The calculation that sizes the pipes, the pump and the tank of a sprinkler installation starts from one question: how is the pipework arranged? In a tree layout water reaches each sprinkler by a single route; in a loop or a grid by more than one. That difference decides where the design area goes, which system types are allowed, whether a pipe-table method may be used and which checks have to be shown.
This article treats NFPA 13-2025 and TS EN 12845+A2 (EN 12845:2015+A2:2026), together with TS EN 12845-2 for ESFR and CMSA, separately. Every rule carries its clause, table or figure number, and nothing is stated that the standards do not say. Where one standard is silent on a point, we say so.
Three layouts: tree, loop, grid
| Layout | NFPA 13-2025 | EN 12845 | Route to a sprinkler |
|---|---|---|---|
| Tree (terminal) | No definition clause; "tree" appears only in explanatory annex material (A.28.2.4.2.3.1, Figure A.28.2.4, Annex B.2.1.1) | Terminal main configuration (3.80): one supply route to each range pipe. Terminal range configuration (3.81): one supply route from a distribution pipe | One route |
| Loop | Looped sprinkler system (3.3.224.6, Figure A.3.3.224.6): several cross mains tied together, branch lines not tied together | Looped configuration (3.40): more than one distribution pipe route along which water can reach a range pipe | Several routes to the branch line, one within it |
| Grid | Gridded sprinkler system (3.3.224.5, Figure A.3.3.224.5): parallel cross mains joined by multiple branch lines; an operating sprinkler is fed from both ends of its branch line | Gridded configuration (3.30): pipe array in which water flows to each sprinkler by more than one route | From both ends |
EN 12845 never uses the word "tree"; its terms are "terminal" and "branched" (3.22, 3.47). EN 12845-2 makes the link itself: "terminal dry-pipe (tree type)" (6.7.2). An array with range pipes on both sides of a distribution pipe is end-centre, with ranges on one side only end-side (3.25, 3.26). NFPA 13 Annex B, which is not part of the requirements, notes that multiple loops and grids keep line losses down at wide sprinkler spacing, while older tree-type configurations cannot benefit from multiple-path flow (B.2.1.1).
Which system type may use which layout?
Both standards tie the prohibitions to the system type, not to the sprinkler. In the tables below ⛔ means prohibited, ⚠ conditional and ✓ no prohibition in the text.
NFPA 13-2025
| System type | Tree | Loop | Grid | Basis |
|---|---|---|---|---|
| Wet pipe | ✓ | ✓ | ✓ | No prohibition |
| Dry pipe | ✓ | ✓ | ⛔ | 8.2.1: gridded dry pipe systems shall not be installed (no exception in the clause) |
| Double interlock preaction | ✓ | ✓ | ⛔ | 8.3.2.7.1 |
| Single and non-interlock preaction | ✓ | ✓ | ⚠ | 8.3.2.7.2: not gridded where protecting high-piled storage |
| Deluge | ✓ | ✓ | ✓ | No layout rule; deluge systems are hydraulically calculated (8.3.3.2) |
| Antifreeze | ✓ | ✓ | ✓ | No layout rule; above 150 L (40 gal) friction is also calculated by Darcy–Weisbach (28.2.2.1.3) |
NFPA 13 has no clause prohibiting a loop in a dry or preaction system; the prohibition is for grids only. The "circulating closed loop" ban in Chapter 31 (31.3.4) concerns a different system type, not looped piping. On gridded systems all branch lines are arranged to facilitate flushing (16.6.4).
EN 12845 and EN 12845-2
| System type | Tree | Loop | Grid | Basis |
|---|---|---|---|---|
| Wet | ✓ | ✓ | ✓ | 11.1.1 |
| Dry, alternate, pre-action | ✓ | ⛔ | ⛔ | 11.1.1: grid and loop in wet pipe installations only |
| Deluge | Not covered | Table 3 note | ||
| Dry CMSA, 141 °C upright standard response | ✓ | ⛔ | ⛔ | EN 12845-2 6.7.2: "terminal dry-pipe (tree type)" |
| ESFR with propylene glycol | ✓ | ⛔ | ⛔ | EN 12845-2 6.5: a tree pipework layout shall be used |
⛔ Grids on dry systems. A gridded dry pipe installation is prohibited in both standards: NFPA 13 8.2.1, EN 12845 11.1.1. EN 12845 extends the same ban to loops; NFPA 13 does not. Under EN 12845 a dry installation is in any case installed only where there is a risk of freezing or where temperatures exceed 70 °C (11.2.1).
