What you don’t know about circuit breaker markings can trip you up.

Most of us pay little attention to markings on circuit breakers. Yet, these markings can be the difference between a safe installation and an unsafe one. So what does the information on this label mean? Let's look at each item in detail to ensure your installation is as safe as it should be.

Overcurrent rating [Sec. 240.83(A)]*. This piece of information appears on the label, but most people understand the overcurrent ampere markings (such as 20A, 30A, or 100A) for the protection of conductors and equipment in accordance with Sec. 240.4, so we won't discuss them here.

Interrupting rating [Sec. 240.83(C)]. Interrupting ratings like 10K, 22.5K, and 65K rms ensure the breaker has an interrupting rating sufficient for the maximum possible fault current available on the line side terminals of the equipment. If the breaker doesn't have a suitable interrupting rating for the available fault current, it could explode while attempting to clear a fault, or downstream equipment could suffer serious damage and endanger people.

Voltage Rating [Secs. 240.83(E) and 240.85]. Breakers are marked with either a slash or straight system voltage rating that indicates their capability to interrupt fault currents.

Voltage markings for breakers with slash voltage ratings are separated by a slash — for example, 208/120V or 480/277V. Each pole of the breaker is suitable when the line-to-ground voltage does not exceed the lower voltage marking, while the higher voltage marking is the value the line-to-line voltage may not exceed. Single-pole breakers are always slash rated.

Although the Code permits you to install a 120/240V slash-rated breaker on a 120/240V 3-phase, high-leg delta system, you cannot install it on the “B” phase (high-leg), because the nominal line-to-ground voltage of the high-leg is 208V. This exceeds the 120V line-to-ground voltage rating of the 120/240V slash breaker.

Be careful when installing slash-rated breakers on a solidly grounded high-leg delta system. Do not install them on corner-grounded, resistance-grounded, or ungrounded systems.

You cannot use a slash-rated 480/277V breaker on a 480V corner-grounded delta circuit because the line-to-ground voltage from two of the conductors would be as high as 480V. This exceeds the breaker's 277V line-to-ground voltage rating.

Two-pole breakers can be either slash or straight voltage-rated, whereas 3-pole breakers are all straight voltage-rated.

Temperature Rating [Sec. 110.14(C)]. You must size conductors to the lowest temperature rating marking on the breaker. For all practical purposes, conductors for circuit breakers are sized based on the 75°C column of Table 310.16.

Installing the wrong breaker can be disastrous, so read your breaker's markings before installation and make sure it's the right one.

* All Code references come from the 2002 NEC.