PowerSwitch

I'm getting my PV panels on the roof in a couple of weeks, and I'm struggling to find somewhere to buy suitable cable.

I'm going to have 1-2m from each of the 6 panels to a junction box. These need to carry just about 2A each, so it's easy to get hold of this cable in a form which is OK for outdoor use exposed to sunlight - i.e. 2-core rubber coated cable.

The problem comes after the junction box, because from there the cable could be carrying up to 15A (the charge controller will be limiting it to this as a maximum). This run will be 10m, so I want cables of 16mm2 cross section. As it will go around the outside of the house and in through an air vent, so it needs to be OK for outdoor use. What I've found so far is:
- there doesn't seem to be 2-core cable of this size that's rubber coated
- there's plenty of armoured cable of this size, but the armouring makes the cable huge (over 20mm across), so hard to get into the junction box and pretty heavy.
- I've wondered about using standard PVC-coated cable, but running it inside some hosepipe to keep the sun off it.

Any thoughts?

I would consider either standard single core PVC cable in plastic conduit, or standard twin and earth cable which is available in large sizes.
Twin and earth is not allways considered suitable for outdoor use, but in practice seems to be fine for at least 20 years.

Welding cable is somtimes used, but is perhaps OTT for only 15 amps.

Thanks Adam, I was hoping you'd give some advice!

Do they have to be insulated? Just use solid metal supported with wood i.e. busbars?

I would advise against bare conductors, a painful electric shock might result in wet conditions, and also leakage currents will pass in wet conditions leading to rapid corrosion.

Insulated will be easier to fix to the house as well I think, no need to worry about sorting it out!

A related question: what's the best way of joining the six pairs of wires from the PV panels to the pair that go to the charge controller and battery? One idea I had was to put eyelet terminals on the ends of the wires, then just bolt them together. But is there a better way? Is there some kibnd of terminal block to to a 6->1 link? Can't seem to find one, but maybe I'm looking for the wrong thing...

The terminal block, or whatever, will all be inside a waterproof junction box on the wall, just under the edge of the roof.

I used terminal blocks from an old fusebox, Mike.

If connecting a number of PV modules in parralel, it would be preferable to fit a fuse to each one.
The will reduce the harm done if one module developes a short circuit.
If each module can produce 5 amps, and 6 are parallelled, then a fault could results in 25 or 30 amps passing.

I would prefer to run a cable from each module to an indoor location, and connect each module to its own fuse in a cheap fuse box.
Then connect heavy cable from the main terminals of the fusebox to the charge controller.

Alternatively cheap fuseholders are available from Maplin, RS and others, mount a suitable number in a weatherproof box with a large terminal block. Better inside though IMHO since weatherproof boxes are not allways as weatherproof as one might hope.

adam2 wrote:weatherproof boxes are not allways as weatherproof as one might hope.

...especially after opening them a few times; it's easy for the seal to move out of place and for it to become wrecked...

This sounds very DIY. I thought FITs schemes had to be installed under the auspices of an MCS installer?

But if it is DIY, I'd go to an electrical wholesaler and ask them if they supply the same cable as the local eletrcicity company uses for drop wires - the brown sheathed, single/multicore 25 mm^2 core stuff. That obviously lasts outdoors.

I got the cable from a company focused on renewable energy in the end. They had the 2-core cable for the PV and I also single core 16mm2 cable for the main run to the controller - I realised if I used 2-core for this it was going to be too fat to go through the glands into the junction box!

Oh, and for the junction itself, I've got a dual busbar intended for marine use on the way, a surplus new item off ebay, so was reasonably priced.

And as for it being DIY, it is, but that's because it's not connected to the grid, and won't be eanring any FITs. I already had much of the equipment anyway, and have just spent the extra on additional panels, roof mounting kit, cable & accessories and more batteries.

Interestingly, I think the ancilliary components are going to have ended up costing nearly as much as the panels! I guess this is the downside of low voltage DC systems - lots of copper...