Which sprinkler on which system?
NFPA 13 contains no restriction linking ESFR, CMSA, in-rack, quick-response, extended-coverage, residential or dry sprinklers to a tree, loop or grid. Its restrictions run through the system type. The EN 12845 main text likewise has no layout-dependent sprinkler restriction; the two layout-dependent rules sit in EN 12845-2.
NFPA 13-2025
| Sprinkler | Rule | Clause |
|---|---|---|
| ESFR | ⚠ Wet pipe systems only, unless specifically listed for dry or preaction systems | 14.2.2 |
| ESFR with antifreeze | ⛔ Not used unless the sprinkler is listed with that antifreeze solution | 8.6.1.2 |
| CMSA | ✓ Wet, dry or preaction systems, in accordance with the listing | 13.2.1.3 |
| Residential | ⚠ Wet systems only, unless listed for dry or preaction systems | 12.1.3, 8.2.2.1.2 |
| Orientation on dry and preaction systems | ⚠ Upright; listed dry sprinklers; pendent and sidewall on return bends in heated areas; horizontal sidewalls that do not trap water | 8.2.2.1.1, 8.3.2.6 |
| Dry-type sprinklers in the calculation | The manufacturer's adjusted K-factor is used | 28.2.4.10.3 |
| Any sprinkler other than K 5.6 | ⛔ Not permitted on a pipe schedule system | 28.5.1.2(1) |
EN 12845 and EN 12845-2
| Sprinkler | Rule | Clause |
|---|---|---|
| Quick response | ⛔ Not used on dry systems or Type A pre-action systems | Table 38 |
| Ceiling, flush, recessed, concealed | ⛔ Not installed in OH4, HHP or HHS areas | 14.2.2 |
| Sidewall | ⛔ Not in HH installations (except corridors, cable ducts and column protection), OH storage areas or above suspended ceilings; flat ceilings only | 14.2.3 |
| Flat spray | ⚠ Only in concealed spaces, above suspended open ceilings and in racks | 14.2.4 |
| ESFR | ⚠ Wet installations only; no dry option (an installation with dry pendent ESFR still counts as wet) | EN 12845-2 clause 4, 6.5 |
| CMSA | ✓ Wet and dry; on dry CMSA systems only dry pendent and upright sprinklers | EN 12845-2 clause 4, 5.1.1 |
| In-rack (ESFR in-rack designs) | ⚠ Wet only; modular in-rack designs are also wet only | EN 12845-2 6.6.2.1.1 c), 6.6.2.2.2 |
EN 12845 also limits the sprinkler by hazard class (Table 37a): K 57 in LH; K 80 or K 115 in OH; for HHP and HHS ceilings K 80, 115 or 160 up to 10 mm/min and K 115 or 160 above it; K 80 or K 115 for intermediate (in-rack) sprinklers.
Where does the design area go?
The most important decision in the calculation is where the design area (area of operation in EN) sits. The two standards think differently here: NFPA 13 looks for a single "most demanding" area, EN 12845 asks for both the most unfavourable and the most favourable area.
NFPA 13: the most demanding area
- Position: for all systems the design area is the hydraulically most demanding (28.2.4.1). The most distant area is not necessarily the hydraulically most remote; where comparison does not settle it, additional calculations are submitted (A.28.2.4.7). Calculations begin at the hydraulically most remote sprinkler (28.2.4.7.6).
- Shape: under the density/area method the area is rectangular, with the side parallel to the branch lines at least 1.2√A and able to include sprinklers on both sides of the cross main (28.2.4.2.1). Fractional sprinklers are rounded up (28.2.4.2.2).
- Tree and loop: in most tree and loop systems the most demanding sprinklers are those closest to the cross main, and a sprinkler left over from a full branch line goes to the position closest to the cross main (A.28.2.4.2.3.1, Figure A.28.2.4 Note 2).
