NEC Table 310.16 Doubt...

[ibarrola]

Hi, quick question...

I'm looking at an example for a coordination study and it mentions that we have a "3-1/C 500MCM". The solution then shows you a reference to table 310.16 (old NEC Ed) where they obtain the ampacity of 380A (circuit is 480v 3PH, installed in conduit).

Does table 310.16 (old NEC Ed) show ampacity for 3/C cables only? And would this apply also for 3 runs of 1/C cables installed in a triangular configuration?


Thanks,
Yogi

 

[ggunn]

Hi, quick question...

I'm looking at an example for a coordination study and it mentions that we have a "3-1/C 500MCM". The solution then shows you a reference to table 310.16 (old NEC Ed) where they obtain the ampacity of 380A (circuit is 480v 3PH, installed in conduit).

Does table 310.16 (old NEC Ed) show ampacity for 3/C cables only? And would this apply also for 3 runs of 1/C cables installed in a triangular configuration?


Thanks,
Yogi

I'm not sure what you are asking. If there is 380A in a three phase circuit there is 380A in each conductor. There is no derate for 3 conductors in a conduit.

 

[infinity]

Is this in a cable tray?

 

[ibarrola]

It doesn't mention being installed on a cable tray, just conduit...

There's a difference in ampacity rating between a 3/C cable and a 1/C cable of the same size. So my question is, does table 360.16 show ratings for a 3/C cable only? Or does NEC have the same ratings for installing either 3 runs of 1/c vs 1 run of 3/c?

Thanks

 

[charlie b]

Does table 310.16 (old NEC Ed) show ampacity for 3/C cables only? And would this apply also for 3 runs of 1/C cables installed in a triangular configuration?

Take a look at the top of the table. It should say something like, "ampacity of not more than three current-carrying conductors in raceway, cable, . . ." The "in raceway" part is talking about three 1/C cables in a conduit. The "in . . . cable" part is talking about a 3/C cable. So it applies to both situations.

 

[charlie b]

There's a difference in ampacity rating between a 3/C cable and a 1/C cable of the same size.

What brings you to think that?

 

[ibarrola]

Take a look at the top of the table. It should say something like, "ampacity of not more than three current-carrying conductors in raceway, cable, . . ." The "in raceway" part is talking about three 1/C cables in a conduit. The "in . . . cable" part is talking about a 3/C cable. So it applies to both situations.

Oh, ok... So NEC has the same rating for either 3 runs of 1/C in a cable tray/conduit or 1 run of 3/C in a cable tray/conduit...

Ok, thanks bunch!

 

[ibarrola]

What brings you to think that?

IEEE-45 ampacity table which has ratings for single conductor and three conductor cables.

Wouldn't a given conductor in a 3/C cable have less heat dissipation then a single conductor in a 1/C cable?

 

[smoothops10]

I agree with Charlie B that the rating is the same for 3/C or 3-1/C in 310.16.

Not to detour the thread but for medium voltage the 3/C cable with overall jacketed conductors has a different ampacity than a corresponding 3-1/C with individual conducors of the same size. Reference T310.60(C)(77) vs T310.60(C)(79) for instance. Most cases it is lower. Interestingly, for some it is higher - 5-35kV 105C MV-105 column.

 

[winnie]

IEEE-45 ampacity table which has ratings for single conductor and three conductor cables.

Wouldn't a given conductor in a 3/C cable have less heat dissipation then a single conductor in a 1/C cable?

The NEC ampacity tables are simplifications/approximations that give the same value to a 3 conductor cable and 3 single conductors in a conduit. They also treat different types of conduit (steel, aluminum, PVC) and different sizes of conduit as being equivalent, even though they clearly cannot be.

This doesn't mean that the actual physics limit ampacity is the same; just that the two situations are treated as equivalent for NEC calculations. The NEC tables are _very_ conservative; conductors usually run much cooler than the table would lead you to expect.

-Jon

 

[ibarrola]

The NEC ampacity tables are simplifications/approximations that give the same value to a 3 conductor cable and 3 single conductors in a conduit. They also treat different types of conduit (steel, aluminum, PVC) and different sizes of conduit as being equivalent, even though they clearly cannot be.

This doesn't mean that the actual physics limit ampacity is the same; just that the two situations are treated as equivalent for NEC calculations. The NEC tables are _very_ conservative; conductors usually run much cooler than the table would lead you to expect.

-Jon

That explains it...
Thanks Jon, and thank you all for your input.

 

[infinity]

It doesn't mention being installed on a cable tray, just conduit...

There's a difference in ampacity rating between a 3/C cable and a 1/C cable of the same size. So my question is, does table 360.16 show ratings for a 3/C cable only? Or does NEC have the same ratings for installing either 3 runs of 1/c vs 1 run of 3/c?

Thanks

Triangular configurations usually apply to single conductors in a cable tray.

Does table 310.16 (old NEC Ed) show ampacity for 3/C cables only? And would this apply also for 3 runs of 1/C cables installed in a triangular configuration?

392.80(A)(2)(d) Where single conductors are installed in a triangu-
lar or square configuration in uncovered cable trays, with a
maintained free airspace of not less than 2.15 times one con-
ductor diameter (2.15 ? O.D.) of the largest conductor con-
tained within the configuration and adjacent conductor
configurations or cables, the ampacity of 1/0 AWG and
larger cables shall not exceed the allowable ampacities of
two or three single insulated conductors rated 0 through
2000 volts supported on a messenger in accordance with
310.15(B).