Four Double Sockets on a 10A Radial.

APHSLTD · 16 Sep 2014

Any issues with this? Having done a quick google have read some say minimum MCB size for sockets on a radial should be 20A (some disagree).

This is for a garage conversion into a kids play room. Total circuit is 22M of 2.5 T&E. Two sockets will be in an external wall and cables will run seperatly each in their own PVC conduit behind 120mm of PIR board.

I will be putting the conduit in the 25mm gap between the OSB & insulation rather than behind the plasterboard, due to not wanting to rebate the insulation or effect the VCL. Due to this I have downrated the cable to 13.5A rather than 20.

Load on the circuit will be TV, XBOX, router, & a couple of table lamps (610W) Have allowed a further 1200W for the hoover being plugged in now & again so 1810W in total 7.9A.

Is this ok or will I need to up the cable size and fit a 20A breaker?

The reason I need two sockets in this wall is that it is the only realistic place to position a bed when it eventually becomes an additional bedroom.

APHSLTD · 16 Sep 2014

A bit more digging 433.1 looks like 20A it is then.

securespark · 16 Sep 2014

Yeah. I would uprate the cable (only pennies in the long run) and put it on a 20A radial.

bernardgreen · 16 Sep 2014

APHSLTD said:
Any issues with this? Having done a quick google have read some say minimum MCB size for sockets on a radial should be 20A (some disagree)..

Why should the regulations state a minimum MCB size for a circuit ?

If someone chooses to use 10mm² cable to carry 3 amps then a 6 amp MCB will work as intended to protect the cable. ( if a very long cable is carrying 3 amps then 10mm² may be necessary due to voltage drop. )..

That said if the room will have a change of use in the future then as said upgrade the cable to a larger size and fit 20 amp MCB for future use.

APHSLTD · 16 Sep 2014

bernardgreen said:
APHSLTD said:

Any issues with this? Having done a quick google have read some say minimum MCB size for sockets on a radial should be 20A (some disagree)..

Click to expand...

Why should the regulations state a minimum MCB size for a circuit ?

If someone chooses to use 10mm² cable to carry 3 amps then a 6 amp MCB will work as intended to protect the cable. ( if a very long cable is carrying 3 amps then 10mm² may be necessary due to voltage drop. )..

That said if the room will have a change of use in the future then as said upgrade the cable to a larger size and fit 20 amp MCB for future use.

Room will be a tv/xbox room for the kids, in the future when they've outgrown it then it will become a spare bedroom so no real increase in load.

The only reason I didn't just extend the ring was the fact I had to put cables behind the insulation.

Was just unsure on the regs & if LABC may have an issue with it.

Thanks for the advice.

JohnW2 · 16 Sep 2014

APHSLTD said:
A bit more digging 433.1 looks like 20A it is then.

As has been suggested, 433.1 does not impose any particular minimum (20A or anything else) on the rating of MCB or cable in a circuit.

If the designer believes (as you appear to in your situation) that it is unlikely that the load will exceed 10A by a small amount for long periods of time, then to have a 10A circuit would be compliant with 433.1.

However, as others have pointed out, 10A is very little for a sockets circuit (less than the demand of some single loads which could theoretically be plugged in!), so I'm not sure how confident you could be that loads a little over 10A for long periods of time were always going to be "unlikely".

The designer obviously always has to do some (hopefully intelligent) guessing with sockets circuits, since (s)he has no control over what will actually be plugged in. Even with a 20A circuit, it would often be hard to be all that confident that it was unlikley that two 3 kW/13A loads would ever be plugged in to the circuit.

Kind Regards, John

Taylortwocities · 16 Sep 2014

Dear APHSLTD

The reason you may be concerned is because you need to understand what the MCB/fuse in teh consumer unit is for.

It is there to protect the cable so that the MCB/fuse will disconnect the supply before the cable melts and catches fire/.

Maybe you are looking at the issue from the wrong end.

The first thing you do is to look at what the operating load will be.
Lets say it is 17 amps:

Select a CPD that is more than the operating load (so 20amps is a standard size)

Then you'll need to look at what cable can carry at least 20amps (given the environmental conditions for the cable). That will probably be 2.5mm² if twin and earth.

Then you'll need to check the voltage drop is within spec. given the operating load and the length of the cable run.

A full explaination can be found
HERE

Hope this helps

JohnW2 · 16 Sep 2014

Taylortwocities said:
The reason you may be concerned is because you need to understand what the MCB/fuse in teh consumer unit is for. ... The first thing you do is to look at what the operating load will be. Lets say it is 17 amps: ... Select a CPD that is more than the operating load (so 20amps is a standard size) ... Then you'll need to look at what cable can carry at least 20amps (given the environmental conditions for the cable). That will probably be 2.5mm² if twin and earth.

I have to say that I got the impression that the OP does understand, even though it may sound as if he is "starting with the cable" ... he estimated his maximum load as 7.9A, selected the corresponding OPD (10A) and then confirmed that, with his installation method, 2.5mm² cable was OK for with that OPD (CCC 13.5A).

As I've been saying, the real question seems to be how confident he can be that the load is never likely to exceed 10A for long periods.

Kind Regards, John

APHSLTD · 20 Sep 2014

Thanks for the info guys. My main concern is that the cables are behind 110mm of kingspan.

They are inside 20mm conduit in a 20mm gap between the kingspan and OSB of the timber frame.

Looking at the tables they should be at the plasterboard side to allow me to downrate to 20A

So I have erred on the side of caution and treated them as fully enclosed in insulation and downrated to 13.5A

The voltage drop worked out at roughly 3.6V so fine.

