Voltage loss question

supply

Definately. At my last place I ran a manual Bridgeport, an Interact 1, J&S 540 & 1300 , Colchester Master and numerous small stuff off a 10mm SWA about 30 foot long. Not all at the same time mind. The lights dimmed a little if I got too enthusiastic.

AWEM

Hi Bob,

If you're going to the bother of digging in the SWA, put in the heaviest you think you could ever need! Nothing worse than having to dig it back out to go up a few sizes...

I ran mine (also about 30 yards) in 10mm buried about 18" below ground level for half its length, the rest clipped to an adjacent garage wall, have a

63A MCB (highest current allowed for the XLPE cable allowing for voltage drop and temperature constraints, otherwise it would have had to be 16mm, and I thought I'd better leave the remaining 17A of the board's fuse for household use!) and 100mA time-delay RCD at the "head end" in a 4-module enclosure fed from a Henley block on the meter tails - at the shed end I have the lighting non-RCD (other than the 100mA at the head end) and everything else on an 80A 30mA RCD - this way the SWA's protected from damage-related faults, and the lights should stay on if anything in the shed trips the local RCD (I didn't fancy groping in the dark with rotating lathe chucks etc. around!).

Anything over 20 yards and you should consider making the shed a TT installation with its own local earth (electrically separated from the elec. board's earth protecting the run of SWA), take a look on the UK-DIY FAQ "taking electricity outside" Wiki for more info

and remember it's legit. to DIY without Building Reg's notification if you're just replacing existing cables etc. with new of the same rating (honest, Guv, it was all there to start with, I just replaced the cables due to physical damage...).

The machinery wouldn't be the main issue for me (max 3HP motors on VFDs on my lathe and prospective mill), the planned next purchase TIG welder will be the big current consumer!

HTH, Dave H.

Many thanks to both of you for your replies. Seems I will need to study this deeper, and go as suggested, biggest cable possible. Its all a "Just in case" ,but sods law dictates if you dont do it, you will have to one day. I actually laid plastic pipe under the concrete base of the shed, and it will continue under the garden path, so threading cable should be a bit easier to do as opposed to burrying underground Thanks Bob

Hi Bob, just one minor point, if you're running the SWA in a conduit you'll need to de-rate it a bit, as there won't be as much thermal conduction into the surrounding earth due to the air in the pipe! Painful, ain't it? More painful, there doesn't seem to be an intermediate size between 10mm and 16mm SWA... If you decide to run anything else in the pipe with the SWA, *all* cables need to be insulated for the highest voltage present too, as far as Wiring Regs are concerned - and electromagnetic pickup between cables may be a problem.

Dave H.

I always like belt and braces, try 5.00mm (it should give a slightly lower voltage).

Bob: It's a question of money, and convenience, in your particular case.

As it is single-phase, you'll need to have a fuse or MCB at the house board to feed it all, and once you go much over 6mm cable then you'd be looking at a 40A or 63A breaker or fuse.

If it was me, I'd go up to 10mm or even 16mm, and look out for a length of either on the cheap from ebay or someone (contractor) at the end of a job with some spare cable to dispose of. The larger sizes are relatively cheaper as they are not as popular as the 2.5mm, 4mm and 6mm.

If you bury it in a 3" ground pipe it won't need derating, especially if it is one of the larger sizes.

16mm might seem a bit of overkill, and it is, but if you can pick it up cheaper than 10mm etc then you might as well have it.

The only other issue is terminating the larger sizes on domestic boards.

Peter

Peter

-- Peter A Forbes Prepair Ltd, Rushden, UK snipped-for-privacy@prepair.co.uk

Is there any point in putting in a 3-phase cable? It'll work for single phase, but if maybe in future Bob wants 3-phase ..?

-- Peter Fairbrother

At 30 yards, voltage drop will be the problem - the Wiring Regs are quite tight on it, as it affects output from lights and possibly safety (particularly with induction motors, they draw *more* current at lower voltages, so more cable heating, winding heating etc.) - 30 yards of XLPE insulated 10mm will have a maximum design current of 70A or so, so the nearest lower breaker would be 63A (nothing to stop Bob fitting a lower rating on a heftier cable, though). As to the ground pipe, it may still need derating, particularly if it's plastic rather than cast iron - time for a read of the Onsite Guide and 17th Edition regs, perhaps, to be absolutely sure!

Teminating anything over 6mm (shower cable size) on a domestic board's tricky verging on a no-no - tee off the meter tails with a Henley block to existing consumer unit and the shed feed's own enclosure (with the RCD and MCB to suit), take the SWA P+N cores straight into the shed's enclosure and use a Piranha (SP?) nut attaching the SWA to a IP54 or better adaptable box on the outside of the wall to take the SWA earth to the board's earth via

16mm stripy green/yellow. That *should* meet the regs *and* be safe.

Peter's suggestion of running 3-phase cable's a good one, getting the local electricity board to install 3-phase might cost more than the rest of the workshop, though... Possibly be worth using 4-core so that the later addition of 2 more phases is (slightly) less disruptive and the neutral's there for 240v?

