Electrical Formulas Reference

Complete collection of electrical formulas for electricians and engineers. From Ohm's law to three-phase calculations.

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Ohm's Law

The fundamental relationship between voltage, current, and resistance.

E = I × R

Voltage = Current × Resistance

I = E / R

Current = Voltage / Resistance

R = E / I

Resistance = Voltage / Current

Where: E = Voltage (volts), I = Current (amps), R = Resistance (ohms)

Power Formulas

Calculate electrical power (watts) using different known values.

P = E × I

Power = Voltage × Current

P = I² × R

Power = Current² × Resistance

P = E² / R

Power = Voltage² / Resistance

I = P / E

Current = Power / Voltage

Where: P = Power (watts), E = Voltage (volts), I = Current (amps), R = Resistance (ohms)

Three-Phase Power Formulas

Formulas for three-phase AC circuits.

P = √3 × E × I × PF

Three-phase power (watts)

I = P / (√3 × E × PF)

Three-phase current (amps)

kVA = √3 × E × I / 1000

Three-phase apparent power

PF = kW / kVA

Power factor calculation

Where: √3 ≈ 1.732, E = Line voltage, I = Line current, PF = Power factor

Voltage Drop Formulas

Calculate voltage drop in electrical conductors.

Single-Phase

VD = (2 × K × I × D) / CM

Two-wire voltage drop formula

Three-Phase

VD = (1.732 × K × I × D) / CM

Three-phase voltage drop formula

Where: VD = Voltage drop (volts), K = Resistivity constant (12.9 copper, 21.2 aluminum), I = Current (amps), D = One-way distance (feet), CM = Circular mils of conductor

→ Use Voltage Drop Calculator

Motor Formulas

Calculate motor power, current, and efficiency.

Horsepower to Watts

P = HP × 746

Motor Efficiency

Eff = Output / Input

Synchronous Speed (RPM)

N = (120 × f) / P

f = frequency (Hz), P = poles

Slip Percentage

Slip = (Ns - Nr) / Ns × 100

Ns = sync speed, Nr = rotor speed

→ Use Motor Sizing Calculator

Transformer Formulas

Calculate transformer turns ratio, voltage, and current relationships.

Turns Ratio

Ep / Es = Np / Ns

Voltage ratio equals turns ratio

Current Ratio

Ep / Es = Is / Ip

Voltage up = current down

Single-Phase kVA

kVA = (E × I) / 1000

Three-Phase kVA

kVA = (√3 × E × I) / 1000

Resistance Formulas

Calculate series, parallel, and conductor resistance.

Series Resistance

Rt = R1 + R2 + R3...

Parallel Resistance

1/Rt = 1/R1 + 1/R2 + 1/R3...

Two Parallel Resistors

Rt = (R1 × R2) / (R1 + R2)

Conductor Resistance

R = (K × L) / A

K = resistivity, L = length, A = area

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