Utility Transformer to 225kva

[Alfont1120]

The local utility has a padmount 13.8 to 480V transformer at the street. Our plans show us coming out of that with 4-wire plus ground and hitting a 300A fused disconnect. From there, we hit a 225kva 480 to 208V transformer which goes to a 600A fused disco ahead of the first panel. The engineered drawing calls for a 4-wire plus ground coming out of the utility transformer but that doesn't seem right. I don't see why we'd need a neutral since we will get one out of the X0 of the 225kva. The more I think about it, even pulling a ground from the utility transformer is starting to seem off as well. Do I just pull 3 phases to the 300A disco and 3 phases and a ground from that disco to the 225kva and bond it there?

 

[augie47]

Assuming the 300 amp disconnect is your service disconnect, Art 250.24 would require you to bring the grounded conductor to the disconnect to establish a ground-fault return path to the transformer {(sized per 250.102(C)(1)}. From that point you would need an equipment grounding conductor.{sized per 250.122]

 

[don_resqcapt19]

The grounding (supply side bonding jumper) conductor between the transformer and the disconnect is not required and most likely will not be connected to anything in the transformer if you install it.

There is no reason to install the neutral conductor between the disconnect and the transformer. It is not needed.

You will need four conductors and a supply side bonding jumper between the transformer and the 600 amp disconnect.
If you are running two sets of 350kcmil between the transformer and the disconnect, you will need a 2 AWG supply side bonding jumper with each set.

The two sets of conductors between the disconnect and PP1 will require a 1 AWG equipment grounding conductor with each set of 350s.

 

[JoeStillman]

The grounding (supply side bonding jumper) conductor between the transformer and the disconnect is not required and most likely will not be connected to anything in the transformer if you install it.

If the Δ-Y transformer is the only load, could you run a bonding jumper instead of a neutral between the padmount and the 400/3?

 

[Hv&Lv]

As long as the utility bonds their X0 you shouldn't have neutral drift.
I would run the 4) 350 conductors and connect one to X0. The #4 is never done on services coming to our transformers since the XO has your grounded conductor and we bond the X0 to our MGN system.

 

[jaggedben]

If the Δ-Y transformer is the only load, could you run a bonding jumper instead of a neutral between the padmount and the 400/3?

Not if it's the service disconnect, 250.24(C) requires you to bring the 'grounded conductor'. Which mostly just means to tape it white (or gray), not green, but still.

 

[JoeStillman]

Not if it's the service disconnect, 250.24(C) requires you to bring the 'grounded conductor'. Which mostly just means to tape it white (or gray), not green, but still.

Either one would provide an adequate path for ground fault currents that hit the equipment grounding system to get back to the neutral.

 

[david luchini]

Either one would provide an adequate path for ground fault currents that hit the equipment grounding system to get back to the neutral.

If either would be fine, then run the grounded conductor. Because the Code tells you to.

 

[don_resqcapt19]

Either one would provide an adequate path for ground fault currents that hit the equipment grounding system to get back to the neutral.

The only difference would be the color identification of the conductor because both the grounded conductor and the supply side bonding jumper are sized per Table 250.102(C)(1) when there is no neutral load. The code requires the grounded conductor to be run to the service equipment so it will have to be identified as such.