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.
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
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.
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.
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.
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