Should I use one or two red wires on a Workhorse 3 to run an 18W SOX bulb? I'm asking because most people say to use two red wires. However, an 18W SOX lamp is most similar in electrical characteristics to a 24" F20T12 fluorescent tube, and Fulham's website says to use only one red wire for an F20T12 tube. So which is it? One or two red wires? If it is two, please explain why.

According to Fulham spec sheets, a WH3 ballast runs a F20T12 lamp at 94% power with one wire connected (almost perfect). Considering that an 18W SOX lamp runs at roughly the same specs (slightly less rated current), then connecting one wire is almost certainly the right way to do it. Connecting two wires would probably be pretty bad. I would expect an 18W SOX lamp to be slightly overdriven, maybe 105% power (just a guess), but overall a good match with the WH3 with one wire connected. I definitely wouldn't do both wires, but maybe there is something I am missing.

Well, a question no one dared to ask yet is what arc current 18W SOX needs when operated on HF. We all know that on 50/60Hz it is 0.35A, and most likely was thought to be homologated with European 230V/18W fluorescent choke impedance. But on HF *likely* SOX arc voltage drops a bit compared to line frequency operation which probably means a bit lower lamp power at 0.35A current than when run on line frequency due to lower cathode drops. For usual fluorescents this is a good thing meaning higher efficiency at the same or better light output. But SOX arctubes are heavily temperature (=power) dependent so *probably* arc current needs to be compensated (up-rated) for HF to work optimally?

By a factor of 2x ?

Not 2x of course, but still probably lamp current needs to be somewhat compensated for HF work against its nominal value at 50/60Hz.

 

The cathode drop dissipation does not affect the cold spot temperature (that is what matter to maintain the sodium vapor pressure inside), the cathodes radiate the heat only on the cap end, the rest of the tube way more follows the general anode column power density.
So from that perspective, assuming the anode column is the same for HF as for 50Hz, the current would need to stay the same 0.35A.