PV wires - unequal lengths?

[Tadmeister]

Hi all,
Apologies if this is a very basic question but is it ok if the positive and negative cables running from a string of panels to the other equipment are of unequal length. I know that keeping the same length is very important with battery +ve and -ve cables but not sure whether this is as important with PV cables...?

I have two strings of four 455W panels and I have 6mm cables running to a fused 'combiner box' about 5m away from the positive ends of the strings/rows. The negative cables have to run about 4m further than the positives as their panel connections are at the other end of the roof. So the negatives would need to be 9m to get to the combiner box whereas the positives are only 5m. Fom the combiner box there is going to be another 6m run to the mppt/inverter/batteries etc (this is two 10mm cables carrying the combined power of both strings - 190V and 18-19A max).

I know that having such a big run is not ideal but I believe that these are the only viable location options.

Should I make the positive 6mm cable run longer (i.e. 9m) to match the negative cable length or is it ok to have unequal lengths?

Thanks in anticipation,

Tom.

[marshman]

Don't worry about it. It won't make any difference. 9m is not a long run. They are relatively high voltage and low(ish) current, so losses are not that great.

The "matched" battery leads only refers to the leads interconnecting between the batteries themselves and are required to ensure the batteries all charge and discharge at the same rate -the actual cable lengths between the batteries and inverter don't matter - the shorter the better to reduce voltage drop and losses. Battery systems are typically "only" 48V and the current is a lot higher.

[Mr Gus]

As per MM's reply.
That is not a massive run by any means when you look at multiple domestic install projects.

Better ask & do once, rather than...

[Tadmeister]

Thanks so much guys - that's reassuring!

[Sunrisemike]

Talking about unequal length of wires, I have a conundrum at the moment. We had to take 12 of the panels off of the roof to renew the felt, and re slate. The other four are on another part of the roof. I turned off the a/c and D/C switches and the roofer removed the panels. After reinstating them, neither he or I can figure out how they are wired up. There are two long leads and two short ones. I am guessing that four of the panels are linked to the other 4. Any ideas how they could be wired up? Any advise will be gratefully received!!
Here's a photo, the other 4 are on the lower roof out of shot just to the right.

Mike

[Mr Gus]

Any commissioning paperwork that went with the job?
(bearing in mind how much detail is demanded) I am surprised there is no schematic for exactly this sort of event?

[ALAN/ALAN D]

During apprenticeship with the C.E.G.B. I was taught very early on how to use a Left and a Right handed screwdriver. Also when ever you disconnect a wire always mark each cable that you take apart. They suggested with a bit of paper with numbers written in pencil wrapped round each core and stick them on with sellotape.
Every one writes differently with a pencil on a flimsy bit of paper. Very difficult to read.
I found out very early that this did not work. When there was a few of you there drinking Very Big Cups of Tea. It didn't take long for someone to bash your foot with a hammer and the hot tea gets spilled onto all the bits of paper. All the paper falls off. No markings left.

Wrap P.V.C. Coloured tape around each cable.
First cable Pick any colour Wrap one turn around first cable that's disconnected on each side.
Wrap two turns around second cable that's disconnected on each side.
Wrap three turns around third cable that's disconnected on each side.
Keep on going. When you had hundreds of cables to take apart. Change the colour of the P.V.C. Tape as you go along.

Found going back to the same box years later you could still see all the cable marks.

16 panels. 4 on one roof and 12 on the other inverted by 90 deg. ?

What make / type of panel.
What is the wattage of each panel.
What is the open circuit voltage of each panel.
How many inverters have you got.
How many MPPT strings on each inverter.
What type of inverter.
Maximum D.C. Input voltage allowed on each string.
Any shading problems / Opptimisers fitted. ?
Are you on the Isle of Wight ?

[Tinbum]

I also take photos of everything- it's so easy now.

[AlBargey]

I am a big fan of the label printer!

[Mr Gus]

Having worn out 2x dymo label printers prior to giving the hell up with ebay incompetence and support to top sellers, I resort to using the left over stickers & an array of sharpie pens, + coloured electrical cable.

Always take digital shots before breaking something down & in situ, = grief saved when troubleshooting.

This I started doing on a sony floppydisc digi cam, ..which was good for 6 shots.