Overhead Submain Cabling

[DibbyD]

What type of cabling do you use for overhead submains?

Residential property, owner wants an overhead connection from the main dwelling to separate garage for a sub-board, single phase.
(owner can't be convinced to go underground)

(3000) 3.12.1 says polymeric insulated or neutral screened - cant find small gauge (6mm) XLPE twin or N.S. at the wholesalers.

Any ideas?

[pluto]

It depends on the span lenght, use PVC conduit if the span is short, or a run of 4x 2 timber. Use 2 + E TPS or 2 core + N/S. If a long span use 2 core + N/S, you have to use larger CSA cable and hard drawn cable for a long run.

Use of the outbuilding provisions for earthing not recommended.

[DougP]

You should be able to find 6mm2 twin NS cable? I have used it before, although it might have been the underground variety.
The other option is to use TPS in a conduit on a catenary (4.13), or build a solid structure between the buildings if they are close. If you build a structure, then the minimum height requirements don't apply.

[DibbyD]

Thanks for the replies - it's quite a long run so will check again with the wholesaler to try find 2C NS 6mm

[AlecK]

TPS on catenary doesn't need conduit.

(use 3.13 catenary instead of 3.12 aerial)

[DougP]

I was under the impression that cable manufacturers recommended that their cables were protected from UV. But on re-reading their information, I see that they mean the insulation of the single cores, and that the sheath provides the UV protection.

I've noticed that lines companies are also sleeving the single insulation these days at pole top connections. And tails from overhead to mains boxes have been required to be double insulated for quite a while now. I thought this was overkill, as it connects to the single insulated lines, but I guess the reason is that the single insulated PVC will break down eventually.

[gregmcc]

Hi @DougP, I do work for a line company, and yes the service connections and the insulation falling apart is a BIG problem, we don't sleeve the PVC, we join UV stable cable on to the end and heavy wall heatshrink the link and the service PVC, this also has the advantage of creating a water barrier to stop water traveling down the cable and coming out at the other end (we use shear off links with blind holes each end).

[AlecK]

Tails from o/h supply to mains entry box are not required to be double insulated; exactly the same rules apply to the tails as for the incoming aerial span. However the insulation material may well be different; therefore requiring different treatment.

These days aerials are generally N/S rather than singles
N/S in smaller sizes tends to be PVC / PVC; but for larger it's PVC sheath over XLPE cores
XLPE is not UV stable, so needs a protective layer. Not only at poletop, but also within the meterbox if exposed to sunlight via the window.

TPS is PVC / PVC, and is generally OK for exposure to sun (certainly better than many corrugated conduit products).
Conduit will decrease the CCC of the cable within - as does exposure to sunlight.
Obviously TPS in conduit AND exposed to sun will be lower than for either factor in its own. The Tables in "3008" only cover one factor or the other; but 3.5.8 provides guidance.

[JamieP]

Isn't the main issue that the people use conduit on a catenary to run at heights lower than those for cables suspended on a catenary?

Cables follow for N/S heights of aerial conductors in accordance with 3.13.3 (a) giving 3 meters for areas not used by vehicles which is usually higher than most spans I see between a home and another structure etcetc

I thought the use of conduit helped negate the need to run a up and across because such only applied to cables on a cat wire outside, not conduits, mostly on short runs when structures are very close

Correct me if I'm wrong

[AlecK]

3.13.3 says anything on catenary (outdoors) uses the clearances for N/S.
There's no height for TPS as aerial conductors; 'cos you just don't do it.

[JamieP]

3.13.3 specifically says cables though

The first word is cables, it doesn't mention anything else on a catenary for those height restrictions

[DougP]

There seems to be some confusion between aerials and catenary?

[AlecK]

Rather than risk chasing down off-topic rabbit-holes; I'll avoid the (non) issue of "conductors" vs "cables" for now; other than simply observing that "cable" is a defined term [1.4.17], and that every cable has at least one conductor.

There are several options for the original situation
[Residential property, owner wants an overhead connection from the main dwelling to separate garage for a sub-board, single phase;
owner can't be convinced to go underground]

Not stated whether the cable route would cross an area used by vehicles, so not possible to determine the column of Table 3.8 that applies.

One option is aerial wiring systems, which must be one of the 4 conductor types specified in 3.12.1; and must be set to minimum clearances as per 3.12.3.

Another option is cable on catenary ; and 3.13 includes 2 rules about height.
3.13.2 (e) requires sufficient height to prevent danger / damage; then 3.13.3 (a) specifies that catenary outdoors, irrespective of cable , conductor type, be at minimum height as for a N/S cable used as aerial wiring.

