Wire derating based on conduit fill
Wire derating based on conduit fill
(OP)
I am relocating a 125 HP DC motor from one plant to another and have a question concerning derating the wiring. The old wiring ran in tray and will be placed in coduit at the new location. If I follow what NEC says that 4-6 current carrying conductors in a raceway must have thier ampacities based on 80% of table 310-16. If I run my Armature wires 2-350 MCM (FLA 205A)in a conduit and then add the 2-#12 (8.4A) field wires, it would seem to me that the code would require me to derate all of the wires forcing me to use 500 MCM to stay in code. This seems a bit unreasonable to me as the #12 are an order of magnitude smaller and would seem to have no effect on the larger conductors. Is there something in the code that I am missing or do I really have to derate if I want to stay in "code"
Thanks,
DOug
Thanks,
DOug





RE: Wire derating based on conduit fill
I don't know of any exceptions to the derating rule that would apply in this case. What are the sizes of the OCPDs?
Also if this was my installation there would be 2 conduits, one for the #12s and a second for the 350s. If this conduit run has any length and a number of bends it would be very difficult to install the #12s in the same raceway as the 350s without damaging the #12s. When using #14 and #12 control wires for power circuits, I never use the same raceway when the power conductors are larger than #3.
Don(resqcapt19)
RE: Wire derating based on conduit fill
If you have XHHW wire that is rated at 350 Amps for 90 deg C you can take 80 % of that 350 x 80% = 280 amps.
I am using a 97 code book (my newer one is not here) but the present code is the same. The code is about adjacent sections of a circuit, the adjacent section that is going to be derated is in the middle of the run ( in conduit) there you can use the 90 or even 105 deg C rating.
You may be able to use 250 MCM
I would run the aramature ckt at # 10 or # 8 just for the mechanical strength. ( cheaper than running a seperate conduit).
RE: Wire derating based on conduit fill
Doug
RE: Wire derating based on conduit fill
Don(resqcapt19)
RE: Wire derating based on conduit fill
RE: Wire derating based on conduit fill
Regardless of the strict letter of the NEC, I don’t believe that the 4-6 conductor derating rule is intended to apply to this type of installation. The heat contribution by the 12AWG field wiring is insignificant to the 350s.
The concern about insulation damage for small-vs.-big conductors in the same raceway is valid. If there is suitable termination space, using 6-2AWG conductors may be appropriate and will only add minor cost the job. Use of a high-quality pulling lube will help limit pinching/snagging. Another approach for limiting damage to small wiring would be substitution of 12-10AWG tray cable in place of individual conductors.
RE: Wire derating based on conduit fill
I agree that in reality that the heat from the #12s will have no effect on the larger conductors and the installation wouldn't be a problem from that standpoint.
Are you suggesting that he use 3 #2 conductors in parallel for each of the aramature leads? If so what about 310-4?
Don(resqcapt19)
RE: Wire derating based on conduit fill
RE: Wire derating based on conduit fill
Table B 310-11 in Appendix B of the NEC is a way to account for diversity of load when derating conductors. It's save me from running extra conduit in a couple of cases.
RE: Wire derating based on conduit fill
I'm sorry. I missread your post. The cost of those larger conductors may well exceed the cost of installing 1/2" conduit with 2 #12s for the field. This might be a good application for cable tray. The armature conductors could be run as single conductors and a 2 conductor #12 could be used for the field.
Don(resqcapt19)
RE: Wire derating based on conduit fill