Cable for Power to Garage

[alb1988]

Detached garage at the bottom of the garden which had power (due to presence of some old bakelite switches and sockets), but appears to now be dead (will get this checked to confirm, but let's assume it is for the purposes of this question). At some point in the next year or two the garage will be overhauled to include power for lights and say 6 double sockets. Will be a consumer unit in the garage fed from the house by an armoured cable.
The lawn is currently being relaid so makes sense to bury the cable now. Therefore what size armoured cable will be required, how deep will it need to be buried, and how should the ends be protected until such time as its connected to the house and a consumer unit is added to the garage end to prevent degradation.

 

[JAWS]

Your electrican will calculate the size of armour you'll require for the requirements you will need, size will take in account many factors. The depth should be no less then 600mm.

I am sure others will advise

 

[telectrix]

several factors to consider.
1. anticipated load ( Amps or kW)
2. total length of cable run
3. Type of earthing in the house
4. any other services (gas/water) expected in the garage.

as for burying the cable, a depth of 15" -18" is considerd average, but there are other factors to think about.... protection of cable from damage; warning tape, etc. if you search the forume you'll find a few threads regarding burial of cable/s.

having said that, the installation should be done by a qualified sparks, so get quotes and let him/her advise on the cable. you may be allowed to dig and bury it (to keep costs down) under supervision.

 

[davesparks]

The best thing to do is bury a duct now so that the cable can be pulled in later.

This avoids you putting the wrong cable in now, any potential damage as the trench is backfilled or while the other work is going on.
Plus it allows you to add other cables in the future if you wish.

 

[Strima]

As above run a conduit in, also feed through a couple of draw cords for future use.

Would you also be considering an electric vehicle charging point in the future?

 

[alb1988]

Thanks for the replies so far, all very useful. I should have thought of a duct earlier, that is a much better solution from a future-proof point of view. I will pursue having that done instead. Having read through other posts here would I be correct in the following spec?
Dig 600mm deep trench, install twin walled duct with draw lines with bend radii of 1000mm, cover with earth to half depth to the ground level, lay printed polythene warning tape, then back fill the other half depth to ground level. Is that correct or would it need to be 600mm to the top of the duct?
Which brings me on to a couple of follow up questions if I may...
1. If you were to come to the site in a couple of years time to install electrics in the garage, what size twin walled ducting would you be pleased to see? The duct run would be in the region of 40m from the house to the garage with 4 bends.
2. Assuming one uses SWA for the power and a SWA cable for data, could these run in the same duct, or would they have to run in two separate ducts?
3. What would be the implications if EV charging point were to be added in the future? A third duct, or just oversize to x mm the duct? (What would x be?)

 

[telectrix]

there's no hard and fast blanket rule about depth. basically, should be deep enough to prevent damage.e.g. in earth a spade may be used, so deeper than any digging is required. if under slabs or other hard standing, a shallower depth would suffice. best to lay duct on a 4" bed of sand.. cover with 4" sand.. then soil with no stones for anothe r4" or so, then backfill with the spoil.
as to size of duct, 75mm - 100mm would be my choice.
you can run all cables in 1 duct ,IMO, but others may prefer separate duct for ELV ( under 50V ) cables.
cable size depends on load and also critically volt drop. ie. 6mm SWA @ 40A over 40m would give you approx. 12V drop, which is borderline for most applications, ( 5% )but too high for lighting ( 3% ). personally I think, with the almost universal use of LED lighting, that 3% figure is obsolete as LED lighting will run perfectly on a lower voltage than incandescent or fluorescent lighting.

hope that gives you some sort of idea.

 

[pc1966]

How deep you bury the duct depends on the risk of it being damaged. Some of the figures mentioned (e.g. 60cm) are those used by the DNO (electricity suppliers) for carriageway use, but it is 45cm for under a footpath (and 1-1.2m for ploughed arable land), and those are not mandatory on private land / private circuit. Depth is to the top of the duct.

You can run CAT-5/6 cable in the same duct safely as the power cable as the steel armour is earthed and acts as a screen for noise, but get cable that is tough enough for outdoor use. For example:

cablecafe.co.uk

www.cablecafe.co.uk

If you really want high speed (above 1Gbit/sec) then putting in fibre would be best, but I don't know much about getting it in a cable form tough enough to pull through (you can, I have seen it, but never had to order any!).

If you plan for an EV charging point then the main difference is the power demands. Typically an out building with a few 13A sockets and lights will only be needing 10-20A and often you would get away with 4mm cable at that distance. But if you plan on having a 7kW charger or more then you are adding 30A to that to begin with and so would need heavier SWA cable, perhaps up to 10mm or so.

You can often pull through a 2nd run if the EV is added later, and you might just want that if separate circuits anyway, so it is something to discuss with the electrician. It is easier to pull all through at once if you can, so even if the EV is delayed having a cable in is just the cost of the wire which is around £4-5/m for that sort of size.

