Workshop Electrics

11 May 2009
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United Kingdom
I have read most strings on garages and workshops and have formulated this proposal. This is a stone built workshop detached from house with 1metre gap. I am not an electrician but an oepsirenced DIYer.

I need:

a supply to 2 Fluorescent lights, 4 x dbl sockets & 2no. 3kw kilns.


Run a 4mm T&E from the main house CU on RCD side of split board on a 32A MCB. Total length through house 10m plus 1m across gap (concealed in a 32mm PVC wastepipe 2.5m abocve ground). Cable will then run a further 3 metres through a coal shed to the garage terminating in a 2 way CU.
This CU (which I have) has a 40A 30mA RCD and a 20A D type MCB and a 6A MCB.
I propose to run a 2.5mm T&E ring from the 20A MCB to the 4 dbl sockets and use 2no. double pole 13A FCUs for the kilns. I will run a 1mm cable from the 6A MCB to the lights (in series off 1g switch)

How does this sound ?

My main issues would be:

Is 32A on a 4mm cable okay?
Would I be better to take the 4mm from the non RCD side of the main house CU?
Is 40A RCD in the garage CU okay if I have a 32A MCB in the house CU?
Is 20A D type okay for the ring ?

Your help comments woul dbe appreciste asap as i intend to do this on my day off on Friday.

Thanks for now.
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6kw = 25A so no - your 20A MCB running everything is not sufficient!
Type D are usually used for things such as welders and require a relatively very low EFLI for effective operation - you'll need to buy the right size MCBs etc. A pair of 16A MCBs (type B) would probably be better - one for each of the FCUs for the kilns.
There is little point in running in a ring and protecting it with a 20A MCB - if you deem 20A to be adequate then you can intall a 2.5mm 20A radial however I'd avoid using type D breakers. Might be better buying an RCBO for the socket circuit - see comment below.
What sort of current will be drawn from the socket circuit?
Lighting will probably be fine on a 6A MCB.
Overall you'll need at least a 4 way consumer unit (more is always good for expansion) - personally I'd think twice about protecting the whole premises by a single RCD hence using RCBOs for the socket circuit may be a better idea.
Using MCBs and a method of installation which doesn't require it to be RCD protected will be better for things such as the lighting and the kilns.
The supply size will definately need to be larger than 32A tho, 4mm will probably be too small tbh.
SWA is probably going to be the better way forwards for you with this.
Do you have any water / gas or other services in the workshop?
At least you don't have to worry about part p! I'd still recommend having a spark help you out as it will need to be tested upon completion to ensure it is safe.
Thanks, your advice is appreciated.

The only reason I proposed using the 20A MCB D type was that I already had it as it had perviously served a mitre saw that kept tripping the B type.

So - I shall use a 6mm off a 45A MCB unprotected at the main house CU then and also change the 20A for a 32A MCB.

The sockets may draw only about another 5kw max between them (fan heater, soldering iron, dehumidifier, radio, table lamp.)

I take it you recommend not RCD protecting the lights so that i can still see when other things trip ? I don't really want to go and buy a split board or to run a separate cable from the house CU so does it really matter ? My current garage CU is only 2 way so why could I not just have the sockets and kilns off the 32A ring I now propose ?

re SWA - isn't this really only needed if it is underground, I only have a 1m exterior bridge 2.5 m above ground to the outbuilding so I thought the pvc pipe as a conduit would be okay ?

There is no gas or water in the workshop.
(5kw + 6 kw) / 230 = 48A ish so adding a tad for the lights will mean you need a 50A sub main.
5kw - 20A> so you'll need a 32A socket circuit.
There is little point in RCD protecting things that don't need to be RCD protected - kilns might cause nuisance tripping.
If lights are RCD protected then they should be on a different RCD to the sockets/kilns, if RCD can be avoided alltogether for the lights/kilns it will be advantageous.
Your kilns will take 25A between them leaving you 7A for everything else if they were to be installed on a 32A circuit so better to do it right first time and have a larger consumer unit installed.

A 50A sub main run for 14m will mean you'll need something like 10mm, if it is concealed in a wall or partition anywhere along its length, subject to direct sunlight or if there is a risk of it getting gnawed by rodents then SWA or metal conduit will be the way to go.
How does the cable run through the house?
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The cable would be running through the roofspace of the main house and then on joists through the coal shed. There would be no insulation material to run under or through. If I understand you correctly I would need the following.

10mm SWA off 45A MCB non protected in main house CU.
CU in workshop (min. 50A non RCD with min. 4 Way)
32A RCBO for socket ring/radial
16A MCB for kiln 1 (with 13A FCU)
16A MCB for kiln 2 (with 13A FCU)
6A MCB for lighting

This sounds better.
Yep, you may be better looking at a 50A MCB if one is available for your CU, or failing that using a switch fuse.
My advice is still as above - get an electrician in to help you.

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