# Traps, Interceptors and Separators: IPC Chapter 10 Requirements

> IPC Chapter 10 requires every plumbing fixture to have its own 2- to 4-inch liquid-seal trap close to the fixture outlet, bans S, bell, drum and crown-vented traps, and requires grease interceptors and oil separators where kitchen or vehicle wastes would harm the sewer.

- Canonical: https://harth.build/resources/design-standards/traps-interceptors-and-separators/
- Last updated: 2026-10-27
- Data source: Trap, interceptor and separator rules are taken from the [2021 International Plumbing Code, Chapter 10, Traps, Interceptors and Separators](https://codes.iccsafe.org/content/IPC2021P2/chapter-10-traps-interceptors-and-separators); trap sizes and drainage fixture units from [Chapter 7, Sanitary Drainage](https://codes.iccsafe.org/content/IPC2021P2/chapter-7-sanitary-drainage) (Table 709.1); and trap-to-vent distances from [Chapter 9, Vents](https://codes.iccsafe.org/content/IPC2021P2/chapter-9-vents) (Table 909.1). Dwelling trap rules come from [2021 International Residential Code Chapter 32, Traps](https://codes.iccsafe.org/content/IRC2021P2/chapter-32-traps). The separator worked examples are calculated by Harth from those sections. These are typical model-code values; verify every size, distance and capacity against the edition adopted by the authority having jurisdiction, local amendments and any sewer-authority pretreatment rules for grease interceptors.

## Key facts

| Fact | Value |
| --- | --- |
| Trap seal depth | 2 inches minimum, 4 inches maximum (IPC 2021 Section 1002.4; IRC P3201.2; deeper seals allowed only for special accessible-fixture designs) |
| Fixture outlet to trap | 24 inches vertical, 30 inches horizontal (Section 1002.1, measured centerline of fixture outlet to centerline of trap inlet) |
| Prohibited traps | S, bell, drum, crown-vented and moving-part traps (Section 1002.3; drum traps allowed as solids interceptors and on chemical waste systems) |
| Lavatory and sink traps | 1-1/4 inch lavatory, 1-1/2 inch kitchen sink (Table 709.1 minimum trap sizes; floor drains and clothes washer standpipes 2 inches) |
| Grease interceptor capacity | 2 pounds of grease per gpm of flow (Table 1003.3.5.1, from 8 pounds at 4 gpm to 200 pounds at 100 gpm) |
| Gravity grease interceptor volume | Peak flow (gpm) x 30 minutes (Section 1003.3.7; designed and tested to IAPMO/ANSI Z1001) |

## Direct answer

The 2021 International Plumbing Code regulates traps, interceptors and separators in Chapter 10. Section 1002.1 requires each fixture to be separately trapped, with the trap weir not more than 24 inches below the fixture outlet and the trap inlet not more than 30 inches from it horizontally; fixtures with integral traps, such as water closets, are exempt, and no fixture may be double trapped. Section 1002.4 sets the liquid seal at not less than 2 inches and not more than 4 inches, and seals that can evaporate must be protected by a trap primer or barrier device. Trap sizes come from Table 709.1: 1-1/4 inches for a lavatory, 1-1/2 inches for a kitchen sink or bathtub and 2 inches for a floor drain. Section 1003.3 requires grease interceptors for pot sinks, prerinse sinks, wok stations and similar fixtures in commercial kitchens, while Section 1003.4 requires oil separators at repair garages, car washes and hydraulic elevator pits.

*Content and cited inputs reviewed by [Harth team](https://www.linkedin.com/company/harth-build) · Published 2026-10-27 · Updated 2026-10-27.*

## What are the key takeaways?

- Trap seal depth: 2 inches minimum, 4 inches maximum
- Fixture outlet to trap: 24 inches vertical, 30 inches horizontal
- Prohibited traps: S, bell, drum, crown-vented and moving-part traps
- Lavatory and sink traps: 1-1/4 inch lavatory, 1-1/2 inch kitchen sink

## What does IPC Chapter 10 require for fixture traps?

A trap holds a water seal between the drainage system and the room so sewer gas cannot enter. Chapter 10 of the 2021 International Plumbing Code governs traps, interceptors and separators, and its core trap rules are short:

- **Every fixture is separately trapped** by a liquid-seal trap (Section 1002.1). A fixture is never double trapped.
- **The trap sits close to the fixture.** The vertical distance from the fixture outlet to the trap weir cannot exceed 24 inches, and the horizontal distance from the centerline of the fixture outlet to the centerline of the trap inlet cannot exceed 30 inches.
- **The seal is 2 to 4 inches deep** (Section 1002.4). A seal shallower than 2 inches is easily lost; one deeper than 4 inches holds solids.
- **The trap is self-scouring** and has no interior partitions unless it is integral with the fixture or made of an approved corrosion-resistant material (Section 1002.2). Slip joints are allowed only on the trap inlet, the trap outlet and within the trap seal.
- **The trap is sized from Table 709.1** and is never larger than the drain it discharges into (Section 1002.5).
- **The trap is set level** with respect to its seal and protected from freezing (Section 1002.7). Building (house) traps on the building drain are prohibited (Section 1002.6).

