# Calculating circuit breaker size for 12 volt panels in paralell - Off Grid?

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Registered Users Posts: 2
edited September 2017 #1
hi. I have a few questions concerning the calculation of the DC circuit breaker to use as a cut off switch for a string of  12 V panels connected in parallel.

please note I have three 200 watt 12 volt panels that I intend to connect in paralell. The Isc rating for each panel is 11.89 amps. I have an inline fuse of 15 amps on each panel.

1. What is the formula for calculating the DC circuit breaker size that I intend to use as a cutoff switch?

2. If you use the ISC rating do you multiply the Isc rating for each solar panel? For example if the isc rating is 11.8 A and you have three panels wired in parallel do you multiply the isc rating by 3 (not including the buffer of 1.25 - 1.56)? For example would the formula be 11.89 Isc x 3 x 1.25 (or 1.56 depending on your Country's rules)

• Solar Expert Posts: 3,854 ✭✭✭✭✭✭
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Since there are fuses for each panel there is no need to use a circuit breaker, a DC rated disconnect, 45A or higher, is all that would be required, probably less expensive too. Don't be tempted to use an AC disconnect in a DC application, arcing will result, I'm assuming this is a convenience disconnect local to the charge controller.
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Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
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• Registered Users Posts: 4,496 ✭✭✭✭✭
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Assuming you're combining the three panels into a single pair of wires to the controller, and are sizing a breaker before the controller to use as a disconnect, it would be sized to protect that pair of wires.

Depending on the length of that wire, you may want to size it larger than strictly needed (in order to minimize voltage drop), in which case you could use a bigger breaker for the oversized wire if you wanted.
Off-grid.
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• Registered Users Posts: 2
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Thank you. all square on your info. Much appreciated.

For or my own self development, and knowledge, re my second question, when calculating DC Circuit breaker size for panels wired in paralell, do you multiply the Isc of all panels.Troutbeat said:
2. If you use the ISC rating do you multiply the Isc rating for each solar panel? For example if the isc rating is 11.8 A and you have three panels wired in parallel do you multiply the isc rating by 3 (not including the buffer of 1.25 - 1.56)? For example would the formula be 11.89 Isc x 3 x 1.25 (or 1.56 depending on your Country's rules)

• Solar Expert Posts: 3,854 ✭✭✭✭✭✭
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Troutbeat said:
Thank you. all square on your info. Much appreciated.

For or my own self development, and knowledge, re my second question, when calculating DC Circuit breaker size for panels wired in paralell, do you multiply the Isc of all panels.Troutbeat said:
2. If you use the ISC rating do you multiply the Isc rating for each solar panel? For example if the isc rating is 11.8 A and you have three panels wired in parallel do you multiply the isc rating by 3 (not including the buffer of 1.25 - 1.56)? For example would the formula be 11.89 Isc x 3 x 1.25 (or 1.56 depending on your Country's rules)

Correct
1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS
Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.
5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
• Solar Expert Posts: 9,583 ✭✭✭✭✭
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I would use a CB 3x the size of the fuse the panels call for.   if the sticker on the panels says 15A, then 45A should work (with the right size wire.)
But now that you have a breaker and a box, why not just use that as your combiner and 2, 15A breakers and forget the fuses.  You can switch a string in/out of the system and compare & troubleshoot.
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