DIY Manual

26 | P a g e 10-543-1 REV A

DIY Manual

you want their full wattage! So, you would want to increase the wire size to #10 AWG, which can

safely carry up to 30A.

Over-Current Protection – Breaker Sizing

NEC 690.9 requires that over-current protective devices (breakers, fuses) be rated to handle not

less than 125% of the maximum currents. If the over-current protective device is rated for

continuous duty (called the 3-Hr Rule), then the amp rating of the device can be at 100% or equal

to the maximum currents.

Using the same diagram we saw before, the

combined maximum current is 19.6A.

Adjusted current is 24.5A (1.25 x 19.6A)

Breaker sizing:

• For non-continuous duty breaker:

1.25 x 24.5A = 30.6A

• For continuous duty breaker:

1.00 x 24.5A = 24.5A

Since MidNite breakers are rated at 100%, we can use a 24.5A breaker, which does not exist. In

the above example, in real life, you would use #10 AWG for the PV homeruns connected to a 30A

breaker.

The NEC allows over-current protection to be located at one end of the PV source circuit. In the

system block diagram below, we have a combiner box, #2, with two 15A breakers serving as OCP

devices. We also have what is called the “PV Input Breaker,” #6. This could also be the OCP

device. In most installations, the combiner is located at the PV array, say a ground-mount system,

some distance away from the house. In that case, the #6 breaker acts as a simple ON/OFF switch

so that you do not have to walk out to the PV array to turn off the combiner breakers. It is

common to select the breaker for #6 slightly over-rated, so it will not nuisance trip. Continuing

with the example PV strings above, with an Isc of 9.8A x 2 strings, we know the adjusted PV amps

78V, 9.8A

78V, 19.6A

#??? AWG

MidNite Solar breakers and fuses are rated at 100% continuous duty!