Engineer Insight

Ask Ian - What is Ohm’s Law?

by Ian Renwick

Electric heater characteristics can be described fairly well with four simple variables. They are wattage (power), voltage, current (in amps), and resistance (in ohms). There is an interesting relationship between these four variables in that if you know any two of them you can calculate the other two. This is called Ohm’s Law and it is important when determining how to set up to use an electric heater or heaters; what kind of dielectric protection is required, how much current will be pulled by a heater when energized, and how to check a heater to make sure it’s good to use are all things you can use Ohms Law to help determine.

Those units above are often abbreviated with letters that appear to make only partial sense. They are P for watts (makes sense, P = Power), E for Voltage (this comes from E meaning Electromotive Force, which is measured in Volts), I for Current (from the French, ‘Intensité de Courant’), and Ω or R for Resistance, where Ω is the greek uppercase letter omega.

First, determine what variable you need to calculate (select one of the two at a time) and that will direct your attention to the correct quadrant of the Ohm’s Law Calculator tool graphic.

Each quadrant contains three wedges, where you will find one of the wedges to contain the two variables that you already know values for.

Perform the math as shown in the wedge to determine the missing value that you need for that quadrant.

First Example.

Let’s say you want to maximize the power output using the voltage source and current available to you. Let’s say you have a 240V circuit available that can handle 15 amps. What is the maximum wattage of heaters that this circuit will allow you to power?

Start in the quadrant in the upper left because you want to know the wattage. You already know the voltage and current, so you’ll use the lower wedge in the quadrant for this example. EI means Voltage (E) times Current (I) so 240 x 15 = 3600 watts. There’s your upper limit. With a 240V supply and drawing no more than 15 amps, you can generate 3600 watts.

Second Example.

You have a heater marked with 1000W and 240V, but you don’t know if it’s any good anymore. A good way to check it is with an ohmmeter or multimeter to measure the resistance. We start in the lower right quadrant of the ohm’s law circle because we want to know the resistance. We know the E and W values so we’ll use the center wedge (of the three) to calculate the resistance using E2/W. That gives a resistance of 2402/1000 = 57.6Ω. If your heater measures a resistance of 57.6 Ω or something close to that you can be confident that your heater has the correct resistance.

You can find a tool on the TUTCO website that will perform these calculations for you. Go to / Engineering Resources / Calculators / and click on OHM’S LAW CALCULATOR on the right side.