Electrical Formula Reference

17 formulas organized around core circuit relationships, AC power, three-phase load screening, voltage drop, and quick equipment checks used in U.S. electrical work.

Updated April 24, 2026

Use formulas for screening, then verify the final design basis. Final conductor, overcurrent, equipment, and voltage-drop decisions still need NEC tables, nameplate data, product listings, and actual project conditions.

Final conductor, overcurrent, equipment, and voltage-drop decisions still need NEC tables, nameplate data, product listings, and actual project conditions.

Core Circuit Relationships

Ohm's Law

Use for resistive or near-steady-state relationships before moving into phase-angle or harmonic analysis.

V = I × R

Variables: V in volts, I in amperes, R in ohms.

DC Power

Quick power check for DC circuits and simple resistive loads where voltage and current are known.

P = V × I

Variables: P in watts, V in volts, I in amperes.

Energy Use

Convert load and runtime into energy use for simple consumption or battery-runtime screening.

E = P × t

Variables: E in watt-hours or kilowatt-hours when units are adjusted, P in watts or kilowatts, t in hours.

Single-Phase AC Power

Apparent Power

Base relationship for single-phase volt-amperes before separating real and reactive components.

S = V × I

Variables: S in VA, V in RMS volts, I in RMS amperes.

Real Power

Use when single-phase load current and power factor are known or can be estimated from equipment data.

P = V × I × PF

Variables: P in watts, V in RMS volts, I in RMS amperes, PF as a decimal power factor.

Reactive Power

Screens the reactive portion of a single-phase load when phase angle is available.

Q = V × I × sin(φ)

Variables: Q in VAR, V in RMS volts, I in RMS amperes, φ as the phase angle.

Power Factor

Shows how much apparent power is converted into useful real power at the load.

PF = P / S

Variables: PF as a decimal ratio, P in watts, S in VA.

Three-Phase Load Screening

Three-Phase Apparent Power

Quick screening relationship for total three-phase apparent power from line voltage and line current.

S = √3 × V_L × I_L

Variables: S in VA, V_L in line-to-line volts, I_L in line amperes.

Three-Phase Real Power

Use for balanced three-phase systems when line values and power factor are known.

P = √3 × V_L × I_L × PF

Variables: P in watts, V_L in line-to-line volts, I_L in line amperes, PF as a decimal.

Line Current from kVA

Screens three-phase current from transformer or load kVA when a balanced line voltage is known.

I_L = kVA × 1000 / (√3 × V_L)

Variables: I_L in amperes, kVA in kilovolt-amperes, V_L in line-to-line volts.

Conductors and Voltage Drop

Single-Phase Voltage Drop

Common U.S. screening equation using one-way length in feet and conductor area in circular mils.

V_drop = 2 × K × I × L / CM

Variables: V_drop in volts, K as the conductor material constant, I in amperes, L in one-way feet, CM in circular mils.

Three-Phase Voltage Drop

Three-phase screening version of the circular-mil voltage-drop equation for balanced systems.

V_drop = √3 × K × I × L / CM

Variables: V_drop in volts, K as the conductor material constant, I in amperes, L in one-way feet, CM in circular mils.

Percent Voltage Drop

Turns a calculated drop into a percentage so branch-circuit and feeder screening stays consistent.

%VD = V_drop / V_source × 100

Variables: %VD as percent drop, V_drop in volts, V_source in source volts.

Transformer and Motor Screening

Single-Phase kVA

Basic single-phase sizing relationship for transformer or load screening before final equipment selection.

kVA = V × I / 1000

Variables: kVA in kilovolt-amperes, V in volts, I in amperes.

Transformer Turns Ratio

Relates primary and secondary voltage to the turns ratio in an ideal transformer model.

V_p / V_s = N_p / N_s

Variables: V_p and V_s in volts, N_p and N_s as primary and secondary turns.

Approximate Three-Phase Motor Input Current

Useful for quick load screening, but final conductor and overcurrent decisions should use NEC tables, nameplate data, and manufacturer instructions.

I ≈ hp × 746 / (√3 × V_L × PF × η)

Variables: I in amperes, hp in horsepower, V_L in line-to-line volts, PF as decimal power factor, η as decimal efficiency.

Synchronous Speed

Relates supply frequency to ideal motor speed before slip is considered.

n_s = 120 × f / p

Variables: n_s in RPM, f in hertz, p as the motor pole count.

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