Hi:

I have a question about paralleling cables. I know the U.S. NATIONAL ELECTRICAL CODE states parrallel cables must all be the same length. however in practice it is not always practical to have them all exactly the same. what is an acceptable allowance on the difference?

the situation I have is 12 parrallel cables per phase 12.5 kv 2410 amps

3 phase 60 cycle. 36 cables total, 12 per conduit. quanity of six, six inch ridgid galvenized conduits. 2 A, 2 B , 2 C cables in each conduit. With a 330 foot length will a difference in length of 6 feet in cables of the same phase cause an unacceptable inbalance of current??I am not an enginer, but an electrican. I have seen people spend an extra 50 or more hours in a situation like this to get the cables all the exact length. Is this nessasary?? From what I read and calculate, using the impedance of the cables at 2410 amps/12 = 200.8 amps per conductor. effective impedance of 500 MCM cable in steel conduit shortest 330 ft. long = .02854 effective impedance of 500 MCM cable in steel conduit longest 336 ft. long = .02906 the rest of the cables are all between this in proportional lenghts 300' 8",

301' 16" 302'4", ect due to the difference in length of the bends in conduit.the formula I have found is

I long cable = I total x Z short cable/(Z long cable + Z short cable) I short cable = I total x Z long cable/(Z long cable + Z short cable) The shortest cable would carry a current of 202.6 amps the longet acurrent of 199 amps Is this the correct way to figure the current inbalance???

I understand all cables must be the same insulaton type, conductor type, same type of conduit ect. If you assume equal temperatures, you will be conservative in the current split. That is, you will calculate more current in the shorter conductor than will actually be the case. The shorter conductors will carry more current because the resistance and inductance will be less. With higher temperature, the resistance will increase, reducing the difference between the short and long conductors.