Adding sockets in the garage

8 Jul 2008
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United Kingdom
I have a garage attached to one side of the house. One of the house ring mains serves two bedrooms and one socket in the garage. It is protected by a 32A MCB and a 30ma RCCB trip. Under part P I think I can add sockets but cannot go back to the consumer unit. I have no spare MCBs anyway.

Can I replace the socket in the garage ( on the ring main) with a Garage Consumer Unit having 16A + 6 A MCBs and run radial spurs from there.? Will these be 'Fused Spurs, if so how many sockets can I put on each and how many can I have? Finally, will the main RCCB still provide good protection or should I buy a CU with a RCD in it. Thanks
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In my opinion you would have been better continuing the topic you started the last time rather than starting up a new topic.
OK , so I am learning how to use the forum. I did not think I would get any more advice on that topic so started another, hopefully with better understanding of the problem. Extending the ring I don't think will give me enough distance before voltage drop is a problem. That seems to be the greater of the limiting factors, not shock protection. Also a 32A MCB I think will allow me 33m of ring, wheras a 20A MCB will allow 55m. But even that is not really long enough having used 30m from the CU to the bedrooms and back. I would have to obtain a Crabtree Type 1 MCB of 20A is not easy and I am probably not allowed to fit it. So the new question was a different approach. I hope I will get some useful advice, not just comments on how to use the forum correctly. But sorry.
Well the only reason I say it is to save everyone asking you the same questions again and you having to answer them again.

I don't understand how you are getting the ring lengths you are talking about. My quick and rough calculations would seem to indicate a worst case at 84m to still be within 3%. How are you working it out?
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Have a look at this extensive article, right at the end in the Appendix, Table A12. 2.5mm2 ring on a 32A MCB Type 1 is listed as Maximum length of 33m and 20A MCB Type 1 53m. Am I reading this information incorrectly?
OK - two things. First my initial quick calc had a basic arithmetic error and was out by a factor of 2. Secondly your link shows a table where the assumption is that the complete 32A load is placed at the middle of the ring - furthest from both ends.

A ring usually has a distributed load so will not have such a large volt drop. Which is why the On-site Guide specifies 84m max. Actually I guess this may be changed in the 17th edition version when it comes out, as the volt drop allowed for non-lighting has increased from 4% to 5%.

However here's the way to work it out. 1 m of 2.5mm(2) cable has a line plus neutral resistance of 14.82 mOhm. So voltage drop at the extremity of a ring where all the load is applied at that point is

(14.82 x L x I) / (1000 * 2)

L is length in metres and I is current in amps. The 2 is because there are 2 cables in parallel back from the extremity to the supply.

In your example (which is not allowed because it has 2 spurs from the same point on a ring, and spurs froms spurs) the max current you could draw at the exremity is 22A (16A+6A).

So the voltage drop would be 0.163 x L.

The maximum length would therefore be 42m at 3% (6.9V) or 70.5m at 5% (11.5V). As this is a ring final it does not really represent a lighting circuit and so this would give 70m.

But ring circuits are based on assuming a distributed load, so it is hard to see why you don't just go with the recommendation of the On-site guide convention circuits. The only caveat may be if the cable gets buried in insulation somewhere.
BS3036, thanks for that detailed answer which I have worked through to understand. You have said the design that I was intending is not allowed so again I look at just continuing the ring main. Using the formula (14.82 x L x I) / (1000 * 2) with the existing MCB rated at 32A I would get a ring length of 38.8m. but 62m is the MCB was 20A. I do not have the benefit of the On SIte Guide nor is it in my local library. From you other comments you said the On Site Guide indicates that a ring of up to 84m is allowed subject to certain conditions. The cables are going to be clipped on a thermalite wall not touching. Will the existing 32A MCB be acceptable with 84m 2.4 T&E final ring main then? Would any individual spurs , one per Socket, be allowable on top of that. If I did need to calculate for voltage drop of single spur lengths presumably I would work out those out as single lengths and add them onto the voltge drop on the ring main ( which is now allowed to be longer that my calculation produced).. That bit is a little confusing.

WIll the main RCCB of 63A 30ms be fine in all this?

Note that afaoct the L in BS3036's calculation is the distance from the supply to the midpoint so you need to double his figures to get the max total length.
Thanks Plugwash, I had it right first time but doubted myself at the last minute! Makes it even easier to be happy with 84m.

