Home Inspection Concepts

serving the Ottawa area for over 30 years!


  • What is electricity?
  • Understanding current, voltage, resistance, and power
  • Putting the concepts together
  • Examples
  • Try it yourself



Electricity is the product of current flow moved past a magnet.

when a wire is moved past a magnet. This movement of a conductor through a magnetic field is the principle of operation of any generator. This is electrical production by mechanical means.



Electricity provides you with daily comfort and convenience. It heats, cools, and lights your home. While using electricity is simple, the process of making electricity is complex.

Producing electricity begins with the burning of fuel such as coal, oil, or gas or operating nuclear reactors at the power plant to produce steam. The force of the steam turns the blades of a turbine. These blades rotate a metal shaft in an electric generator, producing electricity.

Hydroelectric dams use water falling onto turbine blades to create the same effect. The spinning turbines cause a magnet in the generator to spin, creating magnetic fields which generate electricity. The electricity travels to a power transformer where voltage (pressure) is “stepped up” or increased so that it can be carried long distances through transmission systems. Close to its destination, the electricity passes through a set or series of transformers called a substation. This grouping of transformers converts the electricity to a lower voltage to be

carried on distribution lines into neighborhoods where we live.

Here, another transformer converts electricity to a lower voltage again so you can use it in your home. 

As electricity flows to your home, it passes through a meter that measures how much current you are using. Finally, the service panel of circuit breakers or fuses in your home regulates the flow of power through the house and protects wiring and appliances from damage.


Do you understand circuit current? Most home inspectors think they know what circuit current means, but it is always helpful to review the concepts. Do you understand the relationship between current, voltage, and resistance in electrical circuits? Do you know exactly what power is, and can you calculate its value given current, voltage, or resistance? As you examine the electrical systems and equipment in a residence, you may need to complete some calculations as you judge whether these systems and equipment are sufficient. Are you comfortable enough with the mathematics of these relationships to perform these calculations and then report back to your client? 



One way to understand the concept of current is to think of it as the flow of electrons from one point to another over time. As a comparison, you can think of other things that flow: water through a garden hose or the speed of light flowing from distant stars to earth. The flow of water in a garden hose might be measured in gallons per minute. The speed (or flow) of light has been

measured at 186 000 miles per second, 1 000 000 000 feet per second, or 300 000 kilometers per second. Interestingly, electricity or electrical current has been determined to move at the same speed. However, electricians do not measure the flow of electrical current in feet per second, miles per second or kilometers per second. They measure electrical current in amperes (A). 

Current is represented by the symbol I. Examine the following equation. Current (I) = E (Voltage) ÷ R (Resistance)


Voltage is a way of expressing the pressure of the energy flowing through wires in an electrical circuit. When we wash a car with a hose, we can feel the pressure of the water moving through it. We may have to adjust the pressure of the water by changing settings on the nozzle so that we do not damage the finish.

Voltage has been described as the pressure of the electrical energy which is produced by the generator. Voltage is measured in volts (V), and a common device used by electricians, a voltmeter, measures the electrical pressure between two points in an electrical system.

Voltage is represented by the symbol E. Below is an equation to calculate the voltage. Examine the equation. 

Voltage (E) = I (Current) x R (Resistance) 



In our discussion of electrical current, we used the example of water flowing through a garden hose as an illustration. Now we need to talk about the concept of resistance. Sometimes the water in a garden hose doesn’t flow through as

quickly as we expect. We look at the little stream coming out of the hose, feel the reduced pressure, and then look more closely at the hose to see what’s happened. Often, the hose has developed a kink which has reduced or cut off the flow. In this case, the flow of water has encountered resistance. The narrowed hose wall prevents much of the water from flowing. How about the case of a car stuck in the mud? The resistance from the mud keeps the car’s tires from rotating completely and propelling the car forward. 

Electrical circuits, likewise, also contend with resistance.

Putting these three characteristics together (voltage, current, and resistance, or their units volts, amperes, and ohms) into one working relationship was accomplished by Georg S. Ohm (1789-1854). The law that describes their relationship is called Ohm’s Law and is expressed in the following equation: 


This is the same equation we just examined before with a different name. Electrical resistance may be defined as follows: “the ability of a material to oppose current flow”. Different materials offer varying levels of resistance to the flow of current. This type of resistance is measured, as noted above, in ohms (0).

This is why copper is the choice of metal for most electrical wiring. It offers little resistance to the flow of current. These are typical resistance values for three metals. (These are examples of resistance for 1 000 ft. of wire each. The input value of 1 A) 

Copper Wire Output = 1A

Aluminum Wire Output = 0.6A

Iron Wire Output = 0.16A

The first, copper wire, offers the least resistance to current flow. Aluminum wire, next, offers significant resistance. 40% of the incoming current is lost due to resistance. Iron wire offers very high resistance with only 16% of the current being transmitted. 84% of the incoming current is lost to the resistance of the iron wire.

Resistance is represented by the symbol R. Below is an equation to calculate resistance. Examine the equation. 

Resistance (R) = E (Voltage) ÷ I (Current)


Power is both an electrical and mechanical rate of doing work. James Watt (1736-1800), the inventor of the steam engine, was instrumental in defining power as a mechanical and an electrical quantity of consumed energy. Many people think that the power rating of a device is the output from a device. This is rarely correct. The wattage of a device is the power it consumes to do work and is independent of efficiency. Power is measured in watts (W), kilowatts (kW), megawatts (MW), or as a measure of power consumed over time (kilowatt-hours or kWh).

It is a generally held misconception that the power of a device such as a 60 W light bulb is a description of how much light a lamp will give off. A 40 W fluorescent lamp emits about seven and a half times the light of a standard 40 W incandescent lamp. Keeping this in mind will help you establish the relationship between consumed power and output power. Inspectors are concerned with consumed power since this is the load placed on a system. Given the electrical demands (power) of the home, the inspector can calculate whether the electrical system and its components are sufficient for the task or whether they need to be repaired, replaced, or upgraded.

Power is represented by the symbol P. Below is an equation to calculate power. Examine the equation. 

Power (P) = E (Voltage) x I (Current) 


If you know the third any two of current, voltage, resistance, or power, you can calculate the third value with one of the previous equations. 

For example,

P = E x I can be turned around by dividing both sides of the equation by current I, to get E, voltage.

Therefore, P=E x I become E=P ÷ I

Now you know how to find voltage. You can use the same technique to find other values as needed.

The following diagrams will help you memorize all the combinations of these equations. Take a few minutes to study them. You can use them to practice calculations at home and you can bring a copy of these diagrams with you on an inspection so that you can quickly find the values you need as you are preparing your report. 

Tip: When inspecting a home, watch for fuses or breakers rated for 20, 30, or 40 A. Overcurrent devices rated above 15 A require larger wire size.

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Mike Fuller Electric Ltd.

1692 Ortona Avenue
Ottawa ON

Working hours

Monday-Friday: 7AM-4PM
By appointment on Saturday and Sundays


Tel: (613) 225-3249

Email: [email protected]




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