I;m about to fit some under unit lights in our kitchen. Normally they’re brought back to a junction box/lighting ring, but I’m bringing them back to a plug. Each circuit will use 21w…am I right in thinking that 3A or less would be best?

watts divided by volts = amps Lowest fuse you can use is best

The fuse is there to protect the flex/cable. As no cable that I can think of would struggle with 3A, and that is more than enough to run the lights 3A will be fine/best.

I’m not a sparky.

3A is right for 21W lights.

P/V=A
21/240=0.0875
=87.5mA

The smallest fuse for a mains plug is 1A so use that. A fuse isn’t just there to protect the wiring, it’s to limit the current that can be drawn if there is a fault. A 1A (240W) spark is less dangerous than a 3A (720W) spark.

As wilej said you can work out the fuse required for a circuit using ohms law. Current (amps)=watts /volts.
You say each circuit but I presume you mean multiple light fittings on one circuit with a plug on the end. If so, add all your watts together i.e. four lights = 84w and divide by 240 to give you the fuse amperage (.35A).
Anyway , unless you’ve got loads of lights then a 3 amp fuse will do the job.

Pedant alert. Ohms law states
V = I x R.
Although you are correct in saying P = I x V.

Pedant alert 2
UK single phase voltage hasn’t been 240 for over 10 years.

Is there any inductive load? (transformer) as you’ll need to allow for inrush current.

The smallest fuse for a mains plug is 1A so use that.

Never in my life have I seen a 1A fuse in a mains plug. I never knew such a thing existed.

Never seen a 1″ one. You can get loads of different fuse types in non standard sizes.

four lights = 84w and divide by 240 to give you the fuse amperage (.35A).
Anyway , unless you’ve got loads of lights then a 3 amp fuse will do the job.

Even then a 1A is plenty.

Pedant Alert 2.1

UK voltage is still 240v. Euro voltage is 220v.

No one actually generates electricity at the mythical 230v, but it allows western voltages to be harmonised as both 220 and 240 fall within the tolerance provided by the 230v ‘standard’.

😉

Either tin foil, or nail. Those are the only 2 fuses required

Having measured voltages almost daily for the last 13 years I would say that 230 is a lot closer to the norm than 240 😉

But aye… stick a big nail in, what’s the worst that could happen…? I was working on a sewage plant a few years back where the entire system was fitted with chunky nails. It made me a touch nervous.

Cheers Fellas.

Each Circuit has a 13w and an 8w lamp. I had 3A in stock so have used those for the moment, but when I’m next in BnQ, I’ll get some 1A and replace.