Grounding/bonding from Exterior pad-mounted Wye-Wye MV transformer to Exterior 480v Panel

E

Eeyore's Tail

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Location
Tukwila, WA
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Sparky
I've got a 500 kva Medium Voltage Wye-Wye Pad mount Transformer outside with combined H0/X0 on the secondary side - 12,470V to 480V. Outdoor 480V panel located about 20 ft away on the same concrete pad. We installed a Ufer ground with 3/0 bare copper attached to the rebar in 2 places just in case. 2 ground rods installed and a GEC run to the panel before we knew what was going on with the Ufer ground.

Read a bunch of conflicting things on grounding and bonding for this and not sure why they got a Wye-Wye transformer. How would you do ground and bonding between transformer and panel? Crappy sketch to illustrate Medium voltage.jpeg
 
If customer-owned equipment, you are in a pickle. You have to connect the primary neutral for TX to be stable, but doing so secondary is no longer an SDS, meaning you should not bond the secondary neutral to earth ground again. If H0/X0 is connected might need to treat the secondary panel as a sub-panel (no bonding jumper). Essentially relying on the primary neutral-to-ground bond. Careful here cause some “panels” are listed, “For use as Service Equipment Only” (bond is built into enclosure) than your in the ultimate Pickle haha.

If it’s Utility owned, do whatever you want

I’m not 100% here also curious to hear more. Hope this helps’
 
MLove said:
... If H0/X0 is connected might need to treat the secondary panel as a sub-panel (no bonding jumper). Essentially relying on the primary neutral-to-ground bond.
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Are the conductors between the transformer and panel inside of metallic conduit? If not, unless there's bonding jumper in the panel there will be no available conduction path for clearing faults to equipment ground.
 
MLove said:
If customer-owned equipment, you are in a pickle. You have to connect the primary neutral for TX to be stable, but doing so secondary is no longer an SDS, meaning you should not bond the secondary neutral to earth ground again. If H0/X0 is connected might need to treat the secondary panel as a sub-panel (no bonding jumper). Essentially relying on the primary neutral-to-ground bond. Careful here cause some “panels” are listed, “For use as Service Equipment Only” (bond is built into enclosure) than your in the ultimate Pickle haha.

If it’s Utility owned, do whatever you want

I’m not 100% here also curious to hear more. Hope this helps’
Click to expand...
Yeah it’s customer-owned… there’s a similar transformer upstream and I think they just copied the setup that one had going one. The panel’s a Square D I-Line so I think we’re good at least on that score.
 
synchro said:
Are the conductors between the transformer and panel inside of metallic conduit? If not, unless there's bonding jumper in the panel there will be no available conduction path for clearing faults to equipment ground.
Click to expand...
They’re in PVC, I’m thinking pulling a ground through each just in case might be a good idea here.
 
Eeyore's Tail said:
Customer owned 😬
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I think we just have to say that 250.184 (multi-grounded neutral systems) cover the secondary side of a Yg-Yg transformer and hope that no nuns or kittens die. So you have 2 choices: 1. Bond and ground at one end, which would be the transformer because it already is, and have separate neutrals and grounds at the switchboard. 2. Use the exception that allows a bond at both locations as long as there is no metallic paths, which I think would apply to you.
 
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