Electrical Terms and Equations

Ohms Law

One Ohm = The resistance of a column of mercury (At the temperature of melting ice) of a uniform cross section of one square millimeter and a length of 106.30 centimeters.

One Volt = The electromotive force which produces a current of one ampere when steadily applied to a conductor the resistance of which is one Ohm.

One Ampere = The current which when passed through a solution of nitrate of silver in water in accordance with certain specifications, deposits silver at the rate of 0.001118+ of a gram per second.

One Watt = The power expended by a current of one Ampere in a resistance of one Ohm.

One Coulomb = The quantity of electricity transferred by a current of one Ampere in one second.

One Farad = The capacity of a condenser in which a potential difference of one volt causes it to have a charge of one Coulomb of electricity.

One Henry = The inductance in a circuit in which the electromotive force induced is one volt when the inducing current varies at the rate of one Ampere per second.

One Joule = The energy expended in one second by a flow of one Ampere in one Ohm.

The Gram-Calorie is the energy required to raise one gram of water one degree centigrade in temperature.  One gram calorie is very nearly equal to 4.18 Joules.

The Ampere-Hour is the quantity of electricity transferred by a current of one ampere in one hour, and is therefore equal to 3600 Coulombs.

The Circular Mil is the unit of cross-section used in the American Wire Gauge.  The term “Mil” means one thousandth of an inch (0.001 inch).  It is the area of a circular wire having a diameter of one Mil.

The Square Mil is the area of a square each side of which is one Mil (0.001 inch).

The Area of a Square Mil is 0.000001 square inches.

The Circular Mil-Foot is a unit circular conductor one foot in length and one Mil in diameter.

The Resistance of such a unit of copper has been found experimentally to be 10.37 ohms at 20 degrees Celsius.

The BTU or British Thermal Unit is a unit of heat energy and is defined as the amount of heat necessary to raise the temperature of one pound of water one degree Fahrenheit.

The Horsepower is equal to 746 Watts.

BASIC ELECTRICAL SYMBOLS

I = Current in Amperes

E = Volts

R = Resistance in Ohms

P or W = power in Watts

KW = Power in Kilowatts

1KW = 1,000 watts

VA = Apparent Power in Volt-Amperes

KVA = Apparent power in Kilovolt-Amperes

HP = Output Power in number of Horsepower.

EFF = Efficiency, expressed in a decimal fraction (output divided by input)

PF = Power Factor expressed in a decimal fraction, the ratio of true power (P, W, or KW) divided by apparent power (VA or KVA)

EQUATIONS for SINGLE PHASE AC CIRCUITS

I = VA ÷ E or Amps equals volt-amperes, divided by volts.

I = 1,000 x KVA ÷ E or Amps equals one thousand, times kilovolt-amperes, divided by volts.

I = W ÷ E x PF or Amps equals watts, divided by volts, times power factor.

I = 1,000 x KW ÷ E x PF or Amps equals one thousand, times kilowatts, divided by volts, times power factor.

I = 746 x HP ÷ E x PF x EFF or Amps equals 746, times horsepower, divided by volts, times power factor, times efficiency.

P = E x I x PF or Power equals volts, times amps, times power factor.

VA = I x E or Volt-amperes equals amps, times volts.

KW = E x I x PF ÷ 1,000 or Kilowatts equals volts, times amps, times power factor, divided by 1000.

KVA = I x E ÷ 1,000 or Kilovolt-amperes equals amps, times volts, divided by 1000.

HP = I x E x PF x EFF ÷ 746 or Horsepower equals amps, times volts, times power factor, times efficiency, divided by 746

EQUATIONS  for DC CIRCUITS

To find the Voltage, ( E )
E = P ÷ I    Volts = Watts ÷ Amps

To find the Current, ( I )
I = P ÷ E    Amps = Watts ÷ Volts

To find the Resistance, ( R )
R = E ÷ I     Ohms = Volts ÷ Amps

To find the Watts, ( P )
P = E x I     Watts = Volts x Amps

For additional DC Voltage Ohms Law equations, see the chart at the top of this page.