Solar Circuit Sizing Troubleshooting

PV Circuit Failures Quick Answer

Most "the calculator said 10 AWG was fine" surprises come from using Imp instead of Isc, skipping temperature/bundling derates, treating 125% as optional, blaming the inverter when voltage drop is the real loss, or sizing OCPD to load instead of continuous PV current.

1. Sized From Imp — Continuous Current Underrates the Circuit

What it looks like: Wire and breaker look fine for "nameplate operating current." Conductors run warm, fuses open on bright days, or the AHJ asks for Isc-based calcs you never documented.

Likely causes: Imp is the current at maximum power point under STC — useful for production estimates, not for NEC 690 continuous-current conductor sizing. Fault and continuous planning start from Isc (short-circuit current) with the applicable 125% factor(s) for your code edition.

Fixes: Pull Isc from the module datasheet. Re-run the Solar Panel Circuit Sizing Calculator and cross-check the sizing chart Isc→continuous table. Document Isc on the one-line.

2. Calculator Ampacity OK — Derated Ampacity Fails on the Roof

What it looks like: Table 310.16 ampacity clears continuous current in the calculator. Inspection or a peer review fails after ambient correction and more than three current-carrying conductors.

Likely causes: Educational PV calculators often select the smallest AWG that meets base ampacity. Rooftop conduits, attics, and bundled DC strings routinely cut allowable ampacity. The worked example in the NEC 690 guide shows 10 AWG passing before temp correction and failing after — that pattern is common in the field.

Fixes: Take the candidate AWG into the Wire Ampacity & Derating Calculator. Upsize until derated ampacity ≥ required continuous current. Re-check terminal temperature limits on inverters and combiners.

3. Breaker/Fuse Trips or AHJ Rejects OCPD

What it looks like: Series fuses open on clear afternoons, or the inspector rejects a breaker that was rounded to the next size without checking module series-fuse ratings and continuous current.

Likely causes: OCPD below continuous current; device not listed for the DC application; series fuse larger than the module's maximum series fuse rating; treating AC branch-breaker habits as PV DC rules.

Fixes: Continuous current first, then next standard size per 240.6 — and verify 690.9 / manufacturer series-fuse limits. Use the chart's OCPD neighborhoods as a starting map, then confirm listed equipment. Guide walkthrough: NEC 690 wiring guide.

4. "Bad Production" That Is Really DC Voltage Drop

What it looks like: Array underperforms on long home-runs. Inverter input voltage sags under load. Crew blames modules or shading first.

Likely causes: Conductor sized only for ampacity, not for %VD at operating current over the real length (out and back). Thin wire on a long DC path can pass ampacity and still lose several percent of voltage.

Fixes: Measure or calculate VD with the Voltage Drop Calculator. Target roughly ≤3% on DC home-runs when practical. Upsize one AWG even when ampacity already passes. General VD diagnosis: Voltage Drop Troubleshooting.

5. Added Parallel Strings — Forgot to Recalculate the Home-Run

What it looks like: Original two-string design was fine. A third string was added later. Combiner-to-inverter conductors and OCPD were never resized. Heat, trips, or inspection findings follow.

Likely causes: Parallel strings add Isc. Source-circuit conductors for each string may still be fine while the combined output circuit is now underrated. Expansion projects often reuse the original home-run out of habit.

Fixes: Total array Isc = module Isc × parallel string count. Recalculate continuous current, OCPD, ampacity (with derates), and VD for the combined path. Do not assume "same wire as before" after any string-count change.