Some Important Formulas of Electrical Engineering

Electric Power Formulas:

P = V I         (1a)

P = R I²         (1b)

P = V²/ R         (1c)

where

P = power (watts, W)

V = voltage+ (volts, V)

I = current (amperes, A)

R = resistance (ohms, Ω)

Electric Current Formulas:

I = V / R         (2a)

I = P / V         (2b)

I = (P / R)^1/2   (2c)

Electric Resistance Formulas:

R = V / I         (3a)

R = V²/ P        (3b)

R = P / I²         (3c)

Electrical Potential Formulas - Ohms Law:

Ohms law can be expressed as:

V = R I         (4a)

V = P / I         (4b)

V = (P R)^1/2         (4c)

Example - Ohm's law:

A 12 volt battery supplies power to a resistance of 18 ohms.

I = (12 V) / (18 Ω)

    = 0.67 (A)

Electrical Motor Formulas:

Electrical Motor Efficiency

μ = 746 P_hp / P_{input_w}         (5a)

where

μ = efficiency

P_hp = output horsepower (hp)

P_{input_w} = input electrical power (watts)

or alternatively

μ = 746 P_hp / (1.732 V I PF)         (5b)

Electrical Motor - Power

P_3-phase = (V I PF 1.732) / 1,000         (6)

where

P_3-phase = electrical power 3-phase motor (kW)

PF = power factor electrical motor

Electrical Motor - Amps

I_3-phase = (746 P_hp) / (1.732 V μ PF)         (7)

where

I_3-phase = electrical current 3-phase motor (amps)

PF = power factor electrical motor