120/208V wye primary 480V delta secondary bonding question

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buzzbar

buzzbar

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Hello all,

I'm working on a job that has a new 120/208V 4-wire wye service. From that new new service (which is a 1000A fused switch), it feeds a 300 Kva transformer. The transformer is a 120/208V wye primary, and 480V delta secondary. From the 300 Kva transformer, it feeds a 480V panelboard, with a main breaker.

My question is this: How would I properly bond the transformer? At the service disconnect, the nuetral and ground are bonded. Normally, I wouldn't bond the XO terminal at the transformer, since it's bonded at the service. The 480V delta panel has no neutral, so would I simply land the EGC at the transformer case? I landed the GEC at the 480V panel, and of course there is a separate GEC at the 208V main service.

Thanks in advance!!

Andy
 
1-You would have to install ground fault monitors or ground one corner of the delta.
2-There is no need to bring 4 wires into the transformer and I would not bond the neutral point on the transformer to the case.
 
bsaxe43 said:
1-You would have to install ground fault monitors or ground one corner of the delta.
2-There is no need to bring 4 wires into the transformer and I would not bond the neutral point on the transformer to the case.
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The 480V panel has ground fault monitors, but not the 208V primary side.
Explain how you would ground one corner of the delta. I assume that would mean a jumper would be installed from the case of xfrmr to H1, H2, or H3?
also, the engineer specified 4-wire system on primary AND secondary side, which didn't make sense to me.
 
buzzbar said:
The 480V panel has ground fault monitors, but not the 208V primary side.
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You don't need them on the 208V side. It will never know the fault is there. In an ungrounded system the fist fault just establishes a ground reference and lights the detector. The primary side can't see any of that.

Explain how you would ground one corner of the delta. I assume that would mean a jumper would be installed from the case of xfrmr to H1, H2, or H3?
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Yes, and a connection to a grounding electrode just like you would for the XO.
also, the engineer specified 4-wire system on primary AND secondary side, which didn't make sense to me.
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That makes no sense unless he is specing that the equipment ground be a wire and not metalic conduit.
 
And if the primary neutral is connected to the wye point of the transformer, that can cause all sorts of problems, given that the transformer has a delta output.
 
4th wire being the EGC?

There is no need for a neutral on primary side and as mentioned may cause problems if one is connected.

Secondary side there is no neutral period. All you can do is bond one phase at the transformer and then run separate grounded and equipment grounding conductors after that point, or utilize as an ungrounded system which still needs an EGC to connect all non current carrying components together, it just doesn't connect to the transformer winding in any way.
 
Thanks for all of the advice. I have never wired a transformer like this. I've done plenty with the PRI-SEC reversed.

The 4-wire system that the engineer spec'd is actually a 5-wire system, I wasn't including the EGC. I've inserted a copy of the riser diagram that shows the service. All of the equipment is within 10-15' of each other.
1000A service riser diagram WWM JPEG.jpg
 
buzzbar said:
Thanks for all of the advice. I have never wired a transformer like this. I've done plenty with the PRI-SEC reversed.

The 4-wire system that the engineer spec'd is actually a 5-wire system, I wasn't including the EGC. I've inserted a copy of the riser diagram that shows the service. All of the equipment is within 10-15' of each other.
View attachment 11242
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He does have inconsistencies in there. He specifies 4 wires plus ground, yet only mentions 480 volt both at the supply end as well as the load end of that feeder.

Also keep in mind in 2014 NEC you can no longer back feed a transformer unless manufacturers instructions permit it. See 450.11(B)
 
My guess is it's a CAD/design error -- that transformer should be 208V delta-480/277V wye (with a 3-wire feeder on the 208V side. There's no other way you could have a 480V 4W board downstream.

Only issue there is whether or not the TVSS needs a neutral to function.
 
Pharon said:
My guess is it's a CAD/design error -- that transformer should be 208V delta-480/277V wye (with a 3-wire feeder on the 208V side. There's no other way you could have a 480V 4W board downstream.

Only issue there is whether or not the TVSS needs a neutral to function.
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The voltages are correct. Panel MDP needs to have 480V Delta. The incoming power is existing, and it's 120/208V 3-phase wye.

My my plan is to install a 1/0 bonding jumper from H1 to case on secondary side, and not install or not hook up the neutral to XO on the primary side.

My my other question is this: my GEC for Panel MDP goes from the ground rods to MDP. Should I have ran the GEC from the ground rods to the transformer? Or, can I run a GEC from the transformer to MDP (along with EGC)?
 
kwired said:
He does have inconsistencies in there. He specifies 4 wires plus ground, yet only mentions 480 volt both at the supply end as well as the load end of that feeder.

Also keep in mind in 2014 NEC you can no longer back feed a transformer unless manufacturers instructions permit it. See 450.11(B)
Click to expand...

This transformer is designed for a 208V wye PRI and 480V delta SEC, so it's not "back fed".
 
GoldDigger said:
And if the primary neutral is connected to the wye point of the transformer, that can cause all sorts of problems, given that the transformer has a delta output.
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Agreed, and it's also a code violation because by grounding the low-side wye he'd be re-bonding the system neutral in a second spot.
 
big john said:
Agreed, and it's also a code violation because by grounding the low-side wye he'd be re-bonding the system neutral in a second spot.
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If a neutral is truly needed to properly operate the transformer, the proper method would be to run a separate neutral and EGC - with no bond between them other then back at the service equipment.
 
This 480V delta transformer will be feeding four transformers out on the docks of a new marina. The dock transformers will then step it back down to 120/208V.
 
buzzbar said:
This 480V delta transformer will be feeding four transformers out on the docks of a new marina. The dock transformers will then step it back down to 120/208V.
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Not a marina expert, but does the designer want a corner ground or an ungrounded system supplying this? I'd think a wye system would be preferred if it is grounded just to have lower voltage to ground.

Is an ungrounded system permitted to be used for such purposes? I can see some advantages, I also see real dangers if there is no one qualified to monitor ground detection system. Most ungrounded systems are intended for processes that need to be shut down in an orderly fashion instead of sudden interruption should there be a ground fault.
 
The engineers have told me to go the cornered ground route. Just curious, why would a wye configuration be better? I am no expert on this, so I'd like to learn.
 
buzzbar said:
The engineers have told me to go the cornered ground route. Just curious, why would a wye configuration be better? I am no expert on this, so I'd like to learn.
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My reasoning which I mentioned was because of lower voltage to ground. Throw in the fact this is a marina - maybe that is even more important - though 277 isn't very forgiving even when not in a highly conductive environment. But I do not do wiring at marinas and if you need 480 volts this likely isn't for small private boats either.
 
The disadvantage of 480 delta is the voltage to ground is 480, not 277, and creates a higher stress on the insulation, motors tend to be damaged easily as motor windings are the weakest link.
 
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