cable sizes to 3amp fused spur would need to be 10mm ?

[kim_l_fixit]

Hi

I am newbie so sometimes may ask stupid questions.

I need some help with circuit, Manufacturers recomendations the electric boiler is 12KW is fed from distribution board via double pole isolater with a 3 amp spur wired directly to this isolator for control circuit, this means 10mm cable from board to the isolator and onwards to boiler, but I think I would also need to use 10mm to 3amp fused spur incoming ,they say because it only draw 1.5 amps on control circuit the feed cable can be smaller.....You cannot fit 10mm cable into 3 amp fused spur - regs say you must follow manufacturers instructionsView attachment 1869

 

[Megaohm7]

Firstly you are probably thinking of a 13a FCU. You can fuse this down as per manufacturers instructions. You are quite right about 10mm not going onto the terminals in an FCU and it wouldn't be able to carry the current required for the 12KW boiler anyway. which I made it to be 52a. You need to do some cable calcs first. Are you studying at college or have done so? Following manufacturers instructions is always a good idea as if anything goes wrong then at least you have that to fall back on. Let us know how you get on those suggestions. It's all a learning curve and there is nothing wrong with asking.. It's how we all learn mate..

 

[kim_l_fixit]

I agree my info is mis-leading it is a 13 amp FCU with 3 map fuse for controls please see pictur I have added from manufacturers manual I think this expains better my concerns. I was thinking of 16mm cable to double pole and 10 mm to unit myself but again manufacturers say 10 mm but don't think they realise things have changed in 17th regs in relation to insulation etc.

 

[sellers]

Take your 10mm to the water boiler via the isolator. From the load side of that isolator also pop in a 1.5mm and take that to your spur with a 3amp fuse inside. The cable from the isolator to the spur is protected by the fact it cannot possibly draw more than 3 amps.

 

[Sidewinder]

^ as Sellers

 

[m4tty]

I took a spur off the ring once in 1.5mm to a fcu (3a) for an extractor fan and was moaned at saying it should have been run in 2.5mm. I knew that the extractor couldn't overload the 1.5mm. Was I right or wrong to do this? Thanks

 

[Badger]

i think the 1.5 taken from the load side of the isolator is bad advice as 1.5mm cannot possibly take the fault current of 50-60amps that would be required to trip breaker in dist board the control wiring may only be able to DRAW 3 amps max but the FAULT/OVERLOAD side of the 1.5mm is still protected at the source of 50 amp + breaker hence why i reckon that control wiring should be taken from the same rcd side but a 6 amp trip not the actual boiler supply, my twopeneth anyway.

 

[Sidewinder]

I also agree with M4tty & Badge!

It is IMHO acceptable to take the 1.5 from load side, but this must be a short run and unlikely to be damaged, that is IMHO within site of the origin of the cable.

I think this is acceptable in brb if less than 3m and may be where the DNO 3m rule is linked to.

brb upstairs, me too lazy to go and get it!!!

Mind Badge, not sure about the control from a different mcb to the load side, don't like that actually, sorry!

 

[fletchflash]

reg 434.2.1

page 75

(i) not exceed 3m in length, and

(ii) be installed in such a manner as to reduce the risk of fault to a minimum

judging by the 2nd part, probably not a good idea to have the cable plastered in or hidden in any way

 

[Sidewinder]

Thanks Fletch!

As I understand pt ii, I would say it has to be surface mounted, conduit, trunking or clipped direct, if any way concealed then you cannot minimise the risk of fault.

I only do this over very short runs and, where both ends of the cable can be seen from either end if that makes sense?

It is often done on large scale installs and there is no way around it very often.

Say a 2500A incomer, and you are taking a 63A sub main of that directly then you could never get a core rated at 2k5A into a 63a device!

Though, for example this would be in a switch room say then this would be a controlled environment etc.

 

[Badger]

from another angle the control side could be run from a 6amp trip and the boiler run from its original supply 63amp etc but the boiler feed could go through 63amp rated contactor and the 6 amp trip feed the coil on the contactor via the load side of the 3amp spur as well as the the control wiring, and that would still remove all power via the contactor to the boiler anyway, reneergise the 3amp fused spur and the coils renergised and so is the control wiring, job done,

 

[SLIPSHOD & SLAPDASH]

That diagram does not make much sense. Offen electric boilers are wired with a separate feed from the C/Unit to supply its control circuits with normally a 3A fuse. Sometimes the control circuit is wired through a time clock to benefit off peak tariffs .

 

[Badger]

That diagram does not make much sense. Offen electric boilers are wired with a separate feed from the C/Unit to supply its control circuits with normally a 3A fuse. Sometimes the control circuit is wired through a time clock to benefit off peak tariffs .

hopefully this makes things a bit clearer :Blushing :slap

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