# 3-Phase 400A 277/480 service one 4" pipe



## Njelectric (Dec 8, 2011)

Currently have a job that is requiring a 400A 3-phase 480v service. The disco is rated at 400A and the connected load is going to be 260A. Normally, I'd parallel 3/0 conductors in two conduits. Problem is, as stated in the title, I only have one 4" pipe and from what I can tell paralleled 3/0's wouldn't work in this case due to derating from counting the neutral as a current carrying conductor. Am I interpreting the code right on that one?


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## Cow (Jan 16, 2008)

We would run one set of 500 CU for this. If we needed the FULL 400 amp rating, we would run one set of 600 CU.

Paralleling in the same conduit is usually not worth it.


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## Njelectric (Dec 8, 2011)

500's is what I was thinking being that I'm under 380A calculated load. I do know that tends to bring up 500's vs 600's disagreement every now and then when calculating services so hopefully I'm ok with the 500's!


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## Njelectric (Dec 8, 2011)

Also, just so I know I'm a 100% here... a #3 copper ground going from transformer to disconnect would suffice, right? Thanks in advance!


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## PlugsAndLights (Jan 19, 2016)

Njelectric said:


> Currently have a job that is requiring a 400A 3-phase 480v service. The disco is rated at 400A and the connected load is going to be 260A. Normally, I'd parallel 3/0 conductors in two conduits. Problem is, as stated in the title, I only have one 4" pipe and from what I can tell paralleled 3/0's wouldn't work in this case due to derating from counting the neutral as a current carrying conductor. Am I interpreting the code right on that one?


Don't know about the NEC, but the CEC would not include the neutral(s)
as current carrying conductor(s) for the purpose of "number of conductor
derating". Code ref 4-004(3).
YMMV,
P&L


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## micromind (Aug 11, 2007)

According to 310.15(B)(4), the neutral isn't counted as a current carrying conductor unless the major portion of the load is non-linear. 

If there are tons of lights, count the neutral, if most of the load is motors and transformers, don't count it.


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## Julius793 (Nov 29, 2011)

Parallel 3/0 or use 500


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## Cow (Jan 16, 2008)

Njelectric said:


> Also, just so I know I'm a 100% here... a #3 copper ground going from transformer to disconnect would suffice, right? Thanks in advance!


Since you said this is a service, there is no equipment grounding conductor from the utility trans. to your service disconnect. Only a grounded conductor(neutral).

Since the service disconnect is where your main bonding jumper(green screw, strap, buss bar, etc) is installed, this is where your EGC's originate from. Everything on the line side of the service disconnect is bonded to the grounded conductor(neutral). Everything after the service disconnect where the main bonding jumper is installed should be bonded to the EGC.


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## pudge565 (Dec 8, 2007)

Njelectric said:


> Also, just so I know I'm a 100% here... a #3 copper ground going from transformer to disconnect would suffice, right? Thanks in advance!


You need a 1/0 copper or 3/0 aluminum GEC in either scenario.


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## pudge565 (Dec 8, 2007)

Njelectric said:


> Also, just so I know I'm a 100% here... a #3 copper ground going from transformer to disconnect would suffice, right? Thanks in advance!


I think you are talking about the grounded conductor here which minimum per 250.24(C) is the same as what I said in the last post for a single conduit.


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## backstay (Feb 3, 2011)

500 should be good for 320 amps, I don't know why you would go higher.

Pudge is correct. I hit the 60 deg column by mistake.


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## pudge565 (Dec 8, 2007)

Cow said:


> We would run one set of 500 CU for this. If we needed the FULL 400 amp rating, we would run one set of 600 CU.
> 
> Paralleling in the same conduit is usually not worth it.



You can do 500 for the FULL 400 amp rating, you are allowed to protect at the next standard size which is 400 amps since the 500 is rated at 380 amps.

ETA: I think your point is that if your load calc is 400 amps you need the wire to be rated for 400 amps which would me 600s but then I think you'd need a bigger service anyway.


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## pudge565 (Dec 8, 2007)

Njelectric said:


> Currently have a job that is requiring a 400A 3-phase 480v service. The disco is rated at 400A and the connected load is going to be 260A. Normally, I'd parallel 3/0 conductors in two conduits. Problem is, as stated in the title, I only have one 4" pipe and from what I can tell paralleled 3/0's wouldn't work in this case due to derating from counting the neutral as a current carrying conductor. Am I interpreting the code right on that one?


You don't count the neutral as a CCC for the purposes of derating. However, you would still not be able to use 3/0 or 4/0 at the 80% detrating factor for the 6 CCC you do have, you would need parallel 250's with a 1/0 neutral or parallel 3s.



Julius793 said:


> Parallel 3/0 or use 500


See above Julius.


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## Cow (Jan 16, 2008)

pudge565 said:


> You can do 500 for the FULL 400 amp rating, you are allowed to protect at the next standard size which is 400 amps since the 500 is rated at 380 amps.
> 
> ETA: I think your point is that if your load calc is 400 amps you need the wire to be rated for 400 amps which would me 600s but then I think you'd need a bigger service anyway.


Not necessarily. We had a similar install last year for a booster station with a calculated load of 795 amps. We paralleled 600's for the full 800 amp service rating rather than the 760 amp rating with parallel 500's. 

The customer approached us right before irrigation season. There was no time to order 1000-1200 amp switchgear which I would of liked to do, so we stuck with an 800 amp off-the-shelf CT cabinet instead. Not ideal, but it does happen.


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## lighterup (Jun 14, 2013)

I read this whole thread and I noticed there is only (1) 4" conduit
but talk of parralelling. Shouldn't each set of parallels be in seperate
conduit at this voltage(480) ? ..to avoid skin effect?


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## telsa (May 22, 2015)

lighterup said:


> I read this whole thread and I noticed there is only (1) 4" conduit
> but talk of parralelling. Shouldn't each set of parallels be in seperate
> conduit at this voltage(480) ? .*.to avoid skin effect*?


Skin effect is not involved here.

Skin effect shows up in the NEC ampacity tables ... as the larger conductors don't directly scale up in ampacity. The current density in the center drops -- and moves to the skin.

If this scheme is a SERVICE ( Service Lateral ) there is a distinct possibility that the neutral can be down-sized. 

But the OP is blank on this possibility.


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## EJPHI (May 7, 2008)

What he said.

FYI the skin depth for 60 Hz Copper is about 2/3".

So the table account for this and paralleling conductors is the way to go.

EJPHI


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## mbednarik (Oct 10, 2011)

You could do either parallel 3/0 or 500. The neutral is not counted as per 310.15 B 5. I think you would be hard pressed to find many loads with poor power factor to create a harmonic neutral issue. Now 500 mcm is rated at 380 and there would not be any derating. If you used 3/0 THWN-2 (most THHN is dual rated to this) the 90C ampacity is 225. Now take 225 x 80 percent for 6 current carrying wires = 360. Still suitable for a 400 if the calculated load is less than 360.


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## MechanicalDVR (Dec 29, 2007)

Hope the job is in and running by now.


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