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.
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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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SurgePV creates complete OCPD schedules — source circuit fuses, combiner outputs, and inverter disconnects — sized per NEC 690.9 and included in every permit package.
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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.
