Hi mrpotsmoka
The base is the same , the bulbs, 250, 400, 600 all are the same size as far as the screw in side .
It is not recomended to use a different bulb than the ballast is designed for, 250 watt ballast for 250 watt bulb, any other size could cause the bulb to explode.
This is on the box from the company who made my lights.
Would a 250 watt ballast light a 400 watt bulb, probably, would it be safe, probably not.
peace
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