Electrical Calculators
Free electrical calculators for voltage drop, wire sizing, conduit fill, and more.
Calculates voltage drop, load voltage, and NEC compliance for copper and aluminum wire runs.
Open calculator →Calculates recommended wire gauge, ampacity, and voltage drop for any circuit load and run length.
Open calculator →Calculates conduit fill percentage and NEC compliance for any combination of wire gauges and conduit types.
Open calculator →Converts watts to amps or amps to watts for DC, single-phase, and three-phase AC circuits, with kVA and wire gauge.
Open calculator →Calculates total resistance, current, and power dissipation for parallel and series resistor circuits.
Open calculator →Calculates output voltage, load effect, current, and power dissipation for any voltage divider circuit.
Open calculator →Estimates fixture count, grid spacing, total wattage, and circuit breaker size for any room.
Open calculator →Our free electrical calculatorshandle the conductor sizing, voltage-drop, conduit-fill, and load math that the National Electrical Code (NEC) requires on every job — without the slide rules and dog-eared ampacity tables. They are built for licensed electricians, electrical engineers, and informed DIYers who need a code-defensible answer fast, whether you are wiring a sub-panel, running a long landscape-lighting circuit, or spec'ing a three-phase motor feeder. Each tool works in copper or aluminum, AWG or kcmil, and shows the NEC reference behind the result.
Why voltage drop and wire sizing matter
NEC informational note 210.19(A) recommends keeping voltage drop under 3% on a branch circuit and 5% total. That limit drives conductor size more often than people expect: a 20 A, 120 V circuit run 100 feet on #12 copper drops about 3.2% — already over the branch-circuit guideline, so you bump up to #10. Run it 150 feet and you are into #8 territory. Undersized wire does not just trip breakers; it wastes energy as heat and can damage motors and electronics. Use the voltage drop calculator to check a specific run, or the wire size calculator to work backward from load and distance to the gauge you need.
Fill the conduit, not the panel schedule, by hand
Conduit fill is the other place estimates go wrong. NEC Chapter 9 caps fill at 40% for three or more conductors, so a 1/2-inch EMT holds roughly nine #12 THHN conductors before you must upsize. Our conduit fill calculator tallies the cross-sectional area for any mix of gauges and conduit types and flags the fill percentage against code. Start with your load in watts or amps, confirm the conductor material, and you will have the gauge, breaker, and conduit answers in one pass.