# 400amp Service, Derate for a 4wire system



## Wirenuts1 (Jan 23, 2009)

Hello everyone,I am Estimating my first 400amp, 3 phase, (4 Wire) system.When looking at table 310.16 those values are for a MAXIMUM of 3 Current Carrying conductors in a conduit.With 3 phase you have 4 wires. In the code it also says you have to derate those values by 80%....The question is, I keep reading all kinds of wierd numbers in the forum and there not makeing sense according to whats written in the code.


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## Bob Badger (Apr 19, 2009)

Unless this supplies 50% or more non-liner load you will not have to count the neutral as a current carrying conductor.


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## 480sparky (Sep 20, 2007)

Take the type of conductor you are using, find it's ampacity in T310.16, and multiply that by 0.8.


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## electures (Oct 23, 2008)

Wirenuts1 said:


> Hello everyone,I am Estimating my first 400amp, 3 phase, (4 Wire) system.When looking at table 310.16 those values are for a MAXIMUM of 3 Current Carrying conductors in a conduit.With 3 phase you have 4 wires. In the code it also says you have to derate those values by 80%....The question is, I keep reading all kinds of wierd numbers in the forum and there not makeing sense according to whats written in the code.


310.15(B)(4)(c) reads;

(c) On a 4-wire, 3-phase wye circuit *where the major portion of the load consists of nonlinear loads*, harmonic currents are present in the neutral conductor; the neutral conductor shall therefore be considered a current-carrying conductor.​

If a major portion of the load is nonlinear the the neutral is considered current carrying and derating factors shall be applied. The values in 310.16 have to be multiplied by 80%.

700 KCMIL CU is rated at 368A and will work for the ungrounded feeders as long as the calculated load does not exceed 368A. The grounded circuit conductor has to be sized for the calculated load. Then the ampacities in 310.16 have to be derated by 80%.​ 
L8R, Kevin​


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## DipsyDoodleDandy (Dec 21, 2009)

*Building*

What type/use building you have there Wirenuts?


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## Bob Badger (Apr 19, 2009)

DipsyDoodleDandy said:


> What type/use building you have there Wirenuts?


Unless the building is literally a server farm the majority of the load will not be non-linear.


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## DipsyDoodleDandy (Dec 21, 2009)

*Lights*

If it's a Marijuana producing facility your probably 50.000001 or more nonlinear and you'll have to count, if not, you probably won't. That's how I analgolize it.


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## electures (Oct 23, 2008)

Computers, solid state ballasts, switching power supplies are some examples of non-linear loads. The sort of stuff found in large commercial building.


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## Dennis Alwon (May 9, 2009)

electures said:


> Computers, solid state ballasts, switching power supplies are some examples of non-linear loads. The sort of stuff found in large commercial building.


Yes, but I would add that it is a rare occurrence to see the non linear loads equal more than 50% of the loads of the service.


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## electures (Oct 23, 2008)

Dennis Alwon said:


> Yes, but I would add that it is a rare occurrence to see the non linear loads equal more than 50% of the loads of the service.


No arguement here.:no:


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