OCPD on primary only what size equipment ground?

[wolfman56]

Set up as:
440v 3p 60amp breaker to- Delta / Delta transformer to one 220v 3p machine. Code indicates with this I can use the primary OCPD to protect the whole run. Can the whole circuit be wired with a #10 equipment ground? The only breaker ahead of the system is a 60? seems odd to wire for 120 amps and have #10 qualify.

Also a second system is set up the same way except I was handed a Delta / Wye transformer. I disconnected the XO, so there is no neutral or bond. Then I grounded the case, same way as the other. I fear the inspector will nick me on it because code only quotes a Delta / Delta can be done this way only. If he doesnt catch it do any of you see a safety hazard to this?

 

[david luchini]

No, #10 would not be acceptable. If you are leaving the 220V side of the SDS as an ungrounded system, then the equipment on the 220V side should be grounded by a grounding electrode conductor sized in accordance with T250.66 for the phase conductor size. The minimum GEC would be #8.

On a delta/wye with a 220V secondary, the SDS would be required to be grounded even if you are not using the neutral per 250.20(B)(1).

 

[petersonra]

No, #10 would not be acceptable. If you are leaving the 220V side of the SDS as an ungrounded system, then the equipment on the 220V side should be grounded by a grounding electrode conductor sized in accordance with T250.66 for the phase conductor size. The minimum GEC would be #8.

I don't believe there is a GEC involved at all in a delta/delta feeder transformer.

 

[wolfman56]

David,
Are you sure?
The instructions you listed would be for a seperatly derived system that supplies a premises. Looking at the NEC definition of a seperatly derived system and premises, this is not either. The transformer is fed from a sub panel located inside a large building, and is only for one machine, nothing else.

 

[david luchini]

I don't believe there is a GEC involved at all in a delta/delta feeder transformer.

Bob,

See 250.30(B)(1) for an ungrounded separately derived system. "A grounding electrode conductor...shall be used to connect metal enclosures of the derived system to the grounding electrode..."

 

[david luchini]

David,
Are you sure?
The instructions you listed would be for a seperatly derived system that supplies a premises. Looking at the NEC definition of a seperatly derived system and premises, this is not either. The transformer is fed from a sub panel located inside a large building, and is only for one machine, nothing else.

Wolfman, the wiring from the transformer secondary to the machine is a "premise wiring system." See the definition in article 100. "Interior...wiring, including power...wiring together with all their associated hardware, fittings, and wiring devices, both permanently and temporarily installed."

 

[cripple]

OCPD on primary only what size equipment ground?

I think there maybe anther issue if this equipment is European it may not work on 208 volts in this case the delta/wye transformer would not work anyway. On a 480 delta/wye transformer the secondary voltage would 208 not 220, and 440 volts the secondary would be 190 which could be below the voltage tolerance for the equipment.

 

[erickench]

I don't think you can use a primary OCPD to size an EGC on the secondary side because the circuits are isolated.

 

[wolfman56]

Thanks for all the help.
It appears like I'm a little skewed on the definition of "premisis". They ought to just say "any wiring, every location".
It looks like I should pull at least a #8, unspliced, from the transformer case, to the ground buss in the service which is over 100' away. I'll use a #6 so I don't have protection issues.
RAW

 

[don_resqcapt19]

I don't think you can use a primary OCPD to size an EGC on the secondary side because the circuits are isolated.

In general, I agree with that, but in this case the primary OCPD provides the protection for the secondary conductors. I don't think there is anything specific in the code that covers how you size this. It would be my opinion that you use the primary to secondary voltage ratio times the primary OCPD rating and use that result to size the secondary EGC. That would require a #6 EGC for this installation.

 

[erickench]

In general, I agree with that, but in this case the primary OCPD provides the protection for the secondary conductors. I don't think there is anything specific in the code that covers how you size this. It would be my opinion that you use the primary to secondary voltage ratio times the primary OCPD rating and use that result to size the secondary EGC. That would require a #6 EGC for this installation.

The problem with is that you would have two different size EGCs and no separate transformer to match them. Unless you decide to use the larger of the two. But how would ground fault current bypass the transformer and flow back to the utility?

 

[don_resqcapt19]

The problem with is that you would have two different size EGCs and no separate transformer to match them. Unless you decide to use the larger of the two. But how would ground fault current bypass the transformer and flow back to the utility?

I don't see any problem with having different size EGCs on the primary and the secondary. The secondary of this transformer is ungrounded so there is no real fault current with the first ground fault. The second ground fault becomes a phase to phase fault. The EGC will often be part of the path between the two ground faults. It has to be sized large enough to handle the fault current until the fault is cleared. The only reasonable method to size this EGC would be to use the rating of the primary OCPD and the turns ratio of the transformer, and then use this value along with T250.122 to size the secondary EGC. The primary OCPD will clear this fault as the increase in current flow on the secondary will also show up as an increase in current on the primary. The ground fault current will not bypass the transformer and flow back to the utility.