OK. Let’s try to get a consensus here.
From the DTI guide:
2.1.2: Use (total) Voc and Isc to specify ‘components’, ie switches, connectors, cable type, etc, using safety factors of 15% for voltage, and 25% for current.
2.1.4.1 Cable sizing: Apply the safety factors as above (plus any BS 7671 derating). It’s not specified which parameters to use for the calculations though, which I think it should.
As Screwdriver says in his excellent post #5, Imp and Vmp are used here.
I hold the DTI guide is wrong not to state this clearly.
2.1.4.1 also states, clearly, that the Voltage Drop between array and inverter should be less than 3% (one tick therefore a recommendation.) So working on a 2.99 % drop for your dc calculations is fine.
2.1.6 is titled ‘Main d.c. cable’ but is actually (I think) talking about panel interconnects and other local array wiring, which is confusing. Using, again Voc and Isc, it applies only to cable type; it’s nothing to do with cable sizing per se. This is where I went wrong in the beginning.
Does anyone have a referenced formula, in metres and mm[SUP]2[/SUP] (not, like most on the web, AWG & feet) for DC volt drop calcs in underground SWA please?