Voltage drop for a spur would just be added on.

Clipped direct is one of the best ways for heat dissipation.

Sorry for brief answers but I'm supposed to be writing software!
I have done another measure of the amount of ring used so far in the bedrooms and upstairs landings and it could well be about 38m. Ideally I would like to use a further 54m in the garage. Total then would be 92m. Much of this is being used as drops as I prefer to do this, than run horizontally between the sockets across the walls. The On Site Guide gave 84m on a 32A MCB. Is there a figure for a 20A MCB as the load on this circuit is never going to be very heavy at any one time. All the sockets and length is the result of trying to get a good number of sockets, with a variety of placement. I am well within the 100sqm allowance. Is a ring protected by 20A MCB acceptable?
Many thanks for your advice.
The thing about the 'conventional circuits' is that if you meet their limitations then you don't need to feel terribly guilty about not doing the calculations. There is no specified conventional 20A ring main in the On-Site Guide.

So you have to do the calculations to back up your design.

These would be:

- Is the earth fault loop impdeance sufficiently low to cause disconnection within 0.4s?

- Will the cpc withstand the heat generated by an earth fault before the supply is disconnected?

- Will the cable withstand the heat generated by a short circuit before the supply is disconnected?

- Will voltage drops be within specification?

A quick back-of-an-envelope (literally) calculation suggests you would be OK with a 20A MCB. To be really sure I suggest you need a measurement for Ze, to know what type of supply you have; TN-C-S, TN-S or TT, and a manufacturer's graph of let-through energy for the MCB.

I don't know your current thinking on notification, but as you can see it may be worth getting an electrician involved. (though the last electrician I encountered said "I don't know why we have to learn all this b*****ks, we never use it on a job!". I guess he was an exception.)

If there are any electricians reading this, I would welcome corrections.
BS3036 You are being incredibly helpful. Thanks.

I understood some of your requirements, but have been unable to obtain the disconnect graph as the Type 1 is no longer available. For the moment I am unsure of most of the rest, but could get it ascertained. However, surely if one designed a circuit based on 32A protection, using 2.5 T&E and then protected the same circuit with a 20A MCB all your questions would be met. There would be no intention to reduce the cable size to 1.5mm

Considering the voltage drop issue, using your original calculation with 1 m of 2.5mm(2) cable having a line plus neutral resistance of 14.82 mOhm.

(14.82 x L x I) / (1000 * 2)

The max current that could be drawn using a 20A MCB would be 20A so the voltage drop would be 0.1482 x L. Working with the 4% allowance of 9.2 v this would give a length of 62m

Plugwash pointed out that this is the distance from the supply to the midpoint so the whole ring length could be 124m

On the
Table A12 it gives 53m for a 2.5 mm 20A MCB and I presume that is also the distance from the supply to the mid point. All the other Shock voltage and 0.4sec disconnect times allow for noticably longer lengths when the 2.5mm ring circuit is protected with a 20A MCB compared to a 32A one. So the 92m that I am looking for would fall well within either of these calculations or guidance tables. Hopefully I will hear from you in the morning
To summarise:

- You now understand why extending the ring is ok, but trying to add a CU onto the ring is not.

- Yes, I think that in all probability a slightly longer circuit on a 20A MCB would be safe.

- But you are left with the notification problem. Only you, in contact with the BCO, can determine the level of proof they will require, as it seems to be ad hoc.

- No-one else can declare your design safe remotely. There has to be some testing done which involves having calibrated instruments and being there on the spot.

Good luck with it.
BS 3036
I am continuing to think this through.

1. I could shorten this ring further by putting one or two of the sockets on fused spurs. This might bring the total length within the 84m allowed and could then remain protected by the 32A MCB. Would the the voltage drop on the spur would then be added to the voltage drop on the ring2. If so the ring allows for xx the total length of cable compared to a spur., unless I need to work out the voltage drop at the exact spot on the ring that the spur is taken from? SO, advice on the value of using a fused spur for some sockets would be appreciated.

2. I intend to notify the BCO that there will be an outside socket on this design. Do they need to be individually RCD protected as they will be on a house ring (or spur) protected by the 63A 30ms RCCB. If they do I will use a IP55 Twin RCD socket. When I run the cable through the wall and across the cavity are there any special requirements for sleeving the cable.

3. If I run the lighting circuit off a switched spur as you suggested does voltage drop need to be calculated for that as well?

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