Help with 1200A, 480v, 3-Phase Service Conductors

Sam Moore

Member
I do mostly residential and light commercial work, so I'm not real experienced with larger commercial services and I'd like for the more experienced guys to double checked my calc. here:

1200A, 480/277v, 4-Wire, 3-Phase Service
Service will have 2, 600 Amp Services Discos

Each Disco I'm going to have Paralleled 3" RMC's

In Each Parallel I figured 4, 350mcm THHN derated @ 80%= 560 Amps allowing for larger breaker/fuse size of 600 Amp (using Copper)

I figured Electrode Conductor of 3/0 Copper

Then Service Feeder to Main Distribution Panel
I figured 4, Paralleled 350 Thhn's with 3/0 Copper Ground or am I allowed to use a
1awg copper ground for 600amps?

I figured 3" RMC for both runs

I want to make sure I'm right, before I price this out,
Thanks in advance for any help with this!

Dennis Alwon

Moderator
Staff member
Although you probably don't need to count the neutral as a current carrying conductor, I would still run parallel 350kcm conductors.

3/0 is the correct gec.

Now I am confused by the 4 parrallel--If the breaker is 600 amps then you would use #1. If the breaker were 1200 amps then you use 3/0.

Since you split it to 2 --600 amp panels I assume you mean feeders to a sub panel from the 600 amp disconnect. This would only need a #1 egc.

Sam Moore

Member
Now I am confused by the 4 parrallel--If the breaker is 600 amps then you would use #1. If the breaker were 1200 amps then you use 3/0.

Since you split it to 2 --600 amp panels I assume you mean feeders to a sub panel from the 600 amp disconnect. This would only need a #1 egc.
Thanks, the more I think about it, the 3/0 makes me confused too...#1 does make better sense to me, that's why I was questioning myself

erickench

Senior Member
Look at note 1 of NEC Table 250.66. According to this note:

Where multiple sets of service entrance conductors are used as permitted in 230.40, Exception No. 2, the equivalent size of the largest service entrance conductor shall be determined by the largest sum of the areas of the corresponding conductors of each set.

If you're using four sets per phase then the largest size conductor would be
4 X 350 = 1400 MCM. According to NEC Table 250.66 the GEC would be 3/0 copper or 250 MCM aluminum.