What size SWA?

[r.p.mcmurphy]

i want to run some armoured cable down to the garage/workshop...it only has

2.5 flat twin and earth unprotected atm, and only an inch or two under the lawn!! (been like that for 15 years though apparently!) needless to say, when i run a compressor down the garage, the amps draw dims the lights and blows the fuses!

i have a single phase carlift, a compressor (100l-3hp motor), a kilowatt of lighting, freezer, computer and occasional use of the 3kw fan heater.

what size cable do i need to use?

tia

Steve

[John Rumm]

Not a complete answer to your question, but:

up your total load (both what you have now and what you anticipate it is going to be), work out the cable length - page above will do the sums for you.

You will still need to do some work to work out what protection is needed on the source end of the cable... knowing what type of earthing setup you have in the house would help.

[Andy Wade]

Motor rating? In the absence of other info I'll guess 16 A.

Current on rating plate? Another 16 A at a guess.

Fluorescents, presumably. Allow 8 A.

Another 16 A should cover that, and general 'small power'

Total current so far is about 56 A. What diversity can be allowed? The lift presumably only draws current for short periods. Based on the guesses above I'd say a submain rated at at least 45 A is needed here,

63 A if you want to give yourself some spare capacity for future additions.

On current rating alone 4mm^2 SWA (XLPE insulated to BS 5467) would be OK, BUT in practice the size required is usually determined by voltage drop, not current rating. Also you need a good 'stiff' (low impedance) supply for satisfactory motor starting. What's the length of run from the supply intake or main switchboard to this garage? You may well need

10 or even 16mm^2 SWA to do this properly.

[R P McMurphey]

no plate on motor..its an old one! i think its 1hp.

no idea..theres no plate.

yep, what you said.

car lift used for only a minute at a time. compressors kicks in and takes about 5 minutes to fill to full presure but if used for painting etc could be running nearly constantly.

circa 25m from main house supply board.

steve

[Christian McArdle]

That gives 6mm minimum XLPE SWA. I'd consider going higher to avoid voltage drop on those large motor starting transients. However, it probably isn't really necessary if the compressor won't be competing with the lift.

Christian.

[Dave Plowman (News)]

With SWA, the cost isn't really proportionate to the size, so I'd go for

16mm two core, with a local earth. 6 mm looks to be about suitable for your load, but is 1.50 a metre compared to only 2.50 for 16mm - so really not worth skimping on given the work involved in installing it.

[Tim.. (Remove Safetycatch)]

I would also recommend you go to 10 or 16mm to give the supply plenty of 'guts' for motor starting- especially compressors and the like which start under load.

I've just installed SWA to an outbuilding and used 16mm. (only 12m, braid earthed house end, local earth shed end) Whilst its bendy enough when your laying it in a trench etc etc, its not so much fun to terminate!! (allow plenty of spare and do large raduis bends) Use a metal-clad isolator / etc with removable end pieces, so you can assemble and tighten the gland at ground level - cos you'll struggle and curse for ages otherwise!!

Tim..

[Andy Wade]

On that length go for 16mm^2 as others have said. Its voltage drop is

2.8 mV/A/m. For 45 A and 25 m that gives above 3 V drop, which is 1.4% and leaves you a reasonable allowance for voltage drop in the workshop wiring.

The original bit of 2.5 T&E never really stood much chance.

[r.p.mcmurphy]

Wonderful! Thanks for that... what about glands and other fittings? i will be running the cable from one of those external 'lecy board meter boxes on the wall. do i need to fit any kind of circuit breaker or can i link straight into the output of the meter? i have fitted glands a long time back as a yoof slave scheme boy...i remember it not being much fun! can you point me to some products?

Thanks!

Steve

[:::Jerry::::]

plenty of

They shouldn't, either your compressor is faulty or is of a crap design...

[Owain]

You must have a double pole switchfuse between the meter tails (split usign a henley block) and the SWA. Fuse must be rated to protect the SWA.

If the switchfuse is in a metal box remember that both meter tails (L&N) must go through the same hole to prevent eddy currents.

Owain

[John]

I'm trying to visualise why you say that considering the direction of the inducing current and the lack of a conductor path parallel to the original conductor(s) We used to have weatherproof a steel cabinet with four 150mm flexible singles each with their own gland coming out of it to feed (3 phase and a protective earth) shore power from a dedicated substation to a massive compressor on board a ship moored at the jetty alongside our silos in one place where I worked. If we couldn't generate measurable eddy currents at the currents we were passing I can't see why it would be a consideration in a domestic situation. ISTR the old fifties style yellow service boxes below the meter having two seperate holes for the L & N with wooden bushes

[Tim Lamb]

What is the smallest meter tail allowed on a 100Amp supply under the current regs.? Some metal clad switch fuse terminals will only accept

10mm.( MEM excel 32A 3-ph)

regards

[Andy Wade]

25mm^2 to the main consumer unit for the full 100A. 16mm^2 is OK on a short branch from a service connector block to a switch-fuse for a submain, like wot we're talkin' about here.

That's not an appropriate choice then. The Wylex 106/108/110 SP&N types will certainly accept 16mm^2, possibly 25. The 108M (60A metalclad) would be a good choice for this job (IMHO).

[r.p.mcmurphy]

ok, so I'm about to order 50 m of 16mm^2 2 core (me thinks i under estimated distance by 100%!), 2 internal glands, 20 20.9mm cleats (as I'm gonna clip about 20m to the house wall before taking it under ground for the last 20m),

20 meters of underground warning tape, Wylex 108m.... what about the other end (the garage)? can you suggest an appropriate item? i will replace the existing fuse wire CU box.

Steve

[Christian McArdle]

With all that equipment, I'd do it properly with a choice of two options:

Option 1:

1. Earth rod.
2. Main earthing terminal.
3. Insulated split load consumer unit.
4. SWA earth isolated from installation.
5. Main incomer: 63A+/100mA time delay RCD.
6. Split RCD: 63A+/30mA immediate action RCD.
7. MCBs (100mA side): A. B6A - lighting B. B16A - lift C. B16A - compressor D. B16A - freezer radial
8. MCBs (30mA side): A. B32A - socket circuit (will also handler fan heater)

You may need C16A for lift and/or compressor, or even a 20A/25A/32A. Refer to manufacturer's installation instructions.

Requires a 9 way (gross) consumer unit.

Option 2:

1. Earth rod.
2. Main earthing terminal.
3. Insulated non-split load consumer unit.
4. SWA earth isolated from installation.
5. Main incomer: 100A DP switch
6. RCBOs (30mA) A. B6A/0.03A - lighting B. B32A/0.03A - socket circuit C. B16A/0.03A - lift D. B16A/0.03A - compressor

You may need C16A for lift and/or compressor, or even a 20A/25A/32A. Refer to manufacturer's installation instructions. Freezer could share with the socket circuit or have its own 16A/0.03A.

Requires a 6way to 12way (gross) consumer unit, depending on RCBO width and use of separate freezer circuit or not.

Option 2 is more expensive, but not prohibitive. It provides better protection against the effect of earth faults on one circuit affecting another, which can be a safety issue, particularly with respect to lighting.

Christian.

[Tim Lamb]

Umm... a 60A metal clad in TP&N takes up a fair space in the hall cupboard:-(

My problem, not the OP.

regards

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