Parallel feed calculation

[pepevegac]

Hi, I was wondering if someone could help me out how to figure out the wire sizes when running parallel vs single feeders. Is there a formula to figure out parallel feeder sizes or for example if you have a 600A main, do you cut it into 300 and 300 and run wire big enough for this amperage. Am a journeyman and I've seen it on drawings but I really don't know how to calculate the size of parallel. Please advice thanks

 

[chris kennedy]

you have a 600A main, do you cut it into 300 and 300 and run wire big enough for this amperage.

Bingo!....

 

[augie47]

I do not see where you mention parallel in separate conduits or in the same conduit. Separate conduits (2), then yes, 50% in each*. Parallel in the same conduit, derating per 310.15(B)(2) will play a roll.

(*With non-linear loads 310.15(B)(2) could still be an issue.)
(as an example on your 600 amp main, parallel 350s in separate conduits would be 610 amps,
350s parallel in the same conduit -3 phase-4 wire would be 496 amps)

 

[infinity]

Your method will work just fine for two sets of conductors. There are many ways to calculate the number of conduits, and conductor size. First you need to decide what you design parameters are. For example some guys don't like to install conductors larger say than 250 kcmil so you would need to factor that in to your calculation when you need to use multiple sets. Another issue may be conduit size. Some times larger conduit work it not feasible so smaller conduits are used and more sets of smaller conductors are used too.

 

[skeshesh]

Also be careful when planning out the job in stages. In a previous job I showed three (3) parallels running to a 600A ocpd thru an abandoned trash chute in a high rise since they didnt want to use large conductor sizes for ease of running it for something like 20 stories. I forgot to check the terminations of the OCPD (since it had been chosen prior to the issue of three instead of two parallel runs) after going with three parallels and later I found out that I had to use a 800A frame to allow for 3 wires of whatever size it was.

 

[pepevegac]

thanks, this comments wrere very useful. oh and yes it was in two different conduits. that means that if for example for the same 600A main, I want to run 3 sets in three different conduits it's the same thing divide the 600 in three equal amperage and use the wire big enough for that individual voltage?

 

[infinity]

thanks, this comments wrere very useful. oh and yes it was in two different conduits. that means that if for example for the same 600A main, I want to run 3 sets in three different conduits it's the same thing divide the 600 in three equal amperage and use the wire big enough for that individual voltage?

Yes, each of the 3 conduits would require conductors with an ampacity of 200 amps.