Power for fire pumps


Figure 3: The conduit feeding the fire pump motor must be RGS, IMC, EMT, liquid-tight flexible metal, liquid-tight flexible nonmetallic conduit Type LFNC-B, listed Type MC cable with an impervious covering, or Type MI cable. Courtesy: Stanley ConsultantsBoth the NEC (695.3(C)) and NFPA 20 (9.2.2(4)) recognize that multibuilding campus-style arrangements have unique requirements. Many larger industrial facilities, military facilities, and institutional facilities purchase their power at a substation at high- or medium-voltage and then the owner is responsible to distribute power within the campus to the buildings. Providing a dedicated service or a dedicated feeder to the fire pumps is just not feasible. The codes address these common situations by requiring two electric feeds that originate from independent sources. Note that a loop feed to a building with both legs originating from the same substation does not meet this requirement, and an alternate source of power, such as a standby generator, would be required. This part of the codes is subject to various interpretations, and the prudent engineer will seek the authority having jurisdiction’s (AHJ) interpretation upfront.

Having laid down the requirements concerning the source of power, Article 695.4 gives requirements for the continuity of power. The main goal is to maintain power to the pump(s); however, at first glance many of these requirements will feel wrong because they are opposite of how the rest of the code is written. We need to take a step back to think about this. Most of the NEC is written to minimize the risks of fires, and therefore equipment is taken offline when it is in danger of starting a fire. However, here the codes are addressing situations where the fire has already started. Therefore, the equipment must continuously run to protect lives, regardless of the risk to the equipment or additional fires. Keeping this reasoning in mind will help clarify the codes’ intention.

To keep the power available to the motor, minimizing the number of ways to disconnect the circuit is important. Although a direct connection is preferable, the NEC allows one disconnecting means, with overcurrent protection, between the source and fire pump controller or transfer switch. When a standby generator is used as an alternate supply, an additional disconnect, and overcurrent protection, is allowed. The NEC also recognizes that a multibuilding campus-style arrangement is more complex and will require additional disconnecting means, and for only this situation, it does not give a limit to the number of disconnects.

The overcurrent protection requirements (NEC 695.4(B)(2) and (NFPA 20:9.2.3) are important, because the protection will be set to act as though there were a direct wired connection. The only protection allowed is to trip during a short circuit fault. To do this, the overcurrent protection device (OCPD) must be rated for the locked rotor current (LRC) of the largest motor and the jockey pump, and then the sum of the remaining pumps and associated accessory equipment (see Figure 1). New to the codes, when a “listed” fire pump OCPD is used, protection is allowed and specific trip settings are listed in 695.4(B)(2)(b) and 20:

Example 1

In Example 1, we review sizing an overcurrent protective device (Figure 1). At 460 V, the full-load amps (FLA) for the 50 hp motor is 65 amps, but the locked rotor current (LRC) is 363 amps per NEC Table 430.251(B) or NFPA 20 Table We also have a 2-hp jockey pump. Although the FLA is 3.4 amps, the LRC for the jockey pump is 25 amps. Adding the two LRCs together, we have 388 amps. Per NEC 240.6 the next size standard overcurrent protective device (OCPD) is 400 amps.

When a standby generator is used, the OCPD is sized to allow for instantaneous pickup of the full pump room load only, not the LRC. The OCPD is not required to provide any overload protection, only short-circuit protection (see Example 2). Although not required, ground fault detection (not protection) is allowed by code. Providing a local visible signal and a tie to the BAS and/or to the fire alarm system should be considered for maintenance purposes. When connecting to the fire alarm system, NFPA 72: requires that any signal other than a “pump running” signal be supervisory. NFPA 20:10.4.7 requires four signals to be transmitted remote from the pump when the pump room is not constantly attended. They are “pump running,” “loss of phase power,” “phase reversal,” and “alternate source of power connected.” Only the “pump running” signal can be either an alarm or supervisory signal.

Example 2

Let’s look at Figure 1 again and size the OCPD for a standby generator. The FLA for the two motors is 68.4 amps. Per NEC Table 430.52 the maximum rating for the inverse time breaker OCPD is 250%; therefore, the maximum OCPD is 171 amps. The next standard size OCPD is 175 amps. An acceptable range would be 150% to 250%. Therefore, using 175%, the OCPD should be rated for 125 amps.

The disconnecting means for the fire pump system must be clearly identified, be separate from the other disconnects in the facility, be lockable in the closed position, and be monitored in the closed position. All of these requirements are designed to keep the fire pumps online, and prevent mistakenly shutting the fire pump down. Imagine the beginning of the firefighting effort when in the rush to cut power to the building, the fire pump disconnect is mistakenly opened by the very team that is trying to fight the fire. These regulations are in place to eliminate this potential scenario.

Transformers are only allowed when the service voltage is different from the fire pump requirements. If a transformer is required, it must be dedicated only to the fire pump system. It must be sized for 125% of the fire pump motors and jockey pump, and 100% of the remaining loads.

See Example 3 to size a transformer. The transformer can only have primary side overcurrent protection. The overcurrent protection complies with the earlier requirements in that it is intended only for short-circuit fault protection, and is set for the LRC of the largest motor and jockey motor and the FLA of the remaining loads.

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