Section 1002.1 lists four exceptions: fixtures with integral traps, such as water closets; a combination fixture on one trap where one compartment is not more than 6 inches deeper than the other and the outlets are not more than 30 inches apart; floor drains in multilevel parking structures discharging to a storm sewer; and a pot sink served directly by a hydromechanical grease interceptor within set distances.

## Which trap types are prohibited?

Section 1002.3 prohibits six trap types, each because its seal can be lost or fouled:

1. traps that depend on moving parts to maintain the seal;
2. bell traps;
3. crown-vented traps, where the vent connects at the top of the trap bend (Section 909.3 also keeps any vent at least two pipe diameters from the trap weir);
4. traps not integral with a fixture that depend on interior partitions, unless made of an approved corrosion-resistant material;
5. "S" traps, which self-siphon;
6. drum traps, except where used as solids interceptors or on chemical waste systems.

## How are trap seals protected from evaporation?

Floor drains and other traps that rarely receive water dry out and open a path for sewer gas. Section 1002.4.1 requires emergency floor drain traps and any trap seal subject to evaporation to be protected by one of five methods: a potable water trap seal primer valve to ASSE 1018; a reclaimed or graywater primer valve to ASSE 1018; a wastewater-supplied trap primer device to ASSE 1044; a barrier-type trap seal device to ASSE 1072; or a lavatory, hand sink or drinking fountain drain routed below the floor into the floor drain, trench drain or floor sink in the same room (adjacent room for a drinking fountain).

## When is a grease interceptor required, and how is it sized?

Section 1003.3.1 requires a grease interceptor or automatic grease removal device for fixtures and equipment with grease-laden waste in food preparation areas, including restaurants, hotel and school kitchens, hospitals, bars, factory cafeterias and clubs. The listed fixtures are pot sinks, prerinse sinks, soup kettles, wok stations, floor drains or sinks that kettles drain into, automatic hood wash units and dishwashers without prerinse sinks. Only fixtures that can discharge fats, oils or grease connect to the interceptor. Individual dwelling units and private living quarters are exempt (1003.3.4).

- **Food waste disposers** may not discharge to a grease interceptor (1003.3.2).
- **Additives** that emulsify grease, chemicals and enzymes are prohibited; only microbial dispensing systems to ASME A112.14.6 are allowed where the manufacturer permits them (1003.3.3).
- **Hydromechanical interceptors** are sized to ASME A112.14.3, ASME A112.14.4, ASME A112.14.6, CSA B481.3 or PDI G101, need a vented flow-control device terminating at least 6 inches above the flood rim (1003.3.5.2) and must hold the grease retention capacity in Table 1003.3.5.1.
- **Gravity interceptors** are sized by multiplying the peak drain flow in gallons per minute by a 30-minute retention time, and are designed and tested to IAPMO/ANSI Z1001 (1003.3.7).
- **Discharge** from any grease interceptor connects directly to the sanitary drainage system (1003.3.8).

## Where are oil separators, sand interceptors and other separators required?

Section 1003.4 requires oil separators at repair garages with floor or trench drains, car washing facilities, factories producing oily or flammable liquid wastes and hydraulic elevator pits. An elevator pit may use an approved alarm system instead, provided it does not shut down pumps that keep the elevator available to firefighters. A separator that is not listed and labeled must be at least 2 feet deep below the invert of the discharge drain with an outlet water seal of at least 18 inches (1003.4.2.1). Where vehicles are serviced, repaired or washed or fuel is dispensed, capacity is 6 cubic feet for the first 100 square feet of drained area plus 1 cubic foot for each additional 100 square feet (1003.4.2.2). Parking garages without servicing, washing or fuel dispensing need no separator.

Other separators in Section 1003 include sand and heavy-solids interceptors with a water seal of at least 6 inches (1003.5); lint interceptors on commercial clothes washers that stop solids of 1/2 inch or larger (1003.6), not required for washers in individual dwelling units; glass interceptors at bottling plants (1003.7); and separators for slaughtering and dressing room drains (1003.8). Every interceptor and separator is vented under Chapter 9 so it cannot become air bound (1003.9), and access must be provided for cleaning (1003.10).

