Dimmer switch wattages

[tpar]

I'm getting conflicting information on the wattage needed for an installation of low voltage halogen lights. By the way, it's in France and apparently over there they have a (little) transformer per light. I have two configurations: either five or ten light sets, each set made up of 12V/20W bulbs. I'm told by one electrician that I need 1000W dimmer for the set of ten, but by another that for low voltage halogen you just multiply the 20W by the number of lights (i.e. 20X10) add a bit for transformer and wiring losses and that's what is needed, so 400W is easily sufficient.

Can anyone tell me who is right?
Supplementary: Is there a loss of light even at maximum from dimmer switches?
What do I need for the five light circuit?
Thanks in advance for any help here!

 

[breezer]

make sure they have transformers that are suitable for dimming.

 

[tpar]

Thanks for your very quick response! Isn't this a wonderful forum.

Yes I'll do that. But what about the wattage of the switches? This is a real concern for me and I have to decide in a couple of days unfortunately.

 

[breezer]

wasnt that quick.

it depends on make of dimmer. idealy the dimmer should be able to "handle" twice the load of that of the total lamp load

better to get one too big than too small (within reason)

 

[tpar]

I've been told that if the halogens are mains halogen, then you have to double the wattage for the switch. So if ten 50W mains halogens are driven from one switch, you need a 2X10X50 = 1000W switch, but if the halogens are low voltage ones driven through transformers, the factor of two doesn't apply - just add a bit for the transformer and ciruit losses. I think you're telling me that the factor of two does apply to low voltage lights as well.

The problem I have is that 1000W dimmers in France are around £80 each, and I haven't been able to locate any at all in the UK. Sigh.

 

[breezer]

better safe than sorry (you may change your mind) tlc have them

 

[tpar]

tlc doesn't have them at the URL you posted. It's a marketing support organisation.

 

[breezer]

it does now

 

[tpar]

Thanks for redirecting the URL. An interesting site which I've saved for future reference

The only 1000W dimmer on the site is for mains voltage lights, not for low voltage + transformer lights. Maybe this is evidence that 1000W isn't needed for the low voltage configuration I have?

 

[breezer]

no, you always dim the mains side, (primary)

 

[barryo]

On the tlc website it has a link at the bottom of each dimmer product page which explains whether or not you have to de-rate for eg GU10, Halogen, LV etc.