In Episode 24 of No Dumb Questions, we explain electrical resistance—the opposition to the flow of electric current—and how it is quantified in wire and cable design.
Resistance is the inverse of electrical conductivity and is measured in ohms ($\Omega$). In electrical conductors, resistance is dictated by three primary factors:
- Conductor Material: Copper is the standard conductor of choice due to its high conductivity and cost efficiency. Higher-conductivity alternatives like silver and gold provide superior performance but are rarely economical for general wiring. Conversely, metals like aluminum and stainless steel offer significantly lower conductivity and higher resistance.
- Wire Gauge (Cross-Sectional Area): Conductor size directly affects electron flow. A larger gauge conductor (thicker wire) provides a wider path for current flow, yielding lower resistance. A smaller gauge wire constricts current flow, resulting in higher resistance.
- Conductor Length: Resistance accumulates proportionally over distance; longer cable runs have greater total resistance than shorter runs of the same material and size.
A simple analogy is water flowing through a pipe: a wide, clear pipe allows water to pass with minimal resistance, whereas narrowing the pipe or adding internal obstructions restricts flow—similar to using a smaller wire gauge or a less conductive metal.
To measure electrical resistance, a multimeter is applied across a length of conductor to determine the opposition to current flow in ohms.
Key Technical Takeaways
- Definition: Resistance is the opposition to electric current flow, measured in ohms ($\Omega$), representing the inverse of conductivity.
- Material Selection: Copper is the industry benchmark; silver and gold offer higher conductivity at greater cost, while aluminum and stainless steel exhibit higher resistance.
- Gauge Factor: Larger diameter conductors allow greater current flow and lower resistance; smaller diameter conductors increase resistance.
- Length Factor: Total conductor resistance increases proportionally with wire length.
- Measurement: Tested directly using a multimeter connected across the conductor.
There’s more where that came from! Check out Episode 25: What are common cable failures?
Episode 24 Transcript – What is Resistance and How is it Measured?
Welcome to No Dumb Questions
Welcome back to another segment of No Dumb Questions, where the only dumb question is a question you don’t ask. Today’s segment, we’re going to be talking about the question, what is electrical resistance and how is it measured?
Defining Electrical Resistance & Conductor Materials
So for the first part of the question, what is electrical resistance?
Resistance is the opposition to the flow of current. This is the opposite of conductivity. Typically this is expressed in the units ohms. So the resistance or conductivity of a conductor in the wire and cable industry may vary.
Materials like copper have a very high conductivity, and they are typically the conductor of choice. Other materials, such as silver or gold can be used, which have better conductivity but are not economical for wire and cable conductors in most cases. Other materials such as stainless steel or aluminum can also be used. However, those have much lower conductivity.
The Water Pipe Analogy: Gauge & Length
So the gauge size of wire and cable will also dictate the resistance and/or conductivity. A larger gauge will allow more current flow, where a smaller gauge wire will allow less current flow. Therefore, the smaller gauge wire will have more resistance.
A good analogy for electrical resistance would be if you had a pipe with water flowing through it. There would be very little resistance to the flow of water. This is very similar to how if you had a copper conductor, there would be very little resistance to the flow of current. Now, for example, if we took the pipe and we put some cotton balls inside of it, that would then add resistance to the flow of the water, similar to how a material with less conductivity would add resistance to the flow of current.
If we expanded that pipe into a larger inside diameter, it would allow for more flow of water. If we minimize that pipe, it would allow for less flow of water. Increasing the diameter of the pipe or reducing it is similar to increasing or reducing the size of the copper conductor. We can lower resistance or increase resistance by changing the material or the gauge size of the wire.
The electrical resistance of a wire is typically expressed over the length of the wire, so a longer wire will have more total resistance than a shorter wire.
Measuring Resistance with a Multimeter
The second part of today’s question was how do we measure electrical resistance? This is very simple. You take a multimeter and you would apply it across a length of cable or conductor. And you would use that to measure the opposition of current flow. Your reading would be in the unit ohms.
Okay, I got a little bonus for you. Do you know who discovered electrical resistance? It was Sherlock Ohms.
And that’s it for today’s segment of No Dumb Questions, where the only dumb question is the question you don’t ask. If you have any questions, please submit them at the link below. Thank you.