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 = ۷۴۶ P_hp / W_input (5)

where

μ = efficiency

P_hp = output horsepower (hp)

W_input = input electrical power (Watts)

or alternatively

μ = ۷۴۶ P_hp / (1.732 E I PF) (5b)

Electrical Motor – Power

W_3-phase = (E I PF 1.732) / 1,000 (6)

where

W_3-phase = electrical power 3-phase motor (kW)

PF = power factor electrical motor

Electrical Motor – Amps

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

where

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

PF = power factor electrical motor