Solar Breaker Calculations

Solar DC Breakers: 

There are many ways to calculate this , This is easy.

Watts / Volts (array) = Amps

For this exercise, I will be using the same 345W panel in 2s5p configuration, into a solar combiner box, we will be using 5 inputs

2s5p = 2 panels in series with 5 matching parallel strings.

The back of your panel you will see a few figures

Voc (open circuit voltage – doing no work – so no amps flowing)

Vmpp (maximum power voltage – the sweet spot for pulling max power)

Imp (maximum power amperage – Impp x Vmpp = your panel wattage rating)

Isc (short circuit amperage – doing nothing good for anyone except causing fires)

Typical values for a 345W panel are:  Voc=47.46, Vmpp=37.93, ,Impp=9.09, Isc=9.64

The Combiner box should have a 15A breaker per series string

Rule of thumb :

Vmpp is approx. 25% LESS than Voc

Isc is typically 6% MORE than Impp  (we will use 10% for our calcs)

Method 1:

Number of Solar array parallel strings x Impp, example 2s5p configuration. 9.09A x 5 = 45.45A x 1.1  (10%) for Isc rating = 50A,

now times that by 1.25 for safety rating (solar lensing effect – google it) = 62.5A  so 64A or 60A DC breaker

Method 2:

Total Wattage of your solar array  (number of panels x individual wattage) / Voltage of Solar array at the terminals going into the PV Inputs = Amps

2a) Using Vmpp

Example, 10 x 345W  (3450W) panels (47Voc , 38Vmp) – use the VMP Voltage connected in 2s5p would be 94Voc (47×2) or 76Vmp (38×2)

3450W / 75.86Vmp = 45.47A multiplied by a factor of 1.1 (Isc) x 1.25 (safety margin)  = 62.53A (60A or 64A DC breaker)

2b) Using Voc  (open circuit volts)

Panel Open circuit voltage is usually about 25% more than Vmp (voltage at maximum power) 10 x 345W = 3450W / Open circuit volts (47.46 x 2) = 36.34A times a factor of 1.7 (use for Voc) , 36.34A x 1.7 = 63A (64A breaker)