sub panel neutral conductor

[Trouyano]

Good morning

I am converting a 200amp main panel to a sub panel. The ground bars are bonded to the panel . The neutrals were unbounded to the panel.
what size conductor is needed to connect the left neutral bar to the right neutral bar ?

Thank you

 

[kwired]

Good morning

I am converting a 200amp main panel to a sub panel. The ground bars are bonded to the panel . The neutrals were unbounded to the panel.
what size conductor is needed to connect the left neutral bar to the right neutral bar ?

Thank you

It really only needs to be the size of the maximum unbalanced load it will see, though that may or may not be all that easy to determine. You would need to look at what circuits are supplied by the second bar and what maximum imbalanced load may be. If it is a case where only neutrals from breakers on that side of the panel attach to the bar on that side - you could easily have a lower maximum if all your straight 240 volt loads were landed on that side of the panel. If you really don't want to take any chances use same size of conductor as the feeder neutral - though you may need special adapter lugs or maybe a tap connector on the feeder conductor itself because of limited number of lugs that accept larger conductors.


Add: in no case can this conductor be smaller then called for in Table 250.122, I'd have do a little looking for reference if you want to know where this requirement is, but I'm positive there is a section requiring this.

 

[rlundsrud]

I can't imagine why the neutral wasn't bonded if this was a main panel, but since it will be a sub panel it is actually set up exactly as it should be. The neutral should be isolated from the ground.

My question is why aren't the neutral buses connected together? Were they separated so one could be used as a ground bus? If that is the case I would leave it that way. If you did want to reconnect them, you would need to install a seperate ground bus and run your ground wires to that.

 

[rlundsrud]

It really only needs to be the size of the maximum unbalanced load it will see, though that may or may not be all that easy to determine. You would need to look at what circuits are supplied by the second bar and what maximum imbalanced load may be. If it is a case where only neutrals from breakers on that side of the panel attach to the bar on that side - you could easily have a lower maximum if all your straight 240 volt loads were landed on that side of the panel. If you really don't want to take any chances use same size of conductor as the feeder neutral - though you may need special adapter lugs or maybe a tap connector on the feeder conductor itself because of limited number of lugs that accept larger conductors.


I would think that you would have to size it based on the panel main breaker and not the possible unbalanced load. I would always size it based on the main breaker or MLO rating, I believe this is how it was originally installed, doing it otherwise might be a violation of 110.3 (b)

 

[kwired]

It really only needs to be the size of the maximum unbalanced load it will see, though that may or may not be all that easy to determine. You would need to look at what circuits are supplied by the second bar and what maximum imbalanced load may be. If it is a case where only neutrals from breakers on that side of the panel attach to the bar on that side - you could easily have a lower maximum if all your straight 240 volt loads were landed on that side of the panel. If you really don't want to take any chances use same size of conductor as the feeder neutral - though you may need special adapter lugs or maybe a tap connector on the feeder conductor itself because of limited number of lugs that accept larger conductors.


I would think that you would have to size it based on the panel main breaker and not the possible unbalanced load. I would always size it based on the main breaker or MLO rating, I believe this is how it was originally installed, doing it otherwise might be a violation of 110.3 (b)

The conductor in question is the neutral conductor of a feeder, just happens to only carry part of load of the entire feeder neutral. Feeder conductors are sized according to 215.2. Maximum possible unbalanced load on this conductor is the minimum ampacity required, but it must be no smaller then called for by 215.2(A)(2) which tells us it must be at least the size or the equipment grounding conductor even if the load is less - so for a 200 amp feeder 250.122 says 6 AWG copper - that means even if there is only maximum 20 amps of neutral load you still need a 6 AWG feeder neutral conductor.