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NEC 690.8
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 690.8 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
The inverter has to match the array. This app sizes string and central inverters by the DC-to-AC ratio and checks string voltage and current windows against NEC 690.7 and 690.8, so the design stays inside the inverter's limits across temperature.
What it checks
- DC/AC ratio — array DC kW to inverter AC kW, commonly about 1.1 to 1.3 (a little overbuilding of the array is normal and economic).
- Max system voltage — string Voc corrected for the coldest expected temperature must stay under the inverter and 690.7 limit (usually 600 V residential, 1000–1500 V commercial).
- MPPT current window — string current within the inverter's input range.
Worked example
A 9.4 kW array on a 7.6 kW inverter is a DC/AC ratio of 1.24 — within the typical range. The app also confirms a 12-module string's cold-temperature Voc stays under 600 V.
Related calculators
Size the array first with the panel sizing calculator and the conductors with the solar wire sizing calculator.
Inverter sizing FAQ
What is a good DC-to-AC ratio?
Most residential and commercial designs land around 1.1 to 1.3. A modest amount of array oversizing relative to the inverter captures more energy in real conditions without much clipping.
Why correct voltage for cold temperature?
Panel open-circuit voltage rises as temperature falls. NEC 690.7 requires sizing for the coldest expected temperature so the string never exceeds the inverter or system voltage limit.
Does it check the MPPT window?
Yes. It verifies the string voltage and current stay within the inverter's maximum power point tracking range across the temperature extremes.
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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