“Motor contribution” is only used in 240.86(C) in the National Electrical Code (NEC), but the NEC does not define it anywhere. A full explanation of this important electrical application is located in other publications, such as fuse and circuit-breaker protection books and articles. Motor contribution is the current that a motor generates during a short-circuit condition with the motor becoming a power source, adding current into the fault. Further study of this phenomenon is necessary to ensure this additional current does not cause more damage to the electrical system.
Section 110.9 indicates that electrical equipment intended to interrupt current at fault levels, such as circuit breakers and fuses, shall have an interrupting rating not less than the nominal circuit voltage and the current that is available at the line terminals of the electrical equipment. For example, a circuit breaker or fuse, designed to open at 10,000 amperes (A) symmetrical current or 14,000A of asymmetrical current, must be able to do so without exterior damage to the device. As 110.9 states, these devices must not have more available fault at the line side of their terminals; otherwise, the higher fault current could cause external damage to the device. Each device is required to have the maximum available fault current, commonly called the “interrupting rating,” which the fuse or circuit breaker is designed to interrupt, marked on the device.
However, 240.86 permits a circuit breaker to be used on a circuit that has an available fault current higher than its marked interrupting rating. To meet this requirement, the breaker must be connected on the load side of an acceptable overcurrent protective device that has a higher rating and called a series-rated system, but it is only permissible where the circuit breaker meets the requirements in 240.86(A) or 240.86(B) and (C).
The first requirement in 240.86(A) covers series-rated combination devices selected by a licensed professional engineer who is engaged in the design or maintenance of these systems and who understands the concepts of series-rated systems. The professional engineer must document and stamp the series-rated system design. This documentation must be available to the people who are authorized to design, install, inspect, maintain and operate the system. Doing so ensures any repairs or changes will account for the original design concept. The series-combination rating must be field-marked on the end-use equipment, including identification of the upstream device, to ensure everyone knows this is a specially designed system. For any fault calculations, the engineer must ensure that the downstream circuit breaker or breakers that are part of the series-rated system will remain passive during the interruption period of the line side fully rated current-limiting device. If the downstream device were not passive (opening at 6 cycles or longer) and started opening within a normal 1¾ to 3 cycles, the current-limiting device upstream would take longer to open, and the downstream device would be subjected to a much higher fault current than it is rated to handle.
Section 240.86(B) covers tested combinations. Unlike the system in (A) described above, the combination of the line-side overcurrent device and the load-side circuit breaker is tested and marked on the end-use equipment, such as at the panelboard or switchboard, with the test ensuring that the upstream device creates enough impedance during the opening so that the downstream device is not subjected to more fault current than the downstream device is designed to handle. An example of that would be a 50,000-, 22,000-, 10,000A system where the available fault current at the utility company connection is approximately 50,000A. If a fault occurs downstream from the main, say downstream from the 10,000A device, the impedance of the opening of the 50,000A device will ensure no more than 22,000A at the line side of the next device downstream (the 22,000A device), and it starts to open, resulting in enough impedance through the 22,000A rated device to ensure not more than 10,000A will occur at the last device.
Finally, 240.86(C) states that either the 240.86(A) or (B) series-rated system shall not be used where (1) motors are connected on the load side of the higher rated overcurrent device (e.g., a 22,000A device) and on the line side of the lower rated overcurrent device (e.g., a 10,000A device) and (2) the sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower rated circuit breaker. The motor contribution of more than 1 percent can flow back though the lower rated device, causing the fault current to exceed the rating of the lower rated device.