Connecting sockets to an immersion circuit

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This is a real one and might be interesting for some of you,
The boiler had a problem so naturally I went and turned on the immersion switches, I have 2 tanks and 2 boilers that work as one unit.
To my 'delight' I found that the immersion is not working, checked the wires from the MCB to the switches and there is power, but interestingly the wires in the switch that coming from the MCB and the ones that suppose to go the the tanks were shortened (so the switch was not effective), if so the immersion should be on permanently no? but no another exciting discovery was the fact that the wires instead going to the tanks were used for outlet sockets in the loft. I called the electrician that done the work (if I can call him so), and told him the story, he said that I asked for switches for the immersion in an accessible place and he put them in but I did not ask specifically to connect the immersion so he did not do it??????

my problem
I went a got 2 DP 20A switches + 2.5 mm2 flex heat resisting cable (the immersion is 3KW each. I intend to connect the 2 immersions in separate circuits (different wires) but ending in the same MCB/RCBO 32A. The problem is that if I use this MCB/RCBO for the immersion I will loose my outlet sockets in the loft. Is there any reason, assuming that the load is ok, that I could not pull from this circuit a fused spur and connect the outlet sockets to it
Thanks
Albert
 
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Albert

Long time no hear!

What I would do is leave the immersion circuit for the water heating and run a feed for the loft sockets from the upstairs ring main.

What sort of loading on the loft sockets is there?
 
securespark said:
Albert

Long time no hear!

What I would do is leave the immersion circuit for the water heating and run a feed for the loft sockets from the upstairs ring main.

What sort of loading on the loft sockets is there?
Yes I was away for a while, good to hear from you,
I know the option of pulling a connection from the upstairs ring, the problem is that it might require going through the wall, and this is something that I do only if I don't have a choice.
This circuit feeds the immersion (will, not yet), a spare room with one double socket, and the loft (2 double sockets, one feeds the tv aerial booster) as you can see there is not much on this circuit, by the way it is not a ring it is a radial circuit.
In any case as a principle is it unacceptable to have the immersion on the same circuit with other possible loads (outlet sockets)?
 
According to the OSG, a radial in 2.5 cable is permissible, if it has a 20A protective device and covers a floor area of no more than 50m2.
 
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securespark said:
According to the OSG, a radial in 2.5 cable is permissible, if it has a 20A protective device and covers a floor area of no more than 50m2.
If I connect the two immersions as two separate circuits of 2.5mm2 each, to the same 32A MCB/RCBO when the floor area is not more than 50m2:
1) would it be ok? because it is like using 5mm2 cable and the OSG says that we can use 32A MCB with 4mm2 and a limit of floor area of 75m2.
2) If I understand correctly the fact that one of the loads on the circuit is the immersion does not make a difference as long as the load and the rest were taken in consideration. (I know that it is recommended to put the immersion on a dedicated circuit, but the recommendation is not always the law)
 
Albert said:
If I connect the two immersions as two separate circuits of 2.5mm2 each, to the same 32A MCB/RCBO when the floor area is not more than 50m2:
1) would it be ok? because it is like using 5mm2 cable and the OSG says that we can use 32A MCB with 4mm2 and a limit of floor area of 75m2.
Surely it wouldn't be like a 5mm² cable - it would be two separate 2.5mm² radials, each on a 32A breaker. Although shared, each individual 2.5mm² circuit would only have 32A protection - too high.

:?:
 
a failure of 2 hand driers wired through fcus and then with 2.5mm cable back to the same 32A breaker was overruled by the niceic

their reasoning being that the fuse at the far end gave overload protection and the 32A breaker gave short cuircuit protection
 
Albert doesn't have any FCUs at the ends, just immersion heaters and sockets....
 
immersion heaters usually are fitted with fcus in my experiance

and i also ask how is 2.5mm direct off a 32A breaker any different from a 2.5mm spur from a ring (which is definately permitted)
 
It's the old "can't overload a 2.5mm² cable with one socket" routine. Albert's got 3 sockets (spare room + 2 in the loft).
 
yeah multiple sockets in a 2.5mm run of a 32A breaker is defiantely a no-no (actually i think it may be ok in cable tray install not sure)
 
Yeah I'll go with Ban & Plug on this, Albert. If your immersions are 3kW, then you could fit flex-outlet FCU's to fuse them down to 13A each, and that would comply.

Your idea of two 2.5's is not quite the same as 5mm2, because there is no ring as such.

Plug - 2.5 direct from 32A breaker has not got the return path, whereas a spur from a ring has.

The OSG says:

Ring 32A 2.5/1.5 = 100m2 OR

Radial 32A 4.00/2.5 = 75m2 OR

radial 20A 2.5/1.5 = 50m2
 
Friends, it is always a pleasure to hear your arguments,
What i'm going to do:
1) As my CCU in very generous (19 MCB/RCBO's), I can join 2 of the light radials in one MCB (in 2 of them I have a dedicated MCB for one room only, total of 500W).
2) This will free one MCB space. change to 16A MCB and connect one of the Immersions to it, (it is a Radial), this will half the problem.
3) Connect the second Immersion to the existing space in the CCU but change it from 32A to 20A (not 16A because I have one of 20A) MCB (Radial).
4) Now, I don't see a reason for an FCU in the first immersion as it is on it's own circuit and protected by 16A MCB, and activated by DP 20A switch not fused, is it still necessary to fuse it? The second immersion I thought to connect the same way MCB, DP 20A switch, pull from the DP a connection to a 13A spur and connect the sockets to this spur, how bad is it?
Now a $64M question, that every one asks, I know that it is not recommended to connect the immersions to a RCBO/RCD, but from experience, is there a reason why it should not work? All my sockets in the house (even the ones that not required by the regulations i.e upstairs etc. only the lights, Hob and Oven are on MCB) are connected to RCBO.
 
what do you think of this?

Socket outlets require a 0.4s disconnection time whereas the immersion, being fixed current carrying equipment only requires a 5s disconnection time. Also, the On-site guide page 154says a seperate circuit is required for the immersion, although cant find this in BS7671 (section 554-04 refers).

Feeding sockets from a fused spur off an immrsion circuit would also be highly confusing as presumably there are other socket outlets connected on the same floor. Such a circuit would not 'minimise inconvenience in the event of a fault' (bs7671 reg 314-01-01).
 
Albert said:
Socket outlets require a 0.4s disconnection time whereas the immersion, being fixed current carrying equipment only requires a 5s disconnection time.
But there's no reason why fixed equipment shouldn't have a faster disconnection time

Also, the On-site guide page 154says a seperate circuit is required for the immersion, although cant find this in BS7671 (section 554-04 refers).
The OSG is also a guide to good practice.

Feeding sockets from a fused spur off an immrsion circuit would also be highly confusing as presumably there are other socket outlets connected on the same floor. Such a circuit would not 'minimise inconvenience in the event of a fault' (bs7671 reg 314-01-01).
It would, IMHO, be imperative to label the MCB "Immersion heater and sockets in loft & small bedroom". Also, one could interpret 314-01-01 to mean that there should be independent socket circuits on the same floor...
 

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