What hapens when you put a 500w bulb in a 300w lamp?

Question

I bought the wrong bulb, but I wanted to know what would happen. I'm pretty sure it might start a fire, but I'm not 100% sure.

It's a T3-type bulb.

Answer 1

You're right it might start a fire, but maybe not.
The fixture rating is based on the expected operating
temperature with a 300W. lamp installed, in a certain
environment range.
The intent is to limit the surface temperature to a
safe level.
Depending on the type of fixture this may be to
avoid fire, or burns to the operator, or both, or
merely damage to certain parts of the fixture.
If you exceed the rating by 66% as you propose,
you may be sure you're aking for, and will get,
some kind of trouble because of it.

Answer 2

I am not sure what kind of lamp you are talking about, but generally, there are two things that can be limiting when it comes to wattage specifications:

1 - Heat produced (the lamp may not be fire safe is it starts melting the wires or the body of the lamp due to an overly hot bulb)

2 - current overdraw (this CAN start the fire as the wires may overheat. If there is a fuse protection, that should blow first.

That's about it.

Answer 3

The real problem will be in the heat given off by the bulb. If it's in a well ventilated position, that may not be a problem. Try it and keep a check on it for a while.
If your voltage is 110V (US), the amps drawn will be..
500W ÷ 110V = 4.5amps.
Generally, the amperage on a breaker for a lighting system is around 15 amps....as I say, try it and carefully keep an eye on it.
(UK system at 220 V will give around 2.3 amps).

Answer 4

If the lamp is rated for 500w you are gold
If not it will over heat and damage the lamp when wires melt and shorts out.