I'll give you that some multi-wire circuits can be used for lighting, maybe a hair salon and such.
Nope, I have to disagree. In my brother-in-law's first salon location, the lighting (all 2 x 4 troffers) was wired with a shared neutral and two of the three phases used. We (meaning me) were constantly replacing ballasts.
Now in the new shop (see "The case of the missing phase.." thread) I wired all lighting with dedicated neutrals per phase and we have lost ONE ballast in 15+ years. Hmmmm......I seriously doubt that the
quality of the troffers has gotten better over the years.
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In the cinema business, sound racks are frequently served with more than one circuit. By 20 years of experience I can positively state that the racks wired with shared neutrals have more problems with induced noise and premature amplifier failure than those where dedicated neutrals are used. In fact, I will VOID WARRANTY on any sound equipment wired/installed with shared neutrals.
Some people can argue that from a strict engineering/theoretical standpoint that it shouldn't matter, as long as all connections are tight and the circuit is not overloaded.
But electricity, despite many years of application, is a strange beast and will behave in unexpected and unpredictible ways. To quote from a book I have, "Industrial and Control Power Systems Handbook"; {Pub. McGraw-Hill, auth. F.S. Prabhakara, Robt. L. Smith Jr., Ray P. Stratford} :
One prominent systems engineer has stated that is mathematically impossible to tie the {power} grid together, but the work and study of electrical industry engineers has made it possible.
{added text by me} Bold emphasis mine also.
In summary, shared neutrals for branch circuits of any kind are not a good idea. As I've said many times on the forum, just because the Code allows it doesn't mean it's always safest or right.
The arguement of increased cost, especially on large jobs, doesn't wash with me. If you're spending that kind of money on large job, what is the REAL cost of the extra wire? In actual percentages, please? On a 20-screen cinema, I would spend maybe an extra $50.00 on wire. On a total materials bill of over $4,000.00
Just my 25 cents. (Used to be 2 cents, inflation you know.
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edited to fix first quote
[This message has been edited by mxslick (edited 01-19-2006).]
