Grounded conductor required for service?

[greenspark1]

Hello.
I have a utility transformer stepping down from 13.2kV to 480/277V and feeding a building. It is grounded at the transformer. I don't have any single phase line to neutral loads so intend to bring only the phase conductors and a ground from the transformer to the building. This means there would not be a grounded service conductor, but would be a ground. I will drive rods at the buildling I have been looking in article 250 to see if this is allowed or prohibited but can't find anything that addresses this unusual situation. Any experience with this or reasons it is NOT allowed?

 

[david luchini]

Hello.
I have a utility transformer stepping down from 13.2kV to 480/277V and feeding a building. It is grounded at the transformer. I don't have any single phase line to neutral loads so intend to bring only the phase conductors and a ground from the transformer to the building. This means there would not be a grounded service conductor, but would be a ground. I will drive rods at the buildling I have been looking in article 250 to see if this is allowed or prohibited but can't find anything that addresses this unusual situation. Any experience with this or reasons it is NOT allowed?

250.24(C) will require you to bring the grounded service conductor to the service equipment.

 

[tom baker]

From the service you will bring a grounded neutral, not a ground. Ground is a term that means "the earth" and not appropiate for this. The neutral serves two functions, carry unbalanced current (white wire) or serve as the ground fault return path (green wire). Its sized to carry the larger of the two, and in your case is sized based on the size of the ungrounded conductors per Table 250.66.

Without the neutral, there would be no way for the current in a line to case fault to get back to the transformer, if it was not installed.

I suspect there are many installations that are 480 V three phase, no loads to neutral, that do not have a grounded neutral conductor.

 

[winnie]

Prior to the system bonding jumper, aren't the grounded service conductor and the 'ground' the same thing? In other words, the OP's service will have 3 phase conductors and a 'ground'?

-Jon

 

[greenspark1]

Tom- We are planning to bring a ground wire (call it grounded service conductor?) from the ground at the transformer to the ground bus of the switchgear. The switchgear does not have a neutral bus so nowhere to land a neutral wire from the transformer. The ground wire will provide for a low impedance path for any fault current.

Without the neutral bus, we can't use a neutral wire and instead take the GSC to the ground bus. Very curious to hear what others think. Seems to be a gray area, but I can't be the first one to do/attempt this.

 

[david luchini]

Tom- We are planning to bring a ground wire (call it grounded service conductor?) from the ground at the transformer to the ground bus of the switchgear. The switchgear does not have a neutral bus so nowhere to land a neutral wire from the transformer. The ground wire will provide for a low impedance path for any fault current.

Without the neutral bus, we can't use a neutral wire and instead take the GSC to the ground bus. Very curious to hear what others think. Seems to be a gray area, but I can't be the first one to do/attempt this.

Again, see 250.24(C). It tells you what you are required to do. If the neutral point is grounded at the transformer, then you must run a grounded conductor from the neutral of the transformer to the switchgear.

The ground bus in the switchgear could certainly serve as the "grounded conductor bus" required in 250.24(C). That bus must be bonded to the enclosure (again by 250.24(C).) 250.24(D) requires a grounding electrode conductor to connect the equipment grounding conductors, the service enclosure and the grounded conductor to the grounding electrode. 250.130(A) requires the equipment grounding conductors to be bonded to the grounded service conductor and the grounding electrode conductor.

It seems to me that the single "ground bus" in the switchgear meets all of these requirements, unless I'm missing something.

 

[jap]

Tom- We are planning to bring a ground wire (call it grounded service conductor?) from the ground at the transformer to the ground bus of the switchgear. The switchgear does not have a neutral bus so nowhere to land a neutral wire from the transformer. The ground wire will provide for a low impedance path for any fault current.

Without the neutral bus, we can't use a neutral wire and instead take the GSC to the ground bus. Very curious to hear what others think. Seems to be a gray area, but I can't be the first one to do/attempt this.

Since your switchgear does not have a Neutral bar in it, how can it be considered Service Entrance Rated?

 

[dereckbc]

Tom- We are planning to bring a ground wire (call it grounded service conductor?) from the ground at the transformer to the ground bus of the switchgear. The switchgear does not have a neutral bus so nowhere to land a neutral wire from the transformer. The ground wire will provide for a low impedance path for any fault current.

Without the neutral bus, we can't use a neutral wire and instead take the GSC to the ground bus. Very curious to hear what others think. Seems to be a gray area, but I can't be the first one to do/attempt this.

Sounds like you meet 250.24 (C). This is quite common in industrial and commercial applications where there are no 277 or 120 volt loads. You will see this in just about every communication center. For loads that require 277 or 120 a transformer will be used.

 

[augie47]

My thinking is in line with jap's. (post #7)
As noted, since the transformer has a grounded XO terminal, the system would be designated 480/277 and, if the gear was purchased accordingly, it would, from my experience, have a buss for the grounded conductor, a disconnect link as required by 230,.75 and a bonding jumper as required by 250.24 and 408.3(C). In addition it would be marked SUSE as required by 230.66
Since your gear lacks some or all of those requirements, I would suspect it was not supplied as "service equipment".
I have encountered similar situations and the supplier has been required to make the necessary modifications and have the listing agency re-certify the gear as' SUSE.'

You might have the option of the having the utility not connect their XO terminal to ground and consider it an ungrounded system, but that would require detection systems {250.21(B)) and the gear would still need to be properly identified as SUSE and constructed accordingly.