Qualified electricians question....Re: mcb

lone ranger

Novice Member
Hi all, I have a question for any qualified electricians out there as its somewhat a grey area from what i can see,its this....

I have steel armoured cable 2.5mm going from my main unit inside my house to the garage in a ring main,
the run from house board to garage is roughly 10 meters to the junction box where the ring main starts.
There are a lot of double sockets in there but the main pull will be 2 electric heaters/radiators, hoping to put two 2killowatt ones in there plus the usual cinema equipment you would have and a projector and electric screen that it houses...
Now the mcb i am using is an rcbo one so it has its own breaker,but do i put the 20amp 1 or can I put the 32amp 1...

Naturally i would much rather put the 32 and i would have ample wattage then but is it safe...
As I understand its roughly 4.5amps per killowatt used so 20amps would leave me bugger all...Naturally if i can only put the 20amp then smaller rated heaters it would have to be but i aint convinced they are good enough.
I am being told different stories from many people,regulation i believe would be around 25amp but i cant find necessary mcb 4 my board...

help please,i really would like to see some consistent info on this:thumbsup:
 

altern8or

Standard Member
32 Amp is too big for a single run of 2.5mm2 cable, 20 Amps is probably as big as this cable will allow safely.
 
S

Sparkmatter

Guest
Hi! Your best bet is to upgrade the armoured cable to 6mm. Use a 40 amp Type B MCB in your main consumer unit, then fit a 32 amp MCB in your garage to protect the ring circuit. Problem solved! :thumbsup:
 

lone ranger

Novice Member
Hi! Your best bet is to upgrade the armoured cable to 6mm. Use a 40 amp Type B MCB in your main consumer unit, then fit a 32 amp MCB in your garage to protect the ring circuit. Problem solved! :thumbsup:

thanks for the advice my good man, what weve eventually done is just scale down a bit my rads, so we will stick with the 20amp mcb rcbo and just have 2 1kw rads,that still leaves me 2.5kw of juice,and i am sure the av equipment and screen will use no where near that much....:D
 

sjj84

Novice Member
That would only be a guidance rating. There are lots of different factors that would come into calculating if 2.5mm cable would be sufficient, such as grouping, how it's mounted, does it go through any thermal insulation. You also need to work out the voltage drop for the installation, this needs to be within 4% of the supply, so less than 9.2V for a 230V installation. If you put up all the details I can work out what size cable and mcb you should be using.
 

lone ranger

Novice Member
That would only be a guidance rating. There are lots of different factors that would come into calculating if 2.5mm cable would be sufficient, such as grouping, how it's mounted, does it go through any thermal insulation. You also need to work out the voltage drop for the installation, this needs to be within 4% of the supply, so less than 9.2V for a 230V installation. If you put up all the details I can work out what size cable and mcb you should be using.

Hi m8, heres some info,

1) Its on its own,no where near other cables (if thats what you mean)

2) Its goes from my main consumer unit straigh outside then cleated all the way,then into gargage where it is then terminated into a 30amp junction box with twin and earth coming out and suppling the double sockets before returning to the junction box to make the ring main.

3) It does not go through any insulation

4) Voltage drop....eeerrrr you what:rotfl: leave that 1 to you...

I have decided to leave it at 20amp mcb/rcbo....hopefully you tell me this is ok..:thumbsup:
 

chrisboy1981

Standard Member
why not use a/c heat pump unit this you could run of what you have and it woulkd more than do your space
 
Glad you said that. Ive just read this post!!
As the cable is run externally, you MUST have it protected against earth leakage as well as over current......hence the rcbo . A Standard Mcb is not sufficiant. Your best bet is to look at more efficeint forms of space heating to reduce your current draw. A storage heater on a timer maybe a good idea. you could set it to energise for a few hours when your not using the area, or even over night. Then have it release it's heat when you are using the room, leaving you all your available power for your AV/Lighting.
 