## How does the IRC treat traps in a house?

IRC Chapter 32 mirrors the IPC for one- and two-family dwellings. Section P3201.2 requires the same 2- to 4-inch seal, P3201.6 the same 24-inch vertical and 30-inch horizontal limits, and P3201.5 prohibits bell traps, S traps, drum traps, traps with moving parts and partitioned traps other than lavatory traps of plastic, stainless steel or another corrosion-resistant material. The IRC adds one practical allowance: a single trap may serve two or three adjacent like fixtures (kitchen sinks, laundry tubs or lavatories) in the same room, with the trap at the center fixture where there are three and outlets not more than 30 inches apart.

## IPC 2021 Table 709.1 minimum trap size and drainage fixture units

Values are reproduced from Table 709.1 of the 2021 International Plumbing Code. Water closets have integral traps, a bathroom group is a combined fixture-unit total, and urinal traps match the fixture outlet size (note d). Typical model-code values; verify against the AHJ-adopted edition.

| Fixture | Minimum trap size (inches) | Drainage fixture units |
| --- | --- | --- |
| Automatic clothes washer, commercial | 2 | 3 |
| Automatic clothes washer, residential | 2 | 2 |
| Bathroom group, 1.6 gpf water closet | Not separately listed | 5 |
| Bathroom group, water closet over 1.6 gpf | Not separately listed | 6 |
| Bathtub, with or without shower or whirlpool | 1-1/2 | 2 |
| Bidet | 1-1/4 | 1 |
| Combination sink and tray | 1-1/2 | 2 |
| Dental lavatory | 1-1/4 | 1 |
| Dental unit or cuspidor | 1-1/4 | 1 |
| Dishwashing machine, domestic | 1-1/2 | 2 |
| Drinking fountain | 1-1/4 | 1/2 |
| Emergency floor drain | 2 | 0 |
| Floor drain | 2 | 2 |
| Kitchen sink, domestic (with or without disposer or dishwasher) | 1-1/2 | 2 |
| Laundry tray, 1 or 2 compartments | 1-1/2 | 2 |
| Lavatory | 1-1/4 | 1 |
| Shower, total flow 5.7 gpm or less | 1-1/2 | 2 |
| Shower, over 5.7 to 12.3 gpm | 2 | 3 |
| Shower, over 12.3 to 25.8 gpm | 3 | 5 |
| Shower, over 25.8 to 55.6 gpm | 4 | 6 |
| Service sink | 1-1/2 | 2 |
| Sink | 1-1/2 | 2 |
| Urinal | Match fixture outlet | 4 |
| Urinal, 1 gallon per flush or less | Match fixture outlet | 2 |
| Urinal, nonwater supplied | Match fixture outlet | 1/2 |
| Wash sink (circular or multiple), each set of faucets | 1-1/2 | 2 |
| Water closet, flushometer tank, public or private | Integral trap | 4 |
| Water closet, private, 1.6 gpf | Integral trap | 3 |
| Water closet, private, over 1.6 gpf | Integral trap | 4 |
| Water closet, public, 1.6 gpf | Integral trap | 4 |
| Water closet, public, over 1.6 gpf | Integral trap | 6 |

Unlisted fixtures take a trap equal to their drainage outlet but not less than 1-1/4 inches (Section 709.2), and 1 gpm of continuous flow counts as 2 drainage fixture units (709.3).

## IPC 2021 Table 909.1 maximum distance from trap to vent

| Trap size (inches) | Slope (inch per foot) | Maximum developed length, trap weir to vent (feet) |
| --- | --- | --- |
| 1-1/4 | 1/4 | 5 |
| 1-1/2 | 1/4 | 6 |
| 2 | 1/4 | 8 |
| 3 | 1/8 | 12 |
| 4 | 1/8 | 16 |

Self-siphoning fixtures such as water closets are not limited (Section 909.1 exception), and the total fall of the fixture drain cannot exceed its diameter (909.2).

## IPC 2021 Table 1003.3.5.1 grease interceptor capacity

| Total flow-through rating (gpm) | Grease retention capacity (pounds) |
| --- | --- |
| 4 | 8 |
| 6 | 12 |
| 7 | 14 |
| 9 | 18 |
| 10 | 20 |
| 12 | 24 |
| 14 | 28 |
| 15 | 30 |
| 18 | 36 |
| 20 | 40 |
| 25 | 50 |
| 35 | 70 |
| 50 | 100 |
| 75 | 150 |
| 100 | 200 |

Above 100 gpm, double the flow-through rating to get the grease retention capacity in pounds.

