Armoured Cable Sizing

[geoffbeaumont]

So, ordered an armoured cable to supply the new shed. Meant to order 4mm² but what turned up was 2.5mm² - which appears to be my mistake not theirs.

Pretty certain 2.5mm² isn't going to be up to the job, but I'm now having doubts about whether 4mm² is fine or I need 6mm²... I know there are some qualified sparkies on here, so could someone check my work please? 

The shed will eventually have:

• Lighting ring
• 13amp Sockets ring
• Probably at least one 13amp outside socket
• Single 16amp socket (to supply an electric ceramics kiln)

The armoured cable from the house will be approximately 40m long. It won't run directly to the main consumer unit which is on the wrong side of the house - there's a secondary one in the loft that already supplies the garage, it'll run to that. I think the supply to that CU should be adequate for both - but I'll be getting a grown up to hook it up so that'll be checked out by someone who knows what they're doing.

The cable will be buried in the ground.

Obviously the sensible thing would be to get a sparky to spec it all and tell me what he wants trenched in - but I'm not planning on spending the money on wiring it all up just yet (can't afford to at the moment), I just want to run the cable into the shed before I finish the floor so I don't have to rip anything up later.

[pete3000]

40m is a fair run, take into account voltage drop  resistance per metre. When you say 13A sockets ring using 2x2.5mm cable that typically requires 32A RCD. if you add the kiln at 16A another 2.5mm cable and lighting 6A you'd be in the region of 50A fully loaded. This is without adding the 13A outside socket. If you are planning on putting it in once (trenching) then I'd go 10mm minimum.

A compressor and Welder or plasma cutter could get you to 32A and beyond.

6mm table states method D buried or in ducts 6mm max @53A. 8mv/A/m drop. Some of the really cheap brands of cable are a bit light on copper, instead making up with a steel content, this can increase the resistance.

8mvx40mx50A= 16v drop @6mm

4.7mvx40mx50A = 9.4v drop @10mm

Obviously if you half the current max to say 25A then you half the volt drop/metre. Also your sparky will take into account where it's being fed from and if it's up to another 50-60amps.

[geoffbeaumont]

The kiln's the only thing ever likely to be up there that's high current. The garage is my workshop, and anything else demanding will finish up in there. 

If it changes purpose in the future most likely that's going to be to either a summer house or home office - either of which would almost certainly lose the load from the kiln and make the 16amp redundant.

[pete3000]

If you think you are ever likely to change use, upgrade or want to run anything else i.e data/fibre or alarm. Then a duct might be worth doing with a couple of pull cords in.

2.5mm at 40m is 19mV/m/A allows 15 A before 5% 11.5v of volt drop is reached. (this is from your incoming supply point though.)

4.0mm at 40m is 12mV/m/A or around 23A

[geoffbeaumont]

Not sure what the length of run within the house is - probably another 15-20m. It should be specced to handle the load, as it was briefed when the house was rewired.

Sounds like 6mm2 is realistically the minimum sensible.

At the moment (as I'm not planning to trench anything in for at least a few months) I'm thinking I'll sort out some ducting into the shed that I can feed the wire through and sort the actual wire later.

I'll look at what ducting would cost - I can see a possibility of wanting to run CAT6 in the future. Not sure if that's a good idea in the same duct as power? I guess it's shielded so it should be...

[pete3000]

Sounds like 6mm is a minimum, If you are using shielded i.e SWA and cat 5e/6 it wont matter as long as it's not tightly packed. remember to pull the ropes in before you bury it avoid sharp bends. If you pull something else through, pull another length of rope through as well.

[FridgeFreezer]

55 minutes ago, geoffbeaumont said:

I'll look at what ducting would cost - I can see a possibility of wanting to run CAT6 in the future. Not sure if that's a good idea in the same duct as power? I guess it's shielded so it should be...

I ducted my garage, it's not hard and you only have to do it once - I bought a 50m reel of twinwall corrugated ducting with the smooth liner - that's the critical part, there's identical-looking drainage pipe that is the same corrugated exterior but with no smooth liner. Since I only needed ~12m of it I actually sold the rest on ebay to someone doing a similar job I'll admit the builders did the burying as they were busy pulling up the concrete path where the ducting was due to go anyway and extending the garage at the time.