lone ranger

Novice Member
Glad you said that. Ive just read this post!!
As the cable is run externally, you MUST have it protected against earth leakage as well as over current......hence the rcbo . A Standard Mcb is not sufficiant. Your best bet is to look at more efficeint forms of space heating to reduce your current draw. A storage heater on a timer maybe a good idea. you could set it to energise for a few hours when your not using the area, or even over night. Then have it release it's heat when you are using the room, leaving you all your available power for your AV/Lighting.
does the storage heater release its heat in 1 go or is it a slow release,:thumbsup:
 
Slightly older types have flaps to allow the heat release to be regulated. Dimplex make a range called Duoheat (I think) which is alittle more 21st century, but pricey:)
 

sjj84

Novice Member
2.5mm swa cable can carry 28A, so it wouldn't be suitable to use the 32A. As it's only a short run the voltage drop would be acceptable. How hard would it be for you to replace the cable with something more substaintal? I would strongly advise you to do this, as even if it is capable of running what you require you won't have much scope for upgrading in the future if you so desire. Using 4mm would allow upto 38A and 6mm 49A. I'm assuming you have fitted a seperate consumer unit into your garage rather than having everything all running from the same breaker? I.e lights seperate from sockets? Also you will need to get your installation inspected once it's finished.
 

davehk

Active Member
Glad you said that. Ive just read this post!!
As the cable is run externally, you MUST have it protected against earth leakage as well as over current......hence the rcbo . A Standard Mcb is not sufficiant. Your best bet is to look at more efficeint forms of space heating to reduce your current draw. A storage heater on a timer maybe a good idea. you could set it to energise for a few hours when your not using the area, or even over night. Then have it release it's heat when you are using the room, leaving you all your available power for your AV/Lighting.
Not quite. The armour of the SWA must be earthed so that if anything goes through it, e.g. a fork, it causes a short and the CPD operates - an MCB is fine. You can use 2-core SWA and use the armour as the CPC, by installing banjos on the glands each end and connecting to the earth blocks at both CUs, or use 3-core swa and earth both the third core and the armour at the house end, and only the third core to the garage CU earth block.

The RCD/RCBO is only needed to protect sockets that may be used to power outside equipment and is best sited the garage consumer unit. You do NOT want two RCDs inline on the same circuit since this can cause discrimination problems - either or both may trip, and having an RCD in the house means you have to go back to the house to reset it if it trips. If you do have an RCD (or RCBO) on the circuit in the house, then you don't need an RCD in the garage.

So the standard approach is something like a 40A MCB in CU in the house, linked to SWA cable of a suitable size for the 40A MCB rating and voltage drop over the length required at that load, into a small CU in the garage with a 30mA RCD feeding a MCBs for the ring (32A) and lighting (6A).

This work must be notified to LABC under Part P of the building regs, before you start, and they will need to inspect and test it (third party inspection is not permitted under building regs, unless the LABC subs it out - and then they cannot pass the cost onto you, see link below)

http://www.diynot.com/forums/viewtopic.php?t=60746
 

lone ranger

Novice Member
2.5mm swa cable can carry 28A, so it wouldn't be suitable to use the 32A. As it's only a short run the voltage drop would be acceptable. How hard would it be for you to replace the cable with something more substaintal? I would strongly advise you to do this, as even if it is capable of running what you require you won't have much scope for upgrading in the future if you so desire. Using 4mm would allow upto 38A and 6mm 49A. I'm assuming you have fitted a seperate consumer unit into your garage rather than having everything all running from the same breaker? I.e lights seperate from sockets? Also you will need to get your installation inspected once it's finished.
I have not fitted a consumer unit in the garage, the swa from main consumer unit has its own dedicated 20amp mcb/rcbo in it for the sockets and the lights are already there from before which are on the same circuit from the extension lights, but I am thinking about upgrading the cable to 4 or 6mm now.:thumbsup:
 

chrisboy1981

Standard Member

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