DIY Manual

25 | P a g e 10-543-1 REV A
DIY Manual
carry, then friction caused by resistance will raise the wire temperature which eventually leads
to failure of the wire’s insulation (the outer protective covering).
NEC Article 690.8 requires that a PV conductor be rated at 125% of the PV source current. For
example, you want 30A continuous, so you need a wire that can handle 37.5A (125% of 30A).
Instead of using #10 AWG rated at 30A, you use a #8 AWG which is rated for 40A. Thus, your 30A
circuit will flow through the #8 AWG wire without heating it up.
In the diagram above, no OCP is required. The PV current is 9.8A, corrected to 12.25A (1.25 x
9.8A), traveling through a #12 AWG wire rated for 20A. Good to go!
The diagram above presents a different situation. The combined PV source current is 19.6A,
corrected to 24.5A (1.25 x 19.6A). If the wire gauge from the nodes down is #12 AWG, then OCP
is required. A 20A breaker is required to limit the current to the ampacity of the wire. To limit
the current is to limit the solar production, so that would be bad design. You bought the panels,
Isc = 9.8A
9.8A x 1.25 = 12.25A
#12 AWG good to 20A
No over-current protection
required.
Isc = 9.8A
9.8A x 2 x 1.25 = 24.5A
#12 AWG is good to 20A.
Over-current protection
required.
Use a 20A breaker OR increase
to #10 AWG wire.
78V, 9.8A
78V, 9.8A
78V, 19.6A
#12 AWG
78V, 9.8A
#12 AWG