I went to replace an electronic ballast in a standard 2'x 4'troffer. Once I opened it, I realized that it had only three tubes, not the four that I was expecting. The ballast had three blue wires and one red, assuming that the red was common to all three tubes and the each blue went to the opposite end of each tube.

The replacement ballast said on the box that it supports either three or four tubes, however the wiring diagram only showed it wired to four and the supporting documentation was just safety information, etc. This one looked more traditional: Two red wires, two blues and two yellows. My question is how to wire this thing to only three tubes. My experience has always been that tubes on these ballasts work in pairs. Any ideas?

If the wires don't match, you have the wrong ballast. Period.

Some electronic ballasts don't work in pairs. Follow the diagram on the ballast and cap off the extra wire.

There may be more info on the internet. Search on the model number and see if there is an installation guide.

The old ballast is a instant start ballast while the new one is a rapid start. They do make the bulb ballasts or you could use a one bulb and a two bulb ballast. I'd be a bit concern with using a four bulb ballast with out specific documentation. I've seen the end result of a four bulb ballast in a three bulb fixture and the light out put between the three bulbs was noticeably different. I doubt those ballasts were listed for three lights though

A rapid start ballast can be wired in a instant start fixture but it has to be carefully rewired per the ballast diagram. Also, a rapid start ballast has a lag in lighting up. that could lead to a customer complaint. I'd suggest to advise the customer of the start up before hand. That's additional labor meaning added cost to someone. If possible, I'd return it and get a correct replacement ballast if practical

There are a number of electronic ballasts that will operate:

2 lamp ballasts (2 blue leads, 1 red) that will operate 1 lamp, just by capping off 1 of the blue leads.

3 lamp ballasts (3 blue leads, 1 red) that will operate 2 lamps, just by capping off 1 of the blue leads.

4 lamp ballasts (2 red, 2 blue, 2 yellow) that will operate 3 lamps with 1 of either the red or blue leads capped.

To retrofit a 4 lamp ballast into a 3 lamp fixture (originally equipped with a 3 blue, 1 red ballast), cap 1 of the red or blue leads.
Remove 1 of the jumpers from the lampholder to lampholder on the common side (originally red). The 2 yellows will go to the lampholders on the common end, 1 to the (originally red) lampholders, and 1 to the lampholder freed up by removing the jumper.

.........or just buy a 3 lamp ballast


Just like the black and white wires that will operate on 120 to 277 volts, they are meant to reduce the number of different ballasts you have to lug around on a day to day basis.

Thanks, guys. I think I'll just buy the three-lamp ballast for these. I'm not looking forward to having to rewire the sockets, so a direct match will be the easiest solution.

Out my way, such units would be termed "independents."

They are less efficient than "master-slave" set-ups.

Independents are used when the layout doesn't allow a master-slave setup. A solo master would really waste energy.

So, that's the kind of ballast you need to be tracking down.

For these issues, never hesitate to phone/ internet Lithonia or the competition.