Multi-Wire Branch Circuit with NO neutral return.

[glene77is]

Guys,
Cautionary Example !

Came across a Kitchen Circuit, for Dishwasher/Disposal and Kitchen Countertop.
with 240 V on the Countertop Recepticals.

Wired as MWBC, from the Loadcenter to the first Countertop Receptical, then on to the D/D.
Ckt #1 fed the Countertop.
Ckt #2 fed the D/D with Switch on Disposal.
Neutral was "shared" at this junction, and the joint was on the Receptical.

The MWBC fed into the junction box for the Countertop Receptical FIRST,
and then Ckt #2 fed down to the D/D.
The home-run neutral was broken free from the Countertop Receptical.
The Countertop Receptical was used for the neutral joint connection.

Thus the MWBC circuit appeared to be "dead", but was reading 240V.
The Countertop appliances were throwing sparks when turned on.
The Dishwasher had been zapped by the 240V and was not working, and was unplugged.
The Disposal was actually in working order, but without proper power.
The Disposal Switch was in working order, and had been left "ON".

Thus, with the Disposal Switch turned "ON"
the power ran from Phase Ckt #2 directly to the D/D junction box and D/D receptical,
up to the Disposal Switch, back down to the Disposal,
through the Disposal, and out the Disposal neutral.
The neutral carried this power back to the CounterTop Receptical.
Since the Home Run was broken free at the Countertop Receptical,
there was NO Neutral Return.
There was 240V on the Receptical by having both MWBC phases on the Receptical.

Fix:
Wire the MWBC Neutral system as a "Back-Bone" ALWAYS,
just as the current code specifies.

Checked other portions of his house.
This house has "push-in" wired recepticals everywhere.
This house has this same loose wire problem everywhere.
The house owner is unable to comprehend this problem,
and unwilling to accept the prescribed 'fix' of replacing every receptical in the house.
Fixed his kitchen receptical as a deadly safety hazard, and said "good-bye".
.

 

[Dennis Alwon]

The bane of the MWBC has been described by you....

 

[mbrooke]

They save money on wire, but throw a clueless home owner or electrician in there and well the value of them goes out the door. :slaphead:

 

[ActionDave]

They save money on wire, but throw a clueless home owner or electrician in there and well the value of them goes out the door. :slaphead:

I guess that leaves the handling of them to the few of us who are competent electricians.

 

[jmellc]

2 of my least favorite wiring methods; MWBC and backstabbing. What a disastrous combination.

 

[iwire]

2 of my least favorite wiring methods; MWBC and backstabbing. What a disastrous combination.

One of my least favorite things to read.

I use MWBCs as often as I can.

 

[jumper]

I use MWBCs as often as I can.

Ditto. 12/4 MC is a standard item for me.

 

[jmellc]

One of my least favorite things to read.

I use MWBCs as often as I can.

Good idea now that we have to use double pole breakers for them. Problem on 1 ckt trips both. I never had the neutral problem with the ones I used to run, as I made good splices & pigtails. But someone coming in later may not be as careful. I was already losing my fondness for them after troubleshooting a lot of them. When the double pole breaker became a rule, I quit running them altogether.

 

[iwire]

Good idea now that we have to use double pole breakers for them.

We don't, I stock handle ties for the breakers I stock.

Problem on 1 ckt trips both.

Use handle ties instead. But it is a stupid code rule anyway.

I never had the neutral problem with the ones I used to run, as I made good splices & pigtails.

I try to but I am not perfect, but that applies to any of the work we do.

But someone coming in later may not be as careful.

I can't tell you how little I care what someone does after I leave.

I was already losing my fondness for them after troubleshooting a lot of them.

I don't see much difference in troubleshooting.

When the double pole breaker became a rule, I quit running them altogether

Sadly I run less of them due to the code change.

 

[jmellc]

We don't, I stock handle ties for the breakers I stock.


Use handle ties instead. But it is a stupid code rule anyway.

2 SP breakers joined with handle ties are in effect a double pole breaker. 1 ckt trips, the other does too. A great idea if one of them is a refrigerator or freezer full of food and it trips just after the family leaves for vacation. They come back to a batch of spoiled food.

 

[kwired]

We don't, I stock handle ties for the breakers I stock.


Use handle ties instead. But it is a stupid code rule anyway.

2 SP breakers joined with handle ties are in effect a double pole breaker. 1 ckt trips, the other does too. A great idea if one of them is a refrigerator or freezer full of food and it trips just after the family leaves for vacation. They come back to a batch of spoiled food.

Handle ties do not trip both breakers - at least for the reputable brands. You likely will have to turn the other breaker(s) tied together off when resetting the tripped one.

Still a dumb rule other than maybe the MWBC that has receptacles with more than one ungrounded conductor landed on them, but handle ties is the best way to deal with it. A double pole breaker costs about the same as two single poles and a handle tie. A three pole breaker generally costs a lot more than three single poles and handle ties.