Hi, guys (and gals?).
I have a coach lantern-style street fixture on my property that needs a bulb. It currently has a 100w LU100 and it may...MAY...be the wrong size. There's no label in the fixture, and when I tried to open up to look at the ballast I stopped short because the screws are VERY corroded and starting to break off if I go any further. My question is, let's say this fixture IS rated at 100 watts. Will it be ok to install a 70 watt bulb without damaging the ballast?

Thanx

In general no, you may not change the wattage of the lamp without changing the ballast also.

There are some exceptions for energy savings lamps, there are 320 watt MH lamps rated for use with 400 watt ballasts etc.

If it's a We Energies fixture it should have a little sticker on the street side with a "10" for 100w or a "7" for the 70w. That's how they tell.

Duke;

What happened to the original failed Lamp?
I am wondering why you might feel the wrong wattage Lamp had been used originally.

If the original Lamp was "Cycling" on and off for awhile, and gradually cycled off faster and faster until it just would not fully start (only had a bluish light output, then goes out within 30 seconds), this is normal end of life results for the HPS Lamp.

If it will not start now, but started and ran OK just a few days before, suspect the Ignitor.

If the Ballast windings are charred, suspect either incorrect Lamp wattage, failed capacitor, or both.

If you use a lower wattage Lamp on a higher wattage Ballast, it will fail sooner.
The cross over piont is at the 150 watt HPS - as there are 2 types here; 55 Volt and 100 Volt Lamps.
Use the wrong ones here and you will know very soon that there was a mismatching conflict!

Also using a Lamp / Ballast match where the values are very different (like 175 watt Lamp on a 400 watt Ballast, or vise verse) will bring sudden doom!

Scott35