The quick answer is that you can usually get 2/3 of the capacity on single phase.
However with the proper connection, and depending on the load, you may get up to 83% of the three phase rating.
Example;
Generator thermal capacities limit maximum KVA. Prime mover capabilities limit maximum kW
A set is rated for 50 KVA 40 KW, 0.8 PF.
For single phase use 2/3 of 50 KVA is 33.3 KVA, 2/3 of 40 kW is 27 kW, at 0.8 PF. so the quick answer is 27 kW.
BUT
Your prime mover still has enough power to produce 40 kW, so if your load has unity power factor or the load power factor has been corrected to unity, you may safely draw 33.3 kW.
33.3 kW from the original 40 kW is a maximum of 83% of the three phase rating on single phase service under favorable conditions.
When the generator is re-connected for true single phase operation, the highest voltage must be used for maximum capacity.
That is, the 240 Volt connection must be used for full capacity on a 120:240 Volt connected generator.
The common connection schemes for single phase conversion of 12 lead generators are:
The Double Delta connection,
The Zig-Zag connection, (Not to be confused with a Zig-Zag transformer connection.)
and
The Bar Delta or Collin connection. (Useful for 10 lead generator single phase connection but also possible with 12 lead generators.)
An exception is the Double Delta connection. This may be again connected with the two deltas in parallel for a maximum possible single phase capacity of 83% of the capacity of a three phase generator originally rated for 0.8 power factor.
Another possibility is a four lead generator where the load is connected across two line leads.
This would be a 208 Volt load on a 120/208 Volt generator. The quick answer is about 58% of three phase capacity.
Can you upload a good picture of the generator nameplate? What is your intended load?
Bill
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"Why not the best?"
Jimmy Carter