# Electrical Formulas

## The most common used electrical formulas - Ohms Law and combinations

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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

P = V I(1a)

P = R I^{2}(1b)

P = V^{2}/ 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 = V^{2}/ P(3b)

R = P / I^{2}(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) / (18Ω)

= 0.67 (A)

### Electrical Motor Formulas

#### Electrical Motor Efficiency

μ = 746 P_{hp}/ P_{input_w}(5)

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)

#### Electrical Motor - Amps

I_{3-phase}= (746 P_{hp}) / (1.732 VμPF) (7)

where

I_{3-phase}= electrical current 3-phase motor (amps)

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