I have no concerns about nuisance tripping of the MCB as I am fully confident the only things plugged into the circuit will be TV, Xbox hoover etc (while we're living here anyway) so load will always be under 10A.

The reason for the 10A MCB is to protect the cable from overheating due to the insulation.

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Again thanks for the info

APHSLTD · 20 Sep 2014

JohnW2 said:
Taylortwocities said:

The reason you may be concerned is because you need to understand what the MCB/fuse in teh consumer unit is for. ... The first thing you do is to look at what the operating load will be. Lets say it is 17 amps: ... Select a CPD that is more than the operating load (so 20amps is a standard size) ... Then you'll need to look at what cable can carry at least 20amps (given the environmental conditions for the cable). That will probably be 2.5mm² if twin and earth.

Click to expand...

I have to say that I got the impression that the OP does understand, even though it may sound as if he is "starting with the cable" ... he estimated his maximum load as 7.9A, selected the corresponding OPD (10A) and then confirmed that, with his installation method, 2.5mm² cable was OK for with that OPD (CCC 13.5A).

As I've been saying, the real question seems to be how confident he can be that the load is never likely to exceed 10A for long periods.

Kind Regards, John

Yes spot on I did start with the cable but to get to 20A would mean 6mm I would rather exclude those two sockets than run 6mm and just extended the ring for the other two sockets in the room.

JohnW2 · 20 Sep 2014

APHSLTD said:
Thanks for the info guys. My main concern is that the cables are behind 110mm of kingspan. ... They are inside 20mm conduit in a 20mm gap between the kingspan and OSB of the timber frame. ... Looking at the tables they should be at the plasterboard side to allow me to downrate to 20A ... So I have erred on the side of caution and treated them as fully enclosed in insulation and downrated to 13.5A

You may be being a bit pessimistic. If you look at Table 4D5 (not 4D2A) of the regs, you will see that the current-carrying-capacity for 2.5mm² cable for Reference Method A ("enclosed in conduit in an insulated wall") is 20A, and what you are proposing does not sound 'as bad', insulation wise, as Method A. The only catch is that, although I can find no reference to this is any descriptions of Method A, the diagram for the corresponding Method #2 shows the conduit in contact with the back of the inner (indoor) side of the wall. I would be tempted to suggest that Method A would be applicable in your case, but I would welcome other opinions.

APHSLTD said:
The voltage drop worked out at roughly 3.6V so fine. ... I have no concerns about nuisance tripping of the MCB as I am fully confident the only things plugged into the circuit will be TV, Xbox hoover etc (while we're living here anyway) so load will always be under 10A. ... The reason for the 10A MCB is to protect the cable from overheating due to the insulation.

Even using the pessimistic current-carrying capacity figures, the cable would obviously only even try to overheat if the current drawn considerably exceeded your estimate of "always under 10A" - so the issue would only even arise if your estimate were wrong.

You have been through a totally correct design process. Even with the conservative/pessimistic assumptions, the 10A MCB would prevent enough current flowing for long enough to do any thermal damage, no matter what loads were plugged in. The only real question is whether 10A always is going to be adequate for you. If so, then that would seem fine. However, the comments above might enable you to increase the MCB to 16A, or even 20A (depending on what others think!).

Kind Regards, John

ban-all-sheds · 20 Sep 2014

APHSLTD said:
The reason for the 10A MCB is to protect the cable from overheating due to the insulation.

MCBs are for fault protection - you may not use them to limit the load.

You should know that.

JohnW2 · 20 Sep 2014

ban-all-sheds said:
APHSLTD said:

The reason for the 10A MCB is to protect the cable from overheating due to the insulation.

Click to expand...

MCBs are for fault protection - you may not use them to limit the load. You should know that.

I think the OP understands that, but perhaps didn't choose his words all that well. He estimates the load as "always under 10A" and, if you read what he's written, he selected the 10A MCB to protect the cable on the basis of a (I think pretty pessimistic) assumption of a CCC of only 13.5A.

The purpose of an OPD is for over-current protection, which encompasses both fault protection and overload protection. In the case of a sockets circuit, the designer can never be certain that an overload will not arise, even if, as in the OPs case, the anticipated/design load is well within the capabilities of the circuit.

Kind Regards, John

westie101 · 20 Sep 2014

you may not use them to limit the load.

I think that you had better inform all the caravan sites in the UK that limit hook-up loads that way and all the DNOs that use fuses for the same use. To name just two applications.

Surely using an FCU with a 13A fuse to feed sockets is doing just that as well

JohnW2 · 20 Sep 2014

westie101 said:
you may not use them to limit the load.

Click to expand...

I think that you had better inform all the caravan sites in the UK that limit hook-up loads that way and all the DNOs that use fuses for the same use. To name just two applications. ... Surely using an FCU with a 13A fuse to feed sockets is doing just that as well

Indeed, and I think this essentially comes down to fairly pedantic semantics. The official line is that the OPD is there only to protect the cable from overload. However, one of the main ways in which it achieves that is by operating if the load connected to the circuit is such that it would otherwise result in a significant overload of the cable.

I think BAS's intended point was probably the (correct) one that one should not rely on an OPD to limit a 'foreseeable' load to a safe level - i.e. that the circuit should be designed to handle any load 'thought to be likely for a significant period' (which is precisely what the OP in this thread has done). However, as I said to him, with the best will in the world a designer can never be certain what loads may be connected to a sockets circuit - so, in such situations, one of the functions of the OPD is to limit what loads may be applied for significant times to an extent that adequately protects the cable.

So, as I said, I would say "essentially semantics".

Kind Regards, John

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Four Double Sockets on a 10A Radial.

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