Wire derating based on conduit fill 3

[dougjl]

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

 

[resqcapt19]

Doug,
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)

 

[BJC]

Section 310 -15 C lets you use the higher rate ampacity in the table.
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).

 

[dougjl]

Good point about the #12's getting damaged. The run is about 250' one way. Thanks for the replies.

Doug

 

[resqcapt19]

Even when using #10s or #8s with 350s there is a great chance of the smaller conductor being damaged if there are two or more 90s in the conduit run. It is correct that you can derate from the 90°C column, however 430-22 requires that he motor conducotrs have an ampacity of at least 125% of the motor full load. Note that per the code this full load must be taken from Tables 430-147 through 430-150. A 125hp DC motor with a 500 volt armature will have an FLA of 205 amps requiring a mimimum conductor ampacity of 256.25 amps. Using an 80% adjustment factor and the 90°C ampacity the smallest motor conductor permitted is a 350.
Don(resqcapt19)

 

[jbartos]

Suggestion: Since NFPA 70-1999 does not address the conductor sizes for derating purposes based on a number of conductors in a conduit. Therefore, you are obligated to derate them too to comply with the code. The cost of derating of larger conductors suggests that you install the extra conduit or cable for the small sized conductors.

 

[busbar]

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.

 

[resqcapt19]

busbar,
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)

 

[busbar]

Oops, no. I meant use a pair of 6AWG or 2AWG in place of the 12s -- nothing paralleled.

 

[BJC]

Not really related to the original problem, but this may be of intrest to some.
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.

 

[resqcapt19]

Busbar,
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)

 

[jbartos]

Suggestion: It is much better to comply with safety codes rather than to use own theoretical proof; since the theoretical proof, even professionally correct one, may or may not be accepted legally or administratively.