How thick cable?

[Adrian]

Just talking to my next door neighbour. He's currently got a very long extension cable strung across the road, on top of the tarmac, between his house and his lambing shed. Call it 100m to be on the safe side.

He's going to run armour cable through the gulley under the lane, but is wondering what size. Reckons he'll chuck a 20A RCD on the house end - it's not much more than a bunch of light bulbs and the odd heat lamp, but it'd be nice to have a socket there he didn't have to worry about using.

He was thinking 4mm2, but wondering if 6mm2 would be wise. I found a cable calculator on t'web, and it reckoned 10mm2 for 16A or above, 6mm2 for 13A.

What think the assembled multitudes?

[Cursitor Doom]

100m is a fair old run, but OTOH, 6mm^2 is a tidy thickness of cable. Seems OTT on the face of it. Are you sure he's not planning to grow skunk?

[Andy Burns]

As it's a lighting circuit, you're not allowed as much of a voltage drop, and if he's going for a 20A circuit for some sockets, the whole circuit should be designed for the voltage drop at max current.

The TLC calculator reckons 16mm^2 SWA for 100m, expensive about £370

If he dropped the house-end MCB to 10A, he could use 10mm^2 instead, still £240

[Fredxxx]

If you want to stay within the 5% allowable voltage drop it ought to be

10mm.

6mm2 has a total voltage drop of 7.3mV / A / m At 100m and 20A this gives 14.6V which is 6.3%

10mm has a total voltage drop of 4.4mV / A / m At 100m and 20A this gives 8.8V which is 3.8%

He could use a 16A MCB and 6mm2 cable but only just!

I'm not sure if these are such hard and fast rules, the reason I say that is it is the design of a circuit that has to be with these rules, but the choice of MCB is there to protect the cable and associated fittings?

100m x 6mm2 cable is expensive, 2-core is £165 + VAT in TLC! 100m x 10mm2 cable for 2-core is £240 + VAT, in terms of copper it almost seems good value!

[Tim Watts]

has a handy calculator. I do not have my tables at hand, but for a 100m at 20A, he's likely to need 16mm2 3 core SWA.

Why?

Because of either the volt drop or the ability to trip the supply side breaker in quick enough time on a dead short (L-E or L-N).

If he came down to a 16A breaker, he'd be OK with 10mm2.

OK - for his purposes you could say "sod the volt drop" but you do still want the breaker to trip correctly under fault conditions.

If you said sod the volt drop, 6mm2 at 16A should be OK. 4mm2 is too weedy.

Given the large cost difference between differing cable sizes for 100m reels, it might be worth him evaluating if he really needs go that high, or if 6A is good enough. That's still plenty of lamps. Not sure how powerful the heat lamps are?

Given me a day or two and I will have access to my book and could tell you for sure.

[Andy Burns]

3% for lighting circuits.

[John Rumm]

Probably way undersized for a 20A circuit.

For starters have a look at:

There is a section there on cable choice.

If you work back from the allowable voltage drop of 3% of 230 or 6.9V, divide that by the design current and the circuit length, and you should have the maximum allowable voltage drop per amp per metre

So 6.9 / 100 / 20 = 0.00345 so you can afford to drop no more than

3.45mV per meter of SWA cable.

From the table here:

That says you will need 16mm^2

[Fredxxx]

Ah - thanks.

[Bill Wright]

How about stepping it up to 2,400V and then stepping it down at the far end?

I can think of a lot more ways of doing it as well . . .

Bill

[Mike Clarke]

Which I assume is based on output of incandescent lamps falling dramatically with reduced voltage.

I wonder if the regs need to add an extra case for LED or CFL lighting installations where the SMPS built into the lamp is much more tolerant of voltage variations?

[meow2222]

Time to get creative: what if you used a transformer at the head end to add half the cable Vdrop, thus permitting half the xsa of cable lol. Or maybe you could run both conductors in parallel and use a ground return :)

A bit more seriously, wouldn't such a setup fall outside of the 17th edition regs? If so, while tungsten lamp light output is affected a good bit by Vdrops, CFLs, linear fl and heatlamps dont care.

Actually it might really be cheaper to use transformers at both ends, transmit at the cable's voltage rating limit.

NT

[Tim Watts]

As I implied, I would not get too excited over volt drop on a rather special case circuit like this as long as it does not compromise safety.

The main two problems the OP has are ensuring disconnection times and also, given the farming nature of the application, he probably has some additional problems concerning provision of earthing - which is way outside of my knowledge.

Animals are more sensitive to voltage gradients through the ground (partly because they span a greater distance of ground and partly because they are usually standing barefoot in mud).

It might be actually worth subbing this one out to an electrician with knowledge of farm requirements.

The OP's friend could still do the laborious work of laying the cable (after seeking advice on the spec) leaving the electrician to connect up and test.

[Andy Burns]

How sensitive are heat-lamps to voltage drop? Do lambs appreciate CFLs/LEDs?

[spuorgelgoog]

If you use XPLE with a 90degC conductor temperature, and set ambient temp to 15degC for underground, non-lighting circuit, 3kW load, TLC calculator says 6mm is okay.

I'd be interested to see what MICC could do with an even higher conductor temp.

If he can get a few posts up then bare wire on insulators might be possible.

Owain

[meow2222]

Go with Bill's 2.4kV and you could probably use tv aerial wire :)

NT

[meow2222]

Back to a bit more seriousness, could one use 2.5mm^2 with a tap changer at one end?

Re disconnect times, the head end mcb can protect the cable, then a shed end CU can protect the sockets, lights etc with quick disconnect times.

NT

[dennis]

But LEDs don't care if you drop 50%. So if its for lighting why would it matter?

The heat lamps won't care either.

Some motors might complain if the drop is high.

Why do lighting circuits only allow 3% these days?

[Fredxxx]

That was one of my thoughts, well, more of making use of a transformer to make 230-0-230 as most cables are rated for that anyway. That would

1/2 current and 1/4 the cross-sectional area required.

But the cost of 2 suitable transformers would be rather prohibitive.

Another thought was to use 4 core, and to load one pair such that voltage drop was within 3% for lighting, and use the other within a 5% drop for everything else.

If it was any further than 100m it might have been worth considering.

[Bill]

You may need this then Bill, I believe that they have a couple, one for each end. Assuming of course that one could be used in reverse.

[Andy Burns]

Of course LEDs, CFLs etc won't care about voltage, but what if someone changes back to incandescents?