PV, DC cabling & MBC roof

[vivienz]

I'm getting a few quotes in on PV systems for the roof of my MBC build and I have a slightly tricky issue of how to route the DC cabling from the array, underneath the tiles on a south facing 35° inclination roof, over the ridge and down the north facing side, into the single storey flat-roof garage where the inverter will live.

 

I'd like it all sealed and tucked away where it won't have anything nasty done to it by local rodents or birds, but it's whether there is enough clearance for the cabling and all its protection.

 

Any suggestions, please?

[Jeremy Harris]

Our cables run through 25mm flexible conduit, alongside a counterbatten and up and over the ridge and then down the roof the other side and out behind the wall cladding to the externally mounted inverter.  Four cables (for two strings of panels) fitted easily inside 25mm flexible conduit OK.

[SteamyTea]

Has anyone calculated the losses in a long DC cable run.  Bit hard I know as the power is varying, but maybe at 5 power settings, with one being maximum and one at 500W and the others spaced between.

I don't think it is a problem, but have seen it give a 10%ish loss at 4kW on a ground mount system.

[ProDave]

The loss will be negligible. It's better to have long DC cables as the voltage is higher and hence the current is lower on the DC side.

 

If you are losing 10% in the cables, someone has messed up big time and wired them with bell wire.

[SteamyTea]

12 minutes ago, ProDave said:

If you are losing 10% in the cables, someone has messed up big time and wired them with bell wire.

I agree, it was a long run back to the house.  I suggested that they double up the cable size where they could.

Was a dreadful installation.  The inverter was in a greenhouse.

[Jeremy Harris]

Found the calcs I did.  The total voltage drop for the longest run over the roof was under 0.1%, so not worth worrying about.

[vivienz]

Splendid! Thanks, chaps.

[Ferdinand]

Obviously install the conduit before the roof gets on if you can, with a drawstring.

 

(for future readers).

[Jeremy Harris]

4 hours ago, Ferdinand said:

Obviously install the conduit before the roof gets on if you can, with a drawstring.

 

(for future readers).

 

 

You have to install the panel mounts and flashing before the tiles/slates go on, as they over/underlap them, so as long as the installer runs the cables in at the same time (which ours did as a matter of course) then things should be fine.  Fitting the PV panels is best done after the slates/tiles have been laid, to avoid the very real risk of damage to the panels by the roofers.