grounded conductor

[iwire]

I am running a subpanel off of a mdp. the mdp is 1200 amps 480/277 volts. I'm running a 3- 500's which are good for 380 amps as the ungrounded conductors and (1) 1/0 for the grounding conductor. My grounded conductor(neutral) load is about 100 amps. Whats the minimum size grounded conductor I can run and where would I find it in the NEC?

Joe, Dennis already mentioned it but it looks like it went unnoticed.

You cannot use 3 - 500s copper for a 1200 amp feeder, you must have conductors with a rating of 1200 amps or more. You cannot roll up to the next size above 800 amps.

 

[M. D.]

Joe, Dennis already mentioned it but it looks like it went unnoticed.

You cannot use 3 - 500s copper for a 1200 amp feeder, you must have conductors with a rating of 1200 amps or more. You cannot roll up to the next size above 800 amps.

I think the 500s are for the 400 amp feeder to the panel he is putting in ,.. No?
500 ,.he said is good for 380 amps he has three of them,.. a 1/0 grounding.
He is asking about the neutral size as there is under 100 amps of unbalanced load. I think he is asking if he can size the grounded conductor based on the load served . I say yes he can

 

[iwire]

I think the 500s are for the 400 amp feeder to the panel he is putting in?

I know that now, I thought his sub panel was 1200 amps.

 

[augie47]

M.D. Take a look at 215.2 for minimum size.

 

[Dennis Alwon]

I think you are correct M.D. The 500's are for the 400 amp sub panel. Not that that changes my responses for the grounded conductor. What is your take on that?

 

[Dennis Alwon]

Gus how does 215.2(A)(2) fit

 

[augie47]

Into the OP's question, I don't think it fits at all as it states "where the feeder conductirs carry the total load"
My referece was to 215.2(A)(1):The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.

 

[kwired]

From 2008 NEC

215.2 Minimum Rating and Size.
(A) Feeders Not More Than 600 Volts.
(1) General. Feeder conductors shall have an ampacity not less than required to supply the load as calculated in Parts III, IV, and V of Article 220. The minimum feeder-circuit conductor size, before the application of any adjustment or correction factors, shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.
Exception No. 1: Where the assembly, including the overcurrent devices protecting the feeder(s), is listed for operation at 100 percent of its rating, the allowable ampacity of the feeder conductors shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load.
Exception No. 2: Grounded conductors that are not connected to an overcurrent device shall be permitted to be sized at 100 percent of the continuous and noncontinuous load.
The size of the feeder circuit grounded conductor shall not be smaller than that required by 250.122, except that 250.122(F) shall not apply where grounded conductors are run in parallel.
Additional minimum sizes shall be as specified in 215.2(A)(2) and (A)(3) under the conditions stipulated.

My guess is that if there were to be a fault from a phase to neutral that you would want at least the same size neutral as the required equipment ground conductor to protect the conductor.

If on a 400 amp feeder your neutral load was 20 amps and you installed a #12 and had a phase to neutral fault within the feeder, which opens first, the 400 amp OCPD or the #12, depending on conditions it could possibly be the #12

 

[Dennis Alwon]

I see it now. I kept going to (A)(2) and getting puzzled.

It also makes sense that the neutral should be big enough to carry the fault current when you have such a large ungrounded conductor with a small grounded conductor. Thanks all.

 

[iwire]

I agree with Gus.

I will also say it looks like it will be clearer in the 2011 NEC

215.2(A)(2) Grounded Conductor. The size of the feeder circuit
grounded conductor shall not be smaller than that required
by 250.122, except that 250.122(F) shall not apply where
grounded conductors are run in parallel. [ROP 2-296]
Additional minimum sizes shall be as specified in
215.2(A)(2) and (A)(3) under the conditions stipulated.

 

[M. D.]

I'm not sure I agree with Gus . I found this ,. is it wrong?

 

[augie47]

It is correct. I think that is what "Gus" was trying to say
The minimum size is based on 250.122

 

[M. D.]

Then I guess we don't disagree with Gus after all

 

[augie47]

somtimes Gus has a difficult time saying what he means

 

[Dave58er]

I agree with Gus.

I believe 215 (A) (1) was brought up by some dude in post #12.

Not to steal your thunder Gus, but I get extra credit scores every time a moderator agrees with me. Double if its Bob.

Also Dennis, I didn't mean to imply that you would be required to pull a neutral in the situation you described. I have installed power panels that fed only 3 phase loads and and no provisions for a neutral. We ran no neutral at all with the feeders. This is legal right?

What if they were single phase 2 pole loads? What if there were provisions for a neutral?

 

[M. D.]

I believe 215 (A) (1) was brought up by some dude in post #12.

Not to steal your thunder Gus, but I get extra credit scores every time a moderator agrees with me. Double if its Bob. ...........
.....

Now ,..if I could just follow along I would not be out here confusing the issue:roll: Been away a bit,.. guess I'm rusty

 

[Dennis Alwon]

I believe 215 (A) (1) was brought up by some dude in post #12. [/quote
I kept getting confused with the article posted. The correct article is 215.2(A)(1) and I kept going to(A)(2). Logic tells us we would need a grounded conductor at least the size of the EGC otherwise there would be a chance the ground fault from neutral to hot would not trip the breaker.

Also Dennis, I didn't mean to imply that you would be required to pull a neutral in the situation you described. I have installed power panels that fed only 3 phase loads and and no provisions for a neutral. We ran no neutral at all with the feeders. This is legal right?

Perfecetly code compliant but you may get arguments from some AHJ

What if they were single phase 2 pole loads? What if there were provisions for a neutral?[/quote]

I believe if you make provisions for a neutral load then the egc would be the smallest that the grounded conductor could be. Obviously it could be larger base on the cal. of grounded conductors in 220.61. Did I answer your questions?

 

[Dennis Alwon]

Now ,..if I could just follow along I would not be out here confusing the issue:roll: Been away a bit,.. guess I'm rusty

I blame myself for much of the confusion. Sorry.

 

[One-eyed Jack]

What if they were single phase 2 pole loads? What if there were provisions for a neutral?

I believe if you make provisions for a neutral load then the egc would be the smallest that the grounded conductor could be. Obviously it could be larger base on the cal. of grounded conductors in 220.61. Did I answer your questions?[/quote]

Dennis, I believe at one time there was a stipulation that the grounded conductor could not be smaller than the egc. I can not find any ref. to that in the codes that I have access to. It may have been a fpn that said normally you would not.

 

[Dave58er]

Did I answer your questions?

That settles it for me. Thanks