The sizing and selection of the generator breaker(s) feeding the fire pump controller can be a muddled topic, so this article attempts to create greater visibility to NEC 695 requirements resulting in greater market alignment.
To start, however, let’s explore potential contributors to the confusion, including repeated requests for magnetic-only breakers or breakers sized for locked rotor current. Past versions (prior to 2014) of the National Electric Code (NEC) may have fed much of the confusion by interlacing the verbiage for the generator requirements amongst the normal serving utility. Misunderstanding may have also resulted from misinterpreting the following NEC 695 phrasing: “provide short-circuit protection only.”
Unfortunately, this historical confusion has resulted in the misperception that the generator should be designed with a “run to failure” mentality, meaning the generator should run to destruction while trying to carry fire pump locked rotor amps indefinitely. This mindset is potentially life threating considering the same generator carrying the fire pump will often power the building’s emergency system (exit lights, egress lighting, stairwell pressurization), which is the system designed to get people out of a burning building.
Although there may have been confusion, the 2014 version of the NEC does a much better job of identifying the generator requirements separate from the serving utility. Here’s an explanation of key code requirements:
NEC 695.3(D)(1): On-site Standby Generator… Capacity
“The generator shall have sufficient capacity to allow normal starting and running of the motor(s) driving the fire pump(s) while supplying all other simultaneously operated load(s).” On-site standby generators and the associated over current devices are only required to carry the normal starting and running currents. This is different from the utility source which is required to carry locked rotor current indefinitely.
NEC 695.4(B)(1)(c) Continuity of Power… Number of Disconnecting Means… On-Site Standby Generator
“Where an on-site standby generator is used to supply a fire pump, an additional disconnecting means and an associated overcurrent protective device(s) shall be permitted.” The generator is allowed to utilize an integral breaker. The generator is also allowed to feed a distribution panel which incorporates an additional breaker for the fire pump. This is different from the utility source which requires a tap ahead of the service and a single supervised disconnect sized for locked rotor amps indefinitely.
NEC 695.4(B)(1)(c) Continuity of Power… Disconnecting Means… Features… On-Site Standby Generator
“The disconnecting means for an on-site standby generator(s) used as the alternate power source shall be installed in accordance with 700.10(B)(5) for emergency circuits and shall be lockable in the closed position.” The separation of circuits’ concepts like “separate vertical sections” that are employed for emergency systems must also be employed for the fire pump breakers. NEC 695 doesn’t specifically state that the generator fed fire pump breakers are classed as an emergency system and as such can be located within an emergency system vertical section. This topic is open to some AHJ interpretation within the scope of NEC; however, IBC 403.4.9.1 defines a fire pump as an emergency load.
NEC 695.4(B)(2)(b) Continuity of Power… Overcurrent Devices… On-Site Standby Generators
“Overcurrent protective devices between an on-site standby generator and a fire pump controller shall be selected and sized to allow for instantaneous pickup of the full pump room load, but shall not be larger than the value selected to comply with 430.62 to provide short circuit protection only.” NEC 430.62 requires that the protective device be sized per 430.52 which limits the maximum inverse time breaker size to 250% of the motors nameplate rating. This sets the maximum size of the breaker. What about the minimum size of the breaker? What does “provide short circuit protection only” mean? NEC provides for short circuit, ground fault, and overload protection. Thus, short circuit only protection means no protection against ground fault or overload. NEC 430.32 defines overload protection at a maximum of 125% of the motor nameplate rating. Breakers sized larger than 125% provide no overload protection and thus provide short circuit only protection. The result of all this is that any breakers from the on-site generator source to the fire pump must be sized greater than 125% and less than 250% of the fire pump rating and support the temporary locked rotor amps associated with normal starting.
The code recognizes the standby generator has a disconnecting means (breaker) integrated into it. It also states that it is not required to tap ahead of this disconnect means which is a significantly different requirement then a utility service. So we continue to validate that the rules for the generator are quite different from that of the utility feed. What about the size the generator disconnecting means and overcurrent protection? The code clearly identifies the output requirements of the generator along with its internally integrated breaker that is sized for normal starting and running of the fire pump motor. Furthermore, the NEC handbook interpretation specifically states that the generators over-current protective devices are not required to be sized for locked-rotor current indefinitely.
Historically, NEC 695 has presented a significant interpretation challenge to system designers. The 2014 version has minimized much of the market confusion. After reviewing NEC 695, the use of magnetic only breakers, tapping ahead of the generator breaker, and breakers sized for carrying locked rotor amps indefinitely seem to be misinterpretations of previous code versions. The requirement to carry lock rotor amps indefinitely applies only to the utility source. As with all code interpretation issues, please consult with your authority having jurisdiction (AHJ) to ensure a common understanding on these issues prior to installation.