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I've seen light fixtures with varying wattage rating. From 30W to 100W even though the actual socket is the same.

What is the limiting factor, why can't I put say a 75W bulb in a fixture rated at 50W?

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This is related to the amount of current (Amps) the electrical parts of the fixture can take without overheating and causing damage.

$$\text{Watts} = \text{Amps} * \text{Volts}$$

So at 120v a 50W bulb will draw $\frac{50}{120}$ (0.416) Amps and a 75W bulb will draw $\frac{75}{120}$ (0.625) Amps.

All electrical conductors have some resistance, this makes them warm up when you pass a current through them. The higher the current (more Amps) the faster it will get hot and the hotter it will get. There are many factors that determine how hot something can get before it will fail, for example how fast it can dissipate heat, what temperature the insulation can withstand. The rating is there so that you don't put too high a current through the electrical components of the fixture, which would cause it to get too hot and melt, catch fire, etc.

How hot something will get can be predicted mathematically, but ultimately the rating value is determined by testing the fixture in a lab with light bulbs of different powers, and measuring how hot it gets. The rating is set such that it will always remain at a safe temperature, within a safety factor.

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  • $\begingroup$ I would not think that the heat rise of the luminaire wiring would be significant. In my experience the minimum wire size is 1.5mm² for reasons of mechanical robustness; 1.5mm² wire is good for at least 10 amps, which is 1,100W (if you live in a 110VAC country) or 2,400W (if you live in a 240VAC country). The heat produced by the lamp itself, will be far more than the heat produced by the wiring. $\endgroup$ – Li-aung Yip Jun 10 '15 at 14:18
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The rating of a lightbulb fixture is determined by the robustness of the materials used in making it. Putting a lightbulb which uses significantly higher power than the fixture is rated for is dangerous.

As an example, consider a cheap desktop lamp. If you put a high wattage lightbulb into the socket, the amount of current draw will be larger than the wiring of the lamp is designed to handle. Since some power is dissipated in the wiring of the lamp, this excess current will cause the lamp to heat up and potentially catch on fire. A fuse is often included in devices such as desktop lamps to prevent this from happening though.

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The rating on the fixture itself, as opposed to the bulb you put into it, is a safety rating. Above the fixture's rated wattage, the fixture will be unsafe. This will probably be due to heat issues that may breakdown the fixture's materials or even damage an installation's surrounding (i.e. it will set your house on fire if you try to install too big of a bulb in the socket). Other breakdown issues may include an arc hazard, but this is unlikely compared to the heat issue.

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The wattage of an incandescent bulb is simply the electrical power that it dissipates.

Often the limiting factor is that this type of bulb dissipates a lot of this energy as heat so one of the most common reasons for limiting the wattage is that you have something like a fabric or paper lampshade which would be damaged or might even catch fire is a high output bulb was fitted.

Another example is in exterior lights where the fixture is usually sealed and so cannot dissipate heat by convection.

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