Panel to Solar Controller Wire Size
As some of you already know, I'm piecing together a solar system. Controller is a Morningstar 45A PWM. Panels will be two 12V 150W wired in parallel. I'm going to be using a combiner box on the roof, ...
Forum Discussion
How do you know there's still plenty of voltage headroom?
You know that voltage drops about 0.5% per deg.C rise in panel temperature.
If you go with a 2% cable voltage drop, voltage at the panel must be minimum: 1.02 * 14.7V = 15.0V
If the panel temperature rises 40C, then panel voltage at full current must be: (1 + 40C * 0.5%/C) * 15.0V = 1.2 * 15V = 18.0V
Bottom line, this panel will not output close to Isc current when panel voltage is at 18.0V. Charging current will taper long before battery reaches 14.7V. It will probably taper below 13.7V.
The cable sizing exercise boils down to when you want tapering to begin.
Sal
You know that voltage drops about 0.5% per deg.C rise in panel temperature.
If you go with a 2% cable voltage drop, voltage at the panel must be minimum: 1.02 * 14.7V = 15.0V
If the panel temperature rises 40C, then panel voltage at full current must be: (1 + 40C * 0.5%/C) * 15.0V = 1.2 * 15V = 18.0V
Bottom line, this panel will not output close to Isc current when panel voltage is at 18.0V. Charging current will taper long before battery reaches 14.7V. It will probably taper below 13.7V.
The cable sizing exercise boils down to when you want tapering to begin.
Sal
smkettner wrote:
You will not get Voc (23v) at max amps. Closer to Vmp (19.1v) to figure losses from regulating voltage.
Still plenty of voltage to practically ignor wire size from panels to controller.
