Battery-to-Inverter DC Conductor and OCPD (NEC 690.9 / 706)
Sizes the DC conductor and fuse between a battery bank and an off-grid or ESS inverter: DC input current = AC power / (battery voltage x inverter efficiency), then NEC 690.8(B) / 706 / 240.4 size the conductor and overcurrent device at 125% of that current, with the OCPD rounded up to the next standard size (240.6). A 4 kW inverter on a 48 V bank at 90% efficiency draws about 92.6 A, so the conductor is rated at least 115.7 A (a 1/0 Cu at 75 C) on a 125 A DC fuse; a 24 V bank would double the current. Use a listed DC-rated (often Class T, for a battery's high available fault current) fuse and switch, keep the run short and heavy, and terminate at the battery's rated torque. A sizing estimate; the inverter and battery datasheets, the fault-current rating, the AHJ, and the adopted NEC edition govern.
Formula and source
dc_current_a = inverter_power_w / (battery_voltage_v x efficiency_pct/100); min_conductor_ampacity_a = 1.25 x dc_current_a; ocpd_a = next standard size (NEC 240.6) >= 1.25 x dc_current_a.
NEC 690.8(B) (continuous current), 706 (energy storage systems), 240.4 (conductor protection), and 240.6 (standard OCPD sizes), by name; the inverter and battery datasheets and the AHJ govern.
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