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?

It sounds good to me except you need to add an GEC.


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.

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.

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.

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.

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, 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.

Does this help any??

from IAEI/Soares



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

that's what I thought electure.

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!