An overview on 80% vs 100% rated breakers with examples

TL;DR — A low-voltage circuit breaker is designed and tested to handle its entire current rating, yet according to NEC and CEC, continuous loads are restricted to 80% of the circuit breaker's rating (the protection must be not less than 125% of the load). Manufacturers conduct extra tests to provide circuit breakers rated at 100%, enabling compliance with the code exception clause. Understanding 80% and 100% rated circuit breakers frequently leads to significant installation cost savings.

The backstory...

The NEC and CEC require low-voltage loads to be protected by an overcurrent device not less than 125% of the calculated continuous load plus the non-continuous load (NEC Article 210.20(A), CEC 8-104). This means when the code is applied, the overcurrent device is sized for 80% of its rating (80% is the inverse of 125%). The breaker is derated in the application of the code.

However, if a manufacturer has their breaker listed as 100% rated, there is a code exception that the 125% does not apply — there is no oversizing. The protection can be sized to match the continuous plus non-continuous load. A 100A continuous load can be protected by a 100A breaker when the breaker is listed at 100% rated.

So, what makes a breaker 100% rated?

UL489 is the standard for Molded-Case Circuit Breakers. The standard stipulates the testing requirements and it requires all breakers to operate at 100% indefinitely in an open-air environment.

However, there are additional testing requirements for listing as a "100% breaker". This test adds a specific size enclosure to the test. The temperature of various points on the breaker is monitored, including the cable terminations. If the temperature stays within test setpoints, the breaker passes the test and can be listed as 100% rated with specific conductor and enclosure specifications. Typically the cable specifications are 90°C cable using the 75°C ampacity.

I like to think that the 100% is a "real-world" test. The additional testing, however, is increased cost for the manufacturers.

There are a lot more details of the UL 489 breaker testing, but the main point is this: 1) all breakers can operate indefinitely at 100% current rating and, 2) additional testing at specified enclosure sizes and cable specifications allows the breaker to be listed as a 100% rated breaker.

A 100% rated breaker is not required and is not economical when the load is non-continuous. Non-continuous loads do not require 125% protection — they can simply be protected by a standard breaker at the calculated load value. A 50A non-continuous load can be protected by a 50A breaker.

Note: An 80% rated breaker is typically just referred to as a "breaker," with the 80% rating implied. The rating is specified only when referring to a 100% breaker. This article uses verbose descriptions for clarity.

NEC Article 100 and CEC 8-104 3) outline rules for determining continuous and noncontinuous loads. For NEC, continuous loads are where the maximum current lasts for 3 hours or more. For CEC, continuous loads are where the load runs a total of more than 1 h in any 2 h period if the load does not exceed 225 A, or a total of more than 3 h in any 6 h period if the load exceeds 225 A.

Keep in mind, these calculations and code rules are for low-voltage (< 1000V). Medium-voltage is all 100% rated equipment.

OK, so when to use a 100% rated breaker?

It may make economic sense to use a 100% rated breaker when it provides a lower amperage rating than required using a standard 80% breaker. Let's look at some examples.

Example 1: a panel with 100A of continuous load and 150A of non-continuous load

First, for a standard 80% rated breaker, we calculate the overcurrent protection required using 125% for continuous loads and 100% for non-continuous (NEC 210.20).

100A * 1.25 + 150A = 275A

In this scenario, the breaker would be a 400A or 600A-rated frame with a trip rating of 300A (next size up from 275A).

We can evaluate the size required of a 100% rated breaker on this application simply by adding the continuous and non-continuous loads.

100A + 150A = 250A

As such, a 250A 100% rated breaker would be sufficient.

From this point, it is simply a pricing exercise to see if the 250A 100% rated breaker is less expensive than the 300A trip, 400A frame 80% breaker. It likely could be considering the difference in moving up a frame size from 250A to 400A.

Example 2: A 800A non-continuous load

As the load is non-continuous, one can simply use a standard 80% breaker at the calculated load of 800A. There is no requirement for 125% up-sizing the breaker on non-continuous loads.

Example 3: a panel with 130A of continuous load and 550A of non-continuous load

Using a standard rated breaker, the ampacity required is 550 + 130 * 1.25 = 712.5. Rounding up, using Table 240.6(A) Standard Ampere Ratings, as allowed when under 800A as per NEC 240.4 B), the overcurrent protection is 800A.

Using a 100% rated breaker, the ampacity required is 550 + 130 = 680A. This also rounds up to an overcurrent protection of 800A. Note, 700A is a standard rating under Table 240.6 (A), but it is not a common size with most manufacturers, especially for 100% rated breakers.

In this example, both standard and 100% rated cases result in 800A overcurrent protection. It is likely most economical to use the standard breaker.

Tip: There may be cases where a 100% rated breaker makes financial sense for the project, but not for the operation or maintenance. One should consider spare parts management and the future availability of spare 100% rated breakers before designing it this way.

Example 4: 980A continuous load

Using a standard rated breaker, the ampacity required is 980 * 1.25 = 1225A. Rounding up, using Table 240.6(A) Standard Ampere Ratings the overcurrent protection is 1600A. As the protection is over 800A, the conductor ampacity must be equal to or greater than the overcurrent protective device (NEC 240.4 (C)).

Using a 100% rated breaker, the ampacity required is simply 980A. This rounds up to an overcurrent protection of 1200A (or 1000A, if available in 100% rated breakers). Similarly, the conductor ampacity must be equal to or greater than the overcurrent protective device.

In this example, all else being equal, it is likely more economical to procure a 1200A 100% rated breaker than a 1600A standard rated breaker, especially when considering the increased cable costs.