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derating

ICE

ICE

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Jun 23, 2011
Messages
13,897
Location
California
310.15(B)(5)(a)
A neutral conductor that carries only the unbalanced current from other conductors of the same circuit shall not be required to be counted when applying the provisions of 310.15(B)(3)(a).

Contractors and others want to apply this to every shared neutral. That neutral has the potential to carry an unbalanced load as is the case for a multiwire branch circuit that feeds lighting or a duplex receptacle for a dishwasher and garbage disposal.

Alrighty then, is the potential to carry an unbalanced load the same as "only the unbalanced load"? I say no. The logical use of a multiwire branch circuit is that it will usually not carry an unbalanced load. And if it has the potential to carry neutral current from just one line then it does not qualify for elimination when counting the current carrying conductors.

What say you?
 
excuse the bump but I would appreciate hearing other opinions on this.
 
Here's the problem.....I don't understand the reasoning behind the code. If the neutral caries no current as is the case in a shared neutral with a balanced load, that neutral shall not be counted as a current carrying conductor.... since the load is balanced and there is no current flow, that shared neutral is by definition not a current carrying conductor.

A shared neutral that caries an unbalanced load does have a current flow.....but the code says that it is not to be counted as a current carrying conductor.....with a caveat that the wire can ONLY carry the unbalanced load. With almost every mutiwire branch circuit there is a potential to carry the current from just one line. That is the case with any receptacle that is fed by a multiwire branch circuit. Most other instances of multiwire branch circuits could also have current flowing from a single line of a single phase system. The code does not appear to recognize that.

For the purpose of counting current carrying conductors to determine a deration calculation a shared neutral is apparently exempt in all cases wheteher balanced or not. That seems at odds with reality Line A and B conductors can have let's say 10 amps one A and 4 amps on B with the neutral carrying 6 amps. So all three are current carrying conductors. Now let's unplug the heater that draws 10 amps. The neutral caries 4 amps from line B and the conductor from line A caries zero amps. In that scenario the neutral is counted as a current carrying conductor but the line A conductor that is also counted as a current carrying conductor caries no current.

This just goes in circles.
 
An MWBC does not always have a balanced load, only 240v motor, for example has a balanced load providing there are no issues with the windings. You can try to balance it out all you want but you can't control future usage of the load center.
 
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