"ransley" wrote in message
...
I have 100w Mercury Vapor fixtures, can I put in a 50w MV bulb, I want
to save a bit on my electric bill.

Never do this becuase the ballast is designed to deliver a certain voltage
and current to the bulb. The ballast would attempt to drive the same current
into the smaller bulb. The bulb could fail early or even explode. The
ballast may overheat and burn out.

From what I hear, MV bulbs, ballasts and fixtures will no longer be sold in
the US. Replacing it with a CFL fixture may be a better option. MV is not
really a very efficient light source anyway.