## Separator sizing worked examples

| Case | Code rule | Calculation | Result |
| --- | --- | --- | --- |
| Gravity grease interceptor, kitchen peak flow 50 gpm | Section 1003.3.7: peak flow x 30 minutes | 50 gpm x 30 min | 1,500 gallons |
| Hydromechanical interceptor rated 35 gpm | Table 1003.3.5.1 | 35 gpm row | 70 pounds of grease retention |
| Oil separator, 1,500 sq ft repair bay | Section 1003.4.2.2: 6 cu ft for the first 100 sq ft plus 1 cu ft per additional 100 sq ft | 6 + (1,400 / 100) x 1 | 20 cubic feet |
| Oil separator depth and seal | Section 1003.4.2.1 | Fixed minimums | 2 feet below discharge invert, 18-inch outlet water seal |

## Where can you explore related guidance?

- [Explore all design standards guidance](https://harth.build/resources/design-standards.md)
- [Minimum Plumbing Fixture Requirements by Occupancy (IPC Table 403.1)](https://harth.build/resources/design-standards/plumbing-fixture-requirements.md)
- [Storm Drainage Sizing: Roof Drains, Leaders and Secondary Drains (IPC Chapter 11)](https://harth.build/resources/design-standards/storm-drainage-sizing.md)
- [IBC Occupancy Classifications: Groups A, B, E, F, H, I, M, R, S & U](https://harth.build/resources/design-standards/ibc-occupancy-classifications-explained.md)
- [Trap: Building Code Definition](https://harth.build/resources/glossary/trap.md)
- [Trap Seal: Building Code Definition](https://harth.build/resources/glossary/trap-seal.md)
- [Vent: Building Code Definition](https://harth.build/resources/glossary/vent.md)

## Sources and method

Trap, interceptor and separator rules are taken from the [2021 International Plumbing Code, Chapter 10, Traps, Interceptors and Separators](https://codes.iccsafe.org/content/IPC2021P2/chapter-10-traps-interceptors-and-separators); trap sizes and drainage fixture units from [Chapter 7, Sanitary Drainage](https://codes.iccsafe.org/content/IPC2021P2/chapter-7-sanitary-drainage) (Table 709.1); and trap-to-vent distances from [Chapter 9, Vents](https://codes.iccsafe.org/content/IPC2021P2/chapter-9-vents) (Table 909.1). Dwelling trap rules come from [2021 International Residential Code Chapter 32, Traps](https://codes.iccsafe.org/content/IRC2021P2/chapter-32-traps). The separator worked examples are calculated by Harth from those sections. These are typical model-code values; verify every size, distance and capacity against the edition adopted by the authority having jurisdiction, local amendments and any sewer-authority pretreatment rules for grease interceptors.

### Source links

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IPC2021P2/chapter-10-traps-interceptors-and-separators)

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IPC2021P2/chapter-7-sanitary-drainage)

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IPC2021P2/chapter-9-vents)

- [codes.iccsafe.org](https://codes.iccsafe.org/content/IRC2021P2/chapter-32-traps)

## Frequently asked questions

### What is the required trap seal depth?

Each fixture trap must hold a liquid seal of not less than 2 inches and not more than 4 inches under IPC Section 1002.4 and IRC Section P3201.2. Deeper seals are permitted only for special designs serving accessible fixtures. Seals that can evaporate, such as floor drains, need a trap primer or barrier-type device.

### How far can a trap be from the fixture?

IPC Section 1002.1 limits the vertical distance from the fixture outlet to the trap weir to 24 inches and the horizontal distance between the outlet and trap inlet centerlines to 30 inches. Separately, Table 909.1 limits the trap arm from trap weir to vent, for example 6 feet for a 1-1/2 inch trap at 1/4 inch per foot.

### Why are S traps not allowed?

An S trap drops vertically after the bend, so the moving column of water can siphon the seal out of the trap and leave an open path for sewer gas. IPC Section 1002.3 and IRC Section P3201.5 prohibit S traps, along with bell traps, drum traps, crown-vented traps and traps that rely on moving parts.

### When does a restaurant need a grease interceptor?

IPC Section 1003.3.1 requires a grease interceptor or automatic grease removal device for grease-laden fixtures in food preparation areas, including pot sinks, prerinse sinks, soup kettles, wok stations, kettle floor drains, hood wash units and dishwashers without prerinse sinks. Food waste disposers may not discharge to it, and dwelling units are exempt.

### How big does a gravity grease interceptor need to be?

Under IPC Section 1003.3.7, multiply the peak drain flow into the interceptor in gallons per minute by a 30-minute retention time. A kitchen with a 50 gpm peak flow needs a 1,500-gallon gravity interceptor designed and tested to IAPMO/ANSI Z1001. Local sewer authorities may set their own pretreatment sizing, so check local rules too.

### Can one trap serve two sinks?

Under the IRC, one trap may serve two or three adjacent kitchen sinks, laundry tubs or lavatories in the same room, with outlets not more than 30 inches apart (P3201.6). The IPC allows one trap for a combination fixture where one compartment is not more than 6 inches deeper than the other and the outlets are not more than 30 inches apart.

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