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NEC 690.30
Validated against the standard.
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How it works
Enter your values
Type in your inputs — the app fills code tables and constants for you, so there's nothing to look up.
Get a code-checked result
Instant answer with a clear PASS / FAIL and the exact NEC 690.30 reference behind every number.
Save & export
Keep a history of calculations and export a clean PDF report to share or attach to a permit (Pro).
What this calculator does
PV conductors are sized by a special two-step current rule and then derated hard for rooftop heat. This app applies NEC 690.8 and the temperature and conduit-fill adjustments so the wire is both legal and safe in real rooftop conditions.
The 690.8 current rule
- Maximum circuit current = 1.25 × the module short-circuit current (Isc), per 690.8(A).
- Conductor and OCPD sized at not less than 1.25 × that value, per 690.8(B) — about 1.56 × Isc before temperature derating.
- Then apply rooftop ambient and conduit-fill derating, which is significant because conduit on a hot roof can see very high temperatures.
Worked example
A module with Isc = 10 A: max current = 1.25 × 10 = 12.5 A; conductor sized for 1.25 × 12.5 = 15.6 A before derating. After rooftop heat derating, #10 PV wire is a common result.
Related calculators
Check loss with the solar voltage drop calculator and the raceway with the solar conduit fill calculator.
Solar wire sizing FAQ
Why multiply Isc by 1.56?
NEC 690.8 applies 1.25 for continuous-current sizing on top of the 1.25 maximum-current factor, so conductors and overcurrent devices are sized at about 1.56 times the module short-circuit current before derating.
Why is rooftop derating so important?
Conduit on a hot roof can reach very high ambient temperatures, and the high-temperature derating factors sharply reduce conductor ampacity, often driving a larger wire than the bare current suggests.
Does it handle PV wire and THWN-2?
Yes, including the 90°C insulations common in PV, with the correct ampacity and derating for each.
Does it work offline?
Yes, it runs 100% offline with no account. Free with rewarded ads; a one-time $2.99 Pro unlock adds PDF export and saved history.
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