I've searched here on LG for wiring a ballast in "overdrive mode". Didn't find anything except for passing references. What is meant by "overdirve mode" and how is it done with 1 lamp ballasts? And with 2 or more lamp ballasts? Not asking to wire Fulham WH2, WH3, etc. ballasts with 2 red wires connected together. Thanks!

When you put it in overdrive, you'd put the two lamp leads into one lamp. Here's an explanation.
Your ballast has two blue leads and a red. The red is the common wire for the two lamps and each blue wire will power the individual lamp. If you were to connect both blues to a single lamp, then you'd be putting it into overdrive mode. Hope this helps.

Oh I get it, 2 inputs onto 1 lamp. So you can't overdrive with a one lamp ballast? And if the lamp is rated far lower than the ballast rating, then that's overdriving the lamp but not necessarily "overdrive mode".

Ok it's fairly straightforward for an ordinary ballast (1st pic)...

But for a 2 lamp CFL electronic ballast with Red, Blue & Yellow wires, how can I overdrive one lamp?

Those are prob electronic RS, I think omitting the yellow wires and hooking the red and blue to the lamp.

There is one more important detail: The "overdrive" connection is nothing else than a parallel connection of two current sources, so the two currents sump up in vector manner (assume synchronous operation, so mains frequency ballast or both branches running from a common inverter stage).
works only, if those two lamp outputs run exactly in the same phase, so e.g. both outputs of a lag phase. In this way the current from both outputs sums up together, so hence provide double current into one load.
That also means the two lamps are operated in parallel circuits, in series the two outputs are 180degC out of phase, so when connected together, there will be no current flowing into the lamp (and the main ballast will operate in fact into a short circuit).

And the second is the problem preventing the "overdrive" with the depicted Robertson - there the two lamps are connected in series, the yellow wires carry no arc current at all, they just feed the heater filaments (during ignition).
Internally there is one inverter feeding via one ballasting choke the red wires. The blue wires are then connected to the internal "ground" of the electronic circuit. The The arc current then flows from the inverter and the ballasting choke to the red wires, through both lamps in series and returns back to the HF ground of the inverter. So the current in both lamps is the same and as the electronic controller operates the inverter so it feeds a constant current regardless of the load, it works with single lamp, as well as with both in series.

All right,..so I can't overdrive one lamp with a series 2-lamp CFL ballast. Thanks for this Medved, it explains more than what you've commented on. I was testing such a 2lamp CFL ballast, a Triad C2642UNV, on an 18watt sox lamp and found it inadequate, running the sox lamp at 13-14w. That ballast is supposed to run 2-26w lamps but will handle 1-42w lamp, tops. Not enough current.

From what I've seen on other forums, it's apparently popular among some saltwater aquarium enthusiasts to wire parallel circuit electronic ballasts to overdrive a lamp to increase output for the coral, etc. I think some do up to 4x for one lamp, obviously sacrificing lamp life for increased UV output, etc.

I'm not that sure, how much the UV output will increase by that, don't forget the lamp will operate at higher than rated temperature, so mercury pressure, so way more selfabsorption of the primary mercury lines exciting the phosphor. I won't be surprised if the 2x overdrive increase the output just by few percent, so the only net outcome being the more heat generation in the lamp...
Way better would be to place another lamp there and connect it to the second ballast output...

Maybe the motivation is just to save money on the lamps (using just one instead of two appear as a way to get the light cheaper; these are indeed way more expensive than general lighting products), but but forgetting the short life will make the lamp expenses even higher...

However, what about running 1 long (as 36W) lamp instead of 2 short (as 18W) lamps ?

I imagine that may be possible with some ballasts of the "series" setup. And if it works, it will be as efficient as powering the lamp at its correct rating is, with proper lamp life too

However, what about running 1 long (as 36W) lamp instead of 2 short (as 18W) lamps ?

I imagine that may be possible with some ballasts of the "series" setup. And if it works, it will be as efficient as powering the lamp at its correct rating is, with proper lamp life too

Of course, that works well, but only if you connect all things correctly.
With some ballast designs you would have to bridge the wires originally aimed for the middle point, with other types you should keep that wires open, all that really depend on the actual ballast topology (voltage vs current mode heating, where are the resonance capacitors,...), how the lamp EOL protection senses the lamp's EOL, and so on.
And that is the problem: Unless it is rated so already from the manufacturer (so the corresponding wiring diagram is published) or you reverse engineer the ballast (to determine the correct connection), you have just very small chance you will connect it correctly. And that means low chance for the system to work correctly. There is just no universal wiring diagram for that at all.