derating 2 wire circuit condutors in conduit

[sparkymarjon]

A debate came up so I thought I would bring it here, If you have, let's say 6- 2wire s.p. circuits in a conduit do you count the nuetrals for derating purposes. I feel because you have multiple different circuits in the same conduit, yes. However others feel that since they are all 2 wire circuits and the nuetral is carrying back the balance load of the hot leg,they cancel each other out and have no effect on the other conductors. I feel since there are amps in both nuetral and hot leg, there will be a heating effect when mixed with other conductors, which is why you need to count nuetrals for derating.. how do you feel

 

[augie47]

If the neutral is dedicated to it's one associated circuit, it counts as a current carrying conductor (any load that appears on the phase will also be on the neutral--thus 2 ccc).
If the neutral is shared on a MWBC it only carries the unbalanced load of the two (three) phase conductors and does not count UNLESS the majority of the load is non-linear in which case harmonics require it to be counted

All is covered by 310.15(B)(2)(4)

 

[roger]

If you have, let's say 6- 2wire s.p. circuits in a conduit do you count the nuetrals for derating purposes.

Along with Gus's post, you would have 12 Current Carrying Conductors in your scenario.

Roger

 

[charlie b]

However others feel that since they are all 2 wire circuits and the nuetral is carrying back the balance load of the hot leg,they cancel each other out and have no effect on the other conductors.

The magnetic field generated by current flowing one way in the ungrounded (hot) conductor is essentially cancelled out by the magnetic field generated by current flowing the opposite way in the grounded (neutral) conductor. But the derating rules are based on the ability of a conductor to reject heat to its surroundings. So you are right: the heating effects do not cancel each other out, but rather are additive.