So I was wondering, after reading the current ratings on my ballasts, if running a 58W tube on a 70W ballast would overdrive it? And if yes, how much? Would it actually cause any noticeable lifetime degredation of the 58W tube? If we look at the specifications on the ballasts, a Tridonic Atco EC70, and a Vossloh Schwabe L58.718, I noticed that the current ratings are more or less the same. extremely similar. the VS ballast is a standard 58W ballast, and as such, the current is 0.67A like we would expect. It can also power a T12 65W tube, same current, the tube just drops a different voltage making it draw more power. the Tridonic is a standard 70W ballast for a 1800mm tube, and while driving a 70W T8 bulb, it will allow 0.70A to pass. But if we look closer, it will output 0.67A with a 75W bulb. Is the 75W bulb the T12 variant of 1800mm tube? I thought that was a 85W, but I may be mistaken. But anyways, the current seems close enough, and this would allow me to run 70W tubes, or 58W tubes in an install I am doing in my shed. this would make me very happy if possible because then I can run both tube types.

I do have all kinds of test equipment and measuring tools, so I might have a probe around. But I figured I would ask if anyone else has done this, and if so, how did it go long term

No harm in trying and measuring the arc current, the ballasts are close enough that even if it was overdriven on the EC70 it probably wouldn't do any harm to the tube for a test.

On a side note there are some Electronic HF ballasts that state they can drive both 58W and 70W tubes so the specs of the tubes are likely both fairly similar.

From IEC 60081, all 58/65/70/75W fluorescent ballasts share the same impedance of 240 Ohms.


Though, 58/65W operation with this ballast is specified at 220V, 70W - at 240V and 75W - at 235V

Using 240 Ohms 70/75W choke with 58W tube at 230V probably will end up with slight (~5%) overdrive, nothing catastrophic

Very interesting information you guys found! If the impedance is the same, the current will more or less be close enough to where it won't matter! I am guessing the biggest reason the 70W ballast is physically beefier than the 58 ones is that they need to be able to dissipate a little more heat. I will definetely try some 58 watt tubes on these ballasts, once my install is done I will definetely do some measurements, because now I am curious. a 5% overdrive should be nothing to worry about, that can happen in normal fixtures too if the mains voltage is high. It should not affect tube life by much. 

Input voltage matters a lot too, of course! I am very sure that at 220V, it will run a 58W tube all day without even the slightest hint of overdriving. BEcause I have tested one of my 70W tubes already and at 220V, it underdrives the tube according to my wattmeter on the input. It draws around 63 watts at 220V, so that is probably around 58W to the tube at 230V it draws slightly more, but still probably not to the point of where it would overdrive a 58. Still definetely underdriving my 70 watters at 230! I will adjust my inverter voltage (I have a fully adjustable victron inverter powering my shed, its all off grid and solar powered!) to around 240V once I have the two 70 watters wired up in the ceiling I definetely do not wish to underdrive them since that is probably bad for them, much more so than overdriving slightly would be! Underdriving causes electrode sputtering which is not good!

This is definetely interesting, because this means that I should be able to swap my 70 watt tubes for 58 watters and run those just fine too without overdriving them! And this is something I can actually do, since I am using C-clips to hold the tubes , and the sockets are free floating ones that hold themselves onto the ends of the tubes. The ballasts are designed for sun tanning tubes I believe, but I am doing something a bit different with them I have a decent idea how to make this install quite neat, and it will definetely be cool.