Sizing Feeder for 400 Amp Subpanel

[preacceleration]

Hi All!

I'm thinking about incorporating subpanels into our building design and was looking for a double-check on my calculations.

I need to size the feeder to a 400 amp subpanel. We are distributing power at 3 phase 415 wye. The subpanel has six 60A three phase circuits on it. Each circuit will be used continuously at 80% for a total continuous load of 288 A.

What gauge wire do I need, if I'm using aluminum? Can I use more than one conductor per phase for the feeder? What number of conductors is optimal to achieve the greatest amps to cross-sectional area ratio?

Thanks!

 

[augie47]

Your wording is a bit confusing to me.
Lets look at it in a slightly different manner. If you load is 288 amps and it's continuous, then you would need a circuit based on 125% or 360 amps.
With a 360 amp load, 240.4 would allow a 400 amp circuit breaker.
To determine the correct conductor a bit more info is needed. Do the circuits require a neutral ? If so, are the loads linear or non-linear ? If more than 3 circuits are considered current carrying, then derating will take place and the size conductor will vary.

 

[preacceleration]

Your wording is a bit confusing to me.
Lets look at it in a slightly different manner. If you load is 288 amps and it's continuous, then you would need a circuit based on 125% or 360 amps.
With a 360 amp load, 240.4 would allow a 400 amp circuit breaker.
To determine the correct conductor a bit more info is needed. Do the circuits require a neutral ? If so, are the loads linear or non-linear ? If more than 3 circuits are considered current carrying, then derating will take place and the size conductor will vary.

The loads are connected L-N, so a neutral is required. The loads are servers that run continuously at max wattage with power supplies of power factors of 0.99, although in the future we may have servers with power supplies with lower power factors, so if that changes whether the neutral is considered to be current carrying, then I'd appreciate calculations for both scenarios.

 

[preacceleration]

Your wording is a bit confusing to me.
Lets look at it in a slightly different manner. If you load is 288 amps and it's continuous, then you would need a circuit based on 125% or 360 amps.
With a 360 amp load, 240.4 would allow a 400 amp circuit breaker.
To determine the correct conductor a bit more info is needed. Do the circuits require a neutral ? If so, are the loads linear or non-linear ? If more than 3 circuits are considered current carrying, then derating will take place and the size conductor will vary.

What I've calculated is that with Aluminum THHN 90 degrees in a dry location, my best bet is to use 3 parallel 3-phase circuits, in separate conduits, with 1/0 awg gauge for the conductors. This provides an ampacity of 405 amps which is greater than the full load current of 360A. I'm still not sure what gauge the Earth needs to be.

If the neutral is considered to be a current carrying conductor, then it's a tossup between 4 1/0 parallels and 3 2/0 parallels, with the 3 2/0 providing just enough ampacity, with the 4 1/0s providing some head room.

 

[bob]

You can use the 90C rating only when derating is required. The 75C rating of 1/0 AL = 120 amps.
3 conductors = 360 amps if derating is not required. Your load of 288 amps x 1.25 = 360 amps
which would load the conductors to 100% and leave no room for additions.
2--250 AL = 410 amps.