Microwave problems--Microwave and Refrigerator sharing neutral wire.

[sean_n]

In the last 2 years, our microwaves have been breaking very quickly. We suspect that it might be our electrical wiring. I've noticed that the microwave and refrigerator are sharing the same neutral wire. What's the chances of this being the cause of our microwave problems? I believe sharing the neutral wire was common practice in the old days--but is against code. Is this true? The microwave and refrigerator are on circuits that are out of phase--I believe that makes a difference but would like confirmation on this. We've had this setup for the last 17 years and only in the last 2 years we've been having microwave problems. I would hate to have to run another wire unless it's absolutely necessary.

[RBM]

It's called an Edison or multiwire branch circuit. It's perfectly proper and legal. It is essential that the two conductors sharing the neutral be on legs of different potential in the panel, which you seemed to have verified. It's also vital that the neutral be enact and tight or the potential for high voltage backfeed exists

[Joseph Meehan]

The only way it is likely to be a problem is if you have a poor connection on the neutral somewhere. This would be especially likely of the back stab connections were used at outlets. Check all the connections on those lines. Each outlet and at the breaker box.

It might be interesting to set up a voltage meter (better yet a recorder) at the microwave outlet and see what happens when the frig kicks in and out. A recording meter could record data over say a week to see if there are some outside problems.

You also should check the ground at the microwave.

[Mike Dobony]

This is right now legal and acceptable, but will not be in August 2008 as the code is changing.

[Kevin Ricks]

Describe the microwave failure. Shared neutral circuit are fairly common in older homes. I mostly see them on dishwasher/disposal circuits. I believe in Canada they are very common or required in kitchen counter circuits. A full load on both circuits will completely cancel each other and neutral will be at 0 current. A load of say 2 amps on one and 3 amps on the other will leave 1 amp on the neutral etc... As the other reply mentioned, if the neutral gets disconnected down stream then you will get up to 240V at the appliance plug depending on what is plugged into the circuits. This could certainly damage both the refrigerator and microwave. Kevin

[Wayne Whitney]

While there is a change in the 2008 NEC regarding multi-wire branch circuits (MWBCs), it does not make them illegal. In the 2005 NEC, if the MWBC served a single yoke with multiple devices, then a means of disconnecting both legs simultaneously is required. In the 2008 NEC, this is always required. The practical implication for residential work is that you have use a double pole breaker.

Cheers, Wayne

[Wayne Whitney]

That should say that you have to use a double pole breaker with handle tie.

Cheers, Wayne

[RBM]

His, and every other existing Edison circuit will still be legal come August