Question about conductor derating.

[M.Chappy]

I have four sets of motor feeder conductors in a conduit, a total of eleven wires. Two of the motors are interlocked with the other two. In other words, only two will run at any one time. Does this eliminate the necessity of derating all conductors to 50% of their ampacity because of Art. 310 "more than three current carrying conductors in a conduit"? I will never have more that six current carrying wires in the conduit at one time, so, shouldn't I be allowed to only derate to 80%? It is my understanding that the code rule addresses the cumulative affect of all conductors under load and producing heat and therefore reducing the current carrying capacity of the wire. I have not been able to find a code section that directly addresses this situation.

Please let me know your opinions.

Thanks

 

[charlie b]

You're not going to find that answer in the code book. My opinion is that for a conductor to be counted as a "current-carrying conductor," it must be capable of carrying current. If an interlock limits the total number of conductors that are capable of carrying current to six, then you are in an 80% derating configuration, not a 50% one.

Welcome to the forum.

 

[petersonra]

I am inclined to agree. The code never defines what current carrying means, and in the derating section, it uses the following terminology:

Where the number of current-carrying conductors in a raceway or cable exceeds three, ...

To me that means what it says. If it can't exceed three (or in your case 6), it just does not exceed the magic number.

You still have to abide by the conduit fill requirements though.

 

[coulter]

... It is my understanding that the code rule addresses the cumulative affect of all conductors under load and producing heat and therefore reducing the current carrying capacity of the wire. I have not been able to find a code section that directly addresses this situation. ...

M -
Here is a method I have used to address this: (2005)
Look at 220.60, noncoincident loads. This lets you down size feeders when there are interlocked loads. (Not what you asked, but usually gets important)

Next look at 310.15.B.2, FPN1. This sends you to Annex B for more than 3 current carrying conductors with load diversity.

Table B310.11, for twelve conductors, happens to be for 50% load diversity, which is what you have. Table says derate to 70%.

PS: how did you come up with eleven - I'd have figured on twelve.

carl

 

[M.Chappy]

Thanks for the welcome and the reponses. One of the feeders is single-phase, that's why 11 wires instead of 12. Conduit fill is what created this problem. The customer increased the size of the motors so the conduit (that was installed underground) was suddenly too small. I will look at Annex B and see if that will apply to this situation before I talk to the AHJ.

Thanks to all.