The most common misconception people have about sump pumps is that the bigger the horsepower of the pump, the better. Not so, and that is why proper sizing of the pump in the first place is so important. When sizing a residential sump pump, there are a few things to take into consideration.

If you look at the Wisconsin plumbing code SPS 382.36 for stormwater basin sizes, it states the size of each sump shall be no smaller than 16” diameter at the top, 14” diameter at the bottom, and 22” in depth deep. The pump size is listed as being of a capacity appropriate for its anticipated use. For the most part, these minimalist requirements have been adopted and are what most homes are built with.

In most cases, the sump pump would operate properly but an extreme rainfall could mean the pump can’t keep up. In that case, a secondary pump attached to a separate power source or a basic 12-volt battery backup system can prevent water damage.

On the other end of the spectrum, there are homes and buildings that were built with the Code’s minimum sizing with no idea what the anticipated capacity needed to be. You may hear statements from these property owners like “I need a one horsepower pump,” or “I have three pumps in my crock and they barely keep up,” and “Why does my pump only last a year?” If the home is properly graded and the roof, gutters, downspouts, and stormwater drainage is working the way it should, then it is necessary to upgrade the existing sump and pump system to a properly sized basin and redundant pump system that will protect from water overflow.

To properly size a pump system, you must first determine the peak inflow. In the example above, with multiple pumps barely keeping up with the inflow, that could be the best way to determine the inflow. Once you determine what the pumps’ outputs are, by checking pump curves, you now know the number needed to select a pump or pumps.

Once you have a good estimate of what the peak inflow is, it is time to determine the basin size. For stormwater, it is best to size the pump system from the bottom of the inlet (lowest inlet if there is more than one) to the bottom of the basin. The reason for this is that in some homes and buildings, as soon as the water sits in the drain tile you may see water coming through the cracks in the floor or where there may be a low spot in the drain tile. Therefore, the pump should turn on before it touches the bottom of the drain tile.

For example, given variables of 50 gpm peak inflow and a measurement of 10” from the top of the basin to the bottom of the inlet, we recommend a 30” diameter by 36” deep basin. A 30” diameter basin holds three gallons of fluid per inch (versus an 18” basin that holds one gallon per inch). A 10” pump down cycle would allow for 30 gallons of water to be pumped per cycle. The 36” deep basin allows for plenty of reserve and dead zone as well.

The 30” diameter basin would also allow extra room for a true duplex system and the control floats. Those floats would be attached to an alternator that would allow for equal exercise of the pumps and the ability for both pumps to run at the same time if one pump was unable to keep up with the inflow.

For questions on residential or commercial pump sizing, the team at JMI Pump Systems is ready to help. Contact us at 800-234-5490 or sales@jimmurrayinc.com.