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DIY Manual

ManualsBrandsMidNite Solar ManualsSolar & Wind PowerDIY Series Inverter & Charger with built-in MPPT controller
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43 | P a g e 10-543-1 REV A
DIY Manual
MidNite breakers are rated for 100% continuous duty and may be opened under a load (that
means with current flowing through the breaker). Although the fuseholders are rated at 100%
continuous duty, the fuses are NOT. Therefore, the fuseholders CANNOT be opened under a
load. Since MidNite fuses are not rated at 100% continuous duty, the fuse must be derated by
80%, or increased by 125%, when sizing for circuit overcurrent protection.
Example – Determine maximum current, conductor ampacity, and size the circuit overcurrent
protection device (breaker or fuse) for a PV array with a short-circuit current (Isc) of 8.9A.
1. Maximum PV circuit current: 8.9A x 125% = 11.13A
2. Conductor ampacity: 11.13A x 125% = 13.91A
3. Circuit overcurrent protection: 8.9A x 125% = 11.13A
NOTE: If using a MidNite 100% continuous duty circuit breaker such as the MNEPV series of
breakers, the “Maximum PV circuit current” calculation equals the required circuit breaker amp
rating (i.e., a 12A breaker for the 11.13A calculation). If NOT using a MidNite breaker or if using
a MidNite fuse, then multiply the max PV circuit current by 125% again (i.e., a 15A breaker for
the 13.91A calculation).
What does this all mean?
1. The maximum current from the PV array to the charge controller will be 11.13A or less.
2. The proper wire gauge to install will be able to handle 13.91A over the distance of the PV
array to the charge controller.
3. The minimum amp rating of the 100% continuous duty circuit breaker is 11.13A, or a 12A
circuit breaker.
4. The minimum amp rating of the non-continuous duty circuit breaker or fuse is 13.91A, or
a 15A circuit breaker or fuse.
NOTE: It is acceptable to round up to the next size breaker or fuse. That is, this example calls for
a 12A breaker; a 15A breaker is acceptable if a 12A breaker is unavailable.
Let’s go back to the prior example of the 4000W inverter, 24V bank with input of 22V-24V, with
an efficiency rating of 85%. What is the correct circuit breaker size for this system?
1. 4000W / 22V / 0.85 = 214A,
2. MidNite makes a 175A or a 250A DC breaker,
3. Since the inverter can surge to 2-3X its rated continuous wattage, the 250A breaker is the
right choice.
4. And considering the surge potential, increasing the inverter-to-battery bank wire size is a
smart and safe idea. #3/0 or #4/0 AWG wire is best for this application.