In the first installment of this three-part series of articles, featured in the August 2024 print issue and online, we discussed fire pump requirements along with Code-related references in the National Electrical Code (NEC) to assist electrical designers. After nearly 50 years of experience as a consulting engineer, I have learned that one of the most misinterpreted and misapplied NEC requirements in the electrical industry is power service to electric fire pumps. This second installment will specifically address the continuity of power requirement in the Code.
Section 695.4 of the NEC addresses the continuity of power requirements for circuits that supply electric motor-driven fire pumps. It states these circuits shall be supervised from inadvertent disconnection as covered in Sec. 695.4(A) and (B).
As discussed in Part 1, the application of Sec. 695.4(A) [Direct Connection] would mean a connection to the fire pump from the electric utility source ahead of any main disconnecting devices and without a fire pump disconnecting means. I am not aware of an electric utility company that would allow a service connection without a main disconnecting means, as their typical standard is a combination meter/main. This brings us to Sec. 695.4(B) [Connection Through Disconnecting Means and Overcurrent Device].
The requirements for the fire pump disconnecting means and/or overcurrent device are as follows:
“Section 695.4(B)(1) Number of Disconnecting Means.
“(a) General. A single disconnecting means and associated overcurrent protective device(s) shall be permitted to be installed between the fire pump power source(s) and one of the following:
“(1) A listed fire pump controller
“(2) A listed fire pump power transfer switch
“(3) A listed fire pump controller and power transfer switch”
Where a fire pump service disconnecting means is required, the above indicates that there can only be one. However, if the fire pump is also served from an on-site standby generator the following applies, from the same Code reference above, allowing more than one disconnecting means.
“(c) On-Site Standby Generator. Where an on-site standby generator is used to supply a fire pump, an additional disconnect means and an associated overcurrent protective device(s) shall be permitted.”
Continuity of power covers the sizing of the overcurrent protection devices serving fire pumps for the different applications as outlined above. Under “Individual Sources” there are two options, but it is the first option we find most common for standard applications. It states the following:
“NEC 695.4(B)(2) Overcurrent Device Selection. Overcurrent devices shall comply with 695.4(B)(2)(a) or (B)(2)(b).
“(a)(1) Overcurrent protective device(s) shall be rated to carry indefinitely the sum of the locked-rotor current of the largest fire pump motor and the full-load current of all the other pump motors and accessory equipment. Where the locked-rotor current value does not correspond to a standard overcurrent device size, the next standard overcurrent device size shall be used in accordance with 240.6. The requirement to carry the locked-rotor currents indefinitely shall not apply to conductors or devices other than overcurrent devices in the fire pump motor circuit(s).”
The locked-rotor current of a motor is approximately six times the rating of that motor. For sizing purposes, a 50-hp motor is rated at 65A, when operating at 480V. The locked-rotor current for that same motor is 363A. Therefore, the overcurrent protective device for a 50-hp fire pump, from an electric utility source, would be sized at 375A. The reason for the oversized breaker is to allow the fire pump to continue operation, in some of the worst conditions, maintaining sprinkler pressure as long as possible while occupants are egressing a building during a fire event.
The overcurrent device selection of Sec. 695.4(B)(2) includes service from an on-site generator, which reads as follows:
“(b) 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.”
The fire pump service from a generator has the overcurrent device sized for normal motor loading per Sec. 430.62 (short-circuit protection only without ground-fault protection), which is much different than noted above from the electric utility service. The reason is the generator serves other critical life safety system loads in the building (e.g., smoke control system, elevators required for emergency evacuation, egress lighting, etc.). Oversizing this overcurrent protective device could cause potential damage to the generator and its operation. The Code requirement protects the generator from any overload/overheating that could be caused by the fire pump, to preserve the operation of the generator for other life safety systems.
Continuity of power covers the disconnecting means requirements unique to fire pump services. It differentiates the two types of services for both utility power and on-site generator power. The utility power source requirements read as follows:
“NEC 695.4(B)(3) Disconnecting Means. All disconnecting devices that are unique to fire pump loads shall comply with items 695.4(B)(3)(a) through (B)(3)(e).
(a) Features and Location – Normal Power Source. The disconnecting means for the normal power source shall comply with all of the following:
(1) Be identified as suitable for use as service equipment.
(2) Be lockable in the closed position. The provision for locking or adding a lock to the disconnecting means shall be installed on or at the switch or circuit breaker used as the disconnecting means and shall remain in place with or without the lock installed.
(3) Not be located within the same enclosure, panelboard, switchboard, or motor control center, with or without common bus, that supplies loads other than the fire pump.
(4) Be located sufficiently remote from other building or other fire pump source disconnecting means such that inadvertent operation at the same time would be unlikely.”
