How are Engines and Motors Sized for Fire Pumps?

Another question we get pretty frequently here at Steven Brown & Associates, Inc. is how are engines and motors sized for fire pumps? In order to determine the sizing of motors and engines for Aurora fire pumps, we refer to the guidelines put forth by the U.L., as well as in Factory Mutual’s Approval Standard for Split-Case Fire Pumps, and finally the NFPA 20 (2013) 4.7.6.

These standards ensure that the engines and motors supplied with Aurora pumps are designed to operate on any point of the fire pump curve without exceeding the motor nameplate service factor, or engine HP rating. Many of our customers have assumed that motors are sized only to 150% of nameplate capacity, but the reality is that it’s not uncommon for these fire pumps to operate well beyond 150% of rated capacity. This can happen if there is an open hydrant or broken pipe downstream, for example.  The confusion is understandable, as many sections of NFPA 20 talk about 150% capacity as a point used to calculate maximum water velocities (such as Table 4.26), and NFPA 25 requires yearly testing only out to 150% of rated capacity. So it becomes very easy to assume that motors and engines are sized for the same point.

While fire pumps may be sized across the entire range of their performance curve and not just up to 150% of rated capacity, they are allowed to utilize a 1.15 motor service factor upon starting.  So, for example, if the maximum brake horsepower required (BHPr) is 114 HP for a given fire pump rating, you are allowed to utilize a 100 HP motor with a 1.15 motor service factor (100 x 1.15 = 115). For budgetary reasons, this is common practice and permissible by both U.L. and Factory Mutual.

Outside of covering for BHPr across an entire performance curve, there are some exceptions to motor and engine sizing in some specific circumstances. For example, if your fire pump installation is installed at a high elevation above sea level, or in a room with a high expected air temperature, then you may require a larger motor or engine to account for those factors (see NFPA 20 (2013) and, as well as for motors).

Another exception would be in the case of motors used with variable speed drives. To ensure reliable performance, motors intended for use in a variable speed application would not be allowed to use any motor service factor at all. So, while you are still sizing the motor to handle performance across the entire performance curve you are not allowed to utilize the motor 1.15 service factor in accomplishing this.

If you’d like to learn more, or have additional questions about fire pump parts or fire pump service, feel free to reply to this post, call us at the contact number below, or use our service or info request page.