DIY Manual

40 | P a g e 10-543-1 REV A
DIY Manual
ii. In this example, for a PV module with an Isc rating of 9.2A, you need to
ensure the wire its connected to has an ampacity rating of at least 14.4A.
5. Lastly, to select the correct sized circuit breaker, follow these two rules:
i. If you use a MidNite circuit breaker, the second 125% factor does not
apply because our breakers conduct 100% of the rated current
continuously. Use a 12A breaker to protect the 11.5A of current flow.
ii. If you do not use a MidNite circuit breaker, then the second correction
factor (125%) applies. You will therefore need a 15A breaker for the
potential 14.4A of current.
Voltage Drop Considerations
When selecting the proper gauge wire to run between the PV array and the charge controller,
thought should be given to the inevitable voltage drop across that distance. Voltage drop
means the loss of voltage due to wire resistance. To minimize voltage drop, a larger size wire is
needed. As a rule, the PV industry strives for 2% or less voltage drop in system design. Voltage
drop calculators are available online or use a DC wire chart with voltage drops annotated.
How does voltage drop impact MPPT charge controllers?
Voc is the maximum voltage a PV module can produce under test conditions (can be higher in
cold temps). The Vmp is the maximum power voltage the panel will produce during normal
operation. Vmp is on average about 72-82% of Voc. So, what does this mean? Although you
used Voc to determine how to wire the panels in series or parallel and being mindful of the
charge controller’s input voltage limits, the Vmp is the voltage the MPPT charge controller
actually manipulates to make power. Therefore, the voltage drop impacts Vmp.
For your MPPT charge controller to work, it needs to have a Voc at least 133% higher than the
highest battery charge voltage. So, if your system is designed with an input Voc that is close to
the minimums of the 133%, then you probably cannot afford a large voltage drop, otherwise
Voc will drop below the minimum required for the MPPT charge controller.
NOTE: To determine correct wire size to minimize the negative impact of voltage drop, use Voc
in your calculations. No correction multiplier is required. Using Voc in the calculations
increases system efficiency.
E (Volts) = I (Current) x R (Resistance)
If resistance increases and current stays the same, then voltage must lower (drop).