A third "overhead" option - especially for a short span - would be using some sort of rigid support for the cable (as suggested by pluto)
Eg a length of wood, metal, or even conduit. As this is neither "aerial wiring system" nor a catenary system; there is no minimum clearance specified. But clearly the min practicable height would be dictated by the other intended / likely uses of the area the cable crossed above.

And the 4th option is to go underground.
Arguably the best option, except the client doesn't want it.


OP looked at 3.12 (option 1), but then couldn't easily obtain a permitted cable type. Perhaps may have (incorrectly) thought that N/S had to be "polymeric". Unclear whether need for 6 mm2 may have been to comply with 3.12.2.2, or for load / volt drop reasons. But regardless 6 mm2 2C+N/S should be readily available.
pluto also observed that for a long run aerial wiring should be hard drawn conductor rather than the normal type. True it should be, but it's not actually a requirement of "3000".

One of DougP's suggestions was TPS in conduit on catenary; and yes it can be done - but adding the conduit isn't required and doesn't achieve anything compared with TPS on catenary.
Conduit isn't needed to protect the TPS from UV, it doesn't affect the minimum clearance, and there should be no need to provide additional mechanical protection to the span.

To me, underground is generally best. Rigid run for a short span has the advantage of no clearance requirements.
I've used metal conduit to be self-supporting over a couple of meters (and with UPS inside doesn't need to be earthed.
Catenary is the easier option for a longer span, using either TPS or N/S.
I tend to use polyester braid as the catenary; no need to earth it and it is UV-resistant. Polypropylene and nylon sometimes have UV stabilisers added, but still not as good. UV-stabilised cable ties of course.
-----------------

Back to terms

I'd assume "cinductor" would be defined in IEC 60050, in which case it is pulled into interpretation of ESRs by ESR 4(3).
But not into interpretation of Standards; where terms take "the meaning used in that Standard" [ESR 4(2)] regardless of whether specifically defined or not - unless either the Electricity Act or ESRs impose a different definition.
ESR 4 defines "conductor" as "a wire, cable, bar, or tube, used or placed in position for the conveyance of electricity; but does not include the wire of an electric fence".
So there can be forms of conductor that are not part of any cable, eg busbars;
but in terms of any cable, a conductor is any part of a cable that intended to conduct electricity.
The main importance of this definition is that it's crucial to determining whether something is PEW or not.

Wiring Rules use "conductor(s)" where the reference is to the current-carrying part(s) of a cable. That's consistent with the definition in ESRs; so there's no conflict.
But when the reference is to the cable as a whole, including any insulation / sheath / other components, the term"cable is used.

Sometimes the conductor is the only part of the cable (eg bare single conductor); in which case the terms are interchangeable.
More commonly there is also insulation (eg conduit wire), and often a sheath as well (amounting to double insulation); in which case the appropriate term is used for the meaning intended.
For MIMS, the sheath is also a conductor (can be used as N but more often as PEC).
Then there are further protective layers that can be added for various reasons, eg SWA, or the earth screens in VFD cables.
And again SWA can be used as a conductor (only as PEC)

[pluto]

The following statement is incorrect


I quote
pluto also observed that for a long run aerial wiring should be hard drawn conductor rather than the normal type. True it should be, but it's not actually a requirement of "3000".
Quote ends

AS/NZS 3000:2007 table 3.9 details the hard drawn cable requirement for aerial cable to which I refered to.

[AlecK]

Table 3.9 does NOT specify that hard drawn must be used.
The table specifies the maximum spans for various types of cable; and the longest span - 60 m - can apply to several types; not all of which are hard drawn.

[pluto]

But AS/NZS 3000:2007 clause 3.12.4 calls Table 3.9.

[DougP]

3.12.1 specifies the allowable types of conductors. They don't have to be hard drawn. The first item on table 3.9 is annealed copper conductors.

[AlecK]

Exactly.

3.12.4 is about distance between supports.
Yes there's a difference between the max distance between supports for annealed Cu (20 m) and hard drawn (40 m for 2 conductors 6 mm2, 60 m for 16 mm2 and greater) . Which shows that yes longer spans of copper need to be HD. And also shows that the max span for Cu depends not only on annealed vs HD but also on size & number of cores.
But - as I posted before - you can get to the max span of 60 m using other forms of cable ie the various forms of Al.
Where Al with Cu screen sits isn't clear; but I'd put it down as Al 'cos it's the cores that do the work.

And there's an Exception that effectively allows specific design

And since the OP is about a submain in a domestic installation (presumably an outbuilding), the span is very unlikely to be anywhere near long enough for these details to be remotely relevant.