PV Fuse and Breaker Sizing Calculator

How It Works

The string fuse: I_sc × 1.25 × 1.25

A PV module is a current source, and its short-circuit current on the label is measured at 1,000 W/m². Real irradiance exceeds that, on a cold clear day with snow on the ground or under the edge of a cloud, so NEC 690.8(A)(1) sets the maximum current of a PV source circuit at 125% of I_sc. The circuit then runs at that current for hours at a time, which makes it a continuous load, so NEC 690.8(B) requires the overcurrent device to be rated at 125% of the maximum current. The two factors compound:

The calculator applies the exact product rather than the rounded 1.56 that trade practice quotes, because rounding down sizes a protective device fractionally low. Enter I_sc as the label gives it and nothing else; pre-multiplying by 1.25 yourself would size the fuse 25% too high, and entering I_mp instead of I_sc would size it too low.

The ceiling on the label: NEC 690.9(B)

Every module label carries a maximum series fuse rating, typically 15 A or 20 A, which is the largest fuse the module's own conductors and bypass diodes can be protected by. NEC 690.9(B) makes it a hard ceiling: the string fuse must not exceed it, and there is no rounding past it. Enter the figure in the optional field and the calculator compares the two. Where I_sc × 1.5625 rounds up to a size above the label, the design has to change, not the fuse: a module with a higher rating, or fewer strings in parallel so the back-feed current falls.

When a string needs a fuse at all

A string fuse protects the string from current flowing into it from the other strings when it faults, not from its own output. With one string there is nothing to feed it; with two, a healthy string can push at most its own I_sc into a faulted one, which the module's series fuse rating already covers. The requirement bites at three or more strings in parallel, which is where the series vs parallel calculator starts flagging it. Many MPPT controllers carry a single input, so the question is often settled by choosing series over parallel in the first place.

Controller and inverter circuits: continuous loads, applied once

The charge controller's output to the battery and the inverter's DC input are not PV source circuits. They are continuous loads under NEC 210.20(A), and the device is rated at 125% of the circuit current, once. For the controller, the circuit current is its rated output current. For the inverter, it is the DC input current at the inverter's rated output, and at the lowest battery voltage it will run at, because an inverter delivering rated watts draws its highest current when the bank is at its low-voltage cutoff. The inverter sizing calculator reports that figure directly, with the cutoff defaulting to 87.5% of nominal.

NEC 240.6(A) standard sizes

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600

The calculator picks the smallest of these at or above the required rating. Rounding down is never permitted; if the required figure is 17.97 A the answer is 20 A, not 15 A. DC circuits also need a device that is DC-rated at or above the circuit voltage: an AC breaker on a 48 V battery circuit, or a 32 V automotive fuse on a 600 V string, is the wrong part regardless of its amp rating.

Worked examples

A string of modules rated I_sc = 11.5 A, maximum series fuse 15 A:

• Maximum circuit current = 11.5 × 1.25 = 14.375 A (NEC 690.8(A)(1))
• Required device rating = 11.5 × 1.5625 = 17.97 A (NEC 690.8(B))
• Next standard size = 20 A
• 20 A exceeds the 15 A on the label, so the calculator warns under NEC 690.9(B): this module cannot be string-fused at 20 A, and the array needs a module rated 20 A or a layout with no more than two strings in parallel

The DC breaker for a 4,000 W inverter on a 48 V bank, 92% efficiency:

• DC input at rated output and nominal voltage = 4,000 ÷ (0.92 × 48) = 90.6 A
• At the 42 V low-voltage cutoff = 4,000 ÷ (0.92 × 42) = 103.5 A, the figure to enter
• Continuous load: 103.5 × 1.25 = 129.4 A, next standard size 150 A (sizing at nominal would have given 125 A, one size too small)

A 60 A charge controller's output to the battery:

• Continuous load: 60 × 1.25 = 75 A, next standard size 80 A

In every case the cable on that circuit has to carry the device rating, not just the load: size it with the solar cable size calculator after settling the fuse, not before.