Understanding Circuit Breaker Labeling: What Does “20AF 20AT 3P” Mean?

by | Jul 17, 2023 | Design

As a vital safety mechanism in every electrical circuit, circuit breakers play a crucial role in preventing damage caused by overcurrent conditions like short circuits and overloads. But the nitty-gritty details inscribed on the labels of these devices can seem like a foreign language to the uninitiated. One such intriguing label you might come across is “20AF 20AT 3P”. Let’s break down this code and look at some potential variations.

Interpreting the Label “20AF 20AT 3P”

The inscription “20AF 20AT 3P” provides specific details about the circuit breaker’s operational capabilities.

  • “20AF” refers to the Ampere Frame (AF) size. The AF size is essentially the physical size of the breaker and its associated interrupting capacity. In this case, the circuit breaker is designed for a 20-ampere frame size. This defines the highest continuous current load that the breaker can handle under normal operating conditions.
  • “20AT” indicates the Ampere Trip (AT) rating, which is the current level at which the breaker will trip to interrupt the current flow. It acts as a protective measure to prevent equipment damage from excess current. In this scenario, the breaker will trip if the current exceeds 20 amperes.
  • “3P” denotes that this is a three-pole circuit breaker. In three-phase power systems, this means the breaker controls three different circuits that share a common trip mechanism. If a fault occurs on any of the three lines, the breaker will trip and disconnect all three phases.

Variations in Circuit Breaker Ratings

These ratings can and do vary widely depending on the specifics of the electrical circuit they’re designed to protect.

For instance, circuit breakers for large industrial applications might have an AF rating of several hundred or even thousands of amperes to accommodate higher load equipment. Conversely, a circuit breaker designed for residential applications might have an AF rating as low as 15 amperes to protect small household appliances.

Similarly, the AT rating can vary to match the load’s characteristics it protects. A piece of equipment with a large inrush current (like motors) may need a breaker with a higher AT rating, allowing a brief surge of current without tripping. However, sensitive electronics may need a lower AT rating to ensure the breaker trips at a lower current, providing finer protection against overcurrent.

Pole count (“P”) can also differ based on the application. Single-phase systems typically use 1P or 2P breakers, while three-phase systems use 3P breakers. A 1P breaker controls a single line, a 2P controls two lines, and a 4P breaker would control three lines plus a neutral in some specific scenarios.


The seemingly cryptic labels on circuit breakers actually provide a great deal of information about the device’s capabilities and applications. Understanding these labels and the ways they can vary will help electrical engineers and electricians select the right breaker for any given application.

Every installation requires a delicate balance between safety and operational requirements, and these labels are the keys to striking that balance. In the end, the choice of a circuit breaker, be it a 20AF 20AT 3P or any other rating, comes down to understanding the needs of the system it protects.

Understanding the Corona Effect and Plasma in High-Voltage Systems

When discussing high-voltage power systems, two terms that frequently come up are the "corona effect" and "plasma." While they might sound complex, with a little unpacking, these phenomena can be understood more intuitively. 1. The Basics: What is an Ion? Before we...

Hall Effect Sensors: Exploring the Science and Application

Title: Hall Effect Sensors: Exploring the Science and Application In the broad field of electronics, sensors serve as indispensable components in our devices, enabling them to interact with the physical world. Among these, the Hall Effect sensor, with its diverse...

Calculating Total Kilowatt-Hours for a UPS Battery System

An Uninterruptible Power Supply (UPS) is an essential component of any data center or any infrastructure where a consistent and continuous power supply is crucial. Understanding how much power a UPS system can supply is of utmost importance for managing resources and...

Understanding High Voltage Dielectric and Electrical Insulation Resistance Tests: A Comparative Study

In the realm of electrical engineering, ensuring the safety and functionality of equipment is paramount. Two critical tests often used in this process are the High Voltage (HV) dielectric test (or 'hipot' test) and the electrical insulation resistance test (commonly...

Understanding the Dynamics of Out-of-Phase Waveforms in AC Power Systems

AC power systems are at the heart of most of our modern electrical infrastructure. They enable the transmission and use of electrical energy across vast distances, powering everything from household appliances to large industrial machinery. Understanding the...

Transmission considerations of alternating current (AC) and high voltage direct current (HVDC)

In the world of electrical power transmission, alternating current (AC) and high voltage direct current (HVDC) play pivotal roles, each having unique benefits and drawbacks. While AC is traditionally used for shorter distances and grid interconnectivity due to its...

Why DC systems don’t involve reactive power?

The concept of reactive power comes into play in AC (Alternating Current) systems, not in DC (Direct Current) systems. This has to do with the nature of AC and DC power and the types of loads they typically serve. Reactive power (often represented by the variable Q...