... Maybe someone should do the definitive calculation using all the right figures to put us out of our misery.
Here you are, now I understand which figure is which (NO thanks to the DTI guide, but many thanks to others on this forum) it’s easy:
Total current at max power, where panel Imp = 4.94A, 4.94 x 2 x 1.25 =
12.35A
Total voltage at max power, where panel Vmp = 50.6V, 50.6V x 8 x 1.15 =
466V
Total Isc where panel Isc = 5.35A, 5.35 x 2 x 1.25 =
13.4A (As
Screwdriver said, we can all agree 10mm can handle this without getting out our tables!)
Total Voc where panel Voc = 60.5V, 60.5 x 8 x 1.15 =
560V (SWA 600/1000, no problem)
Cable volt drop calculation for the
165m DC run (now measured accurately) on 10mm 4-core (4.7 mA/V/m) is:
Vd = 4.7 x 165 x 12.35 / 1000 v =
9.58V
9.58/466 x 100% =
2.06%
So, as my gut was telling me all along, 10mm is fine.
Compared with running AC for 165m, engineering an AC volt drop of 1% max to prevent over-voltage tripping, it’s a huge saving in copper (and hence cash!).
I rest my case as to which option is better.
I just ordered 4-core 10mm at £4.07/m and some 16mm 4-core (for something else) at £5.68/m. Was I ripped off?
ps. I initially thought one uses a different Vd formula for DC. I was confused, and using Ohms/km rather than mA/V/m, and reckoning you had to double the cable length. The latter already takes account of the return path, and apparently reactance and capacitance in big-CSA cables, which is negligible for the smaller stuff, so the single phase figure can be used for DC. Please tell me if this is wrong!
Also, the calc above does not take account of any cable temp factor. Again, it’s negligible and not worth bothering with in this case since 10mm is clearly within spec.
I’m bored with all this thinking now. I’m going out to dig holes instead
Thanks, gentlemen, for all you helpful comments.