Conductor Derating

[Mark Balan]

NEC 310-15(B)(2)a requires derating of current carrying conductors according to the table. In case of raceway containing motor feeder conductors and control conductors (such as LOS, high temp sw, solenoid valves etc.,) I beleive that derating of the feeder cable is not required (unless it has mutiple pairs per phase etc.,)per exception contained for different systems (300-3). The control conductors are derived from a control power transformer (CPT with 120 V secondary) and would also qualify as Class 1 remote control and signalling circuit (less than 600V).

It stands to reason that there is really no threat to overheating of the phase cables within the raceway due to control cables (say #14) which in worst case can carry the maximum output of the CPT (typically 4 to 5 Amps) before the CPT fuse blows and thus removing all currents from the feeder cables to motor. The NEC is somewhat vague in this matter. The reason for derating is the heating effect of cables within a confined space such as a raceway - but control cables (Class 1) do not really contribute to the heating effect and should not be included in the derating calculations - Can anyone comment on this

Thanks

Mark Balan, PE.,

 

[charlie b]

I can't give you any real help at the moment, as I am nowhere near a code book (and don't intend to look things up on-line). I think you don't have to count control cables as being "current carrying conductors," for the reasons you state.

 

[Mark Balan]

Thanks Charlie - The LOS, space heater, sol. valve circuits etc., do carry some small amount of currents. Can they be considered as non current carrying ? I have a real problem with a contractor here on a jobsite who is looking for a major change order by considering the 14 #14 control conductors for his dearting of the phase conductors by 50%.

Mark balan

 

[augie47]

I feel you might get different opinions depending on your AHJ, but it would seem that Exception 1 to 310.15(B)(2) would support your thoughts.

 

[weressl]

NEC 310-15(B)(2)a requires derating of current carrying conductors according to the table. In case of raceway containing motor feeder conductors and control conductors (such as LOS, high temp sw, solenoid valves etc.,) I beleive that derating of the feeder cable is not required (unless it has mutiple pairs per phase etc.,)per exception contained for different systems (300-3). The control conductors are derived from a control power transformer (CPT with 120 V secondary) and would also qualify as Class 1 remote control and signalling circuit (less than 600V).

It stands to reason that there is really no threat to overheating of the phase cables within the raceway due to control cables (say #14) which in worst case can carry the maximum output of the CPT (typically 4 to 5 Amps) before the CPT fuse blows and thus removing all currents from the feeder cables to motor. The NEC is somewhat vague in this matter. The reason for derating is the heating effect of cables within a confined space such as a raceway - but control cables (Class 1) do not really contribute to the heating effect and should not be included in the derating calculations - Can anyone comment on this

Thanks

Mark Balan, PE.,

You should be OK as far as the Code exception goes.

(I personaly - as an engineer - hate to see control and power cables being run together. :wink: It may technically be OK, even though there are many type of control devices where this could cause a problem, but as long as all conductors have the higher voltage insulation level it's kosher. Just because its technicaly OK it does not mean its the right thing to do. )