NEC 750 KCMIL CABLE, TABLE 310.15(B)(17) / 310.17 VS 310.60(C)(69)

[Tuanmetalworks]

Hello all,

I am doing a small study on LV and MV cable systems and came across a rather confusing item in the NEC. I am looking at 750 kcmil cables in free air for a 600V system and a 5kV system. To my understanding the ampacity of a 90 degrees cable at 600V is 885 Amps at 30 degrees ambient. But as I look at table 310.60(C)(69) for a 2001-5000 Volts cable, the ampacity is 900 Amps for a MV-90 cable at 40 degree ambient.

So this is where the confusion begins. First off, why would a 2001-5000 V cable have a higher ampacity compared to a 600 V cable? If anything I would expect it to have equal to or less because generally in higher voltage cables there is a copper tape shielding that would also conduct heat due to induced voltage/current. And also don't forget about the 10 degrees increase in ambient.

It would be great if someone can shed some light on this, because I am lost as to why there is an ampacity increase even though the amount of copper to carry the current remains the same.

Please and thank you

 

[dpc]

The best and simplest answer is that it doesn't really matter why. You cannot apply logic or the laws of heat transfer to the NEC - you will find it frustrating. It is what it is.

The low voltage table is good up to 2000 V, not 600 V, by the way.

There is a long historical track record for the low voltage conductor ampacities and these very seldom get changed. The low voltage ratings also tend to be more conservative.

The medium-voltage cable construction is somewhat different, and it is conceivable that some of the differences could result in a higher ampacity.

The MV tables were probably submitted to the Code-making panel by cable manufacturers, or NEMA.

I suspect you will not have much luck trying to track down a specific physical reason.