How to calculate parallel runs sizing

[Fred B]

Never had to do this before, how do you calculate wire size for a given amperage if using parallel runs vs single feeds. Say the calculations for single phase show I would need 500kcmil Cu. So if I was to do parallel runs 1. what size wire can I use? It was implied to me I could use 2 sets of 4/0 instead. How is that calculated? Is it a simple rule of thumb or a more variable calculation? And 2. So if I am understanding the process correctly if I use 2 ducts I would pull 2 hots and one neutral in each duct, is that right?

 

[infinity]

Basically you take the size of the feeder and divide by the number of sets or divide by the conductor ampacity for the conductors you want to use.

For example:
800 amp feeder you want to use #3/0 which is a 200 amp conductor 800/200=4 sets. Switch to 600 kcmil which is a 420 amp conductor 800/420=1.9= 2 sets.

In your example you want to use 2 sets for 400 amps that means that each set needs to have an ampacity of 400/2=200 amps or minimum size of #3/0.

 

[Dsg319]

Never had to do this before, how do you calculate wire size for a given amperage if using parallel runs vs single feeds. Say the calculations for single phase show I would need 500kcmil Cu. So if I was to do parallel runs 1. what size wire can I use? It was implied to me I could use 2 sets of 4/0 instead. How is that calculated? Is it a simple rule of thumb or a more variable calculation? And 2. So if I am understanding the process correctly if I use 2 ducts I would pull 2 hots and one neutral in each duct, is that right?

Assuming you have a 400amp OCPD. Smallest size permitted In any case is 1/0 I believe.

1. All you need to do is have equal to or greater than givin ampacity of your calculated load or OCPD which ever it is in this case. Say 400amps. 3/0 copper is 200amp wire. Multiple that by 2 and there’s your 400amp conductors in parallel.(Assuming this is is a service and a EGC Is not needed) if EGC was needed it still needs to be full sized based off of OCPD.

2.Yes 2hots 1 noodle in each raceway. (Assuming it is a ferrous raceway) have seen some pics of pvc raceways with all phases grouped together I suppose to ease the wire bending and termination.

basically said same thing as Infinity I know but he posted before I could finish

 

[Fred B]

Basically you take the size of the feeder and divide by the number of sets or divide by the conductor ampacity for the conductors you want to use.

For example:
800 amp feeder you want to use #3/0 which is a 200 amp conductor 800/200=4 sets. Switch to 600 kcmil which is a 420 amp conductor 800/420=1.9= 2 sets.

In your example you want to use 2 sets for 400 amps that means that each set needs to have an ampacity of 400/2=200 amps or minimum size of #3/0.

Oh, duh I was really overthinking this one, looking for some complicated convoluted formula using circ-mil to come up with sizing. Thanks that's simple.

 

[Fred B]

Assuming you have a 400amp OCPD. Smallest size permitted In any case is 1/0 I believe.

1. All you need to do is have equal to or greater than givin ampacity of your calculated load or OCPD which ever it is in this case. Say 400amps. 3/0 copper is 200amp wire. Multiple that by 2 and there’s your 400amp conductors in parallel.(Assuming this is is a service and a EGC Is not needed) if EGC was needed it still needs to be full sized based off of OCPD.

2.Yes 2hots 1 noodle in each raceway. (Assuming it is a ferrous raceway) have seen some pics of pvc raceways with all phases grouped together I suppose to ease the wire bending and termination.

basically said same thing as Infinity I know but he posted before I could finish

R u saying that using PVC it can be done differently?

 

[Dsg319]

R u saying that using PVC it can be done differently?

I’m not percent sure but I believe so. Hopefully someone else will chime in, I’ve always pulled parallels as you mentioned. But have seen stuff online of pvc raceway going to say a pad mount transformer with large conductors, and have all phase a in one pipe, phase b in another and so on to make it easy on terminations and bending wire.

 

[Dsg319]

R u saying that using PVC it can be done differently?

Combining phase conductors in parallel sets

I am working on a project with multiple 4000 amp, 480 volt electrical services. The contractor has proposed grouping the phase conductors for the parallel sets to make installation easier and also to reduce the stress on the lugs. Is it acceptable to group the phase conductors? I think that...

forums.mikeholt.com

 

[infinity]

It is permitted under a very limited scope. Look at 300.3(B)(1)Exception.