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I got a shop-light fixture, leftover from a construction site. The metal boxes with 4 foot tubes. It has wiring for two lights in it - two of the small rectangular connectors on each end that you twist the tube into. This light is big enough for 4 tubes - it's 4' by 2', and I've seen them that size with four bulbs in the store. It has a black ballast box in the middle that shows how to wire it for two lamps.

So I also acquired another ballast wiring harness that shows wiring for three bulbs. Couldn't I install the three-lamp ballast box next to the two lamp one, put in the connectors and only use two of the three extra spaces? Basically turn the 2 bulb fixture into a 4 bulb fixture? The wiring shows that the three-lamp ballast will provide power "up to" three bulbs, but there's nothing saying I can't install just two of the bulbs and leave the third empty.

Would there be excessive heat build up from two ballast boxes being in there?

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    Since you're modifying it anyway, have you considered LED conversion? There are tubes that take line voltage, eliminating the ballast entirely. – Tim B Oct 12 '17 at 15:24
  • FYI a three lamp electronic ballast can be used to light 1 - 3 lamps. We call them smart ballasts. More FYI you can also purchase LED's that snap right into and replace the fluorescent lamp without removing the ballast. They used to be about $17 ea. contractor price, about the same price I paid for a ballast. I have no idea what retail is. – Retired Master Electrician Oct 13 '17 at 12:59
  • Ledmyplace has direct wire/ ballast compatable lamps for under 10$ I use the 3000 lumen 20watt model much brighter than a standard t8. I think I have installed about 30 of these so far double ended is my preference hot at 1 end neutral at the other, the ones I have will work with a ballast also but I want to reduce failures so I take them out. – Ed Beal Jan 3 '18 at 0:29
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If there's a mounting location for a second ballast, yes. Many 4-lamp fixtures do provide for 2 ballasts, and are built for same.

Try to use modern electronic ballasts. They are plentiful dirt cheap on eBay and Craigslist now that so many people are doing LED conversion.

I would add to this, some electronic ballasts are actually made to drive 4 lamps. The wiring can get a little bit knotty and you have to watch your wiring diagrams.

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