Leaving out Neutral to Connect 480V 3

[joozu6]

Is there any negative results to leaving out the neutral in a 277/480V 3-Ph Y-Y padmount transformer to only utilize the 480V?

Occasionally our customers request this. The only negative aspect I can see is that a fault may cause an imbalance and may not be sensed by the current limiting fuse.

 

[busbar]

Ungrounded 480V distribution has been known for serious ground-overvoltage problems for close to 50 years. There is ample discussion at this site. One “reason” for not wanting Y-grounded service is that NEC 230-95 ground-fault protection is required, and that costs a bit more money. This is very poor, short-sighted thinking. Many utilities will not furnish 3ø 3-wire service, except possibly for a rural isolated, single-motor service.

 

[stevenal]

Here's how we've handled it:
We still ground the neutral bushing in order to prevent overvoltages and meet the code. The transformer and customer grounds should then be bonded together to provide a return path for ground faults. Then we meter it like a four wire wye system just in case someone connects a load phase to ground at some point. The resulting system looks a lot like a four wire wye, except the fourth wire is sized to briefly carry fault current rather than steady state unbalance. I'm also curious how others handle it.

 

[alehman]

The NEC covers this situation. If the system is to be grounded, a neutral conductor must be brought to the service disconnecting means and grounded there. This is required to provide a return path for ground fault current. A grounded system without bonding the customer's distribution system to the neutral is a serious safety hazard beacuse there can be no assurance of adequate fault current to operate the customer's overcurrent protection. The miminum neutral conductor size is 12.5% of the phase conductor.

If the transformer is located outdoors, the NEC also requires the neutral be connected to a grounding electrode outside the building.

 

[joozu6]

Thanks guys. Your input is very much appreciated.

 

[stevenal]

Alehman. Good points. I would call the 4rth wire a ground wire unless it was used for load unbalance. NEC actually calls it the "grounded conductor" which would seem to open up the possibility of corner grounding one of the three phase conductors. Also don't forget that NEC doesn't apply on the utility side of the interconnection point. The serving transformer is likely to be utility owned where NESC would apply.

 

[jghrist]

stevenal,

It wouldn't be a real good idea to corner ground one of the three phase conductors coming from a 480GrdY/277 volt transformer would it? You could avoid any faulty customer mistakes by refusing to provide 3-wire services.

 

[pablo02]

Am I reading this thread wrong?

A four wire system provides the neutral for those systems that utilize 270v (phase to neutral)from the wye configuration and is carried through as a neutral -- although this neutral is bonded to ground at the transformer (as stevenal points out and at the service entrance on 4-wire systems), the neutral is separate from the ground -- just like the neutral and the ground wires in your 120v systems -- the neutral is intended to carry current, the ground only under fault conditions, etc.... we utilize the three wire systems (wye configurations) strictly for the 480v (phase to phase)on our large equipment but carry the ground right along with it (although when bonded to the service entrance, provides similar function as the neutral from the xfmr to the service entrance); then utilize neutrals on the secondary sides of smaller service transformers (480v/270v lighting, 120v/208v service, etc) and maintain the required grounding for these separately derived systems...

The problems I see mentioned may deal with a delta configuration where there in no neutral -- (except maybe through high-impedance reference points) depending upon the requirements of the system, I am most familiar with one corner of the delta being grounded in order to reference all voltages to ground...

So, to answer joozu6, we use the 3 wire systems all the time with no problems -- but the (wye-wye) transformer XO connection must be grounded and the ground carried through and properly bonded to equipment as required by code....

inform me if I am completely off-base, thanks...

 

[alehman]

I only have an old 1993 version of the NEC here at home, but here's what it says. I don't think it's changed much recently-

250-23 Grounding Service-Supplied Alternating Current Systems
(a) System Grounding Connections. A premises wiring system that is supplied by an ac service and is required to be grounded by Section 250-5 shall have at each service a grounding electrode conductor connected to a gounding electrode... The grounding electrode condductor shall be connected to the grounded service conductor at any point from the load end of the service drop or service lateral to and including the terminal or bus to which the grounded service conductor is connected at the service disconnecting means. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made either at the transformer or elsewhere outside the building...

(b) Grounded Conductor Brought to Service Equipment. Where an ac system operating at less than 1000 volts is grounded at any point, the grounded conductor shall be run to each service disconnecting meansand and shall be bonded to each disconnecting means enclosure.... the grounded conductor shall not be smaller than the required grounding electrode conductor specified in Table 250-94... and for service conductors larger than 1100kcmil copper... shall not be smaller than 12.5% of the area of the largest service entrance phase conductor.


I guess it's a matter of semantics, but as stevenal said, it's technically called the "gounded conductor". This would allow it to be a neutral, corner or center-tap.

 

[stevenal]

jgrist,

Yeah, not sure we would allow a corner ground. We've got enough different configurations without introducing another. Looks like code would allow it.

Pablo,

I agree. "Grounded conductor" is a load conductor that happens to be grounded, while the "grounding conductor" carries only fault current. The NEC has confused this distinction by introducing a "grounded conductor" that ends at the service disconnect. In the end, this makes things simpler since I don't need to dwell on the distinction (as in my June 10 post). No 3 wire 3 phase services are allowed. You will need to provide a neutral sized per NEC, which allows a smaller size if it's not loaded. Nice and simple.

 

[peterb]

Also allowed under the code is high impedance grounding, which gets rid of the overvoltage menace and can allow continued operation, with an alarm, for a single ground fault.