ampacity

[treyp3]

test question:
what size thhn cu conductors are required to supply a group of 3 phase , 460v, design B motors rated 10HP, 20HP, 30HP

Heres what I got 50+27+14=91. Equals #4

The book agrees that 91a is the load, however it say that #3 is required. It lists table 310.16 and 310.10 fpn as its reference. When I read 310.10 fpn I am directed to annex B., but I cant figure out how to apply this circuit here. Any help?

 

[Dennis Alwon]

#4 is only good for 85 amps at 75C. #3 is the correct answer-- table 310.16

Remember 90C is only good for derating . You cannot use OCP at that ampacity because the terminals are only rated 75C.

 

[kwired]

T 310.16 says a 75 deg 4 AWG copper conductor is good for 85 amps you have a load of 91 amps. A 3 AWG is good for 100 @ 75 deg and is equal or greater than what you need for a minimum size.

You can only use the 90 deg column for insulation deration or if terminals are rated 90 deg.- they will not be rated 90 deg on low voltage equipment.

 

[treyp3]

Aha! Thanx so much.