Check the ballast, they often list compatible lamps right on the case label or do a google search on the ballast model number for the same info. I think it'll work fine, most ballasts work with a number of different lamp sizes. But it won't be overdrive the lamp unless you rewire it. To "overdrive" a lamp, you have to connect multiple ballast outputs to a single lamp.

Ya, most lamps have different shaped bases/pin configurations for incompatible setups. If it fits and lights up, it probably won't hurt a thing.

Sam

You can also get different endcaps if the lamp bases are incompatible. Pin configuration, at least with compact fluorescents, seems to have more to do with manufacturer/country of origin than with electrical compatibility.

Redseal,

You CAN use a 55w lamp in place of 80w - if you make sure of one thing: The ballast MUST be electronic, NOT magnetic (electromagnetic).

The older type ballasts (magnetic/electromagnetic) use heavy iron-core transformers, operate at 60Hz, and are designed for a SPECIFIC lamp type and wattage.

The newer style ballasts are electronic. They are lighter in weight (some are packed with a weighty material), operated at higher frequencies (24kHz - 44kHz), and can drive a range of wattages.

If you peruse some of the ballast catalogs, they will list the range of wattages the ballast can drive, and the Ballast Factor(BF) for a particular lamp.

The BF is the percentage of light the lamp will output with that particular ballast. A BF of .90 means the ballast will drive the lamp to 90% of the manufacturer's rated lumens for that lamp. This is a typical ballast factor.

Generally, the lower wattage lamps (within the driveable range for a particular ballast) will have a high ballast factor - over 1.00, sometimes 1.10, or even 1.20.

Driving a lower wattage lamp with such a high ballast factor (1.20) is essentially the same thing as overdriving the lamp. If you use the 55w in the 80w fixture, I would guess you might have a BF of 1.4, plus or minus. None-the-less, it will be brighter.

I have used a ballast, initially designed for 55w Power Compacts, to operate 50w, 40w, and 36w lamps. The lower the lamp wattage - the higher the ballast factor. I actually get more lumens from the 50w lamp than the 55w lamp.

The only other roadblock might be the lampholder (a.k.a. socket, endcap). The most common Power Compact lamps in the US have 4 pins, lined up in a row [ÂÂÂÂ]. They use a 2g11 lampholder.

There is a also a "Panasonic" type configuration, consisting of 4 pins in a square [::]. They use a GY10q lampholder.

You can change the lampholder, if you need to, but you have to make sure you figure out the pairing of the wires. Two pins connect to one filament in the lamp, the other 2 pins connect to the other filament.

I hope this helps.

Zink

Thanks for the replies. The ballasts are electronic Osram QT-FQ1x80's and I've got 2G11 lampholders so I guess a 55W should plug straight in and work.. only one way to find out for sure..