In ,
ransley wrote:

Can I put a 50w Metal Halide bulb in a fixture sold for a 100 w metal
Halide light.

Can I put a smaller watt bulb in a 100 w Mercury vapor lamp , maybe a
50 or 75 watt.

Sadly, in HID fixtures, one needs to match both wattage and
"ballast compatibility code".

Exception for need to match wattage, is allowance of slightly-lower-
wattage of high pressure sodium to "retrofit" mercury or metal halide.
The need to find compatibility via "ballast compatibility code" still
exists. Furthermore, some "retrofit sodium" lamps have additional
restrictions to ballast types - please know your fixture and read the
lamp (bulb) packaging in all ways!

Bad is same-"general-type" lower-than-prescribed-wattage "lamp" ("bulb")
for HID (mercury, MH, sodium), tends to overpower the "lamp"/"bulb".

Furthermore, several MH sodium "bulbs" (lamps) come with different
"ballast compatibility code" even whenb wattage matches. You need the
"bulb" ("lamp") and the ballast (or the fixture) to have at least one of
these in common! Otherwise, your lamp may fail to start, or could
overheat, or could fail to warm up past 10-20% of full brightness - even
if rated wattage matches! Mismatch of wattage (other than "retrofit
sodium" slight mismatch in wattage) makes *bad things* more likely!

Good luck!
--
- Don Klipstein )