DC isolator sizing

[PVdesign]

I seem to be getting myself a little confused.
My boss keeps quoting 63A DC isolators on commercial jobs? I thought that when the panels are in series the current stays the same but the voltage increases (is added together) to get Voc and Isc? So I've been quoting 20Amp 800V isolators?

How do I work out the string current and the relevant DC isolators needed on say a string of 23x 250Wp panels with a short circuit of 8.71A?

Thanks in advance

 

[AndyL]

Isc x Number of strings x 1.25 for the current rating
Voc x number of panels in a string x 1.15 for the voltage rating

 

[SibertSolar]

AndyL is spot on. 23 panels is likely to be too many for most inverter MPP trackers though so you might need to double-up on the Isc (parallel strings), or run each string into its own tracker input. With most string set-ups, the individual module Isc + 25% is going to be your minimum current rating on the disconnector. For 8.71A Isc, you should select a switch that is rated upwards of 11A per pair of poles, so your 20A @ 800V unit should be plenty (depending on the Voc+15% of course..). Sounds like your boss is confusing AC isolators with DC disconnectors.....

 

[PVdesign]

Yes it seems he was confusing AC with DC! Thanks for the help!

 

[Gavin A]

we often find it best to parallel 2 strings on the roof to minimise cable runs, number of isolators etc so you'll want to make allowances for this when sizing isolators if this is what you're doing (and maybe have a think about it if you're not doing it).

We'll then up the cable to 6mm2 for the 2 parallel strings, particularly as cable runs tend to be fairly long as well. Possibly you could still get away with 4mm, but IMO it's not worth it.