Circuit breakers and fuseholders have terminals to attach wires to the circuit breaker or fuseholder. The overcurrent device is the beginning of the branch circuit or feeder and there must be a terminal to connect the conductor. In fact, a circuit breaker is an assembly which includes the tripping mechanisms and the means to attach a conductor. A fuseholder is also an assembly which is made to hold a fuse of a certain voltage and current rating and the means to connect a conductor. If the terminal gets overheated it can adversely affect the operation of the fuse or circuit breaker.

To prevent overheating at the terminals, Section 110.14(C) requires the rating of the wire to be equal or greater than the rating of the terminal. The UL marking guide for Molded Case Circuit Breakers explains: All circuit breakers rated 125A or less are marked for use with 60 degree C, 60/75 degree C, or 75 degree C only wire. This marking indicates the proper wire size for termination in accordance with Table 310.15(b)(16) of the NEC. It is acceptable to use wire with a higher insulation rating if the ampacity is based on the wire temperature rating marked on the breaker.

Most terminals today are marked 60/75 degree C or 75 degree C. This means that whatever the ampere rating of the overcurrent device, the 75 degree C rating of the wire, taken from Table 310.15(b)(16), can be used to select the conductor. If the terminal is marked for 60 degree C conductors, the ampacity of the conductor must be selected from the 60 degree column of Table 310.15(B)(16) even if the conductor is rated for 90 degrees.

In general, the 90 degree C ampacity ratings for conductors is only used in de-rating for ambient temperature and when there are more than 3 current carrying conductors in conduit. The 90 degree C ampacity ratings of conductors cannot be used because in wiring for 1000 volts and below there are no overcurrent devices with 90 degree C rated terminals.

Fuses and circuit breakers must be installed in enclosures so that under fault conditions if an arc occurs it will not cause injury to anyone in the vicinity.  Luminaries installed over Class 1 Division 11 hazardous (classified) locations are required to be of the totally enclosed type to prevent sparks from reaching flammable material or gases.  Motors that arc while in operation must be enclosed to protect adjacent material from catching fire.  Switches can produce an arc and must be installed in an enclosure to guard the live parts and shield personnel and  other equipment from the danger of arcing parts.

When the service equipment is modified, or when changes are made that might change the maximum available fault current, the fault current calculation must be recalculated and the equipment marked with the new values and the date when the most recent calculation was done.

The maximum available fault current at a service can also be affected by changes made by the utility company. For example, available fault current is usually affected anytime the utility company changes a transformer supplying a facility. However, changes made by a utility remote from a facility such as relocating a substation to change the length of the primary supplying a facility can also affect the available fault current for all of the facilities, supplied by the substation. The only way to ensure that changes made by a utility do not affect a facility is to maintain communication between the facility engineering staff and the utility, and to have engineering calculations done at the facility to determine maximum available fault current when changes are made on either side of the service panel.

An exception allows industrial installations with qualified personnel to skip the field marking requirement.

The available fault current at the terminals of the utility delivery point should be readily available from the utility. Marking it on the equipment with the date will make it easier to confirm the interrupting rating of the equipment is with the range of the available fault current and how long ago the calculation was made.

An Informational Note clarifies that the available fault current is to be used for determining the required short-circuit current ratings of equipment. It is not to be used to establish hazard risk boundaries or the necessary PPE required by qualified persons when working on energized equipment. NFPA 70E, Standard for Electrical Safety in the Workplace, should be used for personnel safety issues when equipment must be serviced while energized.

The entrance and egress requirements for large equipment are strict because of the hazards workers face in the event of arc-flash accidents. Having ready access to an unobstructed means of egress (or way out) can be a matter of life or death in the event of catastrophic arc-flash incidents.

The switchgear in this illustration is Large Equipment in terms of both amperage and physical size. It is required to have a means of entrance and egress at each end of the working space unless one of two conditions is met:

1. The means of egress is unobstructed or
2. The depth of the working space is twice that required in Table 110.26(A)(1).

Although only one exit is visible in this illustration, another exit door also equipped with a panic bar is behind the camera.  A single exit door would be permitted if the work space in front of the switchgear is doubled.

Dedicated space extends from the floor to a height of 6-ft. above the equipment or the structural ceiling, whichever is lower.  This space is set aside for conduit, cable tray, and other wiring methods to enter the equipment.  No HVAC piping or ductwork, plumbing, or other foreign systems or equipment are permitted to be in or pass through the dedicated space.

If foreign systems or piping are installed immediately above the dedicated space, these systems must include protective equipment to prevent any leaks or drips from entering the dedicated space.

For example, if the footprint of a 7-foot high MCC was 2-feet x 6-feet and the MCC was installed in a room with a 12-foot ceiling, the dedicated equipment space would extend from the top of the MCC, 5-feet to the structural ceiling.  Only electrical equipment associated with the MCC could be installed above the MCC.  However, if the MCC was installed in a high-bay location with 35-foot ceilings, foreign equipment and piping could be installed directly over the MCC provided it was at least 6-feet above the top and included protective equipment to prevent drips or leaks from entering the dedicated space.

As a note:  Drop ceilings are not considered structural ceilings.  Using the example above, if the drop ceiling is located at a height of 12-foot and the structural ceiling at 16-foot, the dedicated equipment space would extend from the top of the MCC, 5-feet to the drop ceiling.  Any foreign equipment as related to HVAC piping or ductwork, plumbing, etc. located in the space between the drop-ceiling and structural ceiling must include protective equipment to prevent drips or leaks from entering the dedicated space between the MCC and drop-ceiling.

In this illustration a drip pan is not required because the piping is routed around the dedicated space.

For Large Equipment (equipment more than 6-feet wide), an entrance is required at each end of the space unless means of egress (the exit path) is unobstructed or the depth of the working space is doubled as permitted in NEC article 110.33(A)(1)(a)&(b).

Personnel door intended for entry into and exit from the working space that are less than 25-feet from the work space are required to open in the direction of egress (the way out) and to be equipped with listed panic hardware that opens under pressure.

For example, the personnel door in this illustration is less than 25-feet from the switchgear work space; it opens in the direction of egress and is equipped with a panic bar rather than a knob.  Listed panic hardware is of critical importance to personnel who may be blinded or have severely burned hands resulting from an arc-flash incident.