Wire Gauge Required Question

pianotuna wrote:
Hi Bend,

The "magic" of MPPT is the ability to swap incoming volts for outgoing amps. Therefore, on MPPT systems voltage drop matters.

On an 17 volt pwm system voltage drop can be horrendous, and the system may still work well. A 14.8 volt target allows for ~14% voltage drop. So on pwm thin wire doesn't matter much. So long as the wire has sufficient ampacity to safely carry the load it will be adequate.

But for MPPT it is best to aim at 1% voltage drop, because once those volts "hit" the controller they are turned into amps (with a 3 to 6% loss inside the controller, depending on input voltage).

It is true that higher voltage allows for smaller wire--but line losses DO matter for MPPT.

Controller to battery for MPPT, may need to be beefed up compared to PWM because more amps may be output to the battery bank for a similar wattage of panels.

Bend wrote:
PT-

You are going to need to explain why. I thought part of the "magic" with MPPT was that smaller gauge wire could be used. At least, from panels to controller due to higher voltages. Controller to batts would remain the same as PWM.

exactly. A MPPT controller is a dc to dc voltage converter. Controller input power time controller efficiencty (95-98%) = controller output power.

So you can have high voltage low current in and low voltage high current out.
An example, 36V and 4.25V in and 13V and 12A out. rough numbers for a 160W panel.


Input power will be the panel power MINUS any power lost in the cable between the panel and controller.

With MPPT you CAN use smaller wire IF you wire panels in series to keep the current down, but you still want to use a wire size that minimizes voltage drop, 1% is a good number, that is a 2% power loss, not bad. You do NOT want to wire panels in series with a PWM controller.

With a PWM controller current in = current out. So as long as the input run voltage drop is low enough to stay above the controller required headroom above battery voltage it doesn't really matter.

However, with PWM you won't get the panel rated solar power to the battery. As an example, my "160" W panel has a Imp of 9.5A. that is
all you will get to the batteries. so 9.5Ax 13V= 123W about 40W less that panel power rating. Going to a MPPT controller, my max current is usually above 10A in bright sun, and has occasionally been close to 12A much closer to 160W.