Get a good length of blue nylon rope (I'd guess >2x the overall length), then use a vacuum cleaner on one end to pull it through - I tied off both ends and then I just use sparky tape to wrap a good number of wraps round whatever I'm pulling through and heave ho. If you need extra suction a carrier bag tied on gives a lot more "pull" for the vacuum cleaner.

Cat5/6 won't care about being near an armoured cable, just make sure you get outdoors-rated stuff. Mine's been in & working great for probably a decade now.

 

I think I used 10mmsq, it may have been that 6mmsq was "OK" from memory but I just went one higher to be sure as I was going to be running the welder. I'll also say it's worth getting the slow-reaction breakers on the house end of the feed or power tools etc. can trip it very easily.

Although you're unlikely to be running all the powerful stuff at once, in winter it's very easy to be running a 3kW heater and a couple of other things without really realising it - I've found a cheapy ~2kW infra-red patio heater bolted to the wall is enough to take the chill off even on low setting (so more like 500-1000W) rather than running cheap 3kW fan heaters etc. as I used to.

 

This is the sort of ducting I used: https://www.easymerchant.co.uk/twinwall-ducting/electrical/x50m-pipe-coils/

You can get all different colours etc. and the warning tape to bury above it if you're being fancy.

[geoffbeaumont]

I've ordered exactly that. What did you do about sealing the ends? We have a rat problem round here - not only do I not wish to create a lovely rat highway between buildings, but if the little buggers get into the conduit they'll chew through everything in it (including the pull cords for added inconvenience).

[Anderzander]

Reading this makes me realise how woefully inadequate my garage wiring is.

[jason110]

My garage is 6mm armoured cable. The run is 15mtrs in total. 
 

I’ve got a ring main on 32A, lights on 6A and the Outdoor sockets on 16A 

her office is on 6mm too. That’s just 2kw heater, lights and sockets. 
 

I’m sure when I looked at it, 6mm was the minimum

[Retroanaconda]

I’d still get a sparky to spec it, I’m sure they’d be happy to do so for a minimal fee.

Mine is a 10mm feed and I had it taken straight off the meter tails (i.e. bypassing the consumer unit),via a 63A cartridge fuse rather than a circuit breaker. Very resilient for loads such as welders and compressors.

[Ed Poore]

@geoffbeaumont probably less relevant if you're putting in a duct but you can get EV cable which is armoured cable + CAT5e cable in the same armouring.

My sister used that to supply her shed as it was clipped to a fence rather than ducted so sorted out the Ethernet and mains in one hit. Bit more expensive than two separate cables but useful to know about.

[FridgeFreezer]

3 hours ago, geoffbeaumont said:

What did you do about sealing the ends? We have a rat problem round here

I didn't seal the ends but now I'm wondering if I should

Looks like end caps are available from the usual suppliers (builders merchants etc.) or you could just stuff something reasonably inedible down there, these days I'd be fancy and 3D print a split  cap that fits in the end with holes cutout for the cables

[Jon White]

59 minutes ago, FridgeFreezer said:

I didn't seal the ends but now I'm wondering if I should

Looks like end caps are available from the usual suppliers (builders merchants etc.) or you could just stuff something reasonably inedible down there, these days I'd be fancy and 3D print a split  cap that fits in the end with holes cutout for the cables

We just use expanding foam at work. Usually the pink fire rated stuff. You can get posh expensive proper silver coloured bladders that you wrap round the cables and then then inflate using compressed gas. Trouble is to add more cables you have to puncture them and then replace them. Needless to say the guys just replace with expanding foam as it’s what they have on the van. Usually scrunch up a bit of newspaper and shove that down first so you don’t just fill the whole duct with foam - you only need a 3-4” thick plug of it, which is then easy enough to smash out when you need to add more cables in the future.

