# Power for fire pumps

07/21/2014

Example 4

You may have noticed that the conductor size for the fire pump system feeder of previous examples has not been calculated yet. So let’s determine the conductor sizes required to feed the fire pump system. We’ll assume that equipment terminals are 75 C and the distance from the transformer to the controller is 200 ft. First, we must determine the minimum cable ampacity. If you remember, the ampacity is based on the FLA, not the LRC. We’ve already calculated this value in Example 3. The sum of FLA x 125% was 85.5 amps. So, from NEC Table 310.15(B)(16), a THWN #3 AWG copper cable is sufficient, until we look at voltage drop.

For the voltage drop, the NEC requires two calculations: one requiring a max drop of 15% during motor starting conditions, and another during running conditions requiring 5% at 115% of FLA. The basic voltage drop equation is below. (This neglects inductance).

For motor starting conditions, the current was calculated in Example 1 as 388 amps.

Per NEC Chapter 9, Table 8, a #3 AWG cable is 56,620 Cmils and is sufficient.

The second calculation is for 5% max voltage drop at 115% FLA.

Again, a #3 AWG cable is sufficient. Therefore, a #3 AWG conductor will be used to supply the fire pump system.

Finally, what size ground conductor is required? This is based on the upstream OCPD, equivalent to 400 amps, requiring a #3 AWG copper conductor per NEC Table 250.122.

These examples have provided some insight into the basic design process of providing power to fire pumps. As we’ve seen, NEC 695 and NFPA 20 are critical to understanding the concepts behind providing power to fire pumps. Be sure to determine up front what the local jurisdictional requirements are, review your design in accordance with the standards, and then coordinate your design with the local AHJ. The uniqueness of designing power for fire pumps will be challenging at first. Keep in mind that when your design is operating, you will be providing life-sustaining protection to victims of fire as well as those men and women risking their lives to stop the fire and rescue the victims.

Ernest E. Leaf is a senior electrical engineer at Stanley Consultants. He has 18 years of experience designing electrical power distribution systems for universities and airports, including numerous fire pump motor installations.

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