MPPT Charge Controller Output Current

ScopeAn MPPT charge controller is sized by the current it delivers to the battery, and that current is set by the array watts and the battery voltage -- not by the array's short-circuit current, which is what the PV Circuit Maximum Current and Ampacity (NEC 690.8, the 156% Rule) calculator works from. Power in is roughly power out, and the output voltage is the battery's, so the output current is the array's STC watts divided by the battery's nominal voltage. The calculator picks the next common rating (20, 30, 40, 45, 60, 80, or 100 A, or 100 A units in parallel above that), reports the array-to-controller ratio -- above 1.0 the controller clips at its limit by design, as manufacturers such as Morningstar state -- and gives the output conductor minimum current at 125% of the controller's rated continuous output under NEC 690.8. A field clinic array of eight 400 W modules (3,200 W) on a 48 V bank draws 66.7 A: an 80 A controller at a 0.83 ratio with a 100 A minimum output conductor. The same array on 12 V is 266.7 A -- three 100 A controllers and cable the size of a welding lead -- and on 24 V 133.3 A; the battery voltage decides the whole DC side. It does not check maximum input voltage against the array's cold open-circuit voltage (the Solar PV String Sizing calculator), the source-circuit current, or array temperature derating, and it does not apply to PWM controllers. Ampacity comes from the NEC tables. The controller manufacturer's specifications, the NEC as adopted, and the AHJ govern.

Formula or tableoutput current = array STC watts / battery nominal volts; controller = next common rating at or above it (20, 30, 40, 45, 60, 80, 100 A; 100 A units in parallel above that) or the chosen rating; array ratio = array watts / (controllers x rating x battery volts), clipping above 1.0; output conductor minimum current = 1.25 x the controller's rated continuous output.

Source and editionArray watts over NOMINAL battery volts, which reads a little HIGH against how manufacturers rate: the Morningstar ProStar MPPT data sheet (rev. 1/2016) puts 350 W on its 25 A unit at 12 V and 700 W at 24 V, about 14 V and 28 V per amp, the charging voltage -- so this method can pick one size larger, the safe direction. The same sheet notes input power may exceed the nominal rating because the controller limits its output; NEC 690.8 cited by number for conductors at 125% of continuous current, subsection to be verified against the adopted edition.

Free accessA division on the user's array and battery figures and a list of common trade ratings; no manufacturer table or NEC ampacity table is reproduced.

What governsEstimate. AHJ and licensed electrician govern. Verify against the NEC edition adopted in your jurisdiction.

API field namesarray_w, battery_v, chosen_controller_a, output_current_a, controller_a, controller_count, array_ratio, min_conductor_a

• Power in about equals power out output current = array STC watts / battery nominal voltagecontroller manufacturer rating convention
• Clipping is by design an array above the controller's nominal power is permitted; the controller limits its outputMorningstar ProStar MPPT data sheet
• 125% of rated output the output conductor and overcurrent device are selected on the controller's rated continuous currentNEC 690.8, subsection to be verified