Pretty sure I know what will happen, but I'm going to ask anyway.
When I took over the maintenance of this office/TV studio building there were a stack of 100 watt PAR 38 MH lamps in the lamp cabinet.  There were 4 fixtures that used PAR 38 MH lamps, and I assumed that all used the 100 watters.  I replaced the two very scary to get to fixtures with LED's and pulled out the dead ballasts, 100 watt versions.  I ordered two new 100 watt ballasts and pulled out the dead 70 watt ballasts) for the two hitting a seating area and put in two of the NOS 100 watt lamps.   They were clearly too bright for the space and have had a sheet of diffuser on them ever since.  I finally stumbled across the drawings for the building and discovered that the seating area lamps should be 70 watt, not 100 watt.

I'm down to one 100 watt lamp now and when they are all used up I'd like to go back to 70 watt lamps in these two fixtures.  Will the 70 watt lamps rapidly burn out if used with the 100 watt ballasts or will they only draw the power that they need to light and run properly?  The lamps themselves are somewhat expensive and I'm not willing to experiment to find out.  The ballasts are somewhat expensive as well and if I pull the 100 watt ballasts there is no other place in the building that I can use them.

Thanks-

Val

At work we have some 100W MH fixtures outside and had a case of 70W lamps, so they got used. The 70W lamps will still operate near 100W since that's what the ballast is designed for. However, the lamp life will be decreased, and the arc tube inside the lamp will very likely explode eventually. With PAR lamps there is almost no risk with an arc tube rupture because the heavy glass will contain the fragments. But with the cost of PAR MH lamps, I'm sure you wouldn't want to burn them up faster.

Are the fixtures reflective inside so you could use standard MH lamps? If so, you could use diffused lamps, which will reduce the intensity of the light.

I don't recommend it, you also run the risk of frying the 100w ballast if you use a lamp rated at 70 watts.

I don't recommend it, you also run the risk of frying the 100w ballast if you use a lamp rated at 70 watts.

The ballast will be OK. Even maybe better off as the 70W lamp, because overdriven, will run at higher voltage, so the current will be a bit lower than with the proper 100W lamp, so the ballast will operate colder. So no worry there whatsoever.

But because of the higher arc voltage, the lamp will operate even higher than 100W.
So I would be more worried about the explosion: The outer bulb is designed to contain the explosion when operated normally. But being overheated all of the time may cause it to lose strength and so the ability to actually safely contain the arctube explosion. Yes, the outer bulb construction is very likely the same for both 70 and 100W (so the heat likely wont be that big problem for the outer) and the 70W arctube is smaller. But the 70W arctube is already designed to operate at higher pressure (to keep the arc voltage at the desired 75V even with shorter arc), so it may mean the energy of the compressed gas within arctube (so the source of the eventual explosion energy) could be even higher in normally running 70Watter, not speaking about when overloaded. So it may exceed what the outer

And there is very high chance the lamp will just start to cycle very soon. First because of the faster wear, but also because the higher arc voltage of the overloaded lamp gives the ballast less OCV headroom, so it takes less of degradation to make the arc unstable and extinguish.

To me definitely not any good idea.