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Posted By Topic: Aerial Cable to Shed

JonnoM
Jan 25 2019 00:58

Hoping for some advice on best way to do this as have had a few opinions from other Sparkies.

Customer has shed/workshop aprox 10m from house he wants power to. Concrete driveway between shed and house so have to go overhead on catenary wire.

Option 1 - 2 core Neutral Screen cable run complete distance from internal switchboard in house, between house and shed, cable tied with UV stabilised ties to Catenary wire and terminated in sub board in shed, screen sleeved green and used for earth.

Option 2 - TPS run complete distance as above but where external between house and shed run in flexi conduit or flexi at either end and straight conduit cable tied to Catenary wire.

Option 1 seems the better option but my thoughts are the cost difference in cable being considerably more and is Neutral Screen Cable UV stabilised/ok to run exposed to sun?

Any feedback on either option would be good or recommendations on a better way to do this.

Cheers
   

DougP
Jan 25 2019 07:37

Either of your options are fine.

For option 1, you can join the NS to TPS at the house if you wanted to. But the time involved would probably outweigh the cost benefit. You'll probably find that the 2cNS will come with red and another colour such as white, but not black. So that will need sleeving at each end.

For option 2, yes there are good UV stabilised conduits available now, such as used for solar work.

Another option would be just to run the NS cable as an aerial cable, rather than as a catenary.

Aerial wiring systems are in section 3.12.
Catenary wiring is in section 3.13
In either case, the clearance height requirements are the same.

I just did a submain for a sleepout with 6mm2 2cNS (underground) and it was $7+ per meter.
   

AlecK
Jan 25 2019 09:10

The term "driveway" indicates use of vehicles;; which means required clearance in middle of span is 4.6 m. There will probably need to be uprights each end to achieve that. Need at least 5 m at each end to get 4.6 in centre, plus required margin for high temperature sag[3.12.3.1]; and there will still be considerable strain unless they are even taller - the less catenary droop, the more strain. So either big, strong, posts, or smaller ones with stays.

As for the cable, I'd be considering TPS on catenary, I see no point adding a conduit as the TPS is already OK for UV exposure (confirm with manufacturer) and the odd sparrow perching doesn't create need for further mechanical protection. Derating for "exposed to sun" is a little more than for "enclosed in air", but in conduit and exposed to sun is obviously even worse conditions so probably should be derated even more (there's no column for that).

If using a cable that needs conduit for UV protection, either conduit needs to be much bigger than normal to allow for adequate ventilation; or derate for cable temp 20 deg higher than ambient [3.5.8) in "3008.1.2"].

Naturally with drip loops and proper weatherproof cable entry to buildings, to comply with Building Code requirements (NOT just a squirt of gunk into the hole!).

Also using a non-conductive catenary to avoid need to earth it [5.4.5]; of the common rope / cord types that means polyester. Nylon is not as good, and ployproplene is almost useless. Avoid the stuff from DIY sheds unless it's clearly labelled 'polyester", most of their cheap stuff if polyproplene.

For any catenary system, need UV-resistant ties. Not all black cable ties are; so check before using whatever happens to be in your truck.
   

Someone
Jan 25 2019 17:35

Black circular TPS is specifically marked as UV rated (at least the Lapp stuff), and not incomparable in price to normal flat TPS.

Also looks a bit more professional than bare flat TPS.