MCB's Protecting Spur

20 Aug 2008
Reaction score
United Kingdom
Hi - I have an attached garage with a double socket fed by a spur in single 2.5mm. Spur is from a junction box in the ring-main (about 4 meters away in the hallway of the house).

I've asked my electrician to make the system safe for a tumble dryer and washing machine to be used in the garage. His solution:

Garage consumer unit with RCD fed by the 2.5mm spur
2 x 16A MCB's in the garage consumer unit, one feeding a socket for the washing machine and another feeding a separate socket for the tumble dryer

My questions are:
1. Does this adequately protect the 2.5mm spur cable to the junction box
2. Is this work notifiable
3. Would connecting both washing machine and tumble dryer to one of the 16A MCB's be OK, assuming tumble dryer and washing machine are unlikely to draw more than 16A long-enough to trip the MCB, and that 2.5mm is OK to handle max. 16A
4. If none of the above are OK, are there any other alternatives other than new ring or radial from the main house consumer unit

Advice very much appreciated.

Ole Biscuit Barrel #7
Sponsored Links
That sounds a bit OTT to me. You could run another 2.5mm T&E cable to the JB and turn the garage spur into part of the ring. Then install 2no single sockets, one for each appliance.

You're allowed 1no double socket per spur so you could just fit 2no single sockets on a dual box to the existing spur (bending the rules a bit), one for each appliance. You shouldn't use a double socket because it's rating is not enough for both appliances.

No it's not notifiable because you're altering existing and your garage is attached

Now for the questions.

What else is fed from the ground floor ring?

Is it RCCD protected.

A better way is a new circuit but that is notifiable.
Thanks Pensdown.

Ground floor ring in the house is RCD protected at the main consumer unit and powers a number of aplliances - TV/HI Fi/Fridge Freezer/Dishwasher and some smaller appliances such as radio/ 3x 60W lamps/telephone/mrodband modem.

Can't extend the ring easily to the garage I'm afraid because the spur cable is routed behind plasterboard and through the ceiling cavity, so additional 2.5mm run would mean cutting new route.

My main concern is that with the electricians proposal, I think up to 32A (2 x 16A) could go through the garage consumer unit before either of the 16A MCB's tripped, and this would be above the rating of the 2.5mm spur cable, which I think is max 27A.

What about having both appliances on a single 20A MCB at 20A, enough to protect the cable and probably enough for washing machine and tumble dryer without frequent trips?

shouldnt a spur only supply 13a outlet?

2 16a mcb will allow 32a through single 2.5?

sure its non-notifiable?
Sponsored Links
ezekiel - I think the answer is certainly yes if it's a spur off a socket.

My question is if the spur is from a junction box with no other load on the junction box, can it supply more than 13A, and what is the maximum rating MCB I can get away with without risking overloading the 2.5mm cable? I think 2 x 16A MCB's does not give sufficient protection, but would 1 x 20A MCB?

Try to ignore the MCB ratings because it's the circuit design current that's important. Once that's been established the cable is selected to carry the current and then the MCB is sized to protect the cable.

So your design current on the spur cable assuming both appliances are running flat out on heating could be something like 20amps. The cable is OK at 27amps (assuming no down rating) but a double socket is only good for around 16amps (IIRC) which is why I suggested 2no singles which are rated at 13amps each.

Once the water is heated washing machines don't draw much current but tumble dryers.

Although in principle it should be OK, using the existing spur cable puts quite a lot of load at one point on the ring. If the freezer and dishwasher are on the same leg back to the CU you could overload that section of the ring.

So without further investigation the only safe suggestion I can make is a new circuit.

And don't go with your sparks. You don't need another RCCD and it's a ridiculous suggestion to have a small CU fed from a spur with 2no 16amp MCB's to feed two sockets.
ezekiel - I think the answer is certainly yes if it's a spur off a socket.

My question is if the spur is from a junction box


but thats aspur still surely ( need ban or sparks me think)
Thanks both.

Appreciate that correct solution would be to extend ring, and that 13A limit on the spur is the recommendation.

I might try 20A MCB which should be sufficient protection for a 4 metre run of 2.5mm. If I start getting trips, I'll have to go for the option to extend the ring, or run a new leg from the main consumer unit.

Washing machine was originally installed in the kitchen with dishwasher and fridge freezer and all off separate 13A FCU's. Tumble dryer has always been on the socket from the spur in the garage - so total loading on the kitchen leg won't have changed with this new set-up. I've just moved the washing machine from the kitchen to the garage.

Thanks again.

In summary.

The advise from your sparks does not meet the requirements of BS-7671 2008 and it is a ridiculous suggestion.

You can feed one double socket from your existing spur within the above regulations without any additional RCCD's or MCB's.

You can plug your tumble dryer into one side of the socket and your washing machine into the other, all within the above regulations.

A spur can be taken from a socket, a junction box or directly from the MCB.

The work is not notifiable unless you're installing a new circuit.

Unless you use single sockets you will overload the double socket if both appliances are used together.

You may or may not overload one leg of ring.
Thanks for the good summary Pensdown.

Having done a little more research I think the first route Ill take will be as follows.

2 separate single sockets through a 16A MCB. I understand type B trip characteristics would mean the MCB would trip within 1 hour with a continuous load of 23A, which will hopefully be enough to handle washing machine and tumble dryer peaks, and provide sufficient protection for the 2.5mm spur able.

20A MCB would trip within an hour at 29A continuous load, which I think may be too much for 2.5mm in a wall.

If I find that the MCB is tripping frequently I'll have to go for ring extension or new circuit/radial from the CU.

Thanks again for good advice.
You could always split the ring to create two equally loaded radials and fit two 20A mcbs in the CU.
What else is on the circuit though? You may end up with the whole kitchen sockets on one of the 20A MCBs.
I don't like the idea of what is being suggested in the OP, it certainly does not meet the requirements of BS7671.
A double socket is not rated at 26A, I for one when desiging look at where the high loads are to be installed and install a single outlet for each of them.
The two x 13A single sockets may be a better option providing they are either fed by individual spurs from the ring or the ring is extended around the garage. What else is on the ring circuit though?
These options will be no good if the garage is detached as a single point of isolation is required. If this is the case then a proper sub main feeding a garage CU is probably the best way, which I am thinking would probably be the best method anyway.

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.

Are you a trade or supplier? You can create your listing free at DIYnot Local

Sponsored Links