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Cable size to workshop

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Cable size to workshop

Postby MattS » 12 Oct 2017, 21:26

Having electrics put in the garage next week. Got a friend's partner doing the fitting so it's all properly signed off but I'm digging and laying cable! He's advised 4mm cable. The run is fairly short only around 7m in length.

I've done a bit of reading and wondering if I should up the cable rating? I am not great with electrical stuff - for future use of machinery will this be sufficient? Screw fix don't seem to to 10m of 6mm which is frustrating!
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Re: Cable size to workshop

Postby Robert » 12 Oct 2017, 22:25

The first hit I found on google

http://www.lightwiring.co.uk/lighting-c ... t-ratings/

Has a current rating for 4mm T&E of 30 amps. The rating varies with how the cable is installed so I'm guessing your use.

30amps at 230V is a theoretical 6.9kW maximum load. Motor loads are different to say electric heaters so downgrade that a bit - maybe 5kW of motors running. There are other factors to consider as well but don't want to add confusion. Is 5kW enough?

My workshop is on 4mm steel wire armoured which has a higher rating than twin and earth.

You could run 2 x 2.5 if you want a cheaper way to have a bit more capacity.
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Re: Cable size to workshop

Postby Doug » 12 Oct 2017, 22:33

4mm doesn’t sound much but it depends on what machinery you are going to have, I started with 6mm but as the machines increased I up’d the cable to 16mm.
I definitely go for a higher spec than you think you need, no one ever said I hate having this spare capacity
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Re: Cable size to workshop

Postby 9fingers » 13 Oct 2017, 08:04

Google for TLC cable calculator.
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Re: Cable size to workshop

Postby wizer » 13 Oct 2017, 09:36

What are the regs on depth under ground? Does it have to be armoured inside a duct?
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Re: Cable size to workshop

Postby MattS » 13 Oct 2017, 14:33

Thanks all, I've checked that calculator and reviewed figures for some machines etc. and then spoken with the sparcy again and decided to stick with the 4mm.

wizer wrote:What are the regs on depth under ground? Does it have to be armoured inside a duct?


Tom it's running along the side of our house fixed to the brickwork and then when we get to the corner there is a junction box before going into armoured cable dug to a depth of 400mm with tape over under the lawn. I believe regs quote a depth deeper than 400mm but sparky advised that would be OK.
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Re: Cable size to workshop

Postby Tusses » 13 Oct 2017, 14:54

remember your Kettle will be up to 3Kw ! .. just 2 of those and you'd need 6mm

As Doug said .. more is better.
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Re: Cable size to workshop

Postby MattS » 13 Oct 2017, 15:41

Tusses wrote:remember your Kettle will be up to 3Kw ! .. just 2 of those and you'd need 6mm

As Doug said .. more is better.


:lol: No kettle, it's a short walk to the kitchen. He reckoned the 4mm in this situation would give around 8.5kW
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Re: Cable size to workshop

Postby paulrees1 » 13 Oct 2017, 16:16

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Last edited by paulrees1 on 13 Oct 2017, 17:52, edited 1 time in total.
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Re: Cable size to workshop

Postby 9fingers » 13 Oct 2017, 16:25

Ok You should be fine with starting a 3hp induction motor with that but you will see some light dimming on start up. Once it is running, no problem as the current drops right off.
It is pretty rare in a one man shop to run more than one big motor at a time.
I would suggest that your sparky fit a type c breaker in the house and on the ring main inside the workshop

For others reading this, it is the short length that is allowing the use of such a lightweight cable.

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Re: Cable size to workshop

Postby Tusses » 13 Oct 2017, 17:20

for me the short run would be a reason to take advantage of the lower cost of upgrading ?

TS , Dust extractor. Compressor May come on. Heating in the winter ....

I guess I'm bias as I'm on a long run and slightly under powered. But there wire was here before me.
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Re: Cable size to workshop

Postby MJ80 » 13 Oct 2017, 18:58

I would put the biggest cable in you can. Not being funny but 4mm is good for diddley. You have your lights, sockets and whatever else you are running.
If you know the length you need any electrical wholesaler will be able to sell you lengths by the meter. If it is gong in ducting it still should be nyyj cable.
Cable really coats nothing. Changes in the future do
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Re: Cable size to workshop

Postby Deejay » 15 Oct 2017, 09:05

wizer wrote:What are the regs on depth under ground? Does it have to be armoured inside a duct?


Morning Tom. Good to see you again.

I don't think the reg's are very specific about burial depths. If I recall correctly from when I laid mine they say something like ' at a depth where it is unlikely to be damaged'.

I laid mine across the lawn about two spits deep in a thin bed of sand to prevent stone damage. I put some warning tape (like http://www.protecdirect.co.uk/Site-Equi ... 0oQAvD_BwE ) immediately on top of the cable and again at one spit down.

SWA doesn't need ducting unless you want to make replacement easy. If you do run ducting, leave a spare fish wire in there.

Cheers

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Re: Cable size to workshop

Postby HappyHacker » 15 Oct 2017, 19:51

For the little extra cost I would go to 6mm or even 10mm. You will only put it in once if you go for the larger size. As has been said SWA can be buried direct but be careful if you have stony ground to bed it in sand. If the sheath gets damaged the steel armour can rust and over a long period of time cause problems.
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Re: Cable size to workshop

Postby RogerS » 15 Oct 2017, 21:26

HH has beaten me to it.

Put in a thicker cable. Over that distance the additional cost is negligible.
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Re: Cable size to workshop

Postby memzey » 15 Nov 2017, 21:03

I used 16mm armoured cable in the run to my shed (about 80’). May seem like overkill but my planer thicknesser draws 96 amps on startup so I didn’t have much choice. I also run a 2hp extractor with machines at the same time so better safe than sorry. You should consider what the highest draw you are likely to need in future is, add 20% to that and use the appropriate cable. It may cost a bit more but it won’t seem pricey when compared to a full rewiring job if you go too low. Just my 2p worth.
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