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Joined: Mar 2005
Posts: 49
What are your thoughts about the advisability of using a portable cable connected 37kva 240/480V 3-phase engine-generator to backup a 120/240vac 1-phase service?

Loads are 120vac and 240vac single-phase.

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Joined: Jan 2005
Posts: 34
Is this for a temporary or permanent installation?

John C. Harvey
IndCom Electrical Estimates
Joined: Mar 2005
Posts: 49
The installation is permanent.

The electrical system architecture is shown on a fairly complete 1-line diagram in a request for proposal (RFP) my company is interested in.

The facility is a small 6HP sewage lift station. The incoming service is 120/240vac 1-phase. The plan is to have a permanently installed 120/240vac 1-phase engine-generator on-site. Automatic transfer switch is included. The is also a manual transfer switch to be used for switching out the service and on-site generator and switching in the receptacle for the portable generator.

The PRF shows the portable E-G set to be 240/480vac 3-phase connected to the system through a 60A/4P connector. The portable E-G is existing. It must be an E-G the city has to be moved to wherever it's needed on a temporary basis.

I'm assuming the RFP is correct about indentifying the portable as a 3-phase unit.

The 1-line diagram shows the 240/480vac 3-phase generator connected directly to the 120/240vac 1-phase load without a step-down transformer. (I'm thinking a transformer will be necessary, both for voltage reduction and to provide a neutral.)

I've never seen a 3-phase generator used directly on a 1-phase load before. The load would be totally unbalanced.

Have you seen such an application before?

Do you have any idea how the generator would be wired for such an application?

Thanks for your reply.

Joined: Nov 2000
Posts: 2,148
240/480? Must be a delta generator. If so there is no way to get 120 without a transformer.

Joined: Jun 2001
Posts: 642
Whoever sent out that type of spec obivously does not have a clue. Send that back and ask for them to employ a Qualified E.E. to review and rewrite it. Then maybe they can get something that would work.

This sounds like something that has no budiness out to bid unless it is a design build and you can charge for the design and engineering needed to do this right.

Joined: Mar 2005
Posts: 49
I think I may have it figured out.

Since the information is in an RFP, we can't contact anyone in the City about technical questions until the project has been awarded. Also, the detailed design has not been done yet. What is presented in the RFP is the design concept.

I think the portable E/G may be easily connectable for either 480vac/3-ph or 240vac/3-phase. In this case a neutral may be brought out with three phase conductors. The neutral may be the center of one of the delta phases. It looks like the City's intent is to connect the generator windings in the 240v 3-ph configuration and use the phase with the center tap (neutral) as a 120/240v 3-wire 1-phase source.

If this is the case, will this arrangement work OK on a single phase 240v and 120v loads?

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