A Circuit Breaker or Not a Circuit Breaker…That is the Question


You start your day with your first inspection and upon entering the site the owner indicates, reluctantly, that there is some equipment that is not certified and that the facility needs to be up and running by tomorrow morning. It’s a simple machine with a few motors and a small industrial control panel. The system runs at 15 amperes and is on a 30 ampere branch.

You decide to try and help by inspecting the machine, but the first device you come across is a recognized circuit breaker and you ask yourself, “Is this circuit breaker acceptable for this application?” and then you realize that it may be a long day.


Chart 1

Many readers can identify with the previous scenario whether they are laboratory field engineers or electrical inspectors. The manufacturer of the equipment usually considers a certified device that looks (and sometimes acts) like a circuit breaker as a circuit breaker. However, there are devices that look like circuit breakers, better known as supplementary protectors, that have limited applications just like any other recognized component.

What is a circuit breaker? The 1999 version of the National Electrical Code defines a circuit breaker as: “A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its ratings”.

With overcurrent defined as “Any current in excess of the rated current of equipment or the ampacity of a conductor. It may result from overload, short circuit or ground fault.”

The molded case circuit breaker standard (UL Standard 489) specifically lists devices that are “intended to provide service entrance, feeder, and branch circuit protection in accordance with the National Electrical Code, NFPA 70.” Some specific application of molded case circuit breakers would be as outlined in Section 210-20, Conductor Protection, and Section 430-52, Rating or Setting for Individual Motor Circuit, of the 1999 Code.

An example of a simple circuit and the possible application and misapplication of a supplementary protector is figure 2: Example control panel with a supplementary protector.

So what is a supplementary protector? Section 240-10 in the 1999 NEC establishes clear guidance on the differences between circuit breakers and supplementary protectors:

“Where supplementary overcurrent protection is used for lighting fixtures, appliances, and other equipment or for internal circuits and components of equipment, it shall not be used as a substitute for branch-circuit overcurrent devices or in place of the branch-circuit protection specified in Article 210. Supplementary overcurrent devices shall not be required to be readily accessible”.

The Supplementary Protector Standard (UL 1077) recognizes devices that are “…intended for use as overcurrent, or over- or under-voltage protection within an appliance or other electrical equipment where branch-circuit overcurrent protection is already provided, or is not required.”

It is important to note that the supplementary protector does not meet the strict definition provided in the NEC as noted above and these devices are intended only to protect components in specific applications (not branch circuits).

This article will help you to identify and understand this sometimes confusing terminology and application.

What are the differences between listed molded case circuit breakers and supplementary protectors? Quite significant as shown at left:

Why are supplementary protectors and molded case circuit breakers often confused? There are a variety of reasons for the confusion as follows:

1. Supplementary protectors are often called circuit breakers for equipment (CBE) or miniature circuit breakers (MCB). The literature and advertising of these devices may refer to them as circuit breakers leading the designer to the incorrect conclusion that this device will always act as a molded case circuit breaker.

2. The supplementary protector manufacturers may not clearly indicate that suitable branch protection or in- line fusing is required in certain applications.

3. European equipment uses supplementary protection as circuit breakers for equipment. Many times this equipment is imported into the United States. Generally, these are used in industrial settings that provide wiring to equipment remote from a control panel (many times a violation of the NEC).

4. Look very similar to molded case circuit breakers and are thought by designers to provide equivalent protection.

How can you tell if a supplementary protector will be acceptable in the use? The use of a supplementary protector is very limited beyond a listed appliance or assembly. The NEC, NFPA 79 Electrical Standard for Industrial Machinery and UL 508A Industrial Control Panels are examples of product standards that provide guidance in determining the acceptance of a supplementary protector. Two flow charts will help you with the analysis for use of these devices, but should not be substituted for careful review of the applicable end product standard.

A flow chart that can assist in the hazard evaluation for a supplementary protector.

An example of a simple circuit and the possible application and misapplication of a supplementary protector is figure 2: Example control panel with a supplementary protector.

Using the decision diagram in figure 1 and assuming the fuse is not initially installed as part of the circuit, the circuit diagram can provide guidance on the proper installation of the supplementary protector.

If there is a short circuit or overload on the load side of the supplementary protector then the only protection provided is through the 100-ampere branch circuit. The 14 AWG conductor is not protected by the 100-ampere circuit breaker in accordance with the NEC.

The fuse provides the needed branch-circuit protection, sized appropriately for the voltage and would need to be sized at 15 amperes. With the added fuse, the supplementary protector would be acceptable for the application above. The branch-circuit conductor on the line and load side of the supplementary protector is protected by the fuse.


Supplementary protectors are devices that must be properly used within equipment and appliances just as any other recognized component. It is very important that if you are designing or reviewing a design that contains a device that looks like a circuit breaker (both in person and on paper), that you evaluate the circuit for proper branch-circuit protection. Supplementary protectors have very limited applications outside of listed appliances or assemblies.

An appliance or assembly designer or manufacturer should always request the conditions of acceptability for the supplementary protector they are considering using. This information will confirm that the recognized component is used properly within its certification limitations.


Electrical Inspector’s Update, CSA International, Volume 4 Number 1, publication date unknown.

Electrical Trade News, Electrical Safety Authority, Safety Notice Number 99-03, publication date unknown.


UL 489, Molded-Case Circuit Breakers, Molded-Case Switches, and Circuit Breaker Enclosures, Ninth Edition, 1996.

UL 1077, Supplementary Protectors for Use in Electrical Equipment, Fifth edition, 1999.

Unpublished Meeting

Presented in Morrisville, NC on August 27, 2001 by Square D