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What's the difference? #183033 12/22/08 10:58 PM
Joined: Jan 2004
Posts: 1,507
George Little Offline OP
Here you go, help me understand this.

I have an installation that has me wondering why the code is the way it is. I am hoping you can shed some light on this to help me understand the code.

Customer owned primary service feeding a transformer outside and by using PVC they intend to bond the XO at both locations. The grounding electrode connection will be located at the service rated transfer switch inside the building. The transformer secondary output is 4,000 amperes at 277/480 volts. The phase and neutral conductor are 10 sets of 600 kcmil. Since we need to provide an equipment bonding jumper per 250.30(A)(2) it is to be sized based on 250.102(C) based on the derived phase conductors. In a parallel installation the conductor is based on the size of the conductors in each raceway. Which would be 1/0 in each raceway.

Here is what makes me feel strange.

A 4,000 amp feeder (10 Paralleled Sets using 600 kcmil conductors) protected at their ampacity by an overcurrent device, 250.122(F) would require a 500 kcmil conductor in each raceway for an equipment grounding conductor.

With the installation I have that has unprotected feeder conductors from the transformer the equipment bonding jumper only needs to be a 1/0 in each raceway.

George Little
2017 / 2014 NEC & Related Books and Study Guides
Re: What's the difference? [Re: George Little] #183038 12/23/08 03:08 AM
Joined: Aug 2007
Posts: 853
leland Offline
-primary service - How does this fall under "seperately derived"?
Just asking coz I like the challenge!

Would this not be a "Service"?

Re: What's the difference? [Re: leland] #183047 12/23/08 12:49 PM
Joined: Mar 2003
Posts: 849
Yoopersup Offline
Primary Service rated switch on transformer required as Transformer belongs to owner. HV 4160, 7200, or 13800.
Every thing after thats a feeder.
Transformer requires Netural bonded & Ground rod on Secondary.
I take it Neutral bonded at Building disconnect.
That if a Equipment grounding conductor installed on seconday to building disconnect creates a Parallel neutral.
(Neutral bonded two places)
If no continious metallic paths between building & transformer.(PVC raceway) according to 250.32 B(2) is a equipment grounding conductor required at all??
Whatca think??

Re: What's the difference? [Re: Yoopersup] #183052 12/23/08 03:51 PM
Joined: Apr 2002
Posts: 7,234
HotLine1 Offline
You under '05NEC??

Re: What's the difference? [Re: HotLine1] #183055 12/23/08 05:37 PM
Joined: Jan 2004
Posts: 1,507
George Little Offline OP
John- We are under the '05 NEC. I suspect that you were looking at the '08 which would require a grounding conductor.

Ernie- The issue here has many facets that I found challenging and I know this is something that you deal with routinely.
First the conductors are unfused at the xformer secondary located outside and run to a transfer switch inside the building. Do we have words in the code that say we need a bonding conductor when running RNMC for this type of installation?
Secondly if the conductors are fused do we need a EGC or a bond wire? In the case of the EGC it is selected per 250 122 and we need a full size EGC in each raceway. If we only use a bond conductor we size it based on 250.66 and that size is related to the conductors in each raceway. Hence a considerable size difference.
I still can't find where we need a bond or a ground for this type of installation in the '05 unless the system has GFPE.

Last question - Under the '08, we need a EGC, so would we also need a bonding conductor?

Last edited by George Little; 12/23/08 05:39 PM.

George Little
Re: What's the difference? [Re: George Little] #183057 12/23/08 06:13 PM
Joined: Jul 2007
Posts: 85
WireNuts29 Offline
ok I have a really dumb question. Where in side the transformer would you land all these ground wires? in All the transformers I've seen there is usually a double lug for a ground ring around the pad then back into the secondary side.

Re: What's the difference? [Re: George Little] #183059 12/23/08 06:52 PM
Joined: Mar 2003
Posts: 849
Yoopersup Offline
240.21(C) or if Supervised Industrial Installation 240.92 (B) . for secondary protection of conductors.
I'd say if you meet rules no protection required at the transformer.
If you do put protection on secondary of the Transformer.
Then I;d say Neutral & Ground would seperate after that to building disconnect switch so EGC would be required & sized by 250.122. (4000Amp = 500KCMIL)in each conduit.

If you do NOT put protection at the transformer location , use PVC & comply with 250.32(B)2.
I;d say no EGC required from transformer just bond at Panel at the Building location.
As far as Grounding or Bonding .I take it your taking about at the XO of transformer. That Does require bonding either way. 250.30 (A).
I take it if you run a EGC to Transfer switch , its a 4 pole right. Otherwise Neutrals bonded both ends ??
I'd say if meets exception of 250.32(B)2 .
Run just Phase condutors & neutral from transformer to Transfer Switch. No protection at transformer.
Then you can use a 3 or 4 pole transfer switch yer choice.
Then at building Treat just like a service as far as grounding requirements.
If 4 pole switch used , Grounding required at Both Generator & M.D.P. if 3 pole just at MDP.(Main Distrubtion Panel)
Seems like in 2008 Ya gotta run the EGC per 250.32(B)unless existing building & meets (1)(2)(3).
If thats true Transfer switch would then HAVE to be 4 pole .
Thats the way I see Open Opinions.
Is this 120/208 or 277/480??

I have Never seen EGC run from Transformer secondary lugs to ist disconnect outside with that size service.

Last edited by Yoopersup; 12/23/08 06:57 PM.


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