Electrical Formulas

The most common used electrical formulas - Ohms Law and combinations

Common electrical units used in formulas and equations are:

Volt - unit of electrical potential or motive force - potential is required to send one ampere of current through one ohm of resistance
Ohm - unit of resistance - one ohm is the resistance offered to the passage of one ampere when impelled by one volt
Ampere - units of current - one ampere is the current which one volt can send through a resistance of one ohm
Watt - unit of electrical energy or power - one watt is the product of one ampere and one volt - one ampere of current flowing under the force of one volt gives one watt of energy
Volt Ampere - product of volts and amperes as shown by a voltmeter and ammeter - in direct current systems the volt ampere is the same as watts or the energy delivered - in alternating current systems - the volts and amperes may or may not be 100% synchronous - when synchronous the volt amperes equals the watts on a wattmeter - when not synchronous volt amperes exceed watts - reactive power
Kilovolt Ampere - one kilovolt ampere - KVA - is equal to 1,000 volt amperes
Power Factor - ratio of watts to volt amperes
Electric Power Formulas
W = E I (1a)

W = R I2 (1b)

W = E2/ R (1c)

where

W = power (Watts)

E = voltage (Volts)

I = current (Amperes)

R = resistance (Ohms)

Electric Current Formulas
I = E / R (2a)

I = W / E (2b)

I = (W / R)1/2 (2c)

Electric Resistance Formulas
R = E / I (3a)

R = E2/ W (3b)

R = W / I2 (3c)

Electrical Potential Formulas - Ohms Law
Ohms law can be expressed as:

E = R I (4a)

E = W / I (4b)

E = (W R)1/2 (4c)

Example - Ohm's law
A 12 volt battery supplies power to a resistance of 18 ohms.

I = (12 Volts) / (18 ohms)

= 0.67 Ampere

Electrical Motor Formulas
Electrical Motor Efficiency
μ = 746 Php / Winput (5)

where

μ = efficiency

Php = output horsepower (hp)

Winput = input electrical power (Watts)

or alternatively

μ = 746 Php / (1.732 E I PF) (5b)

Electrical Motor - Power
W3-phase = (E I PF 1.732) / 1,000 (6)

where

W3-phase = electrical power 3-phase motor (kW)

PF = power factor electrical motor

Electrical Motor - Amps
I3-phase = (746 Php) / (1.732 E μ PF) (7)

where

I3-phase = electrical current 3-phase motor (Amps)

PF = power factor electrical motor