The fire pump disconnecting means is required to be service entrance rated. It is also required to have permanent provisions for locking the device in a closed position. The disconnecting means cannot be in the same electrical equipment enclosure, share a common bus, or supply loads other than the fire pump. This is where I have seen most of the design issues by other engineering firms. They tend to locate the fire pump disconnecting means in the same line-up as the main switchboard but at the opposite side of the incoming pull section from that of the building meter/main device(s). By doing so, it is not only sharing a common bus but is also considered the same enclosure. The disconnecting means must be remote from other building disconnects, so inadvertent operation is not likely.
The reason for the remote location of the fire pump disconnect is due to the responding fire department’s need to shut down power to a building during a fire event. The reason for shutting down the power is to limit firefighter’s exposure to electricity while using water to suppress the fire.
As with the first two Sections of 695.4(B), the third Section also has different provisions for on-site generator disconnecting means. It reads as follows:
“(b) Feature and Location – 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 provisions for locking or adding a lock to the disconnecting means shall be installed on or at the switch or circuit breaker used as the disconnecting means and shall remain in place with or without the lock installed.”
The difference here, from that of the utility power service above, is that the disconnecting means neither requires a service entrance rating nor does it require separation from other emergency devices. The generator distribution board could house all the emergency breakers, legally required standby breakers, and optional standby breakers, in three or more separated vertical sections with a common bus between each of them. The fire pump breaker(s) could be grouped in the same vertical section(s) as other emergency devices. There is no requirement for them to be sufficiently remote from other devices.
To complete the section on continuity of power, there is a requirement for supervision of disconnecting means, under Sec. 695.4(B)(3)(e), which lists three methods. Item (3) is the most likely choice for application on most projects.
“(3) Locking the disconnecting means in the closed position.”
Transformers
In my experience, the only time when transformers are used in a fire pump service is when the service entrance from the electric utility company is at medium voltage. This means the meter and main are medium-voltage rated and tapping ahead of the main disconnecting means a step-down transformer would be required to obtain 480V for service to the fire pump. Any other utilization service voltage provided by the electric utility company should be easily matched by the fire pump vendor, removing the need for a transformer.
This Section of the Code is broken down into three parts, with the first two parts required for normal applications and the third part for feeder source applications per Sec. 695.3(C) [Multibuilding Campus-Style Complexes], which will not be evaluated in this document. This section reads as follows:
“NEC 695.5 Transformers. Where the service or system voltage is different from the utilization voltage of the fire pump motor, transformer(s) protected by disconnecting means and overcurrent protective devices shall be permitted to be installed between the system supply and the fire pump controller in accordance with 695.5(A) and (B), or with (C). Only transformers covered in 695.5(C) shall be permitted to supply loads not directly associated with fire pump system.”
A transformer is required where the service to a building is of a higher voltage than the fire pump utilization voltage. Based on the requirements of this section of the Code and application of paragraphs (A) and (B), the transformer is dedicated for use by fire pump loads only. All the previous requirements of Art. 695 applies as well to the installation of a transformer (e.g., power source, continuity of power, etc.).
For the sizing of the transformer, Sec. 695.5(A) states:
“(A) Size. Where a transformer supplies an electric motor driven fire pump, it shall be rated at a minimum of 125 percent of the sum of the fire pump motor(s) and pressure maintenance pump(s) motor loads, and 100 percent of associated fire pump accessory equipment supplied by the transformer.”
In most applications, we do not find there to be “associated fire pump accessory equipment” in a fire pump room requiring electrical connections. Mainly, it is just the fire pump(s) and the jockey pump(s). The transformer at a minimum must be sized at 125% of the combined pump loads.
As with the electric utility power service to a fire pump, the overcurrent protective device should be sized for the locked-rotor current of the pump motor.
“(B) Overcurrent Protection. The primary overcurrent protective device(s) shall be selected or set to carry indefinitely the sum of the locked-rotor current of the fire pump motor(s) and the pressure maintenance pump motor(s) and the full-load current of the associated fire pump accessory equipment when connected to this power supply. Secondary overcurrent protection shall not be permitted. The requirement to carry the locked-rotor currents indefinitely shall not apply to conductors or devices other than overcurrent devices in the fire pump motor circuit(s).”
If a building service is at medium voltage (e.g., 12kV), it generally implies high-rise construction. This requires a generator for backup power and the fire pump to be connected to the generator as an alternate source of power. The Figure depicts what that looks like with a step-down transformer for the electric utility’s primary source and a generator as the alternate source.
This wraps up Part 2 of this three-part series on electric fire pump power services. Look for Part 3, which will further discuss power wiring requirements in the NEC.
**Reproduced with edits and permission of NFPA from NFPA 70*, National Electrical Code, 2017 edition. Copyright© 2016, National Fire Protection Association. For a full copy of the NFPA 70, please go to www.nfpa.org.
