Since the 1965 NEC^®, electricians have been required by the code to consider equipment short-circuit current ratings (SCCR’s), circuit impedance, the speed at which overcurrent devices operate and other circuit characteristics in order to ensure that any installed overcurrent device can eliminate an overcurrent condition before the equipment being protected sustains extensive damage. This language is in NEC^® 110.10.

The image shows a 20-amp rated time clock with a SCCR of 5,000 amperes (5kA). The 20-amp time clock complies with 110.9 as long as it is used to control no more than 20 amperes of electrical load. In order for the time clock to comply with NEC^® 110.10, it must be installed in a way that it will never be subjected to more than 5,000 amps of available fault current. That being said, is it ok to install the time clock right next to the panelboard through a short offset fitting and supply it with just two feet of 12 AWG copper wire?

A few questions need to be asked in order to provide an answer to the above dilemma:

• If a fault occurs in a circuit the time clock controls, how much fault current is available at the time clock? Is it more than 5,000 amperes? In this installation there was 18,000 amps of AFC at the line terminals of the circuit breakers.
• Is the circuit breaker feeding the time clock listed as “current limiting”? Will it interrupt any fault current downstream of the time clock within the first half cycle of a sine wave?
• If the overcurrent device is listed as a “current limiting” device, it is required by the product listing to be marked “current limiting” on the device. If the breaker is marked as such, during a fault condition, how much current will the breaker let through to the time clock before it trips? Will it be more than 5,000 amps? This specific breaker is not a current limiting breaker. That being the case, it may not be fast enough during a fault condition to trip before letting through more than 5,000 amperes of current which may cause the time clock to explode.
• Consider the type, size and length of the wire feeding the time clock. Two feet of 12 AWG copper conductor through a ferrous metal nipple. Is two feet of this size wire long enough so that there is enough impedance in the circuit to get the AFC of 18,000 amperes at the panelboard down to 5,000 amperes or less at the time clock? The answer is no. There is actually over 10,000 amps of AFC at the line terminals of the time clock which only has a 5kA SCCR. This does not comply with 110.10 and the time clock must be supplied by a longer run of conductor in order to achieve compliance. Another option would be to install a disconnect with current limiting fuses between the panelboard and the time clock. In order to do this, the fuse trip characteristics would need to be known in order to find out how much current the fuse lets through to the time clock under a fault condition.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).

2014-2017 Code Language:

110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics. The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the equipment grounding conductor(s) permitted in 250.118. Listed equipment applied in accordance with their listing shall be considered to meet the requirements of this section.