cable ampacity table NEC

[cmelguet]

Hi,

I am trying to figure out the following:

- NEC article 392.80(A)(2)(c)state that for single conductors installed in a trifoil formation with separation of 1 diameter between individual conductors we should use table 310.15(B)(17) for ampacity. This table for example gives you an ampacity of 620 A for a 500MCM, 75 C conductor at 30 C ambient temp.

-NEC article 392.80(A)(2)(d)state that for single conductors installed in a trifoil formation with separation of 2.15 diameter between conductors we should use table 310.15(B)(20) for ampacity. This table gives you an ampacity of 496 A for a 500 MCM, 75 C conductor at 40 C ambient temp, or 560 Amp at 30 C ambient temperatura (using the corection factors of table 310.15(B)(2)(B).

My question is: Why if i separate the conductors 2.15 diameters, which is more than 1 diameter, the ampacity of the table i have to use is less? This make no sense to me, if this is the case, it will be better to separate conductors just 1 diameter and not 2.15.

Regards,
Cmed.

 

[dpc]

(c) refers to "single conductors" in a "single layer". Not sure what your definition of "trifoil" is, but I don't think (c) and (d) are referring to the same conductor configuration.

 

[cmelguet]

Hi dpc,

With trifoil i mean triangular formation, as showed on the attached file.
Please clarify, I cant installed the cables as per showed in the first drawing in the attached file using (c).

Regards, and thanks again.
Cmed

 

[resqcapt19]

(A)(2)(c) is for a single layer. That is the conductors are all laying flat in the tray with a one diameter seperation between the conductors. It is not for a triangular configuration. The rule in (d) applies to triangular configuration.

 

[cmelguet]

thanks DPC. Regards