October 22, 2026

NFPA 13 Equivalent Lengths for Valves: A Worked 4-Inch Riser Example

In an NFPA 13 hydraulic calculation, valves and fittings are counted as extra pipe length. The standard tabulates them in feet of Schedule 40 steel pipe at a Hazen-Williams C of 120; the designer corrects those values to the actual pipe, adds them to the real length and applies the pipe’s friction loss per foot. At 4 in., the table counts an OS&Y gate valve as 2 ft, a butterfly valve as 12 ft and a swing check valve as 22 ft. Alarm check, dry pipe and deluge valves, backflow preventers and pressure-reducing valves are not in that table; their losses come from the manufacturer’s data.

Valves are also the part of the calc that changes when one is substituted during procurement.

How the calculation treats a valve

NFPA 13 uses the Hazen-Williams formula. In US units:

p = 4.52 × Q^1.85 / (C^1.85 × d^4.87)

Here p is the loss in psi per foot, Q the flow in gpm and d the actual inside diameter in inches. The SI form is p (bar/m) = 6.05 × 10⁵ × Q^1.85 / (C^1.85 × d^4.87), with Q in L/min and d in mm. For black steel, C is 120 in wet and deluge systems and 100 in dry and preaction systems; copper tube, stainless steel and listed plastic pipe use 150. EN 12845 projects in the UK and EMEA use the same formula with that standard’s own C values.

A tee or cross is charged only where the flow turns 90°; straight-through flow carries no fitting loss. Under the 2019 and 2022 editions, flow switches in pipe 2 in. or smaller and strainers must also be included.

Where each valve’s number comes from

Device	Loss value used in the calc	4 in. table value
OS&Y gate valve	NFPA 13 equivalent length table	2 ft
Butterfly valve	NFPA 13 table, or the valve’s own tested data	12 ft
Swing check valve	NFPA 13 table, or the valve’s own tested data	22 ft
Alarm check, dry pipe or deluge valve	Manufacturer’s published loss data	Not in table
Backflow preventer (DCDA, RPDA)	Manufacturer’s flow-loss curve at the design flow	Not in table
Pressure-reducing valve	Manufacturer’s loss data	Not in table

Where the designer used a specific valve’s tested data instead of the table, that exact valve has to be installed.

Worked example: 4-inch wet riser at 400 gpm

A wet-system riser in 4-inch Schedule 10 black steel (inside diameter 4.260 in., C = 120) carries 400 gpm and rises 35 ft. It holds the system’s butterfly control valve and a swing check valve; elbows and tees at either end are left out to keep the focus on the valves.

Unit friction loss: p = 4.52 × 400^1.85 / (120^1.85 × 4.260^4.87) = 4.52 × 65,134 / (7,022 × 1,162) = 0.0361 psi/ft (0.0082 bar/m).
Table lengths: butterfly 12 ft + swing check 22 ft = 34 ft.
Diameter correction: the table assumes Schedule 40 pipe with a 4.026 in. bore. Factor = (4.260 / 4.026)^4.87 = 1.317, so 34 ft becomes 44.8 ft.
Total length: 35 ft of pipe + 44.8 ft = 79.8 ft.
Friction loss: 79.8 ft × 0.0361 psi/ft = 2.9 psi (0.20 bar). The valves account for 1.6 psi, the pipe for 1.3 psi.
Elevation: 35 ft × 0.433 psi/ft = 15.2 psi (1.04 bar), bringing the riser total to 18.0 psi.

Elevation dominates, at more than five times the friction loss. On a tight supply the valve choice still counts: an OS&Y gate valve instead of the butterfly removes 10 ft of table length (13.2 ft corrected), about 0.5 psi or a sixth of the riser’s friction.

Corrections that are easy to miss

Situation	Multiply table values by	Effect on fitting and valve loss if skipped
4-inch Schedule 10 pipe instead of Schedule 40	(actual ID / Schedule 40 ID)^4.87 = 1.317	Understated by 24% (0.4 psi in the example)
Dry or preaction black steel, C = 100	0.713	Overstated by about 40%
Copper tube or stainless steel, C = 150	1.51	Understated by about one third

The C multiplier holds a fitting’s pressure loss constant when pipe roughness changes, so rough dry-system pipe gets shorter fitting lengths and smooth copper gets longer ones. Only the dry-system omission errs on the safe side.

Canadian projects: check the edition before the numbers

The National Building Code of Canada 2020 references NFPA 13-2019 for sprinkler systems (Division B, Table 1.3.1.2), and the Canadian Automatic Sprinkler Association notes that the 2025 NBC still references the 2019 edition. Provinces adopt the NBC with their own amendments. Two differences follow:

In the 2019 edition the calculation rules sit in Chapter 27, Plans and Calculations; the 2022 edition moved them to Chapter 28 and the 2025 edition renumbered them again, so clause numbers in a calc prepared to a newer US edition will not match.
The 2022 edition allowed C = 120 in new dry systems of black or galvanized steel supplied with nitrogen from a listed generator (summary of the change), and the 2025 edition extended the allowance to vacuum and vapor corrosion inhibitor systems. Under the 2019 edition, dry and preaction steel is calculated at C = 100 unless the authority having jurisdiction accepts another value, so a template carried over from a US project needs checking before submission.

Procurement: when a valve change reopens the calc

Replacing an OS&Y gate valve with a butterfly valve, or swapping one alarm check valve or backflow preventer model for another, changes the loss the calc was built on. Before a substitute is released, send the designer its data sheet with the published loss values and the UL/ULC listing or FM approval, and expect a revised calculation if the margin moves. See also our notes on NFPA 13 butterfly valve requirements, alarm check valves and DCDA and RPDA backflow preventers.

Checklist before the calc goes out

Actual inside diameter used for each schedule, with fitting values corrected to it.
C value and C multiplier match the system type and the adopted NFPA 13 edition.
Tees charged only where the flow turns 90°.
Valves outside the table taken from manufacturer’s data, with data sheets attached.
Strainers and flow switches included as the adopted edition requires.
Every valve on the purchase order matches the valve in the calc.