Inverter AC Output Circuit Conductor and OCPD (NEC 690.8(B))
Sizes the conductors and overcurrent device from a PV/ESS inverter to the point of connection.
Example
You enter
- Inverter AC power (W) 9600
- AC voltage (V, line-to-line) 240
- Phases (1 or 3) 1
You get
- Continuous output current 40 A
- Min conductor ampacity (125%) 50 A
- Overcurrent device 50
Checked against published worked examples from: IAEI Magazine (John Wiles).
Details, formula, and sources
ScopeContinuous output current = AC power / (voltage x [1 single-phase, sqrt(3) three-phase]), then NEC 690.8(B) / 705.60 / 240.4 size the conductor and OCPD at 125% of that current, rounding the OCPD up to the next standard size (240.6). A 9.6 kW inverter at 240 V single-phase puts out 40 A, so the conductor is at least 50 A (a #6 Cu at 75 C) on a 50 A breaker; the same inverter at 208 V three-phase is only 26.6 A. Use the inverter's RATED continuous AC output current from its datasheet if given, and check the 705.12 busbar / point-of-connection limit separately. This is the AC side; the DC side uses the separate 690.8(A) 156% rule. A sizing estimate; the inverter datasheet, the AHJ, and the adopted NEC edition govern.
Formula or tablecontinuous_current_a = ac_power_w / (ac_voltage_v x [1 single-phase, sqrt(3) three-phase]); min_conductor_ampacity_a = 1.25 x continuous_current_a; ocpd_a = next standard size (NEC 240.6) >= 1.25 x continuous_current_a.
Source and editionNEC 690.8(B) (continuous current), 705.60 (interconnected-source overcurrent), 240.4 (conductor protection), and 240.6 (standard OCPD sizes), by name; the inverter datasheet and the AHJ govern.
Free accessThe output-current relation is power / voltage (public); the 125% continuous factor and the standard OCPD sizes are public NEC, and the inverter power, voltage, and phase come from the datasheet.
What governsEstimate. AHJ and licensed professional govern.
API field namesac_power_w, ac_voltage_v, phases, continuous_current_a, min_conductor_ampacity_a, ocpd_a
- 125% continuous conductor and OCPD at 1.25 x the continuous output current; OCPD to the next standard size (240.6)NEC 690.8(B) / 240.4 / 240.6
- Busbar limit separate the 705.12 point-of-connection / busbar limit is checked separately (pv-interconnection-busbar)NEC 705.12