Neutral Sizing

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jasons

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What size neutral would be required for a service from power company transformer to a 2000A 480V 3ph breaker. Transformer is grounded "Y" and will have no neutral load. Service will consist of 5 runs of 3 1/2" conduit with 3- 600KCMIL phase conductors and grounded neutral in each conduit.
Please show me how you get this answer.
 
Re: Neutral Sizing

Reference: NEC table 8, Conductor Properties and 250.24.b.2. Parallel total is 5x600KCMIL = 3000.
12 1/2% = 375KCMIL. 75KCMIL each is #1. 250.24b.2 says 1/0 is minimum size.
 
Re: Neutral Sizing

Jason, go to 250.24(B)

Roger
 
Re: Neutral Sizing

Jasons,
In this case you directly apply Table 250.66 based on the size of the conductors in a single raceway. See 250.24(B)(2). Each raceway will require a 1/0 grounded conductor.
Don
 
Re: Neutral Sizing

Don
I am not arguing with you yet :)

Can you explain to me why you use that chart and not 250.24(B)?

Even if you use table 250.66 wouldn't you have to use note 1 ?

Thanks
Jason
 
Re: Neutral Sizing

Jason,

It is all in 250.24(B)(1) and 250.24(B)(2) which tell you to use Table 250.66.

Roger, Don and nvcape have all provided the info. :)

[ October 07, 2003, 05:12 PM: Message edited by: iwire ]
 
Re: Neutral Sizing

Jason,
250.24(B)(2) Parallel Conductors. Where the service-entrance phase conductors are installed in parallel, the size of the grounded conductor shall be based on the total circular mil area of the parallel conductors as indicated in this section. Where installed in two or more raceways, the size of the grounded conductor in each raceway shall be based on the size of the ungrounded service-entrance conductor in the raceway but not smaller than 1/0 AWG.
Click to expand...
Don
 
Re: Neutral Sizing

Don
Thanks for the help. What I am stuck on is the difference in 250.24(B)(2) and the note 1 below table 250.66.

If you use the size of the ungrounded conductors in each conduit then 1/0 is right.

If you use the note 1 method then you would use a 3/0.

If you use 250.24(B)(1) then you would come up with a total of 375KCMIL which would be a 400 KCMIL conductor in each conduit.

I am no longer disputing that you are right :D
Can you explain to me why that you use that instead of the other two? I am just needing to know why for future reference.

Thanks
Jason Skelton
 
Re: Neutral Sizing

Jason,
Note 1 to Table 250.66 does not apply to this application. It is only for sizing the grounding electrode conductor and in this case we are sizing the grounded conductor. If you use 250.24(B)(1) you come up with 375kcmil. This is the total size required for the grounded conductor, but you can split this total among the 5 raceways giving you a requirment of 75kcmil per raceway. A #2 is 66.4kcmil and a #1 is 83.7kcmil. A #1 in each raceway would provide the required total but would be a violation of 310.4. So in this case with the applicaiton of 310.4 both 250.24(B)(1) & (B)(2) require a 1/0 grounded conductor in each of the 5 raceways.
Don
 
Re: Neutral Sizing

Don
That is what I needed to know!
I sure do appreciate the time spent on making me understand.

Thanks Again
Jason Skelton
 
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