Brian (BigB)

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I was hoping to get more feedback on the original topic of the one big neutral for multiple circuits. I was hoping not to start another multiwire debate.

Sorry for going off the original question, I will go back to that. [Linked Image]

The original installation;

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8 120 V hots leaving in pipe along with just one #8 neutral

In my opinion is an NEC violation (240.4) had they used a 6 AWG 'common' neutral it could have been code compliant depending on breaker sizes and positions on the panel buss.

Eight 20 amp breakers all on the same bus could load the neutral 160 amps, the 8 AWG would certainly be toast if that happened. [Linked Image]

Eight 15 amp breakers evenly distributed between the two panel busses could load the neutral to 60 amps, 6 AWG @ 75 C would be fine for that installation under any possible loading of the eight circuits.

As Jon (winnie) pointed out;

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I don't see anything explicitly prohibiting or allowing the use of common neutrals in article 210.

In my opinion, what is not explicitly prohibited is allowed.

Here is a image from the 2002 NEC Handbook,


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Exhibit 225.3 A 120/240-volt, single-phase, 3-wire system (branch circuits rated at 20 amperes; maximum unbalanced current of 80 amperes).

They also show an example of a 208Y/120-volt, 3-phase, 4-wire system with eight branch circuits rated at 20 amperes, maximum unbalanced current of 80 amperes using a 4 AWG common neutral.

As far as proper sizing it seems that we just apply the same rules as always, use table 310.16 with the additional step of determining what the maximum unbalanced current could be.

Bob [Linked Image]

Bob Badger
Construction & Maintenance Electrician
Massachusetts