# 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 1000 volt amperes
• Power Factor - ratio of watts to volt amperes

### Electrical Potential - Ohm's Law

Ohm's law can be expressed as:

U = R I                        (1a)

U = P / I                      (1b)

U = (P R)1/2                (1c)

### Electric Current - Ohm's Law

I = U / R                      (2a)

I = P / U                      (2b)

I = (P / R)1/2               (2c)

### Electric Resistance - Ohm's Law

R = U / I                     (3a)

R = U2/ P                    (3b)

R = P / I2                    (3c)

### Example - Ohm's law

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

I = (12 V) / (18 Ω)

= 0.67 (A)

### Electric Power

P = U I                      (4a)

P = R I2                    (4b)

P = U2/ R                  (4c)

where

P = power (watts, W, J/s)

U = voltage (volts, V)

I = current (amperes, A)

R = resistance (ohms, Ω)

### Electric Energy

Electric energy is power multiplied with time:

W = P t                       (5)

where

W = energy (Ws, J)

t = time (s)

Alternative - power can be expressed

P = W / t                     (5b)

Power is consumption of energy by consumption of time.

#### Example - Energy lost in a Resistor

A 12 V battery is connected in series with a resistance of 50 ohm. The power consumed in the resistor can be calculated as

P = (12 V)2 / (50 ohm)

= 2.9 W

The energy dissipated in 60 seconds can be calculated

W = (2.9 W) (60 s)

= 174 Ws, J

= 0.174 kWs

= 4.8 10-5 kWh

#### Example - Electric Stove

An electric stove consumes 5 MJ of energy from a 230 V power supply when turned on in 60 minutes.

The power rating - energy per unit time - of the stove can be calculated as

P = (5 MJ) (106 J/MJ) / ((60 min) (60 s/min))

= 1389 W

= 1.39 kW

The current can be calculated

I = (1389 W) / (230 V)

= 6 ampere

### Electrical Motors

#### Electrical Motor Efficiency

μ = 746 Php / Pinput_w                 (6)

where

μ = efficiency

Php = output horsepower (hp)

Pinput_w = input electrical power (watts)

or alternatively

μ = 746 Php / (1.732 V I PF)                      (6b)

#### Electrical Motor - Power

P3-phase = (U I PF 1.732) / 1,000                    (7)

where

P3-phase = electrical power 3-phase motor (kW)

PF = power factor electrical motor

#### Electrical Motor - Amps

I3-phase = (746 Php) / (1.732 V μ PF)                  (8)

where

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

PF = power factor electrical motor

## Related Topics

• Electrical - Electrical units, amps and electrical wiring, wire gauge and AWG, electrical formulas and motors

## Search the Engineering ToolBox

- "Search is the most efficient way to navigate the Engineering ToolBox!"

## Engineering ToolBox - SketchUp Extension - Online 3D modeling!

Add standard and customized parametric components - like flange beams, lumbers, piping, stairs and more - to your SketchUp model with the Engineering ToolBox - SketchUp Extension/Plugin - enabled for use with the amazing, fun and free SketchUp Make and SketchUp Pro . Add the Engineering ToolBox extension to your SketchUp from the Sketchup Extension Warehouse!

Translate the Engineering ToolBox!