Install/cable solar panels

[JC_UK]

Hi,

I am looking to connect up my 12x 260w solar panels as follows:

1st string - 4 panels - ~1kw
2nd string - 8 panels - ~2kw

Couple of questions please?
I assume best way to connect the panels will be in series and then to the inverter inputs.
For the above the cable run to the inverter will be approx. 6m for the 1st string and approx. 24m for the 2nd string.

Q1. So for the cable back to the inverter should I use 6mm or 4mm, or other cables?

Also the first 12m or the 24m run for 2nd string needs to go under a shared gravel right of way track/roadway ......
Q2. How deep should the cables be dug in please and protection required for the cables?

Many thanks
JC

[Oldgreybeard]

I'd choose to go for 6mm² just to minimise the voltage drop, especially as the cost difference isn't great. To go under the drive use SWA, buried 700mm down (that's the spec for under public paths or roads). You can bury SWA direct, but I'd be inclined to put it in a bit of 50mm underground duct, in case you ever need to replace the cable, plus I'd add a length of pull rope in it, just in case you need to pull another cable through in future. There also needs to be warning marker tape above the cable/duct, as a warning to anyone that comes along and digs in that area in future.

[Stinsy]

Hi,

I am looking to connect up my 12x 260w solar panels as follows:

1st string - 4 panels - ~1kw
2nd string - 8 panels - ~2kw

Couple of questions please?
I assume best way to connect the panels will be in series and then to the inverter inputs.
For the above the cable run to the inverter will be approx. 6m for the 1st string and approx. 24m for the 2nd string.

Q1. So for the cable back to the inverter should I use 6mm or 4mm, or other cables?

Also the first 12m or the 24m run for 2nd string needs to go under a shared gravel right of way track/roadway ......
Q2. How deep should the cables be dug in please and protection required for the cables?

Many thanks
JC

If it is underground it needs to be SWA. The easy way to do it is "short runs 4mm², long runs 6mm²" the proper way to do it is with voltage drop calculations.

You haven't given the specs of the panels so I'll assume 40Vmp and 6.5A Imp.

The first string will therefore be 160V, 6.5A, wired in 4mm² over 6m gives a 0.23% voltage drop.
The second string will therefore be 320V, 6.5A, wired in 4mm² over 24m gives a 0.31% voltage drop.

Both perfectly acceptable (generally 1% voltage drop fro the DC side and 2% for the AC side are the guidelines). However I'd still lay 6mm² because that gives you the opportunity to swap the panels at a later date.

Interesting choice of panels by the way. Are they very old used ones? Nowadays panels tends to be 350-450W units.

[JC_UK]

Hi,

Yes, the panels are pre-used. Build date 2017, so not as old as the Panasonic panels on our roof.

So for future expansion run in 6mm.
I was thinking of running normal 6mm solar cables through a length of 20mm Blue water pipr for protection.

Do you have a diagram/drawing of how to cable between the panels and inverter in SWA please?
Also what depth to bury to on a driveway please?

Many thanks

JC

[Oldgreybeard]

Hi,

Yes, the panels are pre-used. Build date 2017, so not as old as the Panasonic panels on our roof.

So for future expansion run in 6mm.
I was thinking of running normal 6mm solar cables through a length of 20mm Blue water pipr for protection.

Do you have a diagram/drawing of how to cable between the panels and inverter in SWA please?
Also what depth to bury to on a driveway please?

Many thanks

JC

As already mentioned, cable needs to be SWA, buried to a depth of 700mm, as it's a shared drive, so really needs to be at normal utility depths used for services under public paths or roads.

Plastic water pipe isn't adequate protection for cable that's underground, too easy for someone to put a spade through it.

Also, as mentioned earlier, there needs to be warning tape above the cable so that anyone digging there in future gets an early indication that there is a live cable a bit further down.

To make off the SWA connection to the solar cable needs a junction box, I always use Wiska boxes, as they are threaded to take SWA and nylon cable glands. Best to use piranha nuts on the SWA glands IMHO, they make connecting the CPC a lot easier than using a banjo washer.

[Stinsy]

Hi,

Yes, the panels are pre-used. Build date 2017, so not as old as the Panasonic panels on our roof.

So for future expansion run in 6mm.
I was thinking of running normal 6mm solar cables through a length of 20mm Blue water pipr for protection.

Do you have a diagram/drawing of how to cable between the panels and inverter in SWA please?
Also what depth to bury to on a driveway please?

Many thanks

JC

You can run ordinary solar cables through twin-wall duct, but it has to be proper duct not any-old pipe. There aren't any hard-and-fast "rules" (unless you're going under a public road) but if you really want to do a proper job it needs to be SWA at least 450mm deep covered with 150mm of sand then warning tape before backfilled. However in reality you just need to use common sense. EG don't tack it to a fence where the strimmer is going to get it, don't put it so shallow beneath a lawn that someone shoving a fork in will get it, etc.

To terminate the panels into the SWA then use ordinary solar cable stuffing-glanded in to a Whisk box (or similar), with he SWA also glanded in and make off the joint inside.

[Oldgreybeard]

The regs for under surfaced public access drives etc state at least 600mm minimum. 450mm is the depth allowable under areas like unmade ground and other soft ground. Highways regs for most places now state 700mm minimum for LV under roads (not 600mm as in the graphic below), and my inclination for a shared drive would be to use the highways regs, TBH, as anyone could come along at a future date and start digging there, assuming that any services would be at highways depth. Sand should be 75mm under and over the cable, with the warning tape 150mm above.

cable depths.jpg (90.72 KiB) Viewed 1503 times

Cable MUST be SWA (or protected to at least the same degree as steel armour), it isn't acceptable to use anything else, and definitely not soft cable in duct, twin wall or otherwise, unless it's metallic. From the wiring regs for LV cables (i.e. under 1,000V AC, 1,500V DC):

Buried cable - regs.jpg (40.93 KiB) Viewed 1488 times

Worth adding that the utility companies are immune from these regs, they have their own, hence the reason that they, and they alone, are allowed to bury non-armoured concentric cable anywhere (although they do stick to putting it 700mm down). As our transatlantic cousins might say, "go figure" . . .

[Tinbum]

I had to bury our DNO ducting at 900mm deep across a field to our house.