Tips for designing electrical systems for tricky fire pump installations
When it comes to fire pumps, there's more to designing and installing these units than meets the eye. As projects become larger and more complex, such as those found in campus settings, installation requirements vary and design options increase, which often leads to confusion on the electrical design front. Therefore, it's important to turn to more than Art. 695 of the NEC for guidance — there are other codes, standards, and local regulations you must become familiar with when doing this type of work. Let's start from the beginning, outlining the requirements associated with various fire pump installations.
You receive notification that a particular project requires a fire pump. Although there are other possibilities, you will have one of three basic scenarios to address. The fire pump is going to be fed from:
a single utility service point (the simplest).
a single utility service point and an onsite generator. For example, the International Building Code (Sec. 403.10.1.3) requires this arrangement in a high-rise building (more than 75 feet tall). Another situation could be that the height of the building may be beyond the pumping capacity of the local fire department. In this case, you must follow the rules of NFPA 20, “Standard for the Installation of Stationary Pumps for Fire Protection,” in Chapter 9, Sec. 3.1. Another example is simply that the owner requested this arrangement.
dual feeder sources (where one of the sources may or may not be a generator). This would include a campus setting or a multi-building application and fall under the requirements of NEC Sec. 695.3.
Single utility service point
Let's begin with a review of the situation where you have one building, one fire pump, and one power source (from the utility). The major issue is that you will need to run a dedicated feeder from the service transformer (just after the metering equipment) to the fire pump. NEC 695.3 (A)(1) prevents you from coming off of the main service equipment.
If your feeder is before the metering equipment, the utility will install a second set of meters on it. Initially, this is not a big deal. The owner simply has to pay an additional monthly meter charge. However, add the utility's demand charges for a pump that is usually only operated once per month (for testing), and those ongoing charges become costly. On the other hand, demand charges for testing fire pumps are typically nonexistent for a proper installation because the pump is run during the owner's off-peak hours.
Route the wiring outside the building, similar to service entrance conductors. The installation must follow the requirements of NEC 230.6, which calls for you to either place the conductors below 2 inches of concrete, encase conduit in 2 inches of concrete, or route them in conduit 18 inches below the soil. Size the conductors similar to other motor loads (i.e., follow the requirements of 430.22). Size them for 125% of the motor's full load amps (FLAs). If these cable lengths are long, make sure you check the voltage drop on the circuit. Per the NEC and NFPA 20, 9.4.1, the voltage shall not drop more than 15% under motor starting conditions. If it does, you'll need to increase the conductor size to resolve the issue.
You should locate the pump in an isolated room protected from the rest of the building spaces — and not in the same room as the main electrical distribution center. The pump controller will include a safety disconnect switch. Per NEC 695.12, this has to be located in the same room and within sight of the pump. There is no need to include overcurrent protection devices (OCPDs). In the event of a fire, it's better to let the pump run (even overload or destroy itself) than have an OCPD open and prevent the pump from doing its job (supplying water to extinguish the fire).
If you follow NFPA 20, “Standard for the Installation of Stationary Pumps for Fire Protection,” then you'll want to install emergency lighting in the pump room per Sec. 5.12.4 and remote monitoring per Sec. 10.4.7.1. Even if it's not required, a monitoring system is a sound idea. Installing sensors on equipment and wiring them to either the fire alarm system or building automation system can accomplish the monitoring. Items to monitor include: pump running status, loss of phase power, phase reversal, and if controls have switched over to alternate power source (the next example).
A good way to accomplish this is to use the fire alarm or building automation systems, which you're already designing into the project. Connect these monitoring points to the fire alarm panel. A “pump running” signal can be programmed and designated as either an alarm or supervisory signal; other monitored points, such as “phase loss” or “disconnect switch open” must be designated as a supervisory signal per NFPA 72, 22.214.171.124. Provisions will also be needed for a telephone in the pump room, per NFPA 72, 126.96.36.199.
