What Is Overcurrent Protection?

When I moved into my current home, the kitchen had been recently remodeled. Great! That meant it had at least two 20-amp circuits to handle all the kitchen appliances, countertop and wall receptacles and the refrigerator (though electricians often put the fridge on its own circuit).

Compared to my previous house, built in 1900 and updated when knob-and-tube wiring was the latest innovation, this new kitchen was a dream.

I immediately went out and bought an air fryer. Big mistake. The only place it logically fit was right next to the microwave, and only one of those 20-amp circuits fed that section of countertop.

Why did this matter? In a word: overcurrent. My microwave draws 10 amps, the new air fryer about 14 amps. If I tried to use the microwave and the air fryer at the same time, the circuit breaker would trip because the circuit can only handle 20 amps.

Overcurrent happens when too much (“over”) electricity (“current”) flows through a circuit. The situation in my kitchen describes one type of overcurrent, called an overload. Other overcurrents, called short circuits and ground faults, happen when sudden, unintentional spikes in current occur.

Confused yet? I’ll explain in more detail below. What’s important (and reassuring) to know is that our homes have overcurrent protection to stop these potentially dangerous situations.

What Is Overcurrent Protection?

Overcurrent protection is just what it sounds like: protection against currents higher than a circuit is rated for.

Every circuit in your home has a rating, determined by the circuit’s overcurrent protection. When electricity flowing through a circuit surpasses its rating, the overcurrent protection kicks in. This protection can happen instantaneously or on a predetermined timeframe, depending on the nature of the overcurrent.

What’s being protected? The circuit itself, for one thing. Electricity produces heat, and electrical conductors, aka the wires carrying electricity to your stuff, get hot when current’s flowing. Too much heat can harm the conductors over time.

Then there’s what the conductors are attached to. Equipment — appliances, lights, computers and anything else that runs on electricity — must be protected from the heat of overcurrents, too.

Overcurrent protection also protects people, although that’s not its primary function. Conductors and equipment experiencing an overcurrent situation can cause fires, equipment malfunction and electrical shock, so it follows that overcurrent protection is vital for our safety, as well.

What Is an Overcurrent Protection Device?

Overcurrent protection devices (OCPD) are the things that stop the flow of electricity when an overcurrent situation happens.

Circuit breakers are one type of overcurrent protection. Fuses, found in older homes, are another. Those aren’t the only ones, but they’re the ones most relevant to a typical homeowner. Circuit breakers and fuses (if you have them) are found in your home’s electrical panel.

When a circuit is functioning normally, an overcurrent device works like an open gate, allowing electricity to cycle freely through your home’s wiring to power your lights, appliances and tools. If an OCPD senses too much current on the circuit — like when I tried to use my air fryer and microwave at the same time — it slams the gate, breaking the circuit and stopping the flow of electricity in its tracks.

Luckily, OCPD don’t just detect overloads. They also stop ground faults and short circuits, i.e. dangerously high currents resulting from unintentional contact between conductors and/or grounded parts of the circuit.

Overcurrent Protection vs. Overload Protection

So we know that overloads are one kind of overcurrent, and that overcurrent protection shuts off the circuit when overloads occur. What about other overcurrents, like faults? Do OCPD like circuit breakers handle all overcurrents the same way?

No.

Overload protection specifically handles excess current that if sustained over time could cause overheating of the conductors and equipment. This distinction is important because sometimes a quick overload situation is harmless and even expected.

Think about a tool in your garage like a miter saw, power drill or air compressor. When you pull the trigger, the motor screams on in a fast burst — called an inrush current — before settling in to its regular groove.

If every appliance with a motor — like shop tools, your refrigerator and air conditioner — tripped the breaker when it started up, we’d never get any work done and all our food would spoil. For that reason, breakers can distinguish between overloads and faults and react accordingly. (Motors also have internal overload protection separate from your home’s overcurrent protection.)

In contrast, when we’re talking about short circuits and ground faults, these aren’t 24 amps on a 20 amp breaker, or even a brief rush of current when you fire up the circular saw. Ground faults and short circuits generate thousands and thousands of amps. Overcurrent protection must shut down the circuit immediately when these situations occur.

How To Calculate Overcurrent Protection

Licensed electricians spend years learning the nuances of overcurrent protection, like how to determine the OCPD and the conductor size needed for that OCPD.

What is the load that will be on the circuit? Will the load be on continuously for three hours or more at a time? Will any non-continuous loads be on the same circuit? What is the ambient temperature? Some conductors, like the relatively small ones powering many of your home’s electrical needs, are limited in the size of OCPD that can protect them, no matter what the calculations say.

Because this vital protection has so many variables, overcurrent protection calculations are best left to the pros.