Driving 8ft tubes properly - 85, 100 & 125w

[fluorescent]

Hi all,

Does anyone know the specifics on driving 8ft tubes properly? The info I've read on Google suggest 100w tubes should work fine in any 125w fixture, but I don't think this is quite right, otherwise why would Helvar L100 ballasts require a 7.2uF capacitor for 125w tubes and no capacitor for 100w? Also my 125w GEC Europa does not like driving 100w tubes, only seems to draw about 80w with one fitted.

Also I know 85w tubes can be used in 125w fixtures (they get over-driven at 125w and may have a shorter life expectancy I believe) but can a 125w tube be used in a 85w fixture?

Thanks in advance

[Kev]

The cap is shot in your europa then! The 100W was designed to replace 125W lamps as part of the energy saving same thing with 65W replaces 80W 5' 75 replaced 85 6'

[fluorescent]

That's interesting - I'll try another cap in it when I get a chance. It's a 7.3uF cap which tests about 7.4uF so I thought was OK, it drives 125w no problem. I can't imagine I'll find a 7.3uF replacement anywhere but I have a couple of 7.2uF which I can borrow from other fittings to try.

So the info I've read was correct then, any 125w rated fixture should drive 100w ok?

[Kev]

Na only SS QS doesn't start the 100W for some reason!
As for 85W that's a completely different animal all together the 125W tubes will light on 85W gear but I think it hammers them pretty quickly. Lower voltage higher current

[AngryHorse]

Bit of info I put on ATL for T12 tube running specs.....
125watt = 152volts @ 0.94amp
100watt = 125volts @ 0.96amp
85watt = 185volt @ 0.54amp
75watt = 127volt @ 0.72amp
80watt BCs = 100volt @ 0.87amp
65watt = 113volt @ 0.63amp
40watt = 104volt @ 0.42amp
20watt = 58volt @ 0.36amp (Thorn 1978 data)