085 – Ohm’s Law + Wattage
Calculating power needs is critical in production.
Written by Scott Adamson
Last week, we covered three foundational topics of electricity — current, voltage, and resistance.
Once we understand what these terms mean, it’s equally important to understand how they interact with each other. This relationship is called Ohm’s Law. It was defined in the early 19th-century by German physicist and mathematician Georg Ohm and demonstrates how changing one of these aspects within our electrical system will affect the others.
Ohm’s Law is a formula used to calculate the relationship between voltage, which in electrical terms is represented by the letter E, referencing an old-school term electromotive force, current, which is represented by the letter I, representing intensity, and resistance, which is represented by the letter R or by the Greek letter Omega (Ω).
For our purposes, we’ll use V for voltage, A for current (amperes), and Ω for resistance.
Ohm's law states: voltage = current • resistance. This relationship can be represented as a triangle.
In this triangle, voltage = current • resistance.
We can also see that current = voltage ÷ resistance.
And we can calculate resistance, which is voltage ÷ current.
When we talk about more complicated electronic circuits, this equation is particularly important because we’ll need to calculate these values along any point in the circuit. But for our purposes as live sound engineers, we’re talking about power distribution, which means we’re looking at fixed voltages — either 120V in the United States or 240V in Europe. Plus, when we’re on stage, we’re not often calculating electrical resistance. So while this doesn’t really come into play in our day-to-day workflow, it’s still important to have a solid understanding of how these topics work together.
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Another important topic in electricity is Wattage. This is how we measure the full electrical power harnessed by a circuit, measured in Watts.
Wattage is found with a simple equation: Watts = voltage • current, or W = VA.
When we’re talking about audio, most of the power we need will be for amplifiers. This is what takes an audio signal and boosts it so it has enough power to drive speakers. Every amplifier has a power rating, measured in Watts. This is the amount of power it sends to the speaker at full volume, but this doesn’t mean that the amplifier is going to need that much power from our electrical system. By knowing how much current each of our production devices draw, we can figure out what can fit on each circuit and how much power we need over to make the show happen.
