480 Volt Overhead Service 3 Wire Supply, Can I get 277V at the new Panelboard?

[u2close]

Hi, this is my first post. I work at an industrial facility which has many utility poles supplying power around the plant. I am putting in a new outdoor 250 amp, 480v, 3 phase panelboard. The service is 480V from 3 overhead supply cables on a utility pole. There is no neutral conductor. When I wire the panel I will have the 3 phases coming in from the pole, and I will drive a ground rod and connect it to the ground bus on the panel, but what about the neutral? Will I be able to get 277V from the panel or jut 480V 2 pole and 3 pole breakers?

 

[jaggedben]

From your description ("no neutral") you'd need to install a delta-wye transformer to get 277V.

With that said, you should double check with someone who knows what the service really is. It could be corner grounded, or ungrounded, or perhaps you're not realizing that the bare aluminum traveler is in fact a neutral. It affects how you ground and bond at the service, so you need to know.

 

[kwired]

I have to agree with jaggedben, if there is only three supply conductors and it is three phase you either have ungrounded or corner grounded delta system, or improper installation if it is a wye secondary. The two delta choices do not have a neutral at all. A delta system with mid point of one phase grounded will have a neutral - but is 240 volts to two legs and the other leg is a 416 volt high leg. The high leg system however will have 4 wires, the fact you only have 3 lines is a good indication it is either corner grounded or ungrounded.

 

[u2close]

Thank you both for your help. I am going to trace the line conductors back to the 500kVA Transformer that feeds them and verify if I have an ungrounded or corner grounded system. If it is ungrounded, do I have to do anything special at the panel other than terminate the 3 phases and connect the ground? Does the neutral bar get used at all?

 

[ggunn]

Thank you both for your help. I am going to trace the line conductors back to the 500kVA Transformer that feeds them and verify if I have an ungrounded or corner grounded system. If it is ungrounded, do I have to do anything special at the panel other than terminate the 3 phases and connect the ground? Does the neutral bar get used at all?

Not if there's not a neutral wire coming in, and you cannot connect any loads that require a neutral to the panel.

 

[JFletcher]

Welcome to the forum.

You need a 4 wire service to be able to supply 277V loads like lighting. 3W service would be all 480V 3ph loads like motors, and that would also limit you to 480V lighting without secondary transformers.

Grounding is a bit more than driving a ground rod... check article 250.

 

[kwired]

Thank you both for your help. I am going to trace the line conductors back to the 500kVA Transformer that feeds them and verify if I have an ungrounded or corner grounded system. If it is ungrounded, do I have to do anything special at the panel other than terminate the 3 phases and connect the ground? Does the neutral bar get used at all?

If it is corner grounded the grounded phase conductor is the grounded conductor just like the neutral is on a system with a grounded neutral. It gets bonded to the service disconnect or first disconnecting means for separately derived systems, and should be identified with white or gray color. It can land on multipole breakers because they interrupt all lines simulataeously, it can't land on a device that uses fuses as overcurrent protection.

Ungrounded systems you must have a system to indicate there is a ground fault. You still must run a grounding electrode conductor and equipment grounding conductors as usual in this system to keep equal potential between the items those connect to, there just is no grounded conductor from the supply.

 

[Jraef]

I think that it is IMPERATIVE that you get an ACCURATE determination on EXACTLY what you have before you do anything else on this project. No guessing, looking etc., you must find records on the installation to see EXACTLY what that service drop is. there are many different issues related to what you are intending to do based on what it going on with that service drop. Guessing is going to be potentially very dangerous here.

 

[augie47]

Once you have followed Jraef's golden advice in Post #8, be sure to double check your panelboard and breakers. There are some "480 volt"panelbords and breakers that would be compliant on a 480 ungrounded or corner grounded systems and some that are not.
Take a look at NEC 240.85 in that regard,

 

[rlundsrud]

If it is a corner grounded Delta wouldn't the breakers that were serving three phase loads be two pole (I'm asking other members as I am not certain)? This is something you could check as a general reference. I would still strongly advocate doing as Jraef said with regards to verifying exactly what type of service it is.

 

[GoldDigger]

If it is a corner grounded Delta wouldn't the breakers that were serving three phase loads be two pole (I'm asking other members as I am not certain)? This is something you could check as a general reference. I would still strongly advocate doing as Jraef said with regards to verifying exactly what type of service it is.

Only a two pole breaker would be needed, but a three pole breaker would be allowed since the breaker would always interrupt the ungrounded conductors any time it interrupted the grounded conductor.

If you have a three phase panel to make it clearer how the delta supply is being routed to three phase motors and other equipment it may be more economical or deliver a cleaner wire plan to use three pole breakers instead of two pole breakers and a common grounded conductor (not neutral) bar.

 

[kwired]

Only a two pole breaker would be needed, but a three pole breaker would be allowed since the breaker would always interrupt the ungrounded conductors any time it interrupted the grounded conductor.

If you have a three phase panel to make it clearer how the delta supply is being routed to three phase motors and other equipment it may be more economical or deliver a cleaner wire plan to use three pole breakers instead of two pole breakers and a common grounded conductor (not neutral) bar.

Emphasis on the fact one phase is a grounded conductor and still needs bonded to the service equipment enclosure somehow, or at either the source or first disconnect of a separately derived system.

And because it is a grounded conductor should (make that shall) be identified with white or gray.