Confused... Current Carrying Conductor

[Gene Shults]

Question is on Current Carrying Conductor in Conduit.

For derating purposes, (Table 310.15(B)(2)(a) Adjustment Factors for More Than Three Current-Carrying Conductors in a Raceway or Cable). If I have 8 conductors in a raceway, 4 are neutral and 4 are Hot (lets say 110 vac) which are the current carrying conductors? The neutrals or the hots? I can put a current meter on either the neutral or the hot and still read current. Please help define the current carrying conductor? If I use the neutral as the current carrying conductor (which is 4), I will have to derate the wires by 20%, or if I use the total (which is 8) I will have to derate the wires by 30%. Again please help define current carrying conductor...

Thanks
Gene

 

[LarryFine]

. . which are the current carrying conductors? The neutrals or the hots?

Both. Everything except EGC's. The neutral in a MWBC (if all phases are used) would be an exception.

 

[infinity]

Gene,

In your scenario you have 8 CCC's. The hot and the neutral of each circuit will carry equal amounts of current so they are both current carrying conductors. Since you have 8 CCC's you will need to derate them to 70%. If you are using #14, #12 or #10 THHN conductors the derating probably will not affect the ampacity of these conductors since you will derate from the 90 degree C column. This will provide an ajusted ampacity above the limits required by 240.4(D).

 

[winnie]

The term 'current carrying conductor' is tremendously confusing in this particular code section. This code section requires that you total up 'current carrying conductors', yet permits you to exclude certain conductors from that total. These excluded conductors generally _do_ carry current.

I find it easier to think about the total worst case heat production of a set of conductors. When you have the circumstance of the maximum number of conductors carrying their maximum rated current, you will get the worst case heat production. If you look closely at a few circuit arrangements (the ones where code permits you to exclude some of the conductors from the count), you will find that when the _maximum_ number of conductors are carrying full current, one of the conductors in the set is carrying no current at all. Thus in these circumstances the heat production is the same as if that conductor were not present.

In the real world, where current flow is not perfectly balanced in MWBCs, all the conductors will be carrying _some_ current. But in this case the total heat produced will be less then the situation of the maximum number of conductors carrying full current with one conductor carrying no current.

-Jon

 

[Pierre C Belarge]

Take a look at 310.15(b)(4)