Workshop Power Calculations

[eSCHEn]

Hi there.

I've finally got my workshop *cough* shed *cough* on a nice concrete pad and its roof redone. Next job is to run proper power to it. I know there's a few sparks who post here and wondered if I could pick their brains?

When I acquired the shed it originally was fed from a spur off a spur off a ring via T&E "buried" about 2" down in the soil with no protection. This then fed a four double socket radial and 5' florescent in the shed! :O

I'd like to get a high power socket into the shed as well as some 13A doubles and a few lights. I've been doing the calculations but I'm wondering if I'm trying to get too much out of a household CU.

Circuit 1
32A socket (for welder - http://www.mig-welding.co.uk/forum/threads/auction-bought-tig-anyone-know-what-it-is.48477/ first pic).
Demand: 30A is welder's max draw but socket will be 32A
Diversity: 100%
Total => 32A

Circuit 2
Radial for normal sockets.
Demand: 20A
Diversity: 100% (from http://www.tlc-direct.co.uk/Book/6.2.3.htm)
Total => 20A

Circuit 3
Lighting.
Demand: 200W (not decided yet but /very/ unlikely to be more than 200W total)
Diveristy: 90%
Total => 0A (miniscule, < 10^-4)

I make that 52A total.

I'll be feeding from a spare way in my 10way CU (100A RCD with no split load) via a 20m run of SWA into a small CU in the shed.

My questions are:

1. Do the calculations look right?
2. Can I do this on a household CU?
3. I've calculated the cable as 10mm^2 XLPE SWA (using http://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html), there seems to be conflicting information on whether you can use the armour as the CPC for 10mm. Does anyone know for sure?
4. Looking at the feed to the CU the supply looks like TT, would you export an earth at the shed given that it is 20m away? The ground is unfortunately dry and sandy so it'll be a long earth rod if it's needed (and a mare to do).

Thanks!

 

[mfletch]

I ran 10mm armoured cable with a extra earth it's connected to the cu on a separate fuse and trip it's 32amp supply

You say the welder is 30amp but in reality it will only use 20amp once running I've used my welder and compressor and lighting with no problems

 

[eSCHEn]

The welder's a pain for tripping the current MCB in the outhouse (32A B curve) when you first power it (ie plug it into the socket). One you reset the trip it'll weld up to 60% (the highest I've needed it at) and doesn't trip again. I was going to use a C curve on the new socket to try and stop the tripping from the inrush.

I've given up on a compressor on the same circuit - it's just too much I think. Gutting really as SWMBO really wants one as well so I've got "permission" I've a 3.5HP engine here so was going to make my own compressor from that.

 

[Kayos]

I ran a 16mm straight after my meter, via a breaker to the garage, which has its own board. prob overkill as I only have a 16amp socket as the biggest load but the cable was cheap and I may want a 32 in future, no calculations were attempted before I fitted it

 

[eSCHEn]

Hmmm. I hadn't considered that you could split the meter output and run directly to a new CU. I suppose the DNO isn't bothered as you're not using anything that's not metered and the 100A main fuse before the Henley block protects the supply.

 

[Kayos]

I work with cut outs and meters every day so didn't give it a second thought really, I did check I had 100amp fuse and upgraded all the tails to 25mm as well

 

[Robotstar5]

I work with cut outs and meters every day so didn't give it a second thought really, I did check I had 100amp fuse and upgraded all the tails to 25mm as well

Wish mine was, service fuse is only 60A

 

[Kayos]

Wish mine was, service fuse is only 60A

well I know it's illegal to downgrade a fuse, will check the legalities of putting a bigger one in

 

[brightspark]

Wish mine was, service fuse is only 60A

it can soon be an 80

 

[Bill Edwards]

You are probably calculating that you need far more power than you actually do, and I don't see why a compressor should be any problem.

Am I right in suspecting that there will only ever be one person working in there at a time? If so, you aren't going to be welding flat out using 30a and using a full 20a through the sockets, and running a compressor at the same time.

The welder is by far the big thing, but if you're alone that'll also be the maximum at any point. You just need to make sure you have the compressor switched off when welding as if it cuts in (i.e has a tiny leak) while welding it could end up being too much.

If there will be others in there then yes, you can have to allow for everything at full draw, but if only one man is working then it's damn hard to weld, use an air tool and another 20a of tools all at the same time.

 

[mike os]

on your own you are never likely to be using socket at ful tilt and welding at the same time and have all the lights on...


I would say at most 50% on sockets so 42a if you have a fan heater on and running welder at max...

if compressor starts up at the same time as you start welding I guess you may have a trip, ..... but what are the odds?

 

[Sidewinder]

Just stick a 32A supply in, you'll be fine.
Do not calculate cable ratings @ 90 deg C though, only 70, the cable may do 90, what's connected won't.
I've not checked your links to check though.
Volt drop will be your biggest issue for the welder & compressor.
Just make sure your earth fault loop impedance is low enough for automatic disconnection.
A 16A radial will almost certainly do you socket wise, unless you are going to run heating from it.
As you say the lighting is almost negligible.
TT will probably be easier for you.
SWA as the cpc (earth), up to 95 mm sq you are fine using the SWA, as long as it is correctly terminated.

 

[Cyberprog]

For my workshop here, I was going to install a 10mm2 SWA cable, but I've not gotten round to it yet. I wouldn't use 2-core, I'd use 3-core to provide a separate CPC. At the new house I suspect the run from the board to the new workshop CU will probably be very short, so I may well just run some 16mm2 or 25mm2 depending on what offcuts my local wholesalers have left

 

[Sidewinder]

Why 3 core?

 

[sparkysy]

2 core swa was finished in production years ago 3 core is the most common now! You should still be earthing the steel wire though even with a core as the earth for mechanical protection!

 

[Sidewinder]

2 core swa was finished in production years ago 3 core is the most common now! You should still be earthing the steel wire though even with a core as the earth for mechanical protection!

Wrong.
2 core SWA is still available, and a damn site cheaper, and suitable, if competently installed especially for larger sizes, up to 95mm sq.
The wholesalers may only stock 3 core, but that does not mean 2 core is not available.
However the rest of your post is basically accurate.

 

[brightspark]

i use 3 core up to 4mm or even 6mm anything bigger and its 2 core theres a hell of a price hike when your putting 25mm in@ 120 meters length the cables readily available at cleveland cable company or any wholesalers can get it in within the day

 

[Sidewinder]

To be honest, it depends on the urgency & convenience for me also brightspark.
When you get to 25+ the cost saving is significant.
Mind I designed & installed a job with 2 core 2.5 a while ago as it was to plastic enclosures so the armour continuity had to be retained, "daisy chianed".
Piranah nuts rule!
There was about 2500 m of the stuff over all the circuits, so the saving in labour not terminating the 3rd core, and the saving in Cu was significant.

 

[Kayos]

i use 3 core up to 4mm or even 6mm anything bigger and its 2 core theres a hell of a price hike when your putting 25mm in@ 120 meters length the cables readily available at cleveland cable company or any wholesalers can get it in within the day

Slightly off topic but we used to have a yard near Cleveland cables, immense place with probably millions of pounds in stock, and the security to match!

 

[Cyberprog]

Yep, I'd agree, urgency and convienence are usually a factor, and most isn't urgent and I'm a bit lazy, so I spec 3-core Mind you, I hate SWA with a passion, so try and avoid it wherever possible!