I recently picked  up a Cooper lighting canopy fixture from a building supply shop, and it came with a multi-tap CWA ballast that can run both H39 and M57 lamps. For some reason, when I use a MV lamp the light draws only 146 watts, and 1.26 amps, but when I run a MH lamp, it draws 215 watts and 1.82 amps. Is this normal or is my ballast defective. And also the Mv lamp takes longer to warm up than the MH lamp. UPDATE: after leaving it on for a while, it levels out at around 164 watts, 1.41 amps, and 0.96PF. Lamp is respectfully bright.

Interesting. Usually on these CWA MH ballasts when they under run lamps, it's with all lamp types and the culprit is usually the capacitor losing capacitance. If you have a multimeter that checks capacitance try checking the capacitor. If not, try swapping in one from another ballast. M57 ballasts usually use 10uF 400v.

After letting it run for even longer, it’s now drawing 168 watts. Maybe as it runs the cap is reforming? Btw this fixture is NOS. It was made in July of ‘07.

I went ahead and ordered a replacement cap just to be safe. Is it possible for a cap to be bad and still be completely flat? The light is still running as I’m typing this and is now drawing 171 watts, 1.46 amps and 0.97 PF.

I have a few ballasts that were heavily used that have caps that all underrun lamps. They were the plastic ones so there were still flat but their plastic casings were yellowed from heat and long use. Caps for 175w MH aren't too expensive and hard to find so I probably would have done the same.

My fixture uses a metal oval shaped capacitor. I ordered the new one from 1000bulbs.com

Have you checked the lamp current? Not the mainsinput, but really thecurrent fliwing into the lamp. It is supposed to be around 1.7..1.8A, significantly lower than that means most likely faulty capacitor.

Are the caps polarized?

Have you checked the lamp current? Not the mainsinput, but really thecurrent fliwing into the lamp. It is supposed to be around 1.7..1.8A, significantly lower than that means most likely faulty capacitor.

I do not have the equipment on hand to check the lamp current

Something that came to mind. What were the MH and the MV lamps you tested with.

The reason I ask is I remember one of my MV ballasts don't play nicely with Caster 175w lamps. It runs Sylvania MV lamps perfectly but the Casters seem under run and draw less than rated too. So if you were using a off brand MV, it might be worth it to try using better lamp. Ideally a older MV lamp but one made by a major brand would probably be better.

I heard Feit lamps also behave weirdly due to the arctube deviating enough from a standard lamp too.

I’m using a Chinese GE DX coated from I got from Lowes, a Feit Clear MV lamp, and a GE Multi-Vapor MH lamp that was made in Hungary.

I’ve changed the cap, still getting the same results from the MV lamps. I’m gonna buy some quality vintage lamps.

Yeah I would try it out with a vintage lamp. I believe some batches of Feit 175w lamps had slightly different electrical characteristics. Not sure about the Chinese GEs but the Chinese Sylvanias seem to be fine.

Would I risk harming the ballast and cap by using these cheap lamps for an extended period of time?

If the cheap lamps run with an arc voltage drop that is lower than 120v, you might risk harming the ballast since both 175w H39 mercury vapor lamps and 175w M57 probe start metal halide lamps run with an arc voltage drop of about 135v with an operating current of about 1.5a, which is the same as VHO fluorescent tubes.