Derating quanity of current carrying conductors.

[wireman74]

OK... so is 3 circuits sharing a neutral, 4 ccc's or 3? I think that I read it to be 3, but i want to know for sure. Then if you have a neutral for every circuit, that would be 2 ccc's right? I don't know why this is so confusing. But we have talked about it among our company, and everybody thinks differently. I need to ask the pro's!

 

[roger]

OK... so is 3 circuits sharing a neutral, 4 ccc's or 3? I think that I read it to be 3, but i want to know for sure.

Except in a very rare circumstance it would be three, see 310.15(B)(5) (2011) or 310.15(B)(4) before 2011

Then if you have a neutral for every circuit, that would be 2 ccc's right? I don't know why this is so confusing. But we have talked about it among our company, and everybody thinks differently. I need to ask the pro's!

Yes, in a two wire circuit both conductors carry the same current.

Roger

 

[kwired]

OK... so is 3 circuits sharing a neutral, 4 ccc's or 3? I think that I read it to be 3, but i want to know for sure. Then if you have a neutral for every circuit, that would be 2 ccc's right? I don't know why this is so confusing. But we have talked about it among our company, and everybody thinks differently. I need to ask the pro's!

Assuming the three circuits are on three different phases of a wye system then yes it is 3CCC - the neutral only carries imbalanced current of the phase conductors. Main exception to this is with non linear loads the harmonic currents may produce enough heating within the neutral conductor that it essentially is not carrying only imbalance and must be treated as a CCC.

A neutral for every circuit means heat is produced in every conductor and therefore 2CCC per circuit - making 6 CCC for the three circuits.


A neutral between two phases of a wye system is not carrying imbalance current though and is treated as a CCC.

 

[jcassity]

Assuming the three circuits are on three different phases of a wye system then yes it is 3CCC - the neutral only carries imbalanced current of the phase conductors. Main exception to this is with non linear loads the harmonic currents may produce enough heating within the neutral conductor that it essentially is not carrying only imbalance and must be treated as a CCC.

A neutral for every circuit means heat is produced in every conductor and therefore 2CCC per circuit - making 6 CCC for the three circuits.


A neutral between two phases of a wye system is not carrying imbalance current though and is treated as a CCC.

exactly,, in my typical work situations, i have given up on this and count my N leg as a ccc.

you have to undersand what devices are Linear vs Non-Linear then its easy as pie to figure.

 

[kwired]

exactly,, in my typical work situations, i have given up on this and count my N leg as a ccc.

you have to undersand what devices are Linear vs Non-Linear then its easy as pie to figure.

You may also want to consider just how much non-linear load there is and whether or not it is significant enough to have enough harmonics to be much of a factor on a common neutral conductor. If you have a machine that is linear load but has a very low amount of non linear load in a control circuit you can generally overlook this non linear load.