Solar panel wiring.

[dobbslc]

A quick question on solar panel system wiring to the knowledgeable peeps on this forum. I've had a quick Google but can't find a definitive answer.
I'm helping a friend on his loft extension and while the floors are up and we have access to the stud walls etc what is the best cable to run from the roof to the fuse board downstairs?
There's a new distribution board being installed by the builders in the loft extension but I'm assuming the solar system cabling needs to go directly to the main house D/B/meter tails?
Size wise I'm thinking 4mm T&E.
Any thoughts?

 

[Gareth J]

Where does he want the inverter? How big (kW) will the inverter be? How many kW of panels? What sort of distances between panels, inverter and CU are we talking?

Edit, if the supply cable to the loft extension is large enough, you may get away with plumbing into that. A certified installer may not want to though.

 

[dobbslc]

No idea about most of that
The inverter won't fit where the dist board is as it's by the front door, the total run from roof to dist board is about 15m max.

 

[Gareth J]

Need to know because if you're running DC (raw solar elec) behind the loft boarding, it'll be different cable to the AC

 

[MetalMonkey]

If you are running the cabling for the DC from the panels, really you have to know what sort of system it will be - there are lots of different designs with various configurations of strings, optimisers etc.
Really need to design the system first and then run the cabling. The PV-Ultra stuff from Doncaster is supposed to be quite good and available as a two or four core.

If you are running a normal twin and earth for feeding back the AC from the inverter into the grid, then you need to calculate the size based off the inverter taking into account the usual derating factors and volt drop.
I'd suggest in most cases a 6.0mm will be plenty if you just want to sling in something for future.

You could probably mount the inverter in the loft, but consider temperature as well - lofts can get really hot which isn't great for the inverter.

As to where it goes back to, that will depend on how you are feeding in - presumably there will be a feed in/generation meter.
I always thought it was better to split off the tails and feed via a separate db rather than backfeeding it into the consumer unit which is the way most of them seem to be done.

 

[Wildefalcon]

Put a conduit in?

 

[Chippie]

Put a conduit in?

Doesn’t have to be 20-25 mam either….I ran speaker wires under the floor using 50mm waste pipe…

 

[Gareth J]

Need to know because if you're running DC (raw solar elec) behind the loft boarding, it'll be different cable to the AC

All these things aside though, presuming it's a fairly usual setup, 4mm AC might be ok, 6mm almost definitely. And probably two strings (4x cables) of 4mm DC solar cable.

 

[dobbslc]

Thanks! I've never fitted solar panels before and won't be this time it's just so we can get the required cables installed while the roof is off and floors are up etc.
A conduit is a good idea if I can find a fairly straight run.
I imagine the inverter will be in the roof space near the DC cables. Being just a simple sparky I don't like the sound of running high current DC through the house is that a done thing?

 

[J1nx]

6mm preferred for string cables, panels will be 4mm, I’d run in conduit. Cut off the panel MC4s and install matching pairs, preferably Staubli brand, the newer ‘evo’s’ are nice but require a tool to part.
Most inverters now have approved DC isolator so straight into the inverter (do not add additional), again match MC4 brand if at all possible.

 

[Mikes]

If you are putting a conduit in, put a draw wire through it as you go and save your mate a load of grief at a later date.

 

[Erie Fred]

Question on solar panels....do they have some fusing and/or switching right at the panel ?
or is the wire from the roof, down to the inverter un-protected ?

 

[dobbslc]

Good question^

 

[Gareth J]

I imagine the inverter will be in the roof space near the DC cables. Being just a simple sparky I don't like the sound of running high current DC through the house is that a done thing?

No, I don't particularly like the idea either. Fairly high voltages too. But cables are double insulated and the electrons have to get from the roof to the inverter somehow. Wouldn't surprise me if conduit was mandatory.

 

[J1nx]

Question on solar panels....do they have some fusing and/or switching right at the panel ?
or is the wire from the roof, down to the inverter un-protected ?

Panels have no protection but are self limiting, a short can only produce so much. If you have a combiner box for multiple strings then generally you fuse at this point, usually 15A.

Isolators and MC4s are by far the most common failure point, hence my post above to avoid extra isolators and use matching, branded MC4s

 

[Dave-sparks]

You want to achieve the smallest voltage drop possible between the inverter and the incoming supply, as far as I know 1% is the recommended maximum.

If the cable is being buried in a wall or suchlike then it should have good mechanical protection to prevent damage from drills, screws/nails etc. This circuit is not like a normal circuit as there is a source of supply at both ends of the cable.

Under current regulations a T&E cable buried in a wall will have RCD protection which will trip should an accident occur and someone receive a nasty shock. With the solar circuit an RCD will still trip but the inverter will continue to supply electricity for long enough to ensure that the person receiving the shock is well and truly dead before it shuts itself down due to the supply failure.

 

[MetalMonkey]

With the solar circuit an RCD will still trip but the inverter will continue to supply electricity for long enough to ensure that the person receiving the shock is well and truly dead before it shuts itself down due to the supply failure.

This is why I never understood why it was considered acceptable to backfeed through an MCB into the consumer unit - pretty much makes the RCD redundant as the busbar will still be live for potentially several seconds before the inverter registers the loss of the grid.
I suppose if they are all RCBOs it might not be so bad, but I've seen plenty of single RCD and split load boards done this way.

 

[minimutly]

This is why I never understood why it was considered acceptable to backfeed through an MCB into the consumer unit - pretty much makes the RCD redundant as the busbar will still be live for potentially several seconds before the inverter registers the loss of the grid.
I suppose if they are all RCBOs it might not be so bad, but I've seen plenty of single RCD and split load boards done this way.

Er no, the inverter is (usually) bound by the G83 type approved regulations that mean it trips allmost instantaneously. Not sure if the newer ones are slower, but I wouldn't be suprised though.

 

[Jack_K]

Question on solar panels....do they have some fusing and/or switching right at the panel ?
or is the wire from the roof, down to the inverter un-protected ?

In Australia for many years it was a requirement that a rooftop isolator was fitted (switch attached to mounting rails). It was intended to be used to isolate the wiring for safety however to do that you have to be on the roof anyway. The problem with them is that they are plastic and degrade in the sun and therefore water eventually gets in. 500v+ and water equals fire.

They finally removed the requirement for rooftop isolators a couple of years ago.
As long as the panels are all in series fusing is not needed as the panels cannot possibly output higher current than the short circuit current.

 

[J1nx]

Er no, the inverter is (usually) bound by the G83 type approved regulations that mean it trips allmost instantaneously. Not sure if the newer ones are slower, but I wouldn't be suprised though.

It will be G98/99 now and I think the requirement is 0.5s for loss of mains.

DNOs have a bit of a balancing act, fast safe disconnection is obvious but unfortunately tight tolerances that trip out all local inverters could cascade into a serious problem so they need to consider this and maintain a fault ride through ability.