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...
RCDs aren't current dependant - although you need one of a suitable
current carrying capacity.
I'd guess you mean an MCB?
To work out the load, divide watts by volts, so in the case of your heater,
---- = 7.4 amps
Normally, the MCB in the CU protects the house wiring. The fuse in either
the plug or the FCU (fused connection unit) protects the appliance -
although if there is only the one appliance on the circuit - like say a
cooker or immersion heater, the MCU can protect both.
*It IS as bad as you think, and they ARE out to get you.
Dave Plowman firstname.lastname@example.org London SW 12
An RCD is a device that detects imbalances in the neutral and live flows.
Typically, you use 30mA for a final circuit, or 100mA time delay for whole
house arrangements (only when required). However, I think you really mean
MCB, which is a device that cuts out when a certain level of overload
current or short circuit current has been drawn.
For a resistive load, you divide the 240V power consumption in watts by 240.
The heater specifications should give outputs for 240V and 230V. Assuming
the 1.7kW is rated at 240V, this will give just over 7A. A 10A MCB would be
good here, although 16A MCBs are frequently used. Check the installation
manual to see if it (or its flex) requires protection at a certain level.
The manufacturer's recommendation should override any calculations or
assumptions you make.
The hand drier circuit will also have a reactive element due to the fan
motor. However, this will not be significant compared to the resistive
element of the heater. In any case, the things are typically no greater than
3kW so that they can be fitted on a 13A FCU. Use a radial circuit with at
least 16A capacity, with an FCU connecting between. A device such as this
normally shares a circuit with other devices, perhaps on a 32A ring final
If you really do want a dedicated circuit, then it may be possible to use
the 16A MCB and avoid the FCU. However, you MUST check the installation
instructions to ensure that this provides adequete protection to the dryer.
Many will allow this, as continental European final circuits are often rated
at 16A with no subsequent fuse protection, unlike UK systems, and many
manufacturers prefer to have a single version of an appliance to sell
throughout the European market. You may need a DP switch anyway for
maintenance, so little saving will be made.
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
You're all right, its an MCB. I'm learning, slowly...
I'm leaving the existing 30amp in and then changing it to a 10 after the
weekend. Stefeks right to question the hand-drier positioning and the
bonding which I plan to have handled by a competant electrician who is doing
a lot of work here over the next 2-3 weeks.
The drier I have in mind is a proper commercial jobby from TLC...
Surely these are designed to go within reach of sinks etc?
just "be sensible" wording (i.e. don't put it where it's likely to get
regularly splashed if that'd be bad for safety). They are prescriptive
about minimum distance from nearest edge of bath or shower: need to be
out of arm's reach, specified as 60cm (two of our quaint English feet,
metricated ;-) But your electrician will know all about that.
Cheers - Stefek
Thats what i thought, just thinking of the local McDonalds etc
I'm the poor s*d who's going to be putting the damn thing in! I'll just run
a 6mm earth from it and leave it ready to attach to an existing pipe. I
assume thants whats needed, if so I have a box of earth clamps.
Our local macdonalds has these sinks which are basically like holes in
the wall with a drainhole in the bottom. You stick you hands in and a
(PIR?) sensor detects your hands and squirts some soap on them. A couple
of seconds later it turns on the warm water and gives you about 10
seconds of water. A couple of seconds after that something else in there
starts blowing hot air on your hands. All without having to take your
hands out of the hole in the wall...
Hell, those are the washbasins???? And there I was thinking they were
an automatic bidet..... ;)
Never did figure out how to stop the front of my trousers getting wet!
Sending email to my published email address isn't
guaranteed to reach me.
Seems to be a modern trend for designing whbs and urinals which look
as much like the other as possible.
On one occasion it was only when trying to figure out how and where to
wash my hands that I realised I'd just pissed in the sink!
I'm told - by someone who moves in such exalted circles - that in one
of the Bachelor Rooms at Balmoral that there was a little notice above
the somewhat grubby hand sink saying "Go on! Everybody else does"
(The room of necessity being at the far end of a rather draughty and
apparently uncarpeted corridor")
The Dook's idea of fun?
(Please put out the cats to reply direct)
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