Fire Department Connection sizing
The Fire Department
Connection (FDC) must be sized with one 2½-inch inlet per 250-gpm required to
supply the hydraulically most demanding system’s requirements. An alternate
number of FDC inlets is acceptable provided hydraulic calculations are submitted
indicating that with the friction losses (hose, fittings, valves, etc.) through
this alternate number of FDC inlets, for the required pressure and quantity of
water of the hydraulically most demanding system’s demand, does not exceed the
pressure typically provided by a Fire Department Pumper Truck (approximately
155psi). Provided calculations are submitted which indicate the alternate number
of FDC inlets will provide the system requirements based upon the pressure
typically provided by a Fire Department Pumper Truck, the proposed arrangement
would be acceptable.
FL=0.80(q^2)L [that's 0.80 times q
squared times L]
Where FL=friction loss (psi), q= flow divided by 100 (gpm),
L=hose length divided by 100-ft.
For a required flow of 1850gpm, assuming a
hose length of 100-ft (typical), the loss in each hose to a four way FDC is
calculated to be 17.11psi. The friction loss from the FDC to the base of the
riser is calculated as follows:
18-inches of piping
1 check valve
Total Eq. Length – 19.55-ft
36-inches of piping length
Total Eq. Length – 83.76-ft
A C-Factor of 100 is utilized based
upon the piping being dry pipe up to the check valve. Resulting friction loss
18.41psi thru the 2½-inch FDC inlets plus 10.25psi friction loss thru the 6-inch
FDC manifold and feed to the base of the riser. Combining all calculated
friction loss results in 45.77psi leaving a balance of 109.23psi available for
operation of the sprinkler system which exceed the base of riser sprinkler/hose
demand [i.e. the 155psi pumper truck availble pressure minus the 45.77psi losses
= 109.23 psi available pressure that can satisfy the sprinkler system pressure
demand as long as that sprinkler demand is less than the 109.23psi].
13, 14, and 24 all indicate the purpose of the FDC is to provide a connection
whereby the Fire Department pumper truck(s) can provide an
auxiliary/supplemental water supply for fire protection. The degree of
supplementation is not specifically indicated. Supplementation is a requirement
that can vary – 25%, 50%, 90%, etc., with 100% being the ‘worst case scenario’.
The requirements give a minimum size for the FDC, but do not give any info on
when a 2-way, 3-way, 4-way, or 6-way connection should be selected. An article
written by AFSA noted a light and ordinary hazard building could use the minimum
size, but a storage building would justify using a larger FDC. Manufacturer’s
listing provide info that the FDC should be sized based on 250gpm per inlet.
NFPA 14 (2003), Section 188.8.131.52.3 indicates the maximum demand for any
standpipe system to be 1250gpm, and since a 6-way FDC is designed for a flow of
1500gpm, this would not explain away the reason for such a device as being for
Back to supplementing the flow and pressure, if a water
supply had a static pressure of 90psi and residual of 80 flowing say 750gpm,
when a pumper truck connects-up downstream of the alarm check, backflow, etc.,
and boosts the pressure to say 155psi, something interesting happens. Since the
pressure downstream of the alarm or backflow check valve (FDC side) exceeds the
pressure on the upstream side (city water supply), this forces the check valve
to close. The pumper is now ‘supplementing’ most, if not all, of the
pressure/flow to the system. There has been instances where a small FDC (i.e.
Yard Siamese with 4-inch supply main running hundreds of feet) was used and
proved to inadequately supplement high challenge fires resulting in extensive
loss. NFPA 1142 (2001), Section F.4 indicates one of the uses of an FDC is in
case the valve controlling the water supply to the sprinkler riser were shut
off, the demand of the sprinklers can still be provided thru the pumper.
Frequently, the valve controlling the sprinkler system is closed by the Fire
Department once the pumper is connected to ensure a malfunctioning check valve
between the system and the water supply (i.e. alarm valve) does not rob the
system of the pumper’s pressure.
Closed valves (inaccessible or hidden);
seized-up, improperly maintained, or worn-out Fire Pumps; poorly maintained
private service mains (debris/corrosion in underground pipe, etc.) are but a few
of the reasons the FDC’s supplementary role can change from augmenting a little
pressure to providing almost the total fire protection demand.
is to explain the concerns for facilities using the minimum sized FDC ~ the
minimum size indicated in the codes may not be sufficient for all applications
(especially where high-challenge fire may occur such as high-piled storage and
other extra hazard occupancies where the sprinkler demand is relatively large).
The 2003 International Fire Code (IFC), Section 104.1, authorizes the Authority
Having Jurisdiction (AHJ) to render interpretations to the Fire Codes, such as
when a minimum requirement is insufficient for a particular liability.