Dave H.

These chaps have a handy calculator that isn't far off the tables in the Wiring Regs:-

I used 16mm^2 XLPE 4 core over a similar distance from the house to the shed, just in case I could ever afford to pay the pirates to give me three phase to the meter. That's fused at 63A (the standard supply fuse these days seems to be 100A)

Of course, you'll have to use the modern idiot colour scheme for the cables unless you "just happened" to install it before the Part P building Regs came in...

Mark Rand RTFM

That's what I did. When they changed the meter a few years back I asked if I could put in an isolator when the guy did it. He took one look at my isolator, got a higher rated one out of his van and fitted it for the cost of a cup of tea.

When I wired up the shed, I teed the connection between the meter and consumer unit to a separate breaker for the shed circuit. I didn't use Henley blocks, but used Pratley style connectors in a junction box. Henley blocks would have been neater.

The shed has it's own distribution board at the other end of the cable. Just because I could, I added a kWh meter to the shed circuit so I can see how much it's costing me. I had to label it "for indication only" to stop the buggers trying to read it, since it's the same model as the board's meter!

Mark Rand RTFM

BTW. Supplying current to and wiring in an outhouse, shed, garage, etc is subject to regulations and should not be undertaken by "unqualified" persons (like those who don't know what size of wire to use). It would, also, be subject to inspection under Part P.

Of course, who's to know - so long as you don't electrocute yourself of cause a fire.

Andy

In my instance I have been a "Competent Person" according to the Regs for three decades, but I've not paid the annual bribe to become part P self certifying. Howinhell did the lobbyists get that scam put on the statute book??

Mark Rand RTFM

Howinhell did the lobbyists get that scam put on the statute

Some twatting MP's dopey daughter got toasted by some dodgey wiring and she made it her mission to penalise the rest of us. Most of the really hairy wiring I've come across was done by someone with a ticket to say they were authorised to do it, doesn't mean they were 'competent'. I have installed and tested 275kV cables in power stations in the past, but I'm not allowed to put a friggin socket in the shed, utterly bonkers, but it keeps the money going round which seems the main object of any exercise these days.

Richard

Curious thing is, the fault that killed her wasn't in the house wiring - it was the appliance. Which Part P doesn't address at all.

Part P wouldn't have saved her if it had been in force at the time.

Indeed, it is about money - but not about money for electricians or those who certify them, money for councils.

If the sparkie [1] has to report when a loft, shed etc is wired up for the first time, then the council will know what's been wired up - and they can then adjust the ratable value accordingly.

[1] Part P registered sparkies still have to report such work done. You are still allowed to do it yourself if you are "competent", but you have to get it approved in advance and maybe checked later - both of which cost.

-- Peter Fairbrother

And, in reality, a lot of wiring is still done by non-Part P registered "cowboys". Lets face it. If a Part P registered sparkie quotes =A3X for a job, "Mrs Smith" is going to accept the quote for =A3Y (where Y

And there's me thinking it was all about preventing cash-in-hand jobs so the Gov' didn't lose out on income tax, VAT, NI contributions - what's this safety business about? I think having worked on everything from 500KVA generating plant to 100HP DC servo-motor drives to 20KV 1Amp TWT power supplies I have a fair grasp of safety requirements?

Note that Part P excepts replacement of like with like, as long as you're not working in a "special location" (e.g. bathroom, kitchen, swimming pool or Jacuzzi), so new replacement cable and replacement sockets are allowed as DIY to repair damage etc. Of course my shed had a 63A feed when I moved in, I think they were planning on putting in a Jacuzzi, but the cable was indoor shower cable and only 4" deep and I hit it with a spade and damaged it, honest Mr BCO! Of course, although being a few years old most of it was put in beyond the then-current Wiring Regs so should still be pretty compliant now :)

A quick look on Ebay for "cable old colours" will turn up all the cable you could need, see your nearest slob (or me!) for details on making it look a few years old and dirty...

Of course, the New Big Shed will have to be built *around* the existing wiring ;)

Dave H.

I hadn't actually thought of that one. That could be handy when I replace the existing 50 year old concrete and asbestos cement garage next year. How do I hold up the existing fluorescent lights and sockets while I replace the walls and roof and dig a decent foundation for the replacement?

:-)

Mark Rand RTFM

houstonceng wrote: (Honest Gov'ner

Ah, but you miss their diabolical cleverness -

Council inspector: "Okay, I believe you. So, your ratable value went up by two bands in, well, shall we say 2005, and now you owe us five years difference .."

-- Peter Fairbrother

Apply some Ingenuity and Acumen - a big wooden or plastic box full of wires and accessories, eyebolts in each end, pulleys up convenient trees or bolted to guy-roped scaffold poles, further ropes holding the box up nice and high, hence out of the way of digging and building? Could get an Arts Council grant for that, I'd think?

Of course, you'll have to slip a piece of split duct over the SWA or such that feeds it, and drop the box a bit lower while the concrete slab sets around said duct...

;o)

Dave H.