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Size solar string and combiner box conductors complying with NEC 690 (1.56× Isc factor) while keeping voltage drop under 2%.
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Understanding the NEC 1.56× Solar Multiplier
Under Section 690.8 of the National Electrical Code (NEC), photovoltaic conductors cannot be sized simply for rated operating current. Sunlight bouncing off clouds or snow can briefly elevate solar irradiance above $1,000 ext{W/m}^2$, generating currents beyond datasheet ratings.
The Two NEC 125% Factors
- Maximum Circuit Current (NEC 690.8(A)): Short-circuit current ($I_{ ext{sc}}$) must be multiplied by 125% (1.25) to account for cloud edge enhancement effects.
- Continuous Duty Overcurrent (NEC 690.8(B)): Because solar systems generate power continuously for more than 3 hours, conductors and fuses must carry another 125% (1.25) buffer ($1.25 imes 1.25 = 1.5625$).
Why the 2% Voltage Drop Rule Matters
While electrical code allows up to 3% branch circuit voltage drop for safety, in solar PV systems every volt lost to conductor resistance is revenue and solar kilowatt-hours permanently burned as heat. Limiting voltage drop to under 2.0% maximizes MPPT inverter harvesting efficiency.
Frequently Asked Questions
What type of wire is required between solar panels and the combiner box?
NEC 690.31 requires specialized sunlight-resistant, direct-burial "PV Wire" or "USE-2" rated for 90°C wet environments and 600V to 1,000V DC. Standard building wire (THHN/Romex) cannot be exposed on rooftops.
When are string fuses required in a combiner box?
String fuses are required whenever three or more strings are wired in parallel. If one string shorts out, the combined fault current of the other strings would backfeed into the damaged modules, creating a fire hazard without individual series string fuses.
Does ambient roof temperature require wire derating?
Yes. Conductors installed in rooftop conduits exposed to direct sunlight frequently experience temperatures exceeding 120°F to 140°F (50°C to 60°C). NEC Table 310.15 requires applying temperature derating factors of 0.71 to 0.82 to wire ampacity.