I've seen it a number of times.... say it was originally wired as a mwbc to a receptacle in the living room on the black, then out of that receptacle the black goes out to feed the rest of the living room and entry, and the red circuit goes out to feed the master bedroom and bathroom lights...

Then, a second person wiring the house ran a homerun to the switches at the front door, or maybe they ran a homerun to the toilet room switches...

You would have a double fed circuit. Let's say it was the living room plus entry circuit that got double fed. You would have it fed from the black from the mwbc AND a separate 2-wire circuit to the front door switches.

As long as those two circuits are on the same leg, it's going to work like nothing is wrong. But if somebody goes to work on it and take something apart after turning one of those breakers off, they can get lit up.

Just this week I was doing a panel change out, and I got shocked on the 8/3 double wall oven circuit, coming back. The oven had an auxiliary receptacle on the back for some kind of accessory that was not being used. Apparently, somebody thought they needed to wire for an accessory, so there was also a 12/3 ran. It was mistakenly marked "fridge" on the red and "spare" on the black.

Well, apparently someone thought the wiring for that accessory plug needed to be fed, and they hooked the black from the 12/3 into it. Little did they know, it was internally tapped off the oven wiring from the 8/3

The way it was wired, there was a 240-volt dead short if everything was situated as typical. They got around it by swapping the red and black of the 12/3 to the opposite legs. It didn't blow up. Nobody thought there was any problem. But then I told them they're looking at a time and material bill for me to fix it