Electrical gurus - transformer question...

[Whoaru99]

I want to install an isolation/step up transformer between my generator which supplies only 120V and my breaker panel.

The main purpose of the transformer is to take the 120V and make it split voltage 120/240V so both legs of the panel can be fed properly from just a single 120V supply.

The main problem is I would like to use a 5-10kVA transformer and finding these used having 120/240V - 120/240V windings is proving to be a challenge. However, finding 240/480V - 120/240V seems to be easier.

Basically I'm thnking that using the 240/480V - 120/240V transformer backwards and feeding it 1/2 the voltage on the 240V secondary winding would accomplish my goal.

So, the million dollar question is, do you see any reason why I can't feed 120V to the 240V secondary winding which should then provide the 120/240V split phase/split voltage I'm looking for off the primary?

(Yes, I'm aware of needing transfer switch to prevent backfeeding the utility, etc., so no need to go through that.)

[JohnMickish]

It can be done, but it's not a good idea and the hassle and cost would be prohibitive. It would be cheaper just to get a different generator.

The reason it would be a bad idea is that generator puts out it's rated watts (P), and watts are volts(E) times amps(I).

Anytime you double one you cut the other (P=ExI), so if you want to double your voltage you will cut your amperage by half.

I'd just get the right genset and not go thru the hassle. Northern tool has bypass switches all ready to go if your looking to price things out. I'd just get a Honda generator and a couple of extension cords.

[Whoaru99]

Hmmm...yeah, see your point. Total power (watts) is the same, but now it would be essentially split between two legs.

So, while the total load (sum of both legs) could be the same, the effective available to any given 120V load would be cut in half. For lights and small loads probably not a problem and I get the same total power if both legs would be equally loaded. But, for larger 120V loads like motor starting, this could be a problem because of the 1/2 total per leg availability.

It's not so much a problem with the way the electricity itself works, but moreso what the transformer can handle with current capacity of the windings. I could try to pull the full load from one leg, but it could damage the transformer if that amount was higher than what the transformer could handle.

[pikechaser]

Long story short....Get a larger generator...best way to go money wise & going to perform better

[Whoaru99]

No, it's not really a matter of a larger generator, it's using what I have in the most convenient way that still gets the job done.

Sure, would be nice (in some regards) to have a 20kW generator so I could run everything just like normal, but I don't want to have to keep 500 gallons of gasoline on hand to last me a week if need be. In a situation like that I'm fine with just keeping the house warm, the fridge cool, and having a light or two.

[Surface Tension]

My "cheap" 4000/4500 watt gen has 120/240V AC Load.

I know it won't run everything in the house and like you, I don't intend to either.

Just a couple lights, TV, well, and the fridge.

[JohnMickish]

  Whoaru99 said:

Hmmm...yeah, see your point. Total power (watts) is the same, but now it would be essentially split between two legs.

So, while the total load (sum of both legs) could be the same, the effective available to any given 120V load would be cut in half. For lights and small loads probably not a problem and I get the same total power if both legs would be equally loaded. But, for larger 120V loads like motor starting, this could be a problem because of the 1/2 total per leg availability.

I don't know what size generator you have, but your power (amps) is now devided by two as well. If you have 30 amps total, now you will have 15 per leg (phase).

With the bonding and grounding to extablish a new neutral you would need to get it inspected since you now have created a new service. If not bonded correctly you could have yourself real serious saftey hazard (google US servicemen electrocutions in Iraq).

To answer your question is it possible to do? Yes, but don't do it.

[Whoaru99]

I'll probably experiment with it. Already have the transformer...found a shielded 15kVA Sola/Heavi-Duty unit for a good price. If it doesn't work out for this then it may work out to supply isolated power to a dedicated subpanel for my stereo/HT gear.