Subpanel feeder calculator

How feeder ampacity, breakers, and continuous load work together

That pairing of design load, table ampacity, and catalog breaker is the core of the recommendation you see as feeder_wire and breaker_amps, before distance is considered for voltage drop.

Distance, voltage drop, inputs, and the response shape

Long feeders drop voltage in proportion to current, length, and resistivity, and inversely with cross-sectional area. This tool uses the familiar approximate formulas Vd equals two times K times I times L over circular mils for single-phase, and the square root of three times K times I times L over circular mils for three-phase, with K twelve point nine for copper and twenty-one point two for aluminum, length in feet, and current equal to the entered load amperes. Voltage-drop percent is one hundred times Vd divided by the system volts. When that percent exceeds max_vd_percent—three percent by default—the calculator upsizes the feeder until the drop is within limit or until the table is exhausted, setting upsized_for_voltage_drop when the ampacity-minimum wire was not enough. Required fields are load_amps and distance_ft, both strictly greater than zero. Optional volts defaults to two hundred forty, material to copper, phases to one, continuous to true, and max_vd_percent to three. Aliases such as amps, length_ft, voltage, cu, al, and vd_percent are accepted so forms are not locked to one spelling. Success returns design_amps, feeder_wire, feeder_ampacity_a, feeder_cm, breaker_amps, voltage_drop_v, voltage_drop_percent, vd_ok, headroom_amps, k_factor, and a short formula string. Invalid numbers, non-positive load or distance, unsupported material, phases other than one or three, or a load that cannot be served within the table and drop limit fail with invalid_input and are not billed. There is no network call and no model inference—the same JSON always yields the same result.

Where teams use a subpanel feeder calculator and how pricing works

Homeowners and remodelers use a subpanel feeder calculator when planning a garage shop, ADU, barn, or basement panel fed from the main service so they can discuss wire gauge, aluminum versus copper, and breaker size with an electrician using shared numbers. The interactive tool above runs free in the browser for interactive checks. Source inputs and outputs are retained only for the platform retention window and are never used to train models. Always have a qualified professional confirm final feeder and overcurrent equipment under the electrical code adopted in your jurisdiction, including terminal temperature limits, ambient correction, and conduit fill that this planner does not apply. Electrical estimators plug load takeoffs and measured or plan-scale distances into scripts that pin feeder_wire and breaker_amps next to each subpanel on a job, avoiding hand-built spreadsheets that drift between offices. Solar and EV charger projects often add a subpanel or feeder run where voltage drop at high continuous current matters as much as ampacity; changing distance_ft or max_vd_percent shows when the run forces a larger kcmil size. Educators demonstrate continuous-load factors by toggling continuous while holding load_amps fixed, and show why an eighty-foot 60 A feeder may need more copper than a ten-foot run with the same load. Programmatic calls use the ForHosting KIT prepaid balance at $0.002 per successful request, with no subscription and no charge when validation fails. Because the arithmetic is tiny and fully local to the Worker, latency is dominated by the task queue handshake rather than by computation.