Can you operate a 250 watt HID lamp on a 400 watt ballast of the same type?

No you cannot, hid ballasts are sized for their particular lamp. You also cannot run a mercury vapor lamp in a metal halide ballast of the same wattage. I have even had problems with using different manufacturers lamps in certain ballasts same wattage and type.

You can however change the ballast and the lamp to 250 watts with a retrofit kit. Both those sizes use a mogul base and are readily available.

Silverbk,
This is just theoretical. I don't intend to try it. My thoughts were that the ballast coil would operate satisfactorily with a lighter, but not heavier load. Similar to a regular transformer. Does the ballast itself, or the lamp determine current flow?

It's the ballast's job to regulate the current. (for a very good explanation go to Scott35's "choke transformer" in the Theory section of this site)
BTW, The Mercs do burn w/ a MH ballast, as evidenced by the fact that if you order a replacement ballast for the Merc you will receive a MH ballast. We've replaced them many times. It's not a "bright" idea, though. If you can replace the Merc w/ a MH lamp & ballast, you'll get a much more efficient light.

[This message has been edited by electure (edited 07-02-2001).]

You are correct to some extent there are some MH and MV ballasts that are the same. In respects to a Mercury vapor in a MH ballast. But a Metal Halide will not work in an old Mercury vapor ballast, they require more voltage to start.

The newer MH lamps do not have a third starting electrode, making them and their ballasts incompatible with mercury vapor.

I have also had problems between brands, same wattage, ansi type, etc. Different lamp manufactuers use different materials for their arc tubes making them incompatible with other manufacturers balasts.

In my experience it is best to stick with kits when retrofitting hid fixtures, preferably prewired ones like Sola, takes out any guess work.

Yes, but you said a merc lamp w/ a MH ballast earlier.
Yes, I've experienced the same compatability problems as you have. We always replace the entire guts and lamp when we need to make a ballast change. It's not economical to return with a boom truck or a $300 a day rental boom lift to replace a capacitor or igniter in a fixture that you've previously repaired.

We just replaced the lamps at the factory I work at. We changed from mercury to sodium and Have been replacing ballasts on a regular basis even though the distributer said the ballast would be OK. Easy to say since he is not the one in the boom lift soaked in sweat replacing them.

Glad some one else mentioned that a merc lamp will light in a MH fixture....
My company is slowly replacing all the old merc fixtures with metal halide fixtures, but todate, they refuse to buy metal halide lamps....lol