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'[EE]: Common neutral was Bulb Life -- Burned out..'
2001\07\17@153512 by Paul Hutchinson

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Obviously I did _not_ RC about the NEC.

From what I'm reading on the list it sounds like the NEC definitely allows
sharing a neutral between two 120V branches with opposite phasing.

I'm also seeing that the code may or may not allow two branches with the
same phasing to share a neutral.

Personally I think it's a bad idea but I have seen it approved by the local
electrical inspector in a friends house. In his basement was a single duplex
outlet with the hot side tab removed to make them independent circuits. The
outlets were fed from a 12/3 cable and in the breaker box the hots were both
connected to the same phase. This had been installed by a licensed
electrician and approved by the inspector when the home was built. Years
later when he went to finish the basement my friend and I discovered the
arrangement and thought that it was not a good idea (double current in
neutral). He had the electrical and building inspectors come over before
starting construction and asked the electrical inspector if that arrangement
was OK and the inspector said it was fine.


With the double current problem when using the same phase and, the
subjecting devices to odd voltages if neutral goes up in resistance (or
open) problem with using opposing phases, I think I'll stick with separate
neutrals for all branches.

Paul

> {Original Message removed}

2001\07\17@161010 by Michael C. Reid

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From what I'm reading on the list it sounds like the NEC definitely allows
sharing a neutral between two 120V branches with opposite phasing.


>>  I have seen many homes wired with a 12/3 supplying 2-20 amp circuits,
opposite phases.  This does save wire but if the neutral is lost the circuit
goes to 220 and you can say goodbye to any equipment plugged in that is
rated for only 120.  I owned an apartment building years back in Chicago and
a tenant called to tell me that some of her electrical appliances were
smoking (clock radios, etc.)  Sure enough, the small apartment was fed by a
single 12/3 with 2 circuits.  The neutral had come loose in the breaker
panel and poof!  The previous owner was an electrician who had rewired the
building.  He saved time and money with this wiring job, and I had to fork
out to cover the blown equipment.

This is a tempting way to save money, but not for me, not with this object
lesson in my past!

Another critical part of NEC is that circuits have to be derated 80%.  So a
20 amp breaker should not be loaded to more than 16 amps, or 1920 watts.
Some electricians also state that for a load that is on under 3-4 hours you
can run it at 20 amps, but they still usually derate their circuits at 80%.
In the home automation industry, we always teach this basic rule.

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2001\07\17@212214 by Dwayne Reid

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At 03:34 PM 7/17/01 -0400, Paul Hutchinson wrote:
>Obviously I did _not_ RC about the NEC.
>
> From what I'm reading on the list it sounds like the NEC definitely allows
>sharing a neutral between two 120V branches with opposite phasing.

Absolutely!


>I'm also seeing that the code may or may not allow two branches with the
>same phasing to share a neutral.

It it definitely NOT allowed.  I just don't have the time to look up the
specific numbers in the book.

>Personally I think it's a bad idea but I have seen it approved by the local
>electrical inspector in a friends house. In his basement was a single duplex
>outlet with the hot side tab removed to make them independent circuits. The
>outlets were fed from a 12/3 cable and in the breaker box the hots were both
>connected to the same phase.

Everything sounded just fine until you mentioned that both breakers were on
the same phase - that is a definite no-no and you were smart to catch it
and fix it.

dwayne




Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax

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