loading of immersion heater circuit

[attitude-adjuster]

Hi - I have searched on various 'immersion' heater threads and got some clues to my question, but am not confident in my assessment of my problem - so hoping soemone can advise please.

I have a double 13amp RCD socket in an airing cupboard upstairs. It is fed with 2.5mm cable direct from the consumer unit from a 16amp MCB (type B I think). Currently, a shower pump (300w) and the boiler (3amp fuse required, but sure of wattage rating) are plugged into this. There is also a fused spur off of this socket supplying a security light (500w - 3amp fused spur).

I would like to consider using this circuit to supply an immersion heater element (3Kw) in the hot water cylinder in the airing cupboard. This would be for emergencies - ie if the boiler broke down - but I'm sure this does not matter from an electrical safety point of view - if its done wrong one usage is enough for it to bea problem.

Is it wise to have the immersion on this circuit? Say by using a larger 20A MCB? If I were only using the immersion heater and there was a fault then I would not be protected by using the lowest MCB - right? I think the total current draw of all components currently + immersion heater would be very close to 16amps if not a bit over.

Lastly, although the shower pump is rated at 300w, the manufacturer says use a 5amp fuse?

 

[plugwash]

20*230=4600W
your current load is somewhere below 1500W. A typical immersion heater is 3KW so 20A should be sufficiant and 2.5mm should be fine on a 20A breaker provided its kept away from insulation.

Use a fused connection unit (also known as a fused spur) with a 13A fuse to protect the immersion heater.

Personally i'd be inclinded to get rid of the socket and put everything on fused connection units (you can get RCD protected ones) to prevent someone adding more appliances to the circuit and overloading it.

 

[attitude-adjuster]

I appreciate the fast solution and thanks for the suggestions.

 

[Taylortwocities]

Plug, dont want to rain on your parade but you cannot treat an immersion heater like any other radial circuit.

There's a reg (BS7671 in the van but I'll find it tomorrow) that says that any instantaneous heater for a water vessel over a certain size (I think its 15 litres) must have its own dedicated supply.
Means you are not supposed to add shower pumps, burglar alarms, TV amps etc to your immersion heater circuit.

 

[plugwash]

I think thats only a reccomendation and BS7671 is only advice from the IET (formerlly the IEE) not the law.

also you could argue that the bank of FCUs is essentially equivilent to a CU and the 20A circuit is a submain.

 

[Softus]

There's a reg (BS7671 in the van but I'll find it tomorrow) that says that any instantaneous heater for a water vessel over a certain size (I think its 15 litres) must have its own dedicated supply.

If "instantaneous" really is the word in the regulation, then it doesn't apply to an immersion heater element.

 

[Taylortwocities]

As I said, yesterday. i couldnt check cos regs book in the van and i was in my nightie.

Ok, ref to this is in the OSG page 154 and says
"Water heaters fitted to storage vessels in excess of 15 litres storage capacity ........... are to be supplied by their own separate circuit.
Immersion heaters are to be supplied through a switched cord-outlet-connection-unit complying with BS1363-4.
There's no ref to a reg (damn) and cant find one.

So its guidance for us regular sparks.

Any NICEIC guys care to give a view on what Smashy & Nicey say about this? (Not that they make the regs.......!)

 

[ricicle]

No there are no regs. concerning this but the guidance is there, and as I'm sure you'll agree, it seems silly to take 13 amps capacity off your socket circuits by wiring a water heater to it.
But if you could justify your reasons for doing it..................

 

[jimbob100_99]

I would be inclined to run a new 2.5mmT+E circuit in for your immersion with a 20A dp switch. Its good practice not to power boiler and Immersion from same circuit just in case that circuit ever fails you will loose both means of heating your water. Also i would put a switched fused spur in for your boiler as a boiler should not be left on a switched socket....Just my take on things .......