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#44676 11/09/04 06:52 PM
Joined: Apr 2001
Posts: 449
F
Fred Offline OP
Member
Please check me on this. There has been a lot of discussion on this topic and it gets confusing and I tend to second guess myself. I have a 3 wire w/EGC 480V feed to an unattached building. 3 3/0 hots and a #1 EGC. They terminate in a 480V 200A MB panel which has a #4 GEC connected to a 5/8" x 8' electrode. This panel has a 3 pole 100A CB that feeds a 45kva 480V primary/ 208/120V secondary. From the 480V panel I have 3 #2 cu thhn and 1 #4 EGC going to the tranny and terminated on H1, H2, and H3. The EGC lands in a lug bolted to the tranny frame. From the tranny I have 4 3/0 thhn conductors and 1 #1 ECG. The 3/0 are terminated on X1, X2, X3 and XO. XO being the neutral running to the 208/120V 200A MB panel. The EGC runs from the same EGC lug in the tranny to the EGC bar in the 208/120 panel and is bonded to the neutral bar in the 208/120 panel. Is this correct?

#44677 11/09/04 07:05 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
It sounds good to me except you need to add an GEC.

Quote
250.30(A)(4) Grounding Electrode. The grounding electrode shall be as near as practicable to and preferably in the same area as the grounding electrode conductor connection to the system. The grounding electrode shall be the nearest one of the following:
(1) An effectively grounded structural metal member of the structure
(2) An effectively grounded metal water pipe within 1.5 m (5 ft) from the point of entrance into the building

You should take a look through all of Article 250.30 Grounding Separately Derived Alternating-Current Systems.

In there you will find how to size your bonding jumper and the grounding electrode conductor.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#44678 11/09/04 07:10 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Hey did you mean 45 KVA or is it a 75 KVA?

A 45 KVA would have a rated secondary current of 125 amps and a primary current of 54 amps.


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#44679 11/09/04 07:56 PM
Joined: Apr 2001
Posts: 449
F
Fred Offline OP
Member
Typo. 75kva. It is a wood frame building with no metal water piping. I drove a 5/8" x 8' electrode. That was what I was second guessing myself about. The electrode is connected to the ground bar in the 480 panel where the EGC that comes to the building with the feeder. There is a jumper from the 480 ground bar to the tranny and from the tranny to the 208 panel's ground bar, all #2 thhn. It is bonded to the neutral in the 208 panel.

#44680 11/09/04 08:20 PM
Joined: Jun 2004
Posts: 220
T
Member
sounds good to me fred.You have the bond to the neutral sized right in the secondary panel?I would pound a second rod,just to be sure you have a good(25 ohms or less) ground path.

#44681 11/09/04 08:39 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
According to how the code is written you must run a grounding electrode conductor {250.30(A)(2)} you do not get to use the equipment grounding conductor that runs with the primary's as the GEC. Wish I could do that it would save me a lot of time and copper.

Bob


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#44682 11/10/04 06:47 AM
Joined: Apr 2001
Posts: 449
F
Fred Offline OP
Member
Bob, I don't quite follow. I have a grounding electrode at the second building and a GEC from it to the 480V panel. Are you saying I need to hit the electrode with another conductor directly from the tranny? I have a #2 from the 480 panel ground bar to the tranny running with the primary from the 480 panel to the tranny.

#44683 11/10/04 07:55 AM
Joined: Dec 2000
Posts: 4,287
Member
Does this help any??

from IAEI/Soares

[Linked Image]

I think you'll have to add another supplementalrod unless you have less than 25 ohms to grd

#44684 11/10/04 08:32 PM
Joined: Jun 2004
Posts: 220
T
Member
that's what I thought electure.

#44685 11/10/04 08:45 PM
Joined: Apr 2001
Posts: 449
F
Fred Offline OP
Member
Thank you! The uncertainty on my part arose after a discussion with some other sparkys at the supply house. I did it as shown in the diagram(sorry if I wasn't clear in my post)and others argued that you didn't groung XO in the tranny, you bonded the neutral in the panel. I drove a rod at the building and have a UFER there as well. I did both because the inspector doesn't like/understand UFERs. He wants to "see" a rod. He can't "see" all of the UFER. Thanks for all the help EVERYONE!


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