Wireways and ampacities

[DGrant041]

Greetings!

I've been reading over your forums and haven't found one with the info I'm looking for.

In article 376 it gives all the requirements specific to wireways except which chart to use for conductor ampacities. 376.22 tells us that derating for multiple conductors doesn't begin until you have more than 30 CCC's. But which chart would I derate from? We will be using 90C wire from a 2000KW generator to the switchgear via wireway. The rest of the equation I pretty much understand, it's the cable sizing to start with I'm a little hazy on.

Thank you for sharing your expertise.

 

[pete m.]

Here is a reference to get you started:

110.14(C) Temperature Limitations The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

 

[steve66]

Per 376.22, you use the derating factors in 310.15(B)(2)(a). If you turn to that table, you will see that they apply to "more than 3 current carrying conductors in a raceway or cable.". So you apply those factors to the ampacities given in table 310.16.

Since you are using 90 deg C wire, you may use the 90 degree C column to apply the derating factors to. However, you can not exceede the 75 deg ampacity prior to the derating. (This is because it is almost certain that your terminations will only be rated for 75 deg).

Note that although you don't have to derate at all for 30 conductors, per table 310.15(B)(2)(a), once you hit 31 conductors, you must derate to 45% ampacity!! Than means a 500KCM conductor good for 380 amps drops to only 171 amps!!

Also, remember you can't exceed 20% fill on the wireway...I'm thinking this must be a really large wireway.

I hope that is what you were asking about.

 

[DGrant041]

Thanks so much for your second set of eyes. Apparently I haven't looked close enough BEFORE the table as I should have. 310.16 is what made sense but the reference to the free air charts was just confusing.

Yes. It would be a pretty big wireway. We will probably go with a cable tray this time. I'm not the one who makes that call, I'm just trying to give the-powers-that-be enough information to make an educated decision. I will also keep in mind how to deal with the 75C issue as well.

Pete, Steve-

Thanks!