How to Choose and Size a Sewage Pump for a Commercial Building

To choose a sewage pump for a commercial building, work out four numbers: the peak flow from the fixture units draining to the basin (60 fixture units is about 33 gpm, 2.1 L/s), the minimum flow that keeps solids moving at 2 ft/s (0.6 m/s) in the discharge pipe (46 gpm in 3-inch pipe), the total dynamic head (static lift + pipe and fitting friction + any pressure at the outlet), and the basin volume that gives a pump cycle of at least 5 minutes. Then pick a duplex pair of pumps that each pass 2-inch solids and deliver the design flow at that head, with an alternator and a high-water alarm.

The method and every table below come from manufacturer engineering documents (Zoeller, the Sump and Sewage Pump Manufacturers Association, Little Giant) and from code text published by Los Angeles and Illinois, listed under Sources. Plumbing codes differ between jurisdictions and editions, so treat code limits here as examples and confirm them with your local authority. For how sewage, effluent, sump and grinder pumps differ mechanically, see Pump Types Explained.

Ejector, grinder, effluent or sump?

Solids limits from SSPMA (2011); grinder figures from Zoeller’s Shark series; discharge sizes from the Los Angeles plan-check list (3 in for non-residential ejectors receiving toilets or urinals, 2 in in single-family homes) and Zoeller’s catalog.

• Sewage ejector — the default for a building with toilets or urinals below the sewer. SSPMA and Little Giant both say a sewage pump should handle spherical solids of at least 2 in where water closets drain to it; Los Angeles requires a 2-inch ball to pass in other than single dwellings (1½ in in single-family homes).
• Grinder — when the discharge must go a long way, uphill or into a pressurised sewer. Zoeller’s centrifugal Shark grinders reach 107 ft (33 m) of head and its progressing cavity grinder 240 ft (73 m), through a 1¼-inch discharge.
• Effluent — for liquid that has already settled in a septic or interceptor tank, not raw sewage.
• Sump — clear groundwater only. Keep it separate: the Illinois Plumbing Code says sumps handling subsoil and footing drainage shall not receive sewage, and sewage sumps shall not receive subsoil or footing drainage.

The sizing method in five steps

1. Peak inflow from fixture units.
2. Minimum flow for the discharge pipe size; the design flow is the larger of the two.
3. Total dynamic head (TDH) at the design flow.
4. Pump: one whose curve passes through or just above the design point, with the right solids rating, voltage and phase.
5. Basin and controls: working volume, float levels, simplex or duplex, alarm.

This is the order of Zoeller’s Sewage System Sizing and Selection Work Sheet (FM0551), Little Giant’s Guidelines for Sizing and the SSPMA sizing guide, which use essentially the same charts.

Step 1: Peak flow from fixture units

A fixture unit (FU) is a load factor that lets different fixtures be added up; the peak flow is then read from a curve based on the probability of simultaneous use (Zoeller calls it the “Hunter curve” method and says it suits residential and small commercial buildings). Zoeller’s worksheet assigns these values, with 4 FU added for each flush-valve fixture:

Zoeller credits its chart data to the ASPE Handbook, the Uniform Plumbing Code, Cameron Hydraulic Data and the Plastic Pipe Institute; Little Giant and SSPMA print similar tables. If your code publishes drainage fixture unit values, use those. Peak flow from the same curve (Zoeller FM0551 Figure B):

Read from the log-log curve, so treat each value as ±1–2 gpm. The readings printed with the same curve agree: SSPMA gives 34 FU = 22 gpm and 37 FU = 23.5 gpm, Little Giant 55 FU = 30 gpm. The curve stops at 200 gpm; Zoeller says to consult the factory above it.

Zoeller’s station-design paper notes that the fixture-unit method is conservative (it overestimates peak flow). For high-occupancy buildings such as motels, apartment blocks and large offices, Zoeller offers a larger capacity system chart, and civil engineers use a population (gallons-per-day × peak factor) method, which only works with a large basin.

Step 2: Minimum flow for the pipe size

Whatever the fixtures produce, the pump must move at least 2 ft/s (0.6 m/s) through the discharge line to carry solids. That sets a minimum flow for each pipe size, and the design flow is the larger of this and the Step 1 flow:

Zoeller FM0551 Figure C (2–4 in) and Little Giant Guidelines for Sizing (1¼–3 in); the two agree where they overlap.