- Grid: the extra sprinkler can sit anywhere along the adjacent branch line within the 1.2 length (A.28.2.4.2.3.1). The designer verifies that the most demanding area is being used (28.2.4.5.1), and at least two additional sets of calculations compare it with the areas immediately on either side along the same branch lines (28.2.4.5.2). A single set is accepted where the program shows the peaking of the demand area (28.2.4.5.3). The calculation output includes a diagram of flow quantities and directions for the lines in the remote area (28.4.2.6).
NFPA 13's worked example (Figure A.28.2.4): for a 139 m² (1500 ft²) design area at 11.1 m² (120 ft²) per sprinkler, 1500/120 = 12.5, so 13 sprinklers; along the branch line 1.2√1500/12 = 3.87, so 4 sprinklers.
EN 12845: most unfavourable and most favourable
- Most unfavourable area (3.11): the position at which the supply pressure measured at the control valve set is highest for the design density. Variations in spacing, layout, elevation, range centres, orifice and pipe sizes are considered, and every possible position on the distribution pipes is examined (13.4.2.1).
- Proof in a grid: the area is displaced by one sprinkler pitch in each direction along the range pipes until the position with the highest pressure requirement is found. In a loop the same rule is applied along the distribution pipe (13.4.2.1).
- Shape (13.4.3.1): as near as possible rectangular and symmetrical to the sprinkler layout. In terminal and looped layouts the range sets the remote side, and sprinklers that do not make a full range are grouped close to the distribution pipe (a). In grids the remote side parallel to the ranges is L ≥ 1.2√A (c), or L ≥ 2√A where the ranges run parallel to the ridge of a roof steeper than 6° or along bays formed by beams deeper than 1.0 m (b).
- Most favourable area (3.10): the position at which flow is greatest for a given pressure at the control valve set. The area is as near as possible square, in the hydraulically most favourable position (13.4.3.2, Figures 23–26).
In EN 12845 the 1.2√A and 2√A rules are written for grids only; in terminal and looped layouts the range defines the remote side.
Why calculate the most favourable area?
When an area close to the supply, on short pipe runs, opens, the system draws far more water than the design figure. EN 12845 calls this flow Qmax: the flow where the pressure-flow demand curve of the most favourable area meets the water supply pressure-flow curve, with the suction source at its normal level (3.42). It drives several decisions:
- Pump: in a fully calculated system the pump is rated on the maximum pressure and flow required for the most favourable area (Table 14, 10.7.3, Figure 7).
- Velocity: the limits (6 m/s through valves, flow monitoring devices and strainers, 10 m/s elsewhere) apply in stabilised flow for both the most favourable and most unfavourable areas (13.2.3).
- Tank: in a calculated system the effective tank capacity is Qmax × duration, with 30 min for LH, 60 min for OH and 90 min for HHP and HHS (9.3.2.3).
- Flow meter and valve sign: the flow measuring device covers at least 140 % of the maximum flow demand (8.5.1 d). The pressure-flow requirement of both areas is written on the control valve sign (18.2.3.2), and both areas are shown on the drawings (4.4.3.3).
NFPA 13 has no equivalent of the most favourable area check or of Qmax. EN 12845-2 does not require the favourable area for ESFR and CMSA, and does not allow pipe diameters to be reduced to limit the favourable area demand (6.4).
