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)
