Basic Electrical Calculators
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Electrical Calculators & Unit Converters
Electrical Calculators & Unit Converters
NEC-referenced (NFPA 70, National Electrical Code) calculators for voltage drop, wire gauge sizing, circuit breaker sizing, three-phase power, transformer sizing, motor starting current, and conduit fill — built for electricians and engineers on the job site.
Who This Is For
This category is the job-site set. Voltage drop, wire gauge, breaker sizing, conduit fill, motor starting current, transformer sizing, three-phase power, and the load center calculator answer whether the equipment and the wiring can live on the same circuit. The pages that size conductors and protection are NEC-referenced (NFPA 70, National Electrical Code). They are estimates from published tables and the numbers that are typed in. They are not a permit, and they are not a substitute for the adopted code edition or a licensed electrician.
Use the search box for a name, or read how the eight fit together.
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Wire gauge starts from the load current, the insulation temperature, and the conditions that derate ampacity. Voltage drop asks a different question about the same wire: how much voltage is left at the end of the run. A conductor can pass the ampacity check and still be too long. Breaker sizing pairs the overcurrent device with that conductor. The breaker is not chosen from the wire size alone, and the wire is not chosen from the breaker alone. Conduit fill is the third check. Conductors that are electrically adequate still have to fit, and the fill percentage depends on the conduit type and how many current-carrying conductors are in it.
Motor starting current is larger than running current, and the starting method changes how large. A feeder that supplies more than one motor is not the sum of the starting currents. Transformer sizing begins from the load in kilovolt-amperes, not from a watt guess that ignores power factor. Three-phase power is the supply calculation those loads usually need before the wire is chosen. The load center calculator is the panel view: branch loads, demand, and the main device, rather than one circuit in isolation.
They do not decode a resistor, price a kilowatt-hour, or convert a statvolt. Open Basic Electrical Calculators for Ohm's Law, component values, and energy cost. Open Electrical Unit Converters when the drawing uses a unit prefix or an older unit name and the circuit math has not started. A horsepower figure belongs in the kilowatt-to-horsepower converter only when the task is the unit. When the task is the motor circuit, start with motor starting current.
A 240 volt resistive load and a three-phase motor are not the same start. For the motor, open motor starting current or three-phase power first so the current is the right kind of current. Then wire gauge, then voltage drop along the actual length, then breaker sizing, then conduit fill if the conductors share a raceway. The load center page is last when the question is the panel rather than the branch. Skipping voltage drop because the ampacity passed is the miss this set of pages is built to prevent.
A continuous load, one expected to run at full current for three hours or more, is not entered the same way as a load that cycles. The breaker and wire pages ask for that distinction because sizing under the National Electrical Code (NFPA 70) uses it. Several of these pages also keep a short published list open: standard breaker sizes, common transformer ratings, conduit fill percentages, and similar lookups. Those lists are not a second copy of the calculator. If a row and a calculated result disagree, the nameplate and the adopted code edition decide.
Each page shows the formula or the table it used. Ambient temperature, conductor material, conduit type, power factor, and continuous load are inputs, not hidden constants. Change them when the job is different. Print the page if the numbers have to go to the field. Then compare them with the nameplate and with the edition of the National Electrical Code (NFPA 70) that the authority having jurisdiction has adopted. Related-tool links are there so voltage drop, wire gauge, and the breaker are checked as one job.
EnterPlanet LLC operates the site from Jacksonville, Florida. The About page names the company. Contact is the form if a table or a formula looks wrong.