3 Ph Generator supplying single phase

9 years ago

I live near a little town that has purchased a government surplus 3 phase ( voltage ???) 100kw diesel generator. They plan on using for emergency backup supplying single phase power (120/240) to two separate buildings. Is this a NEC violation?

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ionized_gw

9 years ago

I tend to attempt to find all the pitfalls possible in a project. This could, in fact, be pretty easy. There are just too many unknowns to say at this point. "Little town" means different things to different people. A city slicker might feel that a city of 20,000 is a little town. I grew up in a village of 2,000 embedded in a township of double that population so I feel differently. I envision your two buildings each having a residential-size, 200 amp 120/240 service. That would probably be good. If there is one service to both buildings, terrific. What are the real power needs? on line with a typical suburban residence, or is a well pump involved? How about microwave transmitters?

Then there is the genset. Is it a portable model on a trailer or meant to be permanently installed? Can the output be switched to different voltages? If the latter, transformers are in place and might be useful for you. It may already have the capability built in to power two or more structures.

The instigators of this project appear to be anxious to do it on a low budget. That's great unless they spent too much money on an unsuitable genset already. Maybe you are thinking that you'd be better off buying a couple of inexpensive residential type generators. That could be so. What I do know is this might involve more creativity or experience than a typical residential backup genset installation. If manual transfer is OK you can probably do it for a lot less money. Automatic is always more costly, but it might be just a little more or it could be a lot. That is especially true if you can train the people involved to shed loads manually if they exceed what is available from the genset.

Are these buildings close together if on separate services (meters)? That would be positive. If separate services note that I am not a pro, but if the loads are significant, I'd be looking to move all essential circuits to one leg in one service and use one generator leg to power that building provided that only 120 is needed. The other two can be used to power the larger load building.

One thing that has to be taken into consideration is exercising the genset. Unlike nat gas powered residential backup gensets, a diesel engine running on conventional liquid fuel needs to be brought up to operating temperature and run for some time to avoid sludge buildup. You probably need a signifiant load to do that. I can't say if you have those conditions with a 100 kVA genset and your buildings.

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Comments (12)

ionized_gw
9 years ago
If it is installed correctly, there need not be any code violation. It is likely that it will not supply 120/240, but 120/208. That might or might not be a problem depending on what kind of loads are involved.

In addition, if the buildings are now supplied by single phase 120/240 rather than 3ph, it might be awkward to use a 3ph genset. In order to use the genset to full capacity, the phases have to be loaded evenly. You can only use two of the phases in any single phase panel, not all three. Is the plan to install transfer panels to power only some of the loads to each building or to power both buildings entirely?
tony41
Original Author
9 years ago
Currently the buildings are single phase service. As far as I know they plan on powering the buildings entirely. Besides HVAC unsure what all 240v equipment would be in a police station and disaster shelter.
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ionized_gw
9 years ago
Connecting. A 3 ph genset to a single phase service while using all three windings to their fullest is going to take some creativity and, maybe, added expense. On the other hand, If you only need 60 kW you got no problem unless you need real 240 . If the genset makes more than 120/208 you may also be able to use a transformer to make 120/240. You may need an EE for this.
ionized_gw
9 years ago
last modified: 9 years ago
Here is one way you might be able to do this. If the If the buildings are very dissimilar in terms of the electrical load, you take the smaller building and rearrange all the essential loads on one leg of the service . You connect using a transfer switch and only power those circuits using one winding on your genset. You use the other two windings to connect the other building with a larger load to make 120/208.
tony41
Original Author
9 years ago
I think this will be an expensive and burdensome endeavor.
Vith
9 years ago
Lol perspective. Little town to me is like 300 to 1000 people. :)
tony41
Original Author
9 years ago
I was told yesterday the generator’s voltage is 240/480.
ionized_gw
9 years ago
This is way out of my knowledge at this point and I'd need a lot of details to even begin to figure it out. You really need to have an experienced generator installer look at it. That might be difficult in a rural area. On the other hand, I know that dairy farm installations keep many a contractor busy installing and maintaining gensets that might approach that size, maybe other agricultural industries as well so there might be plenty of help nearby.

Having the manual, model number and any other information would be helpful. Some generators (portable emergency or portable power usually) can be converted by throwing switches. It is possible that the generator can be rewired and some small bits (voltage regulator, perhaps) replaced to get all the 120/240 that you need. It is also possible that it can't be. It depends on how many output leads the generator has. You can get virtually anywhere with an added transformer, but that might be very costly.

I think that a 240/480 genset is unusual. Are you sure it is not 277/480?
tony41
Original Author
9 years ago
I’m unsure that it could be 277/480v.
roseguy
9 years ago
Perhaps your discussion should be with the people involved. Having used generators much larger than this in two of my businesses all I can add is that there is a lot of versatility in most units and to judge from a forum would do no one justice.
tony41
Original Author
9 years ago
Very true!