Why are dimmers de-rated for Halogens?

[Alan]

Been fitting Halogen GU10 50w spots at a friends house over the weekend. Total 300w. Noticed as I went to fit the 400w dimmer I'd bought that it stated that if used with Halogen lights such as GU10's then max is 250w. Why is this? Now I've got to find a 500w dimmer (which supports 300w Halogen) in brushed chrome effect. Any ideas? MK have a white plastic one. May be able to drill out a chrome blanking plate to make the guts from the plastic one fit?

I assume if I did use the 400w dimmer it would run hot and have a short life?

Alan.

[Dave Plowman (News)]

The difficult part for a dimmer is the start current when the filaments are cold - their resistance is very much lower. And certain lamps of the same nominal wattage have a lower cold resistance than GLS types.

[Christian McArdle]

should handle the power. It looks like it would be easy to install onto a blank faceplate.

Christian.

[Christian McArdle]

Usually, although you often need to file out a little slot that engages with a keying protrusion on the pot.

Christian.

[Alan]

Thanks Christian, they look ideal.

I imagine with a hole drilled in the chome plate it should be an easy swap.

Alan.

[Dave Plowman (News)]

You need also to check that no 'live' bits touch the metal plate. While it may be a design that's also fitted to metal plates, I wouldn't count on it.

[Alan]

Good point, will do.