Single utility service point and onsite generator
This project requires design of an onsite generator(s) sized for the fire pump's alternate power source. All the requirements of NFPA 110, “Standard for Emergency and Standby Power System,” should be met in designing and sizing the generator system. The generator(s) must be sized to start and run the fire pump, along with any additional loads connected to it. As per NEC 695.3(B)(1), it does not need to have the capacity to handle the pump's locked rotor current. Nor does the tap for the generator feeder have to be located ahead of its disconnecting means. The generator does have to have enough fuel capacity to provide eight hours of operation at 100% of the pump's rated load (NFPA 20, 188.8.131.52). The transfer of power (from normal to standby and vice versa) needs to take place within sight of the fire pump (NFPA 20, 9.6.4 and NEC 695.12(A), which means the transfer switch needs to be located in the same room as the pump.
As with the first example, it's best to specify a packaged unit. In this case, a combination automatic transfer switch, disconnect switch, and fire pump controller is the best choice. You'll also need to route two feeders to the fire pump room. Exercise care with the routing, and be sure the cables are properly protected to ensure a successful installation, just as you did in the first example.
Dual feeder sources
This is probably one of the most difficult arrangements you'll run across. In this example, the project is a campus environment (i.e., college, medical center, or industrial complex), consisting of multiple buildings with a centralized power distribution system. This scenario falls under the requirements of NEC 695.3(B)(2) with two or more feeder sources supplying power to the pump(s). These sources could be separately derived utility services or a combination of a utility service source and a campus generator bank.
In this situation, you must deal with two additional items: 1) A disconnect switch will most likely be needed, along with an OCPD (somewhere on the feeder or feeders); and 2) Fire-rated cables will need to be specified because you'll most likely be unable to bury the cables as noted in the previous examples. If you are able to route the cables in the concrete slab or underground, then follow the requirements previously mentioned.
Locate the OCPD in an electrical room, preferably in a vault with 3-hour rated protection. Size it to carry the locked rotor current of the fire pump, as per NEC 695.4(B)(1). You must obtain the locked rotor current directly from the motor's nameplate or from NEC Table 430.251(B) based on the motor's design letter. This current can be five to six times the motor's full load amps rating.
For example, a 100-hp motor's full load amps rating may be 124A, while its locked rotor amps rating is 725A. In this example, sizing upward to the next largest standard size OCPD requires an 800A overcurrent protection device. Remember, equipment ground-fault protection is never required for fire pumps, even when the OCPD is rated 1,000A or higher. However, you must meet other requirements of NEC 695.4(B)(2) through (5), such as the OCPD must be listed suitable for service use and be remotely located from other distribution equipment; it must be lockable in the closed position; the disconnecting means shall have an identification placard with 1-inch height letter reading “Fire Pump Disconnecting Means;” the pump controller needs to also have a placard stating the location of the disconnecting means and location of the key, if it is locked. Following the requirements of NFPA 20, 9.2.2(4)(e), this OCPD must also be selectively coordinated with any other supply site OCPD, which means it must trip first under a fault condition.
In reference to the conductors, except in the electrical and fire pump rooms, you're required to follow the requirements outlined in NEC 695.6(B). Due to cost and infeasibility, chances are you would not want to enclose the conductors in 2 inches of concrete. One option is to specify MI cable, with its 1-hour or more fire rating and rated equipment grounding sheath. However, MI cable is expensive, difficult to work with, only comes as single conductors in the sizes you will need, and the outer copper sheath and grounding conductor may not be rated for the OCPD sized in the previous paragraph.
For example, per NEC Table 250.122, an 800A OCPD requires the use of a 1/0 copper grounding conductor, or equivalent. If the MI cable outer sheath is not equivalent to this, then what? An alternative is to specifying fire-rated MC 3-conductor cable, which is less costly, easier to install, and the outer metal sheath equivalent grounding conductor, in most cases, is equal to the inner current-carrying conductors. For example, the grounding conductor rating of 2/0 MC 3-conductor cable is 3/0 AWG, which is good for up to an 1,000A OCPD.
There are certainly other scenarios and twists that will require further engineering, but these examples cover many of the situations you'll typically encounter when designing an electrical system for a fire pump installation. The most important thing to remember is always verify your design with all applicable state and local codes, as well coordinate your efforts with your local authority having jurisdiction.
Granle is an electrical section manager with HDR/Jordan Architects in Rochester, Minn.