This electrical calculator suite provides seven professional-grade tools covering the most common NEC code calculations used by electricians, electrical engineers, and contractors. Each tool uses current NEC code formulas and table values to give you accurate, code-compliant results.
Suppose you are running a 20 A, 120 V branch circuit to a workshop that is 75 feet from the panel using 12 AWG copper wire (6,530 CM). Is the voltage drop within the NEC 3% limit?
You need to pull three 12 AWG THHN wires and two 14 AWG THHN wires into a 3/4-inch EMT conduit. Is this within the 40% fill limit?
For a standard 5 HP, 230 V single-phase motor, NEC Table 430.248 gives FLC = 28 A. Applying NEC Article 430 rules:
The 80% rule applies to any load that runs for 3 hours or more. Common examples: commercial lighting, EV chargers, HVAC units, and data center equipment. If you have a 16 A continuous load, you need a 20 A circuit (16 ÷ 0.80 = 20 A). The breaker must also be sized at 20 A minimum, even though the load only draws 16 A, because continuous loads must not exceed 80% of breaker rating. This is one of the most frequently misapplied NEC rules in residential and commercial installations.
Important: These calculators are provided for educational and planning purposes only. All electrical work must be performed by a licensed electrician and inspected by the authority having jurisdiction (AHJ). Always verify calculations against the current adopted edition of the National Electrical Code (NEC) in your jurisdiction. Local codes may have additional requirements that supersede NEC minimums.
This free electrical calculator provides a comprehensive suite of professional electrical tools in one place. Use the Ohm's Law calculator to find voltage, current, resistance, or power from any two known values. The voltage drop calculator determines conductor losses over long runs to ensure NEC compliance. Wire sizing tools let you select the correct AWG gauge for any load and circuit length.
Additional tools include conduit fill calculator, three-phase power calculator, motor full-load current (FLC) lookup, and residential load calculation. Whether you are a licensed electrician, electrical engineer, or a DIYer planning a home improvement project, these professional electrical tools online give you the numbers you need quickly and accurately — without needing expensive software.
The NEC recommends no more than 3% voltage drop on a branch circuit (NEC 210.19 Informational Note No. 4) and no more than 5% combined on feeder plus branch circuit. For a 120 V circuit, 3% equals 3.6 V of allowable drop. When voltage drop exceeds 3%, the fix is usually to increase wire gauge by one or two AWG sizes.
Multiply the continuous load by 1.25 (NEC 210.19), then look up ampacity in NEC Table 310.16. Apply derating for more than 3 conductors in conduit and for ambient temperature above 30°C. Use the Wire & Breaker tab above — it applies all derating factors automatically and recommends the correct AWG and breaker size.
Standard breakers are rated for continuous operation at 80% of their ampere rating. A 20 A breaker should not carry more than 16 A continuously (loads lasting 3 hours or more). Divide your continuous load by 0.80 to find the required minimum breaker size. Breakers rated for 100% continuous duty are an exception, but are uncommon in residential panels.
Single-phase uses two conductors and delivers one sinusoidal power wave. Three-phase uses three conductors offset by 120° and delivers roughly 1.73× (√3) more power for the same conductor size, with smoother delivery. Three-phase is standard in commercial and industrial facilities. The 3-Phase Power tab converts between kW, kVA, kVAR, and line current for any power factor.
Aluminum wire is permitted but requires CO/ALR or AL-CU rated devices, proper anti-oxidant compound at all terminations, and torquing connections to spec. Because aluminum has higher resistivity than copper, it requires a wire roughly two AWG sizes larger for the same ampacity. For example, 2 AWG aluminum provides similar ampacity to 4 AWG copper.
NEC Chapter 9, Table 1 limits conduit fill to 53% for one conductor, 31% for two conductors, and 40% for three or more conductors. The Conduit Fill tab sums wire cross-section areas from NEC Table 5 and compares them to the conduit's internal area from NEC Table 4, flagging any over-fill condition instantly.
Use Full Load Current (FLC) from NEC Tables 430.248 (single-phase) or 430.250 (three-phase) — not the nameplate amps. Size the conductor at 125% of FLC (NEC 430.22), the overload relay at 115–125% of FLC (NEC 430.32), and the short-circuit protection up to 250% of FLC for an inverse-time breaker (NEC Table 430.52). The Motor Sizing tab handles all four steps automatically.
Power factor (PF) is the ratio of real power (kW) to apparent power (kVA). A PF below 1.0 means conductors carry more current than the useful power alone would require, increasing line losses and conductor heating. Utilities often charge demand penalties for commercial accounts with PF below 0.90. Capacitor banks are commonly added to correct low power factor from inductive motor loads.
AWG (American Wire Gauge) is used for conductors from #18 through #4/0 — lower numbers mean larger wire. Above 4/0, conductor sizes are expressed in kcmil (kilo circular mils), a measure of cross-sectional area. Common sizes are 250, 350, 500, and 750 kcmil. The voltage drop formula uses CM (circular mils) directly: for example, 4/0 AWG = 211,600 CM.
New circuits, panel upgrades, and service changes require a permit and inspection by the Authority Having Jurisdiction (AHJ) in virtually all jurisdictions. Simple repairs like replacing a receptacle typically do not. Always check with your local building department before starting. Unpermitted electrical work can void homeowner's insurance and create obstacles when selling the property.