Duct size for cables

13 Mar 2016
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United Kingdom

Im hoping to get a company to 'mole' in some SWA cables from my house to my garage - approx 25 metres length. These are going to be 6mm cables with a nominal outside diameter of 17.4mm, there will be two of them. Alternatively I mioght only need one 6mm and one 4mm (outside diameter 14.9mm).

Could anyone comment on what size ducting I should be looking to install to take these cables at a later date? the moling company is keen on not putting in anything over 37mm but not sure thats technically big enough as I understand youre not supposed to pack the cables too tight.

Many thanks in advance
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No way will 2x 17.4mm will pull into a 37mm duct, a 50mm will not be easy, i'd go for a size or 2 up on 50mm like 63 or 75.
Thanks for that - yes it did seem a bit tight to me.

Ive actually worked out Im going to need three cables (one for a 7kw EV car charger, and two to carry roughly 3kw each from future solar panels taking DC curent to the inverter stored in the garage, and one back the other way from the inverter /battery carrying AC back to the consumer unit). From calculations done on line I think I should actually be able ot use 2.5mm SWA for the non EV charger cable - which are 13.8mm in diameter.

The limitation I have is that because theyre going to mole the ducting under the garden they can really only put in a 38mm. They could put in a larger one but it would need them to bring different equipment which theyve implied would be more of a hastle. So what Im wondering is can I put the chunky 6mm cable down one duct, and put two 2.5mm down the other one? Or would I be best off putting three ducts in? The guys doing the work have suggested just getting the mole to drag the cable through the ground without any ducting - only problem being I dont yet have the cable and they are on site now doing some other work for my neighbour, and they want to do this tomorrow, I also might need to be put a data cable through the duct also.....so think ducting will be less hastle and easier to replace the cable in future if need be..
Even 2x 2.5mm² in a 37mm duct can be tight, depending on bends and length.

I doubt the DC cable for solar will be only 2.5mm², especially over such a distance. 3KW at 36V = 100A = 25mm²(or 48V = 60A)

If 37mm is the only feasible size they can put in, Yes have a number put in, one each for your 3 cables and if you thing there is a chance of data requirement add a 4th.
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Learning on the go.
Just been doing some calcs. As the panels will be installed in series rather than parallel the voltage will be 216v, current will be max of 11amps.
According to an online calculator 4mm2 (OD 5.2mm) DC cable is needed to take that current, so will be two of those (10.4mm total) to connect the panels on the house to the inverter in the garage.

Maximum design output of the PV panels will be 4.8kw AC back from the garage to the CU in the house, or 3kw tops at night from the battery storage so 4mm cable should suffice, which would be 14.9mm diameter

So my plan would be:

Duct 1 - 6mm SWA cable for EV charger
Duct 2 - 4mm SWA 14.9mm + 2 x 5.2mm dc cables

The ducts are totally straight, actually about 22m long
I've had little experience of PV panels and so far only encountered ELV (36V being typical) at 216V it's a different ball game.

I don't know where you have 11A from. 3KW at 216V is 14A and 4.8KW is 23A and this assumes no losses anywhere.

By the time the cables are run between roof and control kit in garage (assuming I understand your description correctly) allow 2m in garage, 22 for duct, 7m to get to roof and say 9m on roof = 40m. At 23A the volt drop is around 9V or ~4% and I imagine the installers will be wishing to use something more than 4mm² over that length. Additionally I'd imagine they'd be intending to install an armoured cable underground.
The 3kw figure was a guesstimate before I checked the specs of the panels that have been suggested. They are 405w panels at 37v/ 11A each. So 6 in series actually results in 222V, at 11A thats 2.4kw. In reality you will rarely (if ever) get 2.4kw out of them at any point even at mid day in mid-summer.

4.8 kw is the total capacity of the whole system of 12 panels. 6 will be on the garage roof and no need for ducting cables underground as they will be surface clipped.

Sunray - I suspect youre right in saying that 6mm may be better given the distance - although thats not what the solar company have quoted for (virtual survey of course....). Having said that on one of the online calculators its coming out at 4mm still over 40m (15 m being about right for the distance from the array to the inverter).

Ive managed to persuade the moling chaps to put three 37mm ducts in, so my plan is:

Duct 1 - 6mm SWA for EV charger

Duct 2 - 4 or 6mm SWA to connect solar array to inverter

Duct 3 - 4mm SWA to connect DC-AC inverter from solar system back to consumer unit *should be able to handle the 4.8kw peak output with ease) + 2 core energy monitor cable for car charger (this may not be necessary as can actually get a 6mm SWA with this built in although rather pricey).

I think that feels better.... thanks fo the advice
Thanks for all previous advice

Had one other question relating to this

What would people recommend I use to physically attach the SWA to the blue poly rope in the ducts? I imagine it needs to be something that gain take alot of weight. Was thinking some kind of bolt on clap?? Any sugestions?
You need a chinese finger or towing sock to attach the draw cord to the SWA


If possible use a braided cord rather than poly rope. Poly rope is twisted and when under tension it tends to untwist and this can twist the SWA. Braided cord does not untwist when under tension. Poly rope can be used if there is a swivel connector between rope and chinese finger

Twisting the SWA by itself is not a significant problem but if you are also pulling in a new draw cord along side the SWA then the new draw cord will be wrapped around the SWA and not be much use as a draw cord.
Presumably the size of the chinese finger quoted needs to match what cable/ item you want it to grip on to? So 13-16 for a 15.8mm od cable say?
Do they mean mole plough or directional drill? Moving can mean either

If they intend to plough them in are you sure there are no other services they can hit with the plough.

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