Take care when replacing circuit breakers and fuses in existing services and panelboards at single-, two- and multifamily homes. Older single- and two-family homes with 100A to 200A, 120/240V single-phase services were usually guaranteed by the local electric utility companies to have maximum available fault currents of 10,000A rms (root means square or average) with 14,000A asymmetrical. The utility companies use either pole- or pad-mounted transformers that have enough impedance to limit the fault current to a maximum of 10,000A.
Multifamily apartments and single-family homes with services larger than 200A usually have higher available fault currents and require different designs to limit the fault currents at the service and downstream at panelboards within apartments and at multiple panelboards for large homes.
Understanding the applications of circuit breakers and fuses at dwellings helps the electrical contractor and service electricians understand the limitations of new and replacement overcurrent devices, such as circuit breakers and fuses, and the interrupting ratings of these devices.
Circuit breakers and fuses are intended to interrupt fault current at the level of fault current that is delivered to the line terminals of the overcurrent device. Each overcurrent device will normally have an interrupting rating shown on the circuit breaker or fuse. Every circuit breaker having an interrupting rating other than 5,000A must have that rating marked on the breaker. In other words, if there isn’t an interrupting rating on the circuit breaker, then the circuit breaker is only rated at 5,000A and cannot be installed in a panelboard with more than 5,000A of fault current. Fuses will be marked with the interrupting rating whenever the rating is higher than 10,000A.
Where the available fault current delivered to the service or the panelboard is in excess of the interrupting rating of the fuse or circuit breaker and a fault occurs, massive damage can occur, causing the fuse or circuit breaker to blow apart.
As I travel around the country lecturing on the National Electrical Code, I hear about utility companies that are installing transformers for single-family homes with 100A–200A service sizes with available fault currents exceeding 10,000A and, in some cases, up to 22,000A and higher fault current. Where higher fault current values are encountered, a few methods can be used to reduce the available fault current so as not to exceed the interrupting ratings of the existing circuit breakers and fuses.
One solution is to use circuit breakers designed and tested as a series-rated system. For example, a series-rated system may have a 22,000A rating on the service main circuit breaker that has been tested to provide protection for downstream 10,000A branch circuit breakers in the panelboard. When a fault occurs below the branch circuit breaker, the fault current on the line side of the main circuit breaker will cause the main to start to open. As the main breaker starts to open, the impedance in the arc internal to the 22,000A interrupting-rated breaker will restrict the amount of fault current downstream to no more than the 10,000A rating of the branch circuit breaker. Series rated combinations are tested as a system and the service equipment and downstream panelboards must be field marked to indicate that they are special systems. Any replacement breakers must be series rated by the manufacturer of the main breaker and the downstream devices. These series-rated systems are commonly found at apartment complexes and larger residential services where proper replacement of series rated breakers is critical to the safe operation of the electrical system.
Another method of reducing the available fault current downstream of the main service is to install current-limiting fuses or circuit breakers at the service main to ensure that the fault current at the downstream branch circuit devices will not exceed the interrupting rating of these downstream devices. Current-limiting fuses or circuit breakers are defined in the NEC as “a device that, when interrupting current in its current-limiting range, reduces the current flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device was replaced with a solid conductor having comparable impedance.”
In other words, a current-limiting device must have enough fault current going through the device in a fault to react and clear a fault within the first half cycle of the fault. The service equipment or panelboard must be marked to indicate that any replacement fuse or circuit breaker must be of the same type of current limitation.
Take care that proper overcurrent devices are installed for the original and replacements for maximum safety of the system.