Choosing a bigger pipe to cut friction therefore raises the flow the pump must deliver. Codes can add their own floor: the Los Angeles plan-check list requires sewage ejectors to discharge at least 20 gpm. For larger pumping stations Zoeller recommends a force-main velocity between 2 and 5 ft/s, and a force main of at least 4 in where the pumps pass 2½-inch or larger solids.

Step 3: Total dynamic head

• Static head is the vertical lift from the minimum water level in the basin (the pumps-off level) to the highest point of the discharge pipe. For a large wet well, Zoeller suggests using the midpoint between “lead pump on” and “all pumps off”.
• Friction head is found by turning every fitting into an equivalent length of straight pipe, adding the real pipe length and multiplying by the loss per 100 ft at the design flow. Zoeller’s station paper gives the Hazen–Williams equation behind such tables, with C = 150 for PVC, 140 for polyethylene and 130 for new cast iron or copper; the Pipe Pressure Drop Calculator works it out for any pipe with the Darcy–Weisbach method.
• Sewer pressure applies only when you discharge into a pressurised main: multiply psi by 2.31 to get feet.

Zoeller FM0551 Figure D. Multiply by 0.3048 for metres. Little Giant’s table matches except the 2½ in swing check (20.3 ft).

Friction loss in schedule 40 plastic pipe, Zoeller FM0551 Figure E (a ratio, so it is also m per 100 m). Schedule 40 steel loses about twice as much: 1.24 ft per 100 ft for 3 in at 50 gpm.

Worked example: lower-level restrooms in an office building

A small office building has restrooms and a break room on a lower floor below the building sewer. Everything there drains to an ejector basin.

1. Fixture units. 4 flush-valve water closets (4 × 7 = 28), 2 flush-valve urinals (2 × 8 = 16), 4 lavatories (4 × 1 = 4), 1 service sink (3), 1 drinking fountain (1), a break-room kitchen sink (2) and dishwasher (2), and 2 showers (2 × 2 = 4):

2. Pipe size and design flow. The building is not a single dwelling and the basin takes toilets and urinals, so under the Los Angeles rule the discharge piping and valves must be at least 3 in. A 3-inch line needs 46 gpm to reach 2 ft/s, which is more than the 33 gpm peak:

(Where the code allows 2½-inch discharge, the minimum would be 30 gpm and the design flow 33 gpm.)

3. Static head. From the pumps-off level in the basin to the top of the discharge pipe, where it turns horizontal to reach the gravity drain: 14 ft (4.27 m).

4. Friction head. 14 ft of vertical pipe plus 76 ft horizontal is 90 ft (27.43 m) of 3-inch schedule 40 PVC. Fittings, from the 3-inch column:

5. TDH. The discharge enters a gravity drain, so there is no sewer pressure:

The design point is 46 gpm at 15 ft. The fluid power is 46 × 14.91 ÷ 3,960 = 0.17 hp (129 W); the motor is larger because small solids-handling pumps are far from 100% efficient — read the shaft power off the curve. Notice that in a short building run the static lift is 94% of the head; friction only dominates on long force mains.

6. Pump. Two identical pumps, each passing 2-inch spheres and each delivering at least 46 gpm at 15 ft, with the point near the middle of the curve. Zoeller warns not to select a pump if the TDH is below the minimum head shown on its curve, and FM0551 asks you to consult the factory for any system under 5 ft of TDH. The Los Angeles plan-check list also asks for the system curve drawn on the pump curve to find the actual flow.

7. Downstream drain. Los Angeles loads the gravity drain receiving the ejector at 2 fixture units per gpm pumped: 46 × 2 = 92 FU, which the building drain must be sized for.

8. Basin. See the next sections: 57.5 gal (218 L) of working volume, 13 in (330 mm) between lead-pump-on and all-pumps-off in a 36-inch basin.

Simplex vs duplex, alternator and alarm

A simplex system has one pump; a duplex has two in the same basin, run by a control panel that alternates them. SSPMA lists the advantages: the pumps share the wear, the lag pump starts if the lead pump fails or jams, and both run together when the inflow is unusually high. For commercial buildings the guidance is consistent:

• SSPMA: simplex is adequate in most homes, but for public and commercial use “Duplex System is essential.” Little Giant: commercial optional depending on the business, public or industrial essential.
• Zoeller’s station paper: if fixtures must always stay in service, specify a duplex so one pump carries the load while the other is repaired.
• Los Angeles (2017 LAPC plan check): “Provide dual pumps each capable of handling the load independently.”
• Illinois Plumbing Code §890.1360(d): sumps receiving the discharge of more than six water closets or 60 drainage fixture units need duplex equipment.

