Ungrounded delta delta w/ 3wire bus duct 2ndary

I

Isaiah

Senior Member
Location
Baton Rouge
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Electrical Inspector
We have a 3phase, 3wire bus duct coming from the secondary side of an ungrounded delta-delta transformer, 13.8kV to 480V. Shouldn’t this be a 4 wire secondary to account for the SSJB connection NEC 250.30?


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250.30(B) and (C) if outdoor source.

You need the Equipment Ground but IDK if you would call it a "secondary". It has no electrical connection to the transformer until a fault occurs, at which time it becomes a Corner ground. Still no issue until the second fault occurs. Reason why there should be a monitoring system of some sort.
 
ptonsparky said:
250.30(B) and (C) if outdoor source.

You need the Equipment Ground but IDK if you would call it a "secondary". It has no electrical connection to the transformer until a fault occurs, at which time it becomes a Corner ground. Still no issue until the second fault occurs. Reason why there should be a monitoring system of some sort.
Click to expand...

If there is no direct grounding connection (EGC) to the delta corner (B ph, for example) does it still become “corner grounded” upon ground fault?


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More than I know...and I'm sure someone will correct me. It won't be the first time.

A fault in a motor winding to EG, say midpoint between A-B connection would result in 240 Volt A to EG and B to EG. C to EG would be about 415. Assuming 480 Delta un grounded supply. Moving the fault closer to A would increase the voltage B to EG, C to EG and lessen A to EG.
Eventually resulting in A to EG = 0, with B&C at 480. Corner Ground.
 
Without a corner ground:
Your first fault grounds any of the corners. That rotates your voltages. So, 480V between the phases BUT one phase will measure 0V to ground. Everything will still work as intended.

Your second fault, if not on the same phase, will result in two lines to ground and that will blow everything up. More or less. This is why grounding lights are important for ungrounded services. 250.21(B)

I would say that a SSBJ is required on this install but that conductor, sized by 250.102, would terminate, or land on the duct if they wanted. Its goal is to carry fault current on the line side of an overcurrent device. 250.30(B)(3) and 250.30(C) for outdoor source would point you back to that. The duct would need to be rated for fault current, which I believe they all are.

So if we go from, 250.30(C), to 250.30(B), to 250.30(A)(2), to 250.30(A)(2) exception that points to 250.30(A)(1) exception 2, that says

"If a building or structure is supplied by a feeder from an outdoor separately derived system, a system bonding jumper at both the source and the first disconnecting means shall be permitted if doing so does not establish a parallel path for the grounded conductor. If a grounded conductor is used in this manner, it shall not be smaller than the size specified for the system bonding jumper but shall not be required to be larger than the ungrounded conductor(s). For the purposes of this exception, connection through the earth shall not be considered as providing a parallel path."

But you won't have a system bonding jumper as defined in the NEC. You will however have a SSBJ used to bond all metal parts likely to be energized on the line side of the OCPD.
 
Isaiah said:
Shouldn’t this be a 4 wire secondary to account for the SSJB connection NEC 250.30?
Click to expand...
Yes, it needs a supply side bonding jumper, however the metal housing of the bus duct may provide this connection. Not sure I have seen any marked as a SSJB, but have seen many where the metal housing is the EGC

In general, the only difference between the ground and bonding of a SDS that is ungrounded compared to one that is grounded, is that the grounded system has a system bonding jumper and the ungrounded system does not. Everything else is the same.
 
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