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What size breaker are using on this circuit?
I assume you are using copper wire in metal conduit.
The load is 250A and you are using 250A fuses? Or is the actual load less. This makes a difference on the size wire you can use. It might save you some money.
Ok then you have to size for the max amperage.
For this install you need to use 250mcm copper wire with THHN insulation. You will need to use a #4 copper ground wire.
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