Two Radial Circuits - one mcb

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My house has two immersion heaters (two separate tanks), each heater with its own circuit and 16 amp mcb on the consumer unit. I have no more spare spare outlets left on the CU and need one to run an outside supply.
Can I combine the two immersion heater circuits on to one 32 amp mcb provided that I fit each circuit with its own FCU (13 amp) ? At the moment these mcbs are on the non RCD protected section of the main CU. So this would allow the outside circuit to route via a dedicated 30ma RCD rather than from the RCD side of the main CU with risks of nuisance trips to the main system. ( I do not want to change the main RCD to 100ma with the subseqent outside RCD at 30 ma ).
Am I missing something?
Thanks in advance
hereward
 
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Yes, so long as the wiring between each FCU and the 32A is rated to carry at least 32A - 6mm2 should be sufficient.
 
I'd say it was allowed, but bad practice (and in contravention of the guidence about heaters in more than 15L of water on own circuits) you don't need 6mm² as you arn't pulling 32A through it (Think spurs on ring final circuits) - the FCU protects against overload and the MCB against short circuit and earth faults in the cable before the FCUs

This is obviously if you have two separate cables, if tyou daisy chain them you'll need 4mm² / 6mm²

This is notfiable work btw :) (you are changing chararistics of the circuits)
 
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I seem to remember a regulation regarding fixed heating circuits must be fed on their own circuit, but I can't see it at the moment. Can anyone say eiter way on this?

Oh and sorry to contradict you Adam, but I would have thought it is not allowed to feed two 16A radials of one 32A MCB
 
page 154 of blue OSG ( I am too poor at the moment to afford the brown copy!! :LOL: ) in section water and space heating says that each water heater over 15 lts are to be supplied by their own separate circuit

is that what you meant RF?
 
It is indeed still in the brown book, the question is how you interpret it, the heaters are supplied by their own separate circuit ;)

And the two FCU radials on the 32A breaker on separate runs of 2.5 while not a 'nice' solution, I beleive it complies, the breaker protects the cable from shorts and earth faults, the FCU protects it against overload, I haven't yet bought my copy of the regs, so I can't look up numbers

If Lec is still around?, I think he had a hand-dryier situation a bit like this a long while back, but I can't find it via search
 
Yep. Thats the one. It is on page 148 of the yellow OSG. I am also too poor <read tight> to buy the brown ones.
 
Adam_151 said:
It is indeed still in the brown book, the question is how you interpret it, the heaters are supplied by their own separate circuit ;)

But they aren't. one fuse = one circuit.

And the two FCU radials on the 32A breaker on separate runs of 2.5 while not a 'nice' solution, I beleive it complies, the breaker protects the cable from shorts and earth faults, the FCU protects it against overload

You are not allowed to protect the cable with a fuse downstream of the cable.

Hereward, Why not get a small seperate consumer unit installed for the outside supply with it's own RCD. Don't go bodging the rest of your installation. Get it done properly.
 
iirc at the start of the appendix it says something like the a suitably qualified designer can make changes to those, I am sure I made the mistake in the past of thinking this was a hard fast regulation.
 
RF Lighting said:
You are not allowed to protect the cable with a fuse downstream of the cable.

Providing overload current protection is provided downstream and the adiabatic (?sp) equation can be met for fault current then it is permissible to fuse downstream, a common example being a spur from a ring final.
 
RF Lighting said:
Adam_151 said:
It is indeed still in the brown book, the question is how you interpret it, the heaters are supplied by their own separate circuit ;)

But they aren't. one fuse = one circuit.
Indeed

OSG said:
Water heaters fitted to storage vessels in excess of 15 litres capacity, or [snip] are to be supplied by their own circuit

they are supplied from their own circuit, but just not their own individual circuits

as opposed to:

OSG said:
Water heaters fitted to storage vessels in excess of 15 litres capacity, or [snip] are to be supplied by their own circuits

or even

OSG said:
Water heaters fitted to storage vessels in excess of 15 litres capacity, or [snip] are to be supplied by their own individual circuits

[/quote]


I beleive the purpose of it all is to keep the heaters off socket circuits, so I'm not actually sure whether their is much significance in which way its interpretted

EDIT: Sorry, that came out a bit brash and arrogant, I think :oops: , it wasn't intended :oops:
 
RF Lighting said:
Oh and sorry to contradict you Adam, but I would have thought it is not allowed to feed two 16A radials of one 32A MCB

why? i dont see a problem with a 32A feeding 2 13A FCUs for water heaters. if that doesnt comply, then what does? seperate feed to the house since they will be fed from the same fuse (service head) as the rest of the house, therefore not 'on their own circuit'?
 
I can’t see a problem with a suitably sized dedicate radial for the two water heaters. From an electrical design point of view what you have is 1 water heater circuit rated at 6kW. This could feed 1 tank with 2x3kW elements or 2 tanks with 1x3kW. As long as the radial is sized for the total load and each element has it’s own DP switch/FCU it is within the regs
 
Yeah, I'll go with that, as long as area is not a problem. Think about it:

According to table 8A in the brown OSG (on P151), a 32A radial circuit wired in a minimum of 4/2.5 is permitted, providing floor area served does not exceed 75m3.

And for that matter, it is no worse than a cooker circuit:

The green guide states that a pvc cable (6/2.5) with a 32A type B breaker installed to methods 1, 3, 4 6 and 15 can supply a cooker (with no socket outlet) up to 40m away, assuming a maximum Zs of 1.2 Ohms.

Why should the set-up suggested by Pens be incorrect?
 

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