Hi all, first time here.
I wire tilt-up buildings ranging from shells to full-blown manufacturing facilities. Often these buildings have a 480/277 service and require the use of one or more indoor, dry-type transformers, 480 delta to 120/208 wye, from 75 to 750 kva.
These transformers are typically fed from underground PVC conduits, with the secondaries leaving underground in PVC also. Usually everything over 75kva uses parallel secondaries, and over 150kva has parallel primaries as well.
For years we've grounded these the same way, pulling full sized equipment grounds in each PVC, provide ufer to transformer, and bond XO in transformer. This week I was sizing conduits for a 750kva installation and started thinking. (that's where I usually get into trouble)
Our 750 KVA will have 4 - 3" with 3 250 mcm's ea., the secondary will have 6 - 4" with 4 400 mcm's ea.
Add up the ground terminations in that transformer, there?s a bunch of ?em, especially since that?s where we?ve been bonding the neutral.
My questions are the following:
Why does the primary 3-wire circuit in PVC conduits fed from the main service (PVC the entire lenght) length need an equipment ground? The service that feeds it is grounded, and the transformer frame would be bonded via the GEC.
Why do the secondaries need equipment grounds? Exhibit 250-14 of the 2002 NEC
shows the neutral bond at the first disconnecting means, (our low voltage distribution main), with the transformer frame being bonded with the GEC from what would be our distribution.
If a GEC bonding the transformer is sufficient, and I can?t think of a reason why it wouldn?t be, can the GEC be run as a single 3/0 exposed? Remember, there?s 6 secondary conduits looping the transformer secondary with the distribution, and the two are within 4? of each other.
Lastly, where would this GEC between the transformer and main be terminated? It seems to me it should just be between the ground buss of the main and the frame of the transformer, or should it be to the grounded neutral buss in the main where the GEC from the ufer is terminated?
Thanks in advance, kinda long winded but I wanted to paint a clear picture of what I?m talking about. I?ve read other threads here on transformer bonding but nothing addressing the use of PVC.
Bob
[ June 17, 2004, 12:31 PM: Message edited by: sdbob ]
I wire tilt-up buildings ranging from shells to full-blown manufacturing facilities. Often these buildings have a 480/277 service and require the use of one or more indoor, dry-type transformers, 480 delta to 120/208 wye, from 75 to 750 kva.
These transformers are typically fed from underground PVC conduits, with the secondaries leaving underground in PVC also. Usually everything over 75kva uses parallel secondaries, and over 150kva has parallel primaries as well.
For years we've grounded these the same way, pulling full sized equipment grounds in each PVC, provide ufer to transformer, and bond XO in transformer. This week I was sizing conduits for a 750kva installation and started thinking. (that's where I usually get into trouble)
Our 750 KVA will have 4 - 3" with 3 250 mcm's ea., the secondary will have 6 - 4" with 4 400 mcm's ea.
Add up the ground terminations in that transformer, there?s a bunch of ?em, especially since that?s where we?ve been bonding the neutral.
My questions are the following:
Why does the primary 3-wire circuit in PVC conduits fed from the main service (PVC the entire lenght) length need an equipment ground? The service that feeds it is grounded, and the transformer frame would be bonded via the GEC.
Why do the secondaries need equipment grounds? Exhibit 250-14 of the 2002 NEC
shows the neutral bond at the first disconnecting means, (our low voltage distribution main), with the transformer frame being bonded with the GEC from what would be our distribution.
If a GEC bonding the transformer is sufficient, and I can?t think of a reason why it wouldn?t be, can the GEC be run as a single 3/0 exposed? Remember, there?s 6 secondary conduits looping the transformer secondary with the distribution, and the two are within 4? of each other.
Lastly, where would this GEC between the transformer and main be terminated? It seems to me it should just be between the ground buss of the main and the frame of the transformer, or should it be to the grounded neutral buss in the main where the GEC from the ufer is terminated?
Thanks in advance, kinda long winded but I wanted to paint a clear picture of what I?m talking about. I?ve read other threads here on transformer bonding but nothing addressing the use of PVC.
Bob
[ June 17, 2004, 12:31 PM: Message edited by: sdbob ]