11.2 Fire Department Connections (FDC) Arrangement & Sizing
Key Takeaways
- The Fire Department Connection (FDC) serves as an auxiliary water supply inlet allowing fire department pumper apparatus to inject supplemental water and pressure into automatic sprinkler and standpipe networks.
- FDC placement must be on the street side of the building, highly visible, unobstructed, and positioned between a minimum of 18 inches and a maximum of 48 inches above finished grade.
- NFPA 13, 14, and 24 mandate raised or cast signage with minimum 1-inch-high letters indicating the system type (e.g., 'AUTOSPRINK', 'STANDPIPE'), the building zone served, and the required design demand pressure if it exceeds 150 psi.
- FDC sizing requires a minimum of one 2.5-inch internal threaded inlet per 250 gpm of total system demand, or listed 4-inch or 5-inch Storz quick-connect couplings with 30-degree downward angles and locking mechanisms.
- The FDC piping assembly requires a listed check valve to prevent system backflow, an automatic 3/4-inch ball drip drain valve installed at the low point between the outside connection and check valve to prevent freeze rupture, and must connect on the system side of the main water supply control valve.
Last updated: August 2026
Fire Department Connections (FDC) Arrangement & Sizing
A Fire Department Connection (FDC)—commonly referred to in field practice as a Siamese connection—is an essential auxiliary component of water-based fire protection systems. Governed by NFPA 13 (sprinklers), NFPA 14 (standpipes), and NFPA 24 (private fire service mains), the FDC enables responding fire department pumper apparatus to pump high-pressure, high-volume water directly into building sprinkler and standpipe piping.
The FDC does not replace the permanent municipal or private water supply; rather, it acts as a critical redundancy that supplements water volume, overcomes municipal pressure drops during massive municipal draft, and provides emergency pressurization if an on-site fire pump fails.
Spatial Location & Mounting Height Criteria
Proper architectural placement and physical access to the FDC are paramount for rapid fire department deployment.
+---------------------------------------------------------------------------------------------------------+
| FDC LOCATION & PHYSICAL PARAMETERS |
+-----------------------+---------------------------------------------------------------------------------+
| Street Orientation | Must be located on the street side of the building, clearly visible from |
| | approaching fire apparatus, with unobstructed access. |
+-----------------------+---------------------------------------------------------------------------------+
| Mounting Height | Centerline of hose inlets must be between MINIMUM 18 inches (457 mm) and |
| | MAXIMUM 48 inches (1,219 mm) above finished grade or pad. |
+-----------------------+---------------------------------------------------------------------------------+
| Hydrant Proximity | Typically located within 100 feet (30.5 m) of an approved fire hydrant to allow |
| | short 5-inch supply hose lay from hydrant to pumper to FDC. |
+-----------------------+---------------------------------------------------------------------------------+
| Clearance Envelope | Minimum 36-inch (914 mm) radial clearance maintained around the FDC, free of |
| | shrubs, landscaping, fences, architectural posts, or bollards. |
+-----------------------+---------------------------------------------------------------------------------+
+-------------------------------------------------------------------------+
| FDC MOUNTING HEIGHT BOUNDARIES |
+-------------------------------------------------------------------------+
Exterior Building Wall
|
| +-----------------------------+
| | FDC SIGN (1" Letters) |
| +-----------------------------+
| |
| +--------+--------+
| | [2.5"] [2.5"] | <--- Inlets w/ Drop Clappers
| +--------+--------+
| |
| | --- MAX: 48 Inches (1.22 m)
| | |
| | | Permissible Inlet
| | | Centerline Zone
| | |
| | --- MIN: 18 Inches (0.46 m)
=======+======================+============================================
| Finished Grade / Ground Level
Signage & System Identification Requirements
Firefighters arriving at a burning building must immediately identify what system the FDC supplies, what pressure to pump, and which section of a sprawling facility is connected.
