An older office's 4-foot T12's are to be replaced with direct wire LEDs because the ballast cost + labor can be avoided. The instructions indicate that the hot is wired to one end and the neutral is wired to the other end. Because they are T12 fixtures, I believe they are non-shunted tombstones (sockets)

The new wiring configuration will be augmented with a sticker to indicate that only LED bulbs can be used and not to use T12 bulbs.


If the sticker is ignored and a T12 is installed in the new wiring configuration: 120V AC applied to each end of the bulb, will this result in a safety or fire hazard when the 120AC power is applied to the T12?

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A fluorescent light is an arc-discharge light (like neon, sodium, mercury vapor and metal halide). There's a tube of gas and cathodes on each end. It takes a high voltage to strike the arc, and a less-high voltage to sustain it.

4-foot T8 and T12 tubes have 2 pins on each end of the tube. They connect a small incandescent style filament, whose job is to preheat the cathodes to reduce the strike voltage and ease wear-and-tear on the tube. Instant-start ballasts ignore this filament and use an extra high voltage to strike the arc.

Some LEDs want 120V on 2 pins on the same end, and the other end is a dummy. On those, if you put a real fluorescent in there, you can expect a Pop! as the filament burns out.

Since you got the powered on opposite ends type of tube, I would expect nothing to happen. The strike voltage of a 4' fluorescent tube is significantly above 120V, especially without preheating.

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  • There is no filament in a fluorescent tube. But it has a negative impedance, or more precisely "S" type – SX welcomes ageist gossip Jul 8 '17 at 22:45
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    @Fizz not on 96" tubes, no. However I am presuming OP is talking about common <=4' T12 fixtures, which certainly do, and they look like this. en.m.wikipedia.org/wiki/Fluorescent_lamp#/media/…. Have you taken apart (read: broken, heh) any T12 tubes? – Harper - Reinstate Monica Jul 8 '17 at 22:54
  • Yes, for this conversation: 4ft tubes – gatorback Jul 8 '17 at 23:49
  • After checking the amazon link he gave he is indeed talking about those 4' preheat/bipin T12s.These "SHINE HAI" things say "Please note that in the case of ballast incompatibility a simple bypass of ballast may be require." Ahem. A preheat fluorescent tube will blow if used like that, with filament burning out and possibly loss of vacuum as well with a certain but small chance of mercury contamination of the environment. I'm not aware of fires, but who knows... – SX welcomes ageist gossip Jul 9 '17 at 0:04
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    @fizz These are not same-end-powered LEDs like the common ones you may be familiar with. These are dual-mode, that can work in plug-n-play OR ballast bypass. Since they want to be compatible with instant-start ballasts, they draw hot and neutral from opposite ends. Weird, huh? Since they do, they do not need to worry about shorting vs non-shorting lampholders, or live vs. dummy ends. Really, it's better all around. – Harper - Reinstate Monica Jul 9 '17 at 2:28

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