paralleled feeder conductors

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tower

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say you have a continuous feeder loaded of 724 amps and the conductors are paralleled 4 times per phase to 75 degree terminals. after the calculations you come up with the phase conductors being sized at 4/0 awg. with out going into calculations, my question is. how is it that the amperage is assumed to be evenly divided amongst the paralleled conductors.say 230 amps per phase for a total of 920 amps.is there a capacity at which each 4/0 will realize.can one phase over heat.our is it like working with 4 electricians roughing in a office space.you no every body doesnt pull there fair share one way or another,and the poor bastard that is`nt freinds with the project manager is giong to have to carry the lazy fat slub if he wants to keep his job.its relative is`nt it? or is there a mathamatical engenering answer ?
 
Re: paralleled feeder conductors

The code requires that the conducors be of the same type and length, so each will carry 1/4 of the power. Be sure that each raceway will have a fully sized equip grounding conductor.
My only comment about your anology is that I'm only pulling my fair share on even numbered days, when there is a full moon expected that night :) .
 
Re: paralleled feeder conductors

I think your mathematical answer would be Ohm's law. You are taking two conductors of the same size, insulation, length, conductivity, ampacity, etc...therefore you have two conductors with the same resistance. Voltage is constant. I=E/R, right? your voltage (E) is the same in all sets of feeders. Your resistance (R) is the same in all sets of feeders, therefore your amperage (I) is the same in all sets of feeders.
 
Re: paralleled feeder conductors

Just as the others have said if all the conductors have the same properties the current will divide equally.

If you had a bucket of water with 4 identical holes in the bottom of it each hole would pass the same amount of water.

It is often said that electricity takes the path of least resistance, that is not entirely correct.

Electricity will take all available paths, the path of least resistance just ends up with more current on it.

Luckily conductors do not have personality's like workers so day in day out they all work together in the most efficient way. :D
 
Re: paralleled feeder conductors

it is very important to order all the wire at the same length and the wire be installed of the same phase and the same length. we had to build an emergency 4000 amp feeder due to a main switchgear fire. the run was eighty feet long and we did not check the wire lengths from the supply house - the wire was cut by the supply house manager and some runs were five to six feet difference in length. this caused the amperage on the shorter runs to carry (500 mcm in air) 700 to 800 amps while the longer lengths were carrying only 350 to 450 amps. the phases can be different lengths but the phases must be the same....
 
Re: paralleled feeder conductors

ok lets take this a little futher....lets say that you are pulling 3 parallel sets of 4/0 in a conduit. 3 wire per phase. do you have to derate for having 9 current carring conductors. or do you count the 3 wire of each phase as one conductor?
 
Re: paralleled feeder conductors

Electricbeaver, yes you have to derate the conductors. Go here for more information.

Roger
 
Re: paralleled feeder conductors

yes if there are more than three conductors in the raceway - but you do get quite a bit of amperage capabilities using parallel setups. we just did a press feeder and used parallel 1/0 in lue of 350 mcm - press manufacturer didn't like it but it was legal and operates fine. we made sure the wire lengths were the same and the currents are shared between the wires.
 
Re: paralleled feeder conductors

It would be rare for us to run parallel conductors in the same raceway.

Most of the time we run a separate raceway for each set so we do not have to take a derating hit of 80% or even 70%. :eek:
 
Re: paralleled feeder conductors

Just to let you know i run paralleled conductors in different runs also this just happened to be existing and the ability to run new pipe was out of the quistion
 
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