High-water alarm. Fit an audible and visual alarm on its own float above the lag-pump-on level. Zoeller puts the high-water alarm at or below the lowest incoming sewer line and, for mercury float switches, spaces the floats at least 6 in apart; Los Angeles requires the high water level to be at least 2 in below the lowest inlet. The lowest stop level must be no lower than the top of the pump housing or the maker’s minimum submergence, whichever is higher.

Basin sizing and cycle times

A basin that is too small makes the pumps start too often, which shortens motor and switch life. The pumping-station rule Zoeller quotes from the Water Pollution Control Federation’s Manual of Practice FD-4 is that each pump’s cycle should not be shorter than 5 minutes, nor the average cycle longer than 30 minutes; the same paper notes that detention under 30 minutes at average flow avoids most odour problems. The shortest cycle happens when the inflow is exactly half the pump rate, which gives the working volume between pump-on and pump-off:

Check it in the example: with no inflow the pump empties 57.5 gal in 57.5 ÷ 46 = 1.25 min (75 s); at the worst case (23 gpm in) it fills in 2.5 min and pumps down in 2.5 min, a 5.0-minute cycle; at the 33 gpm peak it fills in 1.74 min and pumps down in 57.5 ÷ (46 − 33) = 4.42 min, a 6.2-minute cycle.

Where the guidance differs. Zoeller’s technical brief TB0005 asks for “at least a 30-second pump cycle and no more than 6 starts per hour” without saying whether that is per pump or per basin. With an alternating duplex, a 5-minute basin cycle is 12 starts an hour shared between two pumps, 6 each. Applied to the basin as a whole, 6 starts an hour means a 10-minute cycle and double the volume, 115 gal (435 L). Little Giant’s basin chart assumes run times of 15 seconds to 4 minutes. The 5-minute station rule is the more demanding on run time and the 6-starts-per-basin reading the more demanding on volume; follow the pump maker’s figure for the motor you choose.

Computed as π/4 × D² × 12 in ÷ 231 in³ per gallon for an empty cylinder; pumps, floats and pipes take some of the volume. These match Little Giant’s chart except at 24 in, where it prints 23.9 gal/ft.

The basin depth then stacks up from the bottom: pump height to the all-off level, the 13 in working band, then lag-on and alarm floats (6 in apart each if you use Zoeller’s spacing: 13 + 6 + 6 = 25 in), then at least 2 in to the lowest inlet under the Los Angeles rule, 27 in (686 mm) above the off level in total. Zoeller’s FM0551 basin chart recommends at least a 24-inch diameter at 46 gpm and refers to its FM0541 sheet for simplex and duplex basins; a duplex needs room for two pumps, so check the maker’s duplex basin sizes.

Venting, check and gate valves, connection

• Gas-tight cover and vent. The Illinois code requires sewage to be lifted from “a gas-tight, covered and vented sump”; Los Angeles requires an airtight cover and a vent pipe extending through the roof. Size the vent to your code (Illinois uses its Appendix A Table K).
• Check valve and shut-off valve. Los Angeles requires an accessible check valve and gate valve on each pump discharge, with the gate valve on the discharge side of the check and both outside the pit. Ohio amends §712.2 of the 2021 International Plumbing Code with an exception that requires “only a check valve” on the ejector discharge in houses under its residential code, which implies the base section asks for more than a check valve elsewhere — read the edition your jurisdiction has adopted.
• Air. Zoeller says to always drill a vent (relief) hole in the discharge pipe between the pump and the check valve, because trapped air can stop the pump moving water; long horizontal runs may need air-release valves, and force mains need them at high points.
• Water hammer. Check valves slamming shut cause pressure surges; Zoeller recommends spring-loaded or lever-and-weight check valves where the operating head exceeds 35 ft.
• Connection. Los Angeles requires the discharge to enter the horizontal gravity drain from the top through a wye.

