⚡ Electrical Formulas Reference

Comprehensive reference guide for electrical formulas used in electrical engineering, installations, and calculations.

📋 Variables & Units

Voltage

Volts (V)

Current

Amperes (A)

Resistance

Ohms (Ω)

Power

Watts (W)

Voltage Drop

Volts (V)

Resistivity Constant

Ohms (Ω)

Length (one-way)

Feet (ft) or Meters (m)

Circular Mils

Power Factor

Unitless (0-1)

Energy

Kilowatt-hours (kWh)

Time

Hours (h)

Supply Voltage

Volts (V)

Forward Voltage

Volts (V)

Forward Current

Amperes (A)

Apparent Power

Volt-Amperes (VA)

Reactive Power

Volt-Amperes Reactive (VAR)

Impedance

Ohms (Ω)

Reactance

Ohms (Ω)

Ohm's Law

Voltage

V = I × R

Voltage equals current times resistance

Voltage (from power)

V = P / I

Voltage equals power divided by current

Voltage (from power and resistance)

V = √(P × R)

Voltage equals square root of power times resistance

Current

I = V / R

Current equals voltage divided by resistance

Current (from power)

I = P / V

Current equals power divided by voltage

Current (from power and resistance)

I = √(P / R)

Current equals square root of power divided by resistance

Resistance

R = V / I

Resistance equals voltage divided by current

Resistance (from power)

R = V² / P

Resistance equals voltage squared divided by power

Resistance (from current and power)

R = P / I²

Resistance equals power divided by current squared

Power Calculations

Power (DC)

P = V × I

Power equals voltage times current

Power (from resistance)

P = I² × R

Power equals current squared times resistance

Power (from voltage and resistance)

P = V² / R

Power equals voltage squared divided by resistance

Power (Single Phase AC)

P = V × I × PF

Power equals voltage times current times power factor

Power (Three Phase AC)

P = √3 × V × I × PF

Power equals square root of 3 times voltage times current times power factor

Voltage Drop

Voltage Drop (Single Phase)

Vd = 2 × K × I × L / CM

Voltage drop for single phase circuits

Voltage Drop (Three Phase)

Vd = 1.732 × K × I × L / CM

Voltage drop for three phase circuits

Voltage Drop Percentage

Vd% = (Vd / V) × 100

Voltage drop as percentage of source voltage

K (Copper)

K = 12.9 Ω

Resistivity constant for copper at 75°C

K (Aluminum)

K = 21.2 Ω

Resistivity constant for aluminum at 75°C

Wire Sizing

Wire Size (Ampacity)

I = A × Correction Factor

Current capacity with temperature correction

Circular Mils

CM = d²

Circular mils equals diameter in mils squared

Wire Resistance

R = K × L / CM

Resistance equals resistivity times length divided by circular mils

LED Calculations

LED Resistor

R = (Vs - Vf) / If

Current limiting resistor for LED

LED Power

P = (Vs - Vf) × If

Power dissipated by current limiting resistor

Series LEDs

Vtotal = Vf1 + Vf2 + ... + Vfn

Total forward voltage for LEDs in series

Resistor Calculations

Series Resistance

Rtotal = R1 + R2 + R3 + ...

Total resistance of resistors in series

Parallel Resistance (2 resistors)

Rtotal = (R1 × R2) / (R1 + R2)

Product over sum for two parallel resistors

Parallel Resistance (multiple)

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

Reciprocal sum for parallel resistors

Energy & Cost

Energy (kWh)

E = P × t / 1000

Energy in kilowatt-hours

Cost

Cost = E × Rate

Cost equals energy times electricity rate

Daily Cost

Cost/day = (P × hours/day × Rate) / 1000

Daily electricity cost for appliance

Annual Cost

Cost/year = Cost/day × 365

Annual electricity cost

AC Circuits

Apparent Power

S = V × I

Apparent power in volt-amperes (VA)

Real Power

P = S × PF

Real power equals apparent power times power factor

Reactive Power

Q = √(S² - P²)

Reactive power in volt-amperes reactive (VAR)

Power Factor

PF = P / S

Power factor equals real power divided by apparent power

Impedance

Z = √(R² + X²)

Impedance equals square root of resistance squared plus reactance squared

⚠️

National Electrical Code (NEC) Compliance

These formulas are based on standard electrical engineering principles and NEC guidelines. However, always:

Verify calculations with local electrical codes
Consult NEC Article 210 for branch circuits
Consult NEC Article 215 for feeders
Apply appropriate safety factors
Consider environmental conditions (temperature, conduit fill, etc.)
Get approval from licensed electrician or electrical engineer for critical installations

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