Electrical Terms and Equations

Common Electrical Terms and Definitions

Dear Mr. Electrician:  Can you recommend books on electrical terms and equations that include definitions?  I found some technical books in the library, but the books use words and terms I do not understand.  Do you have any suggestions?

Answer:  Many electrical books about electrical terms and equations I read as a youngster are no longer published.  They belonged to my father and were excellent for learning about electricity.  A lot of them are still available as used books online.

NOTE: Some text links below go to applicable products on Amazon and eBay.  As an Amazon Associate, I earn from qualifying purchases.  Using my links for Amazon and eBay helps keep this website FREE.

One of the best publishers of electrical books was Audels.  I have a few that belonged to my dad and some that I bought for myself in the 1970s.  The pictures are old, but the information is excellent.

Audels also published books for many other trades and engineers.

Table of Contents:

If you plan to do electrical wiring in your home, you should understand wiring methods and code requirements.  Article 100 in the National Electrical Code (NFPA 70) is entitled Definitions.  It defines all electrical terms used for electrical installations.

See below for a short list of some standard electrical terms.


Ampere is The unit of current strength.  It is the current which, when passed through a solution of nitrate of silver in water following certain specifications, deposits silver at the rate of 0.001118+ of a gram per second.  Electrical current flow is measured in Amperes or Amps using an ammeter.

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

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.  BTU’s x 2.93 + 10,000 = Kilowatt hours.

Top Of Page

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

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

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

Farad – The capacity of a condenser in which a potential difference of one volt causes a charge of one Coulomb of electricity.

Flush Thrust Unit (FTU) – A fictional unit of measurement fabricated by John Grabowski.  A Flush Thrust Unit is for determining the flushing capabilities of a toilet.  The calculation to determine a toilet’s Flush Thrust Units includes water pressure, water temperature, tank volume, the diameter of the discharge tube, the height above sea level, and last night’s supper.  This is more of a plumbing calculation than an electrical one.

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.

Ground or Earthing refers to a wire known as the grounding electrode conductor connected directly to the planet Earth via water pipes and/or ground rods or other means.  The GEC has no color code and can be a bare, uninsulated wire.  This provides lightning protection and voltage stabilization.

In the USA, the ground is also connected (Bonded) to the neutral in the main electrical disconnect for a home or building to provide short circuit protection, also known as fault protection.  In your outlet and switch boxes, the ground wire is known as the equipment grounding conductor (EGC) and is either un-insulated bare or green in color.

Article 250 in the National Electrical Code covers all types of grounding.  The equipment grounding conductor color is green or can also be an un-insulated bare wire.  Article 100 for definitions is also helpful.

Top Of Page

Henry – The inductance in a circuit where the induced electromotive force is one volt when the inducing current varies at one Ampere per second.

Horsepower is equal to 746 Watts.  A one-horsepower electric motor consumes approximately 746 watts.

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

LINE or HOT refers to the current-carrying conductors (Wires) that have electricity flowing to power things such as lights and appliances.  These wires are also called ungrounded conductors.  Wire colors for hot conductors can be anything but white, gray, or green.

LOAD is what is powered by electricity.  Anything that consumes electricity to operate is a load on the electrical system.  Lights, appliances, machinery, and computers are all examples of a load.  A typical single-pole wall switch that operates a ceiling light will have a LOAD wire on one screw and a LINE wire on the other screw.

On some switches, motor starters, and disconnects, the LOAD connection will be labeled with a T, while the LINE will be an L.  You may see L1 and L2, and below them, T1 and T2.

Sometimes a switch or motor starter will have contacts labeled NO and NC.  NO is Normally Open, and NC is Normally Closed.  When a contact or switch is open, no electricity flows.  When it is closed, electricity flows.

Neutral or Neutral Conductor refers to the return path of electricity back to the utility company power transformer.  In the National Electrical Code, this wire is called the grounded conductor because it is bonded to earth ground in the main disconnect for the home or building.  Sometimes electricians will call it the return.  The wire color for the neutral in the USA is usually white, but gray is sometimes used.

A typical electrical receptacle outlet in your home will have a Neutral and a LINE connected to it.  Anything that is plugged into the outlet is considered a LOAD.

CLICK HERE to See Electrical Wiring Books on eBay

 Top Of Page

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.

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

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

The Area of a Square Mil is 0.000001 square inches.

Switch Leg refers to the LOAD wire that comes from a switch.  The switch leg has no power when the switch is off or in the open position.

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

Watt – The power expended by a current of one Ampere in a resistance of one Ohm.  Watts is the amount of power consumed.  It is a unit of electrical power required to do work at the rate of one Joule per second.  It is the power expended when one ampere of direct current (DC) flows through a resistance load of one Ohm.

Watts is called true power.  The electric meter on your house measures Kilowatt hours which is how many thousands of Watts are used per hour.

A Kilowatt is 1000 watts.  Kilowatts x 1.341 = Horsepower.

Top Of Page


I = Current in Amperes

E = Volts

R = Resistance in Ohms

P or W = power in Watts

KW = Power in Kilowatts

1 KW = 1,000 watts

VA = Apparent Power in Volt-Amperes

KVA = Apparent power in Kilovolt-Amperes

HP = Output Power in the number of Horsepowers.  One horsepower is equal to 746 watts.

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)

Top Of Page


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, power factor, and 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

Top Of Page


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 equations, see the circular Ohms Law chart at the top of this page.  Click here for an online FREE Ohms Law calculator.  Insert two values and press “Calculate” to get the other values.

You may also be interested in all of my posted electrical wiring diagrams

Information about electric motors is available in my blog post here.

To see all of the categories of electrical topics discussed by Mr. Electrician, click here.

Click here for a FREE copy of my book “Almost Everything You Need To Know To Repair a Bathroom Exhaust Fan In Your Home.”

Visit my Link Tree for my social media connections and merchandise.

Top Of Page