 

Jon

[geoffbeaumont]

13 hours ago, Jon White said:

We just use expanding foam at work. Usually the pink fire rated stuff. You can get posh expensive proper silver coloured bladders that you wrap round the cables and then then inflate using compressed gas. Trouble is to add more cables you have to puncture them and then replace them. Needless to say the guys just replace with expanding foam as it’s what they have on the van. Usually scrunch up a bit of newspaper and shove that down first so you don’t just fill the whole duct with foam - you only need a 3-4” thick plug of it, which is then easy enough to smash out when you need to add more cables in the future.

 

Jon

Rat'll go through expanded foam in seconds - in fact they love the stuff... If I need to block a hole up with that I embed copper mesh in it.

[geoffbeaumont]

14 hours ago, FridgeFreezer said:

I didn't seal the ends but now I'm wondering if I should

Looks like end caps are available from the usual suppliers (builders merchants etc.) or you could just stuff something reasonably inedible down there, these days I'd be fancy and 3D print a split  cap that fits in the end with holes cutout for the cables

I could only find what looked like clip on glands to take a single large cable, or rubber caps (not specifically for twinwall but available in suitable sizes) secured with a jubilee clip. The latter would work well in most respects (just drill holes in it for the cables, add as necessary) but wouldn't stop a rat.

I might have to make something from stainless mesh.

[Les Brock]

Hi Geoff,

Based on 40A load ....... Quick one as home time so made some guesses, can swap bits as needed ...... 10mm XLPE Armoured using 30mA RCD

 

Geoff Calc Sum An.pdf Geoff Calc Sum.pdf

Geoff Calc.pdf

[Jon White]

On 5/9/2024 at 1:09 PM, geoffbeaumont said:

Rat'll go through expanded foam in seconds - in fact they love the stuff... If I need to block a hole up with that I embed copper mesh in it.

I have cats. No rat problems around here 🤣

[geoffbeaumont]

16 hours ago, Les Brock said:

Hi Geoff,

Based on 40A load ....... Quick one as home time so made some guesses, can swap bits as needed ...... 10mm XLPE Armoured using 30mA RCD

 

Geoff Calc Sum An.pdf 136.32 kB · 9 downloads Geoff Calc Sum.pdf 88.27 kB · 9 downloads

Geoff Calc.pdf 114.29 kB · 5 downloads

Thanks Les - that's really helpful!

[Gazzar]

Go large, do once.

[Gazzar]

On 5/9/2024 at 1:09 PM, geoffbeaumont said:

Rat'll go through expanded foam in seconds - in fact they love the stuff... If I need to block a hole up with that I embed copper mesh in it.

Stainless steel wire kitchen scourers are very inexpensive. 

[Gazzar]

On 5/8/2024 at 8:57 PM, Ed Poore said:

@geoffbeaumont probably less relevant if you're putting in a duct but you can get EV cable which is armoured cable + CAT5e cable in the same armouring.

My sister used that to supply her shed as it was clipped to a fence rather than ducted so sorted out the Ethernet and mains in one hit. Bit more expensive than two separate cables but useful to know about.

Very helpful. @Ed Poore Do you know if there's a plug and socket that can take mains and data in one? For my other project......

[geoffbeaumont]

6 hours ago, Gazzar said:

Stainless steel wire kitchen scourers are very inexpensive. 

The copper mesh is fine and very soft - it tangles in their teeth rather than being too tough to chew through. But scourers might do that too...might have to try them next time I need to block a hole up!

[Ed Poore]

9 hours ago, Gazzar said:

Very helpful. @Ed Poore Do you know if there's a plug and socket that can take mains and data in one? For my other project......

I'm not sure. My sister got a single gang Ethernet wall plate from TLC which you don't even have to crimp the wires into a connector. Just push them into something akin to a scotchblock behind the panel, there is a tool but I just used a screwdriver.

https://www.tlc-direct.co.uk/Products/GU7071.html

I've seen mains and Ethernet but they use power line in the back rather than dedicated cat 5.

Edit

I didn't know what to expect but the Cat 5 cable is completely separate from the mains within the armour so it's basically a "normal" but shielded Cat 5 bundled with the normal mains wiring.

[Gazzar]

OT @Ed Poore two sockets is the simple, but untidy answer, I'd like to avoid two cables, or a single double headed cable, going in to the wagon. Don't think what I want exists at reasonable money.