In uk.d-i-y, enuff wrote:

Is there a formula I can use to calculate the appropriate RCD size for a
circuit? I am about to use an old electric shower supply to feed a 1.7Kw
storage heater. Its the only thing on the circuit (or is it a spur :-) and
currently has a 30amp RCD on it.

I assume 30amp is way too much for a little storage heater but how do
calculate what to replace it with? Same applies to a circuit that I'm
putting a hand drier on and so on...

Doesn't seem like you mean RCD (residual current device), which is a safety
device protecting against current which has been sent out of the live
conductor going anywhere other than back along the neutral, but rather an MCB
(miniature circuit breaker) - modern version of a fuse, wot you've probably
got a row of in your consumer unit (modern-speak for 'fuse box').

Yes, there's a simple formula relating watts to current; watts = volts *
amps, so your 1.7kW heater will pull about 7A. (Easier still rule of
thumb: each 1kW means 4A near as dammit).

*But* if you're confused about RCDs versus MCBs, unsure whether the
thing feeding the storage heater is a circuit or a spur (a bit like
asking whether something is a tree or a sessile oak), you should probably
spend a while in the library or a tenner or so at Amazon to read up some
more on electrickery-as-she-is-practised before getting too ambitious.
(That hand drier you mention - will it be in a bathroom? If so, how about
its supplementary bonding? you know that the minimum conductor size for
that depends on your earthing arrangements, don't you? which Zone will it
be in? This little litany is not meant to intimidate, but to help you
calibrate where your current level of competence is compared to that needed
to do the works you have in mind).

HTH - Stefek