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Hi, apologies for what may be a dumb question!

I'm planning to upgrade a broken dimmer switch which was rated up to 250w with an LED-compatible one (the Varilight JQP401W), which is apparently only rated up to 120w.

I initially assumed this would be fine as the switch only controls a single light fixture with a 100w bulb (and presumably the bulb's wattage will be even lower in the future if I'm using an LED one). However, all the lighting in my house is on the one circuit (i.e. one switch in the fuse box controls all of it), so the total wattage on that circuit is fairly high.

With this in mind, is moving to a dimmer with a lower rating likely to cause problems? Or am I right in thinking that only the wattage of the bulbs the switch controls is what matters, regardless of what else is on the circuit?

Any advice is appreciated!

The wattage on the switch is the maximum that switch can control, if the switch only controls 1 pendent / light fixture with single bulb then the max rate bulb it can service is 120w
if you have a fixture with 5 spot lamps (gu10 for example ) the highest lamp rating would be 24w per lamp (120÷5) or if the switch controlled 2 pendants (wall lights or a larger room) then each lamp / bulb max rating can be 60w

If your looking down the led lamp / bulb route make sure the dimmer switch is led compatible, not all are.

You are confusing, and you must not confuse, Watts with Amps, look it up.

The switch wattage is related to the item(s) it is alone switching, 120W is OK for one 100W incandescent bulb but should not be used to switch two; and as you say, it's a dimmer intended for LED & the wattage will be a lot less even if there are several fixtures.

The MCB in the distribution board, for all lighting, is most likely 6 Amp & cover a total of about 1500 watts, after that you go to two 6A MCBs & two lighting circuits, usually upstairs and downstairs.

just to emphasize what scotty says
you must choose both a dimmer designed for leds and an led that will work with a dimmer
also some dimmers need a minimum load to work properly if the load is too small the bulb will still light up but may not dim or dim incorrectly

also worth mentioning with an led bulb quoted as say 3w you will often get quoted as compatible with say 40w incandescent 10w le bulb choose the lowest as the actual wattage at 3w as the led is the most efficient

Just saying.............
The Varilight JQP401W dimmer switch has been designed to operate with LED lamps, now all SRJDIY has to do is to get some dimmable LED lamps.

Thanks for the advice guys! :)

I wish it was that easy, I have just bought a Lidl Smart colour change LED bulb (£3), thought good to be able to dim bulb, but did not read the packet, it says "Requires a Zigbee compatible gateway for use" ups, thought £3 was cheap, now need to spend another £20 to use it.

I use electronic switches, not actually dimmer switches, but I had these bulbs, clearly marked dimmable, but they to begin with flashed when turned off, and I had to fit a load capacitor the problem is getting them to fit, and then found the lights shimmer when on.

Wife got these bulbs, they don't need load capacitor and don't flash when turned off, but should not be sold in UK as not marked with wattage or lumen, also covers will now not fit, but they work well, one did fail and I opened it, the smoothing capacitor to stop if flickering was bigger than the whole other bulb, actually found fault and fixed it so working again.

Just got a dimmable bulb for wife's table lamp, £2.99 Homebargains against £1.29 for non dimmable.

I think I need to buy a Zigbee compatible gateway, it's simply a nightmare trying to find bulbs that will work with electronic switches.

Zigbee is on the way out. (Allegedly)