The 2011 National Electrical Code (NEC) provides a controversial and much-discussed new 110.24 involving field-marking of available fault current on service equipment. The new requirement will not be required for dwelling units, and an exception has been added to the new text that permits a major portion of the electrical industry to bypass it. A total discussion of the pros and cons will permit a better understanding of the issues involved in 110.24.

Existing Section 110.9 requires electrical equipment that is intended to interrupt current at fault levels to have an interrupting rating that is not less than the current available at the equipment’s line terminals at the circuit’s nominal voltage. The definition of interrupting rating, as used in 110.9, is the highest current at rated voltage that a device is identified to interrupt under standard test conditions. There is an informational note under the definition and a second paragraph in 110.9 that states equipment intended to interrupt current at other than fault levels may have its interrupting rating implied in other ratings, such as horsepower or locked rotor current. For example, a motor-disconnecting means may be required to open or be opened during the locked rotor current of a motor during motor startup.

Existing Section 110.10 requires overcurrent-protective devices, the total impedance of the electrical circuit, electrical equipment short-circuit current ratings, and other electrical characteristics be selected to provide protection for the electrical components of the system. Any overcurrent-protective device must be selected and coordinated with other circuit-protective devices to be able to clear a fault without extensive damage to the electrical equipment of the circuit. Other than fuses and similar devices, if the electrical component in the circuit is damaged beyond use, it is a violation of 110.10. Listed equipment applied within the listing requirements is considered to comply with the requirements in 110.10 and the listing standard, as well as the NEC, may require the available fault current to be marked on the equipment.
As can be seen by the discussion here, ensuring electrical equipment can withstand the amount of available fault current is already part of the listing of the equipment and is currently required by both 110.9 and 110.10. New Section 110.24(A) in the 2011 NEC states, “Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved.”

The new 110.24(C) states, “When modifications to the electrical installation occur that affect the maximum available fault current at the service, the maximum available fault current shall be verified or recalculated as necessary to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) shall be adjusted to reflect the new level of maximum available fault current.”

Most large electrical service equipment, such as switchboards, motor-control centers, and distribution equipment, will have a label applied at the factory indicating the maximum fault current applied during the testing to ensure the equipment bracing is sufficient for the fault current. This factory label is not the label indicated in 110.24, since 110.24 requires a separate, dated, field-installed label, indicating the level of fault-current at the equipment. When the service gear is first installed, the fault-current bracing of the equipment and the calculation should be the same.

However, there are many different reasons for the available fault current to change. If a utility transformer is changed for any reason and the utility does not replace the transformer with the same or a higher impedance transformer, the available fault current may go up. An additional change could occur where the utility has two different substations supplying the service equipment with periodic transfer from one substation to the other with a different available fault current from each substation. All new and existing facility owners should ensure the utility supplying power provides notification of any significant changes that might affect the available fault at the facility service equipment.

As I state in the opening paragraph, an exception has been added that exempts these field-marking requirements for industrial installations where conditions of maintenance and supervision ensure that only qualified people service the equipment. The implication is that most large industrial facilities will ensure that electrical service equipment is updated regularly and fault current marking will be changed on the equipment. Electrical contractors should inform all their large industrial customers to ensure the maintenance personnel are aware of the fault current levels supplied to their facility.


ODE is a staff engineering associate at Underwriters Laboratories Inc., based in Peoria, Ariz. He can be reached at 919.949.2576 and mark.c.ode@us.ul.com.