How to calculate RCD "size" ?

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Hi All
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...
Thanks
Tony
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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,
1700 ---- = 7.4 amps 230

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.
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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 circuit.
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.
Christian.
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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
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Thanks all
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...
http://www.tlc-direct.co.uk/Products/MRE88ACS.html
Surely these are designed to go within reach of sinks etc?
Tony
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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
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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.
Tony
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bonded to other metalwork in the bathroom...
cheers, Stefek
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Also, if like MacDonalds, it is a toilet room only with no bath or shower, then supplementary equipotential bonding is not required.
Christian.
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Christian McArdle wrote:

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...
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wrote:

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! ;)
PoP
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PoP wrote:

Just give it a quick shake before you put it away...
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wrote:

It's easier to kick the drops off.....
PoP
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Don't you mean "spin dry" ?
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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!
--
Niall

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wrote:

Bachelor flat was it ?? I'm told that that is normal behaviour in those :-)
.andy
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wrote in response to
"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? Barley Twist (Please put out the cats to reply direct)
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Aston Mathews in Islington had a combined WHB/Urinal in their showroom, which must be the ultimate bachelor flat accessory.
--
Andrew Gabriel

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On 20 Jan 2004 21:58:18 GMT, snipped-for-privacy@cucumber.demon.co.uk (Andrew Gabriel) wrote:

Wasn't this the thing where the WHB was above and the water from washing the hands went down and flushed the urinal?
.andy
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That's it -- all one piece of porcelain though. If you're careful, you could have a wash and piss at the same time, but it might take a little practice...
--
Andrew Gabriel

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