What does NEC 690.9 require for overcurrent protection in solar systems?

NEC 690.9 requires overcurrent protection for each ungrounded PV source circuit conductor and PV output circuit conductor. The minimum OCPD rating is 125% of the maximum circuit current. The OCPD must be rated for DC operation at the system voltage — standard AC breakers are not suitable for DC circuits. Listed fuses rated for PV use are the most common OCPD for source circuits.

When can overcurrent protection be omitted for a solar circuit?

NEC 690.9(B) permits omitting overcurrent protection when the conductor ampacity equals or exceeds the sum of all parallel source circuit currents — meaning the conductor is inherently protected by its own capacity. This typically applies when only one string connects to the inverter (single-string system) and the conductor is sized to carry the full array fault current without protection. Most modern string inverters list specific conditions for OCPD omission.

What fuse rating do I use for a solar source circuit with Isc = 9.58A?

Minimum fuse rating = 125% × 9.58A = 11.975A. Round up to the nearest standard fuse size available for the DC voltage rating required. Standard PV fuse sizes are typically 10A, 12A, 15A, 20A. A 15A fuse would be appropriate for this circuit (nearest standard size above 11.975A).

What is a combiner box and why does it need fuses?

A PV combiner box aggregates multiple string source circuits into a single output circuit. Each string input has its own fuse (the source circuit overcurrent protection). The combiner output then connects to the inverter as the PV output circuit. Combiner fuses are DC-rated, typically 10A–30A per string, and housed in a listed PV combiner box with DC-rated fuse holders.

Solar Fuse & Breaker Sizing: NEC 690.9 OCPD Calculator Guide

Q: Can I use standard AC breakers in solar DC circuits?

No. DC arcs do not self-extinguish at zero crossing the way AC arcs do. Standard AC breakers are not designed or listed for DC interrupting duty at solar PV voltages and currents. All OCPDs in PV source and output circuits must be specifically listed for DC operation at the applicable system voltage. Use DC-rated fuses (for combiner boxes) or DC-rated circuit breakers (for string inverters and AC panels with DC input).

NEC 690.9 requires overcurrent protection for solar PV circuits — and the specific equipment requirements matter. Standard AC breakers are not allowed in DC circuits. The 125% sizing rule applies. And combiner box fuses have their own selection criteria.

Why DC OCPDs Are Different

DC fault current behaves differently than AC. In an AC circuit, current crosses zero 120 times per second — at each crossing, the arc temporarily extinguishes and can be interrupted by the breaker. In a DC circuit, current is steady — there’s no natural zero crossing.

A standard AC breaker can’t interrupt DC fault current at the voltages and currents in a solar system. The arc would sustain, potentially burning through the breaker. DC-rated fuses and breakers use different internal designs to interrupt DC arcs.

All OCPDs in PV source and output circuits must be:

Specifically listed for DC operation
Rated for the DC voltage of the circuit
Rated for the DC current level

The 690.9 Sizing Rule

Minimum OCPD rating = 1.25 × Maximum circuit current

Circuit Isc	× 1.25	Minimum OCPD	Standard Fuse Size (next up)
8.0A	10.0A	10.0A	10A
9.0A	11.25A	11.25A	12A or 15A
9.58A	11.975A	11.975A	12A or 15A
10.0A	12.5A	12.5A	15A
11.0A	13.75A	13.75A	15A
12.0A	15.0A	15.0A	15A
13.5A	16.875A	16.875A	20A
14.5A	18.125A	18.125A	20A
15.0A	18.75A	18.75A	20A

OCPD Voltage Rating

The OCPD must be rated for at least the maximum circuit voltage per NEC 690.7:

System Type	Maximum Voltage	Required OCPD Voltage Rating
Residential (1-2 family)	600V DC	600V DC minimum
Commercial/other	1000V DC	1000V DC minimum
Listed 1500V systems	1500V DC	1500V DC minimum

Using a 600V-rated fuse in a 1000V DC system is a code violation — the fuse may not safely interrupt fault current at 1000V.

When OCPD Can Be Omitted (690.9(B))

NEC 690.9(B) allows omitting source circuit overcurrent protection when:

The circuit has only one module or one series string, AND
The maximum current of all other source circuits connected to that conductor does not exceed the conductor’s ampacity

In practice, this applies to single-string single-inverter systems where the inverter is the only load and the string current never exceeds the conductor’s derated ampacity without an additional OCPD. Many modern microinverter and single-string string inverter systems meet this condition.

Check the inverter installation instructions — most manufacturers specify whether an external source circuit OCPD is required or whether the inverter’s internal protection is sufficient.

OCPD Omission Is Inverter-Specific

The conditions for omitting source circuit OCPDs depend on the specific inverter listing and installation instructions. Do not assume OCPD can be omitted without confirming with the inverter manufacturer’s documentation and the AHJ. Some AHJs require source circuit OCPDs regardless of the inverter listing — ask before omitting.

Combiner Box Fuse Selection

A combiner box aggregates multiple source circuits. Each input requires a listed DC fuse:

Combiner fuse rating per string:

String fuse minimum = 1.25 × module Isc
String fuse maximum = conductor ampacity of the string source circuit wire

Example: 4-string combiner, 9.58A Isc modules

Minimum fuse: 1.25 × 9.58A = 11.975A → use 12A or 15A DC fuse
String wire: 10 AWG THWN-2 (but check derated ampacity for the location)
Maximum fuse: cannot exceed derated conductor ampacity

Combiner output circuit:

Output circuit Isc = 4 strings × 9.58A = 38.32A
Minimum output OCPD: 1.25 × 38.32A = 47.9A → use 50A DC-rated breaker or fuse
Output circuit wire must be rated ≥ 50A derated ampacity

Listed PV Fuse Products

Common listed DC fuses for PV combiners:

Littelfuse KLKD, KLLU series (600V DC, 1A–30A)
Mersen (Ferraz Shawmut) PV series (600V, 1000V DC)
Eaton/Bussmann PV series (600V, 1000V DC)
Midnite Solar MNPV fuses (various ratings)

All must be installed in fuse holders listed for DC use at the applicable voltage. Standard AC fuse holders are not acceptable for DC circuits.

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Frequently Asked Questions

What does NEC 690.9 require for OCPD sizing?

Minimum OCPD rating = 1.25 × maximum circuit current. For source circuits, that’s 1.25 × module Isc. For output circuits with multiple parallel strings, it’s 1.25 × (sum of all string Isc values). The OCPD must also be rated for the DC voltage of the circuit.

Can I use AC circuit breakers in DC solar circuits?

No. DC arcs don’t self-extinguish like AC arcs do. Standard AC breakers are not listed for DC interrupting duty at solar voltages. All OCPDs in PV source and output circuits must be specifically listed for DC operation at the applicable system voltage.

When can I omit overcurrent protection for a solar source circuit?

NEC 690.9(B) allows omitting source circuit OCPDs when the conductor is inherently protected by the maximum available fault current — typically in single-string systems where the conductor ampacity exceeds the maximum current that can flow. Verify with the inverter manufacturer’s installation instructions and confirm with the AHJ before omitting.