Peak output

Earthstore · Jul 7, 2012

While they are refitting your inverter, get them to clip all the cables as they really should not be hanging from the switches just relying on the terminal connection.

A few clips only cost pennies, mind you they can not have much profit left in your job, but they should have got it right the first time.
I know you have been persistent but they do keep coming back so they are due a little credit.

wstevew · Jul 7, 2012

Hopefully they will continue to do the right thing and learn from their errors. But they are big enough to know better.

scottliv · Jul 8, 2012

I also have a 3.75kw system using an sma 4000tl. It has a 15m ac run using 2.5mm swa. I am currently in discussion with the installer to have it changed to the correct 6mm as stated in sma guidance. He says the 2.5mm cable is within spec according to the BS, but does that not also state that the system must be installed iaw manufacturer guidance /instructions. If it does is it possible to post the reference to this. Sunny design and the draka cable calculator both say the cable is undersized for a 1% drop.

scottliv · Jul 8, 2012

Quick update from the post above. I have just watched the grid voltage at the inverter change significantly. It is a day with bright sunny intervals. Voltage at 3.9kw was 247v dropping to 240v when generating only 400w. This must be an indication of the inverter overcoming the volt drop in the cable at higher currents. Does anyone have any views on this?

Gavin A · Jul 8, 2012

SMA say there are 2 reasons for needing the 6mm2 cable instead of the 2.5mm2 as per 17th edition.

1 - Volt drop needs to be limited to 1% along the cable to avoid the inverter having to kick out at too high a voltage relative to the grid voltage to avoid it tripping out the G83 over voltage settings too often.

2 - The 17th edition cable sizings are based on the maximum load only being used for relatively short periods of time, whereas a 4kW PV system could be kicking out 4kW continuously for 5-6 hours on end, so the cable would be more prone to overheating.

it's sheer incompetence IMO to be using 2.5mm cable when both the manufacturers written instructions and their design tool state that this isn't acceptable, though I know a lot of companies seem to have been doing it.

this is an issue that will become more and more problematic the more other systems get installed on the same phase from the same transformer, as in peak sunlight levels they will push up the local grid voltage, and it will be the inverters on the cable run with the highest resistance that will end up dropping out first. From your figures, it looks as if you're will cut out when the incoming voltage is around 246, which is utterly ridiculous given that the 10 minute cut out voltage SMA use is 253V.

get them back.