Cable under garden for solar PV on garage roof

I have a standalone garage with a flat roof about 15m from the house. I am thinking about solar PV for the roof and my rough calculations suggest maximum 2kWp.

Before I go any further it only makes sense if I can bring the power back to the house to take advantage of the feed-in tariff. There’s a cable that runs under the garden from the house that is about 10mm in diameter. Googling tells me that a cable of this size is safe for 50 amps or so. If I’m getting 2000 W at 220 volts that’s only about nine amps, so I’m guessing the cable should be well able to handle the power. Are my calculations way off here? Is there much transmission loss with this kind of set-up?

Otherwise the cable is anything between 10 and 30 years old. Is there any risk with something so old? The arithmetic changes a lot if I have to dig a new trench and lay an armoured cable of course.

Power consumption in the garage itself is minimal, an electric garage door and a few fluorescent lights. In daytime the power output of the solar PV should be enough to run these but what about at night? Can a system be rigged for the power to flow both ways? If not, would I need a small battery to power the lights and electric door?

"10mm in diameter"

Do you mean?
1) The outside diameter of the entire cable (including 3 cores, insulation etc.) is 10mm.
2) The diameter if one individual copper core is 10mm.

Electric cable is usually sized and rated by the cross sectional area of an individual core/conductor and by the number of cores/conductors. eg: 3 x 2.5mm, known as 2.5mm twin and earth is used for socket circuits in domestic installations and has a current carrying capacity of approx 25 amps (depending on installation conditions).

It might be best to get a Solar installer to "survey" your property.

Can a system be rigged for the power to flow both ways? If not, would I need a small battery to power the lights and electric door?
1). Yes.
2) No.

Jetblue said:

1) The outside diameter of the entire cable (including 3 cores, insulation etc.) is 10mm.

Click to expand...

The external diameter of the whole cable is 10mm including cores and insulation, it's cylindrical. I haven't cut it to look but I presume this would take 2x2.5mm and 1x1.5mm (earth) including core and insultation?

Jetblue said:

It might be best to get a Solar installer to "survey" your property.

Click to expand...

I will do this in due course. I am at very initial phase and am thinking through the basics first.

If the whole cable is 10mm, it is probably 1.5mm per core, fine for lights and socket In shed.

Any reason not to put the panels on the roof of the house? Unless you're self-installing, the frame etc required to mount the panels on the shed roof may not be all that different cost wise from putting the panels on the roof of the house.

Let's say the cable is not suitable...sacrifice the existing cable in the existing trench to pull through 2 new cables in the existing trench.
1 for the Solar PV & 1 to replace the current cable to service lights etc.

Like is mentioned above ask the professionals. Don't ask them if your cable is suitable. Ask them what is the cabling requirement for the proposed system.

Zenith63 said:

Any reason not to put the panels on the roof of the house?

Click to expand...

Pitch of roof and orientation aren't super. The flat-roofed shed seemed a more obvious starting point but I will investigate both.

Horatio said:

Let's say the cable is not suitable...sacrifice the existing cable in the existing trench to pull through 2 new cables in the existing trench.
1 for the Solar PV & 1 to replace the current cable to service lights etc.

Click to expand...

I've no objection in principle but it would add an awful lot to the cost. Am not sure at all of depth or position of existing trench, at the moment it enters the house through a concrete wall a little below ground level (no armour visible) so any replacement would probably need a new access point.

It would be good to figure out definitively what the cable you already have is. Where does it end in the shed, is there a junction box or socket you can pop off to take a look? If it's buried directly in the ground it is likely a black cable with armour beneath this black sheath, then another layer of plastic, followed by your three cables (which will have coloured sheaths on them as well). You might even find the spec of the cable stamped into the outside of the black sheath every meter or two.

It's possible you could take the existing cable as the AC feed to/from the inverter and power the shed from the inverters AC output if it has one (mine did). According to calculators like this - https://www.elandcables.com/cable-calculator - even a 1.5sqmm cable is capable of carrying 2kW over 25-30m. The bit I'm not sure about is that you need to connect an automatic fireman switch in between the panels and the inverter, so that if the mains electricity goes off the panels stop producing power to prevent them electrocuting somebody working on the overhead wires or a fireman who thinks they've disconnected your house then starts spraying water in or chopping away at things - I don't know if the feed to this fireman switch needs to come on a separate cable from your main fuseboard.

Zenith63 said:

It would be good to figure out definitively what the cable you already have is. Where does it end in the shed, is there a junction box or socket you can pop off to take a look?

Click to expand...

I popped the junction box open and it's a black 10mm-width cable with live earth and neutral. There is a metre or so of cable visible and unfortunately nothing written on it.

The more I think about it the more I think I'll probably need a higher-rated cable or cables long term. I'd like to get an EV eventually and the garage is the only place for us to charge one. For decent charging for sure the existing cable would be far too slow. That means some combination of use of the existing trench if possible and probably some excavation near the house too. As it happens the trench seems to be shared with a water pipe - they come out at the same spot in the garage - and am not sure if this helps or hinders things.


Thanks to all for the answers.