Power supply and controls

Little Giant’s checklist before choosing the pump: what voltage, phase and frequency are available, whether the pump will share a circuit, whether the building has breakers or fuses and whether their rating is enough for the motor. If the building has three-phase power, check that the pump comes in that voltage and phase (see three-phase basics). Size the supply for both pumps running together, because the lag pump starts alongside the lead pump at high inflow. The duplex control panel holds the alternator (Zoeller offers mechanical and electrical versions) and the float and alarm connections; put the alarm where staff will see or hear it.

Maintenance access

Zoeller’s station paper stresses that even small stations need access for maintenance crews and equipment: a hoist to lift the pumps out, ladders for wet wells and valve vaults, ventilation to clear dangerous gases, and electrical security. In a building, that means:

• Guide-rail systems (Zoeller and others) guide a pump up to the surface and back down onto a fixed base elbow on the discharge pipe, so it can be pulled for service from above.
• Check and gate valves outside the basin (required in Los Angeles) so a pump can be isolated and pulled while the other keeps working.
• An access cover above each pump and headroom for a hoist or lifting device.
• No dead corners in the basin where solids can settle; Zoeller recommends eliminating them or providing a way to stir the solids back into suspension.

Sources

• Zoeller Company, Sewage System Sizing and Selection Work Sheet, FM0551 (rev. 03/06, © 2012) — fixture units, FU–gpm curve (Figure B), minimum gpm by pipe size, fitting equivalent lengths, friction per 100 ft, basin diameters, psi × 2.31. https://zoellerpumps.com/wp-content/uploads/sites/3/2023/10/fm0551.pdf
• J. Zoeller, PE, Design of Industrial/Commercial Sewage Pumping Stations, Zoeller Company — duplex, 2–5 ft/s force mains, 4 in minimum with 2½ in solids, static head, Hazen–Williams and C values, V = tq/4 and the WPCF FD-4 5–30 minute cycle, float spacing, 35 ft check-valve rule, access and safety. https://www.zoellerengineered.com/wp-content/themes/zoeller/content/literature/Design-of-Industrial-and-Commercial-Sewage-Stations.pdf
• Zoeller Company, General Information – Sizing a Sewage Pump, technical brief TB0005 (web page, 2026) — Hunter-curve method, 2 ft/s, 30-second cycle and 6 starts per hour, vent hole. https://zoellerpumps.com/2026/03/24/general-information-sizing-a-sewage-pump/
• Zoeller Company, Pumps and Systems catalog FM0532 (rev. 11/2016) — Shark grinder heads, 7/32 in shredding, 1¼ in discharge, guide-rail systems. https://www.cooneybrothers.com/userfiles/ad/documents/Zoeller_2640003_Catalog.pdf
• Sump and Sewage Pump Manufacturers Association, Sizing for New or Replacement Sewage Pumps, 2011 (presented by M. Rousseau at NOWRA) — solids sizes, 34 FU = 22 gpm, duplex guidance. https://www.nowra.org/Customer-Content/www/CMS/files/22Mega_Rousseau.pdf
• Little Giant (Franklin Electric), Wastewater: Guidelines for Sizing, form 993391 (01/2013) — minimum flows by pipe size, 55 FU = 30 gpm example, basin chart, simplex/duplex, electrical checklist. https://s3.amazonaws.com/s3.supplyhouse.com/product_files/509081-%20Sizing%20Guide.pdf
• City of Los Angeles Department of Building and Safety, Plan Check Correction Sheet for Sewage Ejectors and Sump Pumps, 2017 LAPC, PC/MECH/Corr.Lst.031 (rev. 01-01-2017) — dual pumps, 2 in ball, 3 in discharge, valves, vent, high-water level, 20 gpm minimum, 2 FU per gpm. https://dbs.lacity.gov/sites/default/files/efs/forms/pc17/supplemental-correction-sheets-for-sewage-ejectors-sump-pumps-%28for-2017-codes%29.pdf
• Illinois Department of Public Health, Illinois Plumbing Code, 77 Ill. Adm. Code 890.1360 Sanitary Wastes below Sewer (amended 38 Ill. Reg. 9940, 2014). https://www.ilga.gov/ftp/JCAR/AdminCode/077/077008900J13600R.html
• Ohio Board of Building Standards, Ohio Adm. Code 4101:3-7-01 Sanitary drainage (amendments to IPC 2021 Chapter 7), effective 15 October 2025. https://codes.ohio.gov/ohio-administrative-code/rule-4101:3-7-01