Also mentioned is earthing and your power supply type, if there is metalwork in the out building and you are on a TN-C-S supply then you need either 10mm earth bonding cable (achieved with the EV-sized cable, or an additional earth wire if smaller used) or you put in an earth rod and have a TT supply at the outbuilding. Hence the real need to discuss the details with your electrician before committing to cable installed.
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In the ideal case you would run just one power cable big enough for everything you might need and have separate circuits at the garage CU. That leads to the lowest overall installation cost and greatest selectivity in faults not taking out healthy circuits.

That might need a CU bigger than the usual "garage" sort that only has 2 breakers and an RCD, so it is something to plan from the start as you electrician could easily fit a small home-style CU with things like surge protection as well as plenty of space for the future use.

 

[davesparks]

You can run CAT-5/6 cable in the same duct safely as the power cable as the steel armour is earthed and acts as a screen for noise, but get cable that is tough enough for outdoor use. For example:

cablecafe.co.uk

www.cablecafe.co.uk

The steel armour on SWA is not a fully effective screen as it has gaps in it and so it doesn't block all interference that may be coming from the conductors.

However I would be happy running duct grade cat5/6 in the same duct as mains SWA

 

[pc1966]

The steel armour on SWA is not a fully effective screen as it has gaps in it and so it doesn't block all interference that may be coming from the conductors.

However I would be happy running duct grade cat5/6 in the same duct as mains SWA

No, it is not a perfect screen. However, the armour is enough to guarantee no power cross risk so you don't need the network cable to be segregated for safety. It just has to survive in a duct!

The network cable is already sets of twisted pairs to provide fairly good rejection of noise anyway.

 

[davesparks]

No, it is not a perfect screen. However, the armour is enough to guarantee no power cross risk so you don't need the network cable to be segregated for safety. It just has to survive in a duct!

The network cable is already sets of twisted pairs to provide fairly good rejection of noise anyway.

What do you mean by power cross risk?

 

[pc1966]

Someone damaging the duct & cable and shorting the mains power to the low voltage network cable.

 

[davesparks]

Someone damaging the duct & cable and shorting the mains power to the low voltage network cable.

I've never heard of that term before, and as far as I know there's nothing in the regulations about that.

 

[pc1966]

I've never heard of that term before, and as far as I know there's nothing in the regulations about that.

I think the regulations say you must segregate cables if they are not all rated at the highest voltage, and I doubt network cable is mains rate (only ever seen 100V working, though no doubt the sheath would be good to more than that for short periods).

But with the earthed shield there is no risk of power-to-signal shorts.
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Actually a mouse/rat getting in to the duct and chewing through insulation is a higher risk! Again the SWA stops the risk of live network cables and avoids any need for RCD protection of the sub-main.

 

[davesparks]

I think the regulations say you must segregate cables if they are not all rated at the highest voltage, and I doubt network cable is mains rate (only ever seen 100V working, though no doubt the sheath would be good to more than that for short periods).

All conductors in the same wiring system must be insulated to the highest voltage present yes.
But insulated and sheath cable in itself is its own wiring system so any insulated and sheathed cable can be run next to ELV cables.

 

[Strima]

Take a look at 528, basically if running in a duct and the comms cable is outer sheath is either not mains rated or the earthed shielding is not capable of taking fault current then the comms cable should be sub-ducted.

 

[pc1966]

Yes, basically my point that if the mains is SWA in the duct you can run normal (but outdoors rated) network cable along side it.

I have seen SWA CAT-5 cable, not sure if you also get it in CAT-6, but not only is it more expensive, it would be a PITA to deal with as basically you would need it glanded in to earthed patch panels at each end.

One network cable should do, though they could pull in a 2nd if they wanted, as it makes more sense to put a small Ethernet switch in the out building for having multiple devices there. More so if they want any PoE stuff like web cameras, environmental sensors, etc.

 

[James]

SWA Loose Tube Fibre Cable

Steel Wire Armoured Loose Tube Fibre Cable, Available from 4 - 24 Core

www.fibrefox.co.uk

Cat6 SWA Cables - Comtec Direct

Buy F/UTP Cat6 4x2x23AWG LSZH SWA cable suitable for outdoor use of LAN and analogue & digital video and direct burial applications

www.comtecdirect.co.uk

Note, the fibre would need terminating, not something to do diy, also you would need a switch at each end with a fibre port or a media converter.

 

[pc1966]

Thanks for those links, and yes, terminating fibre is very much a specialised job!

I suspect the normal outdoor CAT-6 cable will do, but as they will probably need to buy a reel of it they might as well pull through two runs and terminate/test them out even if only one is used - if a fault later they can swap and see if its the cable.

Also if they get fancier switchs than a £20 amazon special then they should be able to bond the pair for higher speed / greater reliability. I really doubt that would be needed though!