+---------------------------------------------------------------------------------------------------------+
| FDC SIGNAGE SPECIFICATIONS |
+-----------------------+---------------------------------------------------------------------------------+
| Letter Height | Minimum 1-inch (25 mm) high raised, cast, or engraved letters on a durable |
| | weatherproof metal plate or sign. |
+-----------------------+---------------------------------------------------------------------------------+
| System Designation | Must state system service: |
| | - 'AUTOSPRINK' (Automatic Sprinkler System) |
| | - 'STANDPIPE' (Class I/II/III Standpipe System) |
| | - 'AUTOSPRINK & STANDPIPE' (Combined System) |
| | - 'MANUAL STANDPIPE' (Manual System requiring pumper pressure) |
| | - 'DRY STANDPIPE' (Dry Standpipe System) |
+-----------------------+---------------------------------------------------------------------------------+
| Pressure Designation | Where the system design demand pressure exceeds 150 psi (10.3 bar), the sign |
| | must explicitly state the REQUIRED DESIGN PUMPING PRESSURE (e.g., '175 PSI'). |
+-----------------------+---------------------------------------------------------------------------------+
| Building / Zone ID | If the FDC serves only a specific portion of a building or a multi-zone high- |
| | rise (e.g., 'LOW ZONE - FLOORS 1-15'), the coverage boundary must be indicated. |
+-----------------------+---------------------------------------------------------------------------------+
Sizing Criteria & Inlet Configurations
NFPA 13 and NFPA 14 dictate the number of hose inlets and the internal pipe diameter based on total system flow demand.
+---------------------------------------------------------------------------------------------------------+
| FDC INLET & PIPE SIZING MATRIX |
+-----------------------+-----------------------+-----------------------+---------------------------------+
| System Demand Flow | 2.5-Inch Inlets Req'd | Alternative Storz | Minimum Connecting Pipe Size |
+-----------------------+-----------------------+-----------------------+---------------------------------+
| Up to 500 gpm | Two (2) 2.5" Inlets | Single 4" or 5" Storz | 4-inch (100 mm) nominal pipe |
| 750 gpm | Three (3) 2.5" Inlets | Single 5" Storz | 6-inch (150 mm) nominal pipe |
| 1,000 gpm | Four (4) 2.5" Inlets | Dual 4" or 5" Storz | 6-inch (150 mm) nominal pipe |
| 1,250+ gpm | Six (6) 2.5" Inlets | Dual 5" Storz | 8-inch (200 mm) nominal pipe |
+-----------------------+-----------------------+-----------------------+---------------------------------+
+---------------------------------------------------------------------------------------------------------+
| INLET SIZING CALCULATION PRINCIPLE |
+---------------------------------------------------------------------------------------------------------+
| Standard Rule: |
| Number of 2.5" Inlets = Total System Demand (gpm) / 250 gpm per inlet (rounded UP to next integer) |
| |
| Minimum Mandate (NFPA 13 & 14): |
| Every standpipe system and every sprinkler system with more than 20 sprinklers MUST have at least |
| ONE FDC with a MINIMUM OF TWO 2.5-inch female inlets (or one listed large-diameter Storz coupling). |
+---------------------------------------------------------------------------------------------------------+
1. Standard 2.5-Inch Threaded Inlets (Siamese Body)
- Features internal National Hose (NH/NST) female swivel couplings with rocker lugs or pin lugs.
- Each 2.5-inch inlet is equipped with an internal clapper (swing check gate) that automatically swings closed over an unused inlet if only a single hose line is initially connected and pressurized. This prevents pumped water from blowing back out into the operator's face.
2. Large Diameter Hose (LDH) / Storz Connections
- Many modern fire departments utilize 4-inch (100 mm) or 5-inch (125 mm) Storz quick-connect couplings.
- Storz connections are sexless (quarter-turn lock) and eliminate thread mismatch problems.
- Must feature a 30-degree downward angled snout to prevent hose kinking, a locking latch mechanism, and a listed aluminum blind cap with a steel security chain.
Piping Arrangement, Check Valves & Automatic Ball Drips
The piping connecting the exterior FDC to the interior sprinkler or standpipe riser must be engineered with specific trim appurtenances to ensure hydraulic integrity and freeze protection.
