Originally Posted by Shadowcatche
It is a Suntech STP185S open circuit voltage is 45V (I have seen it higher?)
First, get the electrical specs for your panel:
Look for these values (names in parenthesis are the terms Suntech uses) in the panel's specs:
= the "nominal" panel voltage = 24 V. This is the value people typically use when referring to a panel. However, output voltage is typically higher -- as you saw with a 45V output. That's the Vopen-circuit voltage (see below).
(or "Optimum Operating Voltage") = 36.4 V
(or "Optimum Operating Current") = 5.09 A
(or "Open - Circuit Voltage" = 45.0 V
(or "Short-Circuit current") = 5.43 A
= Vmax voltage x Imax current = 36.4 x 5.09 = 185.3 W
Second, use these values to compute fuse size. This is the fuse that should
be between a PV array and the controller (and the batteries).
Fuse size = 1.56 x Ishort-circuit current
= 1.56 x 5.43 = 8.5A (a 10 or 15A fuse will do).
If you based wire size on just fuse size amperage (15A), you'd only need 14 gauge wire.
However, you need to figure line loss for a low voltage system like this.
Third, to figure line loss, I use this online "VDI" calculator because it lets you pick the percentage of line loss:
How to Size Wiring and Cabling for Your System - AltE
This forumula is used to compute a VDI value which is then used to select the wire size:
VDI = (AMPS x FEET) / (% VOLTAGE DROP x VOLTAGE)
For your panel, figuring a 1% voltage drop at 12 feet:
VDI = (5.43 x 12) / (1 x 24) = 2.7 = 3 (rounded up for greater wire size)
For a VDI = 3, a 12 gauge wire is recommended for a 24V PV with a 1% voltage drop over 12 feet.