Parallel 4160 volt cable sizing

[jcroley]

I need some help sizing 4160 volt parallel cables. I have a 6000 HP, 734 FLA motor load fed with 105 degree C cable. I want to feed it in an underground duct bank. Article 430.22 says that my conductor has to supply 1.25 times the FLA or 917.5 amps. Divided by two results in about 459 amps. Here's the question - do I use the one-circuit chart or the 3-circuit chart in table 310.77 giving me two answers - two 500MCM or two 1000MCM? Sorry for the silly question, but since I don't have a single or a triple circuit, I'm confused. Thanks to everyone in advance!

Jim

Thanks,

Jim

 

[dpc]

The NEC tables can't cover every possible circuit configuration (although why they didn't address double circuits is beyond me). In your case, you will need to calculate the necessary derating using the Neher-McGrath calculation method, use a software program to do it for you, or talk a cable manufacturer into doing it for you.

You can use the three circuit ampacity - that should be conservative - but keep in mind the ampacity depends on the thermal resistivity of the material around the conduit, the conduit and the soil around the duct bank. As well as the ambient soil temperature.

If you provide adequate spacing between the two conduits, per the NEC, no derating is necessary.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)