As an industrial generator rental business owner, I always cringe when a customer tells me that they want to power a single phase load with one of my three phase generators. It is an inefficient operation for both the electricity producing generator as well as the (typically diesel) engine. It is however, entirely possible and very easy for anyone with a measure of mechanical and mathematical aptitude to accomplish. Fair warning though, if you do your math incorrectly, you can easily end up causing expensive damage to the generator, and the engine can suffer as well.

You can’t buy a dedicated 1 phase generator that will produce more than about 10kw. It would be cost prohibitive to build and market because beyond that level, 3 phase machines are smaller and cheaper to make, and only a relatively small number of people would need more than 10kw single phase anyway.

Sometimes the generator manufacturer will stamp a larger generator as 1 phase, but it is actually a 3 phase generator that has been rewired and derated to power single phase loads. People who require more than 10kw usually have a three phase electrical service installed as it is much more efficient. Mostly I’ve found the folks who require larger than 10kw single phase to be farmers who power grain dryers or barns, and occasionally machine shops or small restaurants that need temporary power.

THE GENERATOR MATH…

You NEED to derate the generator when powering single phase loads. Lets say the nameplate on the machine labels it as a 208 volt, 3 phase, 20 kw generator. You will need to subtract 1/3 of the nameplate kw to get a single phase rating. In this case 13.2 kw is your single phase output. Keep it below this level and you’re golden.

THE DANGERS…

If you use more than 13.2 kw, the machine will still work great all the way up until the engine begins to labor. That’s going to happen at around 20 kw. The problem is that in order to produce single phase power, only 2 of the 3 windings are used (that’s why we derated by 1/3). Those 2 windings are now doing the work of 3. The generator will not immediately fail, but heat will most definitely build up in the windings as more amps than rated are forced through them. This will lead to premature failure of the generator.

Assuming a diesel engine is used, there is no real danger operating at single phase, but diesel engines work best between 50% and 90% capacity. Anything below a 10kw load (on our example) will cause a condition called wet stacking to occur. This causes a loss of output ability, oil leakage around exhaust components, oil burning, and sometimes soot coming out the exhaust. Use a load bank or increase the load to capacity to correct this condition.