Electrical Formulas

Step-by-step electrical formulas with worked examples for every calculation type. Each formula page walks you through the math with clear examples and links to the interactive calculator.

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Every Electrical Formula in One Place

Electrical calculations all come down to a few core relationships: Ohm's Law (V = IR), the power equation (P = VI), and their derived forms. The formulas below cover every common scenario, from basic watts-to-amps conversion to three-phase power, voltage drop, wire sizing, and energy cost calculations. Each formula page includes the formula itself, step-by-step worked examples, common mistakes to avoid, and a link to the interactive calculator.

The Two Foundational Equations

Nearly every electrical formula on this site derives from two equations:

Ohm's Law: V = I × R. Voltage equals current times resistance. This describes how much current flows through a given resistance at a given voltage.
Power Equation: P = V × I. Power equals voltage times current. This describes how much work a circuit does per second.

Combine these two and you can solve for any unknown given any two of the four variables (voltage, current, resistance, power). Every other formula on this page is a special case or extension: three-phase adds a √3 factor, AC adds power factor, voltage drop accounts for wire resistance over distance, and so on.

DC vs AC: When the Formulas Change

The basic formulas work for DC circuits as-is. For AC circuits, two additional factors come into play:

Power Factor (PF): In AC circuits, voltage and current can be out of phase. Power factor (0 to 1) represents how much of the apparent power is doing real work. Resistive loads have PF 1.0; motors and computers have lower PF.
Three-Phase (√3): Three-phase systems deliver power on three conductors 120° apart. The line-to-line voltage is √3 (1.732) times the line-to-neutral voltage, which appears in every three-phase power formula.

This is why you see three versions of many formulas: one for DC, one for AC single-phase (with PF), and one for AC three-phase (with PF and √3).

Frequently Asked Questions

What is the formula for converting watts to amps? expand_more

For DC circuits: Amps = Watts ÷ Volts. For AC single-phase: Amps = Watts ÷ (Volts × Power Factor). For AC three-phase: Amps = Watts ÷ (Volts × √3 × Power Factor). See the Watts to Amps Formula page for worked examples.

What is Ohm's Law? expand_more

Ohm's Law states that voltage equals current times resistance: V = I × R. Combined with the power equation P = V × I, you can derive 12 formulas that relate voltage, current, resistance, and power. See the Ohm's Law Formula page for all 12 forms.

How do I calculate voltage drop? expand_more

Voltage Drop = (2 × Length × Current × Wire Resistance) ÷ 1000. The percentage is then (Voltage Drop ÷ Source Voltage) × 100. NEC 210.19(A) Informational Note 4 recommends branch-circuit drop at or below 3% and total feeder+branch drop at or below 5% as performance targets. See the Voltage Drop Formula page for details.

Standards & References

This page cites the following electrical codes and standards. Always consult the current edition of your local adopted standard for authoritative requirements.

NFPA 70 (National Electrical Code). The overarching US standard for safe electrical installation. Adopted (with amendments) by every US state as the legal code for residential, commercial, and industrial electrical work. Revised every three years by the National Fire Protection Association.
National Fire Protection Association. Reference →
IEC 60364. Low-voltage electrical installations. The international counterpart to the NEC, covering voltages up to 1000V AC / 1500V DC in residential, commercial, and industrial installations.
International Electrotechnical Commission. Reference →
NIST SP 811: SI Unit Definitions. Definitions of the volt, ampere, watt, ohm, coulomb, and joule under the International System of Units (SI). Authoritative reference for unit conversions and precision.
National Institute of Standards and Technology. Reference →

Disclaimer: The information on this page is provided for reference. Always consult a licensed electrician and the current edition of your local adopted electrical code before performing electrical work.