This Is my first post on lg i was just wondering at work they have just replaced the metal halide bulbs in our fixtures and i got one of the old bulbs its still new but they have a contract to get them replaced every year i have a metal halide ballast at 160watt but the bulb i got is a 400watt what would happen if i ran a 400watt metal halide bulb on a 160watt ballast would damage occur or would it just not work
 

I think ballast is at risk of overheating, when operating 400W lamp in 160W ballast.

By the way 160W MH wattage look strange to me (i know this wattage number only with SBMV, but SBMV does not use external ballasts). With MH i know the 175W probe start MH spread in US and 150W pulse start in both US and Europe. Both have arc voltages around 100V (175W about 120..140V, 150W about 90..100V).

I assume all MH's have similar rated arc voltage (~100..140V) at full rated power, so the ballast would be designed for arc voltage in this range.
But when 400W lamp would be run in 160W ballast, the lamp would not warm up, so would operate at lower arc voltage (could be easily as low as 30V, depend on exact lamp design).
This lower arc voltage may cause higher then rated current in the ballast (by far not enough for the lamp to warm up, so it will stay in this condition), what may cause ballast overheat after some time.
If it really actually overheat or not depend on many factors:
- Hoe the ballast current, so power dissipation depend on the arc voltage
- What is the ambient temperature (if the elevated dissipated power would be able to exceed temperature limit)
- How well is the ballast heat-sinked (with good heatsinking the ballast temperature does not really have to rise above the limit)
- If the ballast contain thermal protection
- How long it will stay in this condition

But such operation may even damage the lamp, as it may operate for long time with aggressive MH salts only molten, but not evaporated, what may excessively corrode arctube internal components.

But in any case the system operation would be far from rated configuration and no one (nor lamp, nor ballast/fixture manufacturer) would accept any warranty...