High rise apartment distribution

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joelsandel

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I am working in a high rise apartment complex and the way the apartments are fed is by running a set of 7 wires inside an EMT. Each apartment is 120/240V single phase 100amp, There are 2#2 conductors per apartment and they all share 1 neutral conductor which is a #6 wire. I know that there is a section in the code that talks about it but I can?t seem to find it, please help me finding it and if you are familiar about this code please explain me in theory how the neutral works.
thanks.
 
215.4 Feeders with Common Neutral Conductor.
(A) Feeders with Common Neutral. Up to three sets of 3-wire feeders or two sets of 4-wire or 5-wire feeders shall be permitted to utilize a common neutral.
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Give me a minute, I think this is out of the 2011.

No, thats still in the 2011???
 
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Thanks for your time and I appreciate your answers.

I know about the common neutral code. But I see that contractors have been doing the common #6 neutral for 3 apartments in high-rise buildings for the past 40 years. I'd like to find out the theory behind it, and which part of the code I can find it.

Thanks,

Joel
 
LarryFine said:
Joel, welcome to the forum! :)

I seems that a #6 is too small for a single unit's neutral, let alone three units sharing one. :confused:
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Perhaps not a single apartment, but most likely for three. The common neutral has to be sized to carry the maximum possible neutral current of all three apartments.
 
A neutral can often be smaller than the phase conductors.
Remember that most 240 volt appliances draw no current from the neutral, or a very small current.
Lighting and small appliances draw the rated current from the neutral, but should be balanced between the 2 hots, with a reduced neutral load.

There are however limits to this downsizing of the neutral, which must be sized for the worst reasonably forseeable conditions, not average or likely conditions.
What if the central A/C or heat breaks? and the H/O uses a number of portable 120 volt heaters or A/C units.

In this case, the downsizing may be greater than permitted by code, though it will probably work fine.
Is "probably" good enough though?
 
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