+---------------------------------------------------------------------------------------------------------+
| FDC PIPING TRIM APPURTENANCES |
+-----------------------+---------------------------------------------------------------------------------+
| Listed Check Valve | A swing check valve installed in the FDC supply line to prevent water from |
| | the municipal main or fire pump from backflowing out through the open FDC. |
+-----------------------+---------------------------------------------------------------------------------+
| Automatic Ball Drip | A minimum 3/4-inch (20 mm) listed automatic ball drip drain valve installed at |
| Valve (Velocity Drip) | the lowest point of the dry piping between the check valve and the exterior FDC.|
+-----------------------+---------------------------------------------------------------------------------+
| Piping Pitch | All horizontal pipe between the exterior FDC and the check valve must pitch |
| | downward toward the automatic ball drip valve (minimum 1/4" per 10 ft). |
+-----------------------+---------------------------------------------------------------------------------+
| Tie-In Location | The FDC line must connect on the SYSTEM SIDE of the main water supply control |
| | valve and backflow preventer. |
+-----------------------+---------------------------------------------------------------------------------+
[ FDC PIPING TRIM ARCHITECTURE ]
Exterior Building Wall Interior Heated Building Space
=================================================================================
|
[ 2.5" Siamese Inlets ]
|
| (Exposed to Freezing Outside Air)
|
v
+------+----------------------------------------------------+
| Dry Piping Section (Pitched Downward to Drain) |
+------+----------------------------------------------------+
| |
v v
[ Auto Ball Drip Valve ] [ LISTED CHECK VALVE ]
(Open @ 0 psi: Drains Water) (Flow TOWARD Riser Only)
(Closes @ 5-10 psi when Pumping) |
| v
(Drain Cup to Waste) +-------------+-------------+
| Sprinkler / Standpipe Riser |
+-------------+-------------+
|
[ Main Water Supply Control ]
|
[ From City Main / Pump ]
Operational Physics of the Automatic Ball Drip Drain Valve
- Static / Standby State: The piping between the exterior FDC inlets and the interior check valve contains air. Small amounts of rainwater, melting snow, or seepage past the check valve clapper enter this piping. An internal brass ball inside the automatic ball drip rests at the bottom of its housing, leaving an open drainage orifice (minimum 3/4-inch diameter). Liquid drains by gravity into an open drain receiver or floor drain, keeping the pipe 100% dry.
- Freezing Protection: If water were allowed to accumulate in this pipe section, sub-freezing winter temperatures would freeze the trapped water column, splitting the cast-iron/steel pipe and rendering the FDC completely inoperable during a fire.
- Active Fire Pumping State: When the fire department hooks up and begins pumping water at high pressure (100 to 200+ psi), the sudden rush and velocity of water forces the internal brass ball forward onto a machined sealing seat. This automatically seals the drain orifice, preventing pumped fire water from discharging onto the floor. When pumping ceases and pressure drops to near atmospheric, the ball drops away from the seat, once again draining any residual water.
CRITICAL CODE MANDATE (NFPA 13/14/24): No control valve of any kind (shutoff valve, OS&Y, butterfly, or ball valve) shall ever be installed in the piping between the exterior Fire Department Connection and the check valve. The FDC supply line must remain permanently open and unobstructed at all times.
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Test Your Knowledge
What are the minimum and maximum allowable mounting heights for the centerline of Fire Department Connection (FDC) inlets above finished grade per NFPA 13, 14, and 24?
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Test Your Knowledge
A standpipe system in a commercial facility has a total calculated hydraulic demand of 750 gpm. According to standard NFPA 14 sizing rules, what is the minimum number of 2.5-inch threaded inlets required on the FDC, and what is the minimum connecting pipe diameter?
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D
Test Your Knowledge
What is the primary operational purpose of installing an automatic ball drip drain valve on the FDC piping trim between the check valve and the outside connection?
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B
C
D
Test Your Knowledge
Where must the FDC supply piping connect into a fire sprinkler or standpipe system relative to the main water supply control valve?
A
B
C
D