Electrical safety and the National Electrical Code (NFPA 70)
What the exam tests
Electrical safety and the NEC domain tests Article 690 and related articles as they apply to PV: maximum voltage and current calculations, disconnects, overcurrent protection, rapid shutdown, grounding, and safe work practices.
Core rules
- Treat PV source and output circuits as energized whenever the array is illuminated. There is no single upstream breaker that de-energizes the DC side; use lockout/tagout, insulated tools, and rated PPE.
- 690.7: maximum PV system voltage is the temperature-corrected Voc sum. 690.8(A): maximum current is 125% of Isc (or the sum for parallel strings). 690.8(B): conductor ampacity is at least 125% of that maximum current before adjustment. 690.9: required overcurrent devices are also sized from the 690.8(A) current with the 125% factor.
- 690.12 rapid shutdown: controlled conductors must be reduced to a safe voltage within 30 seconds after shutdown; the threshold depends on location relative to the array boundary (commonly 80 V inside, 30 V outside under the 2017/2020 NEC; verify the edition in force).
- 690.13 and 690.15 require disconnecting means for PV circuits; 690.31 governs wiring methods, and 690.47 addresses grounding electrode connections. Article 250 and 310 apply for equipment grounding and ampacity.
- Only qualified persons should work on PV electrical systems; use a properly rated meter and verify zero energy before touching conductors.
Worked example
Two parallel strings each have Isc = 10 A. Under 690.8(A), maximum current = 2 × 10 A × 1.25 = 25 A. Conductors must have an ampacity of at least 125% of 25 A = 31.25 A before adjustment; any required overcurrent device is selected with the same 125% factor. Do not apply the 125% factor to Voc; voltage and current calculations use different inputs.
Traps
- Applying the 125% factor to voltage instead of current.
- Assuming opening the inverter DC disconnect de-energizes the array conductors.
- Confusing rapid-shutdown voltage and time thresholds, or forgetting the array boundary.
- Using AC breaker sizing rules for DC circuits without checking DC ratings.