All,
Ever since graduation I've been using the 75 degree column of the cable ampacity table to size wire, even though I almost always use 90 degree rated THHN, due to the fact that almost all of our MCC's have 75 degree rated equipment. However, someone told me about how you can calculate using the 90 degree column for de-rating purposes. I am trying to apply it to this example: I need to size conductors for a 400A service:
Old Method I Used:
Six 250MCM wires in one conduit - 255*2*0.8*1.00 = 408A (all values taken from 75 deg column, ambient temp correction is 1.00)
New Method Someone Told Me About:
Six 4/0 wires in one conduit - 260*0.8*1.00 = 208A (less than 75 degree ampacity of 230 for 4/0), so I can use 90 degree ampacity for 4/0, so total ampacity becomes 208 x 2 = 416A
Something about this doesn't seem right to me, can someone explain am I using this method correctly? Incorrectly?
Ever since graduation I've been using the 75 degree column of the cable ampacity table to size wire, even though I almost always use 90 degree rated THHN, due to the fact that almost all of our MCC's have 75 degree rated equipment. However, someone told me about how you can calculate using the 90 degree column for de-rating purposes. I am trying to apply it to this example: I need to size conductors for a 400A service:
Old Method I Used:
Six 250MCM wires in one conduit - 255*2*0.8*1.00 = 408A (all values taken from 75 deg column, ambient temp correction is 1.00)
New Method Someone Told Me About:
Six 4/0 wires in one conduit - 260*0.8*1.00 = 208A (less than 75 degree ampacity of 230 for 4/0), so I can use 90 degree ampacity for 4/0, so total ampacity becomes 208 x 2 = 416A
Something about this doesn't seem right to me, can someone explain am I using this method correctly? Incorrectly?