Calculation parameters side by side
| Item | NFPA 13-2025 | EN 12845 |
|---|---|---|
| Friction formula | Hazen–Williams; SI: p = 6.05 × 10⁵ Q^1.85 / (C^1.85 d^4.87) bar/m (28.2.2.1.2) | Hazen–Williams in the same form (13.2.1); velocity pressure may be ignored |
| C value, steel | Wet black and galvanised steel 120; dry (including preaction) 100; 120 with nitrogen, vacuum or a vapour corrosion inhibitor (Table 28.2.4.8.1) | Mild and galvanised steel 120 (Table 22) |
| C value, other | Plastic, copper, brass, stainless 150; cement-lined cast or ductile iron 140; unlined cast or ductile iron 100 | Cast iron 100; ductile iron 110; cement-lined cast iron 130; stainless, copper, glass fibre 140 |
| Velocity limit | None: velocity is not limited with Hazen–Williams or Darcy–Weisbach (28.2.1.4) | Valves, flow monitoring devices, strainers 6 m/s; elsewhere 10 m/s (13.2.3) |
| Junction balance | 0.03 bar (0.5 psi) (28.2.2.4.1) | ±1 mbar at junctions; algebraic sum of losses round any closed hydraulic loop 0 ± 1 mbar; flows at a junction 0 ± 0.1 l/min (13.2.5.2) |
| Minimum sprinkler pressure | 0.5 bar (7 psi), or the listing if higher (28.2.4.11) | LH 0.70; OH 0.35; HHP/HHS 0.50 bar; in-rack K 115 1.00, K 80 2.00 bar (13.4.4) |
| Area on dry systems | Dry and double interlock preaction: area +30 %, density unchanged (19.2.3.2.6) | Larger dry/alternate area from Table 3 (OH1 72 → 90 m²); LH not allowed dry, use OH1; OH4 not allowed dry, use HHP1 |
| Smallest pipe | Steel 25 mm (1 in.); copper, brass, stainless, listed non-metallic 20 mm (¾ in.) (28.2.1.2) | LH 20 mm; OH/HH single K ≤ 80 sprinkler pipe 20 mm; otherwise 25 mm (Table 36) |
NFPA 13 also bans orifice plates for balancing a system (28.2.4.9.1) and caps the working pressure at 12 bar (175 psi) for extra hazard systems and systems designed to Chapters 20–26 (28.2.4.12). In EN 12845 pipe diameters on the installation side of the control valve set may decrease only in the direction of flow, grids and loops excepted (13.4.5).
Pipe tables or full calculation?
EN 12845: pre-calculated and fully calculated
The designer may choose between pre-calculated systems, where diameters come partly from tables and partly from calculation, and fully calculated systems, where every diameter is calculated (13.1). A full calculation is always used for:
- ⛔ gridded or looped layouts (13.1)
- ⛔ layouts with intermediate level HHS sprinklers (13.1); in-rack sprinklers and their associated ceiling sprinklers (7.2.3.4)
In the pre-calculated method the pipework downstream of the design point is sized from the tables and upstream of it by calculation (3.20). The design point lies downstream of the 3rd or 4th sprinkler in LH (Table 25), at the connection of the range carrying the 17th or 19th sprinkler in OH and the 49th in HHP and HHS (Table 26). Pipe diameters may not increase in the direction of flow to any sprinkler (13.3.1). In OH the pipes between the design point and the control valve set are sized so that total friction loss at 1000 l/min does not exceed 0.5 bar (13.3.4.2).
NFPA 13: pipe schedule
Pipe schedules are used only in existing systems and in new systems or extensions described in Chapter 19 (28.5): new systems of 465 m² (5000 ft²) or less, or larger ones where the Table 19.2.2.1 flows are available with at least 3.4 bar (50 psi) residual at the highest sprinkler (19.2.2.3). The following shall not be pipe schedule systems (28.5.1.2):
- ⛔ systems using sprinklers other than nominal K 5.6
- ⛔ systems using listed pipe outside Table 7.3.1.1 or listed fittings outside Table 7.4.1
- ⛔ Extra Hazard Group 1 and Group 2 (28.5.4: extra hazard occupancies are hydraulically calculated)
- ⛔ exposure protection systems
Pipes to in-rack sprinklers are always sized by hydraulic calculation (28.2.5.1).
Where do in-rack sprinklers and hose streams enter the calculation?
| Item | NFPA 13-2025 | EN 12845 / EN 12845-2 |
|---|---|---|
| In-rack + ceiling | In-rack demand is added to the ceiling demand at the point of connection and balanced to the higher pressure (28.2.5.2, 25.1.10.5) | Simultaneous flow and pressure of ceiling and intermediate level sprinklers calculated together (13.4.1); 3 hydraulically most remote sprinklers per level, up to 3 levels (7.2.3.3) |
| ESFR in-rack (EN 12845-2) | — | In-rack demand added to the ceiling and the two balanced where they connect; in-rack sprinklers have a separate supply pipe (6.6.2.1.1, 6.6.2.1.2). Not added for modular designs (6.6.2.2.1) |
| Outside hose | Added at the connection to the city main or the hydrant closer to the system riser (19.1.6.2, 28.3) | No NFPA-style hose allowance table; the main must cover extra flow for manual firefighting such as hydrants and hose reels (9.2) |
| Inside hose | Added in 190 L/min (50 gpm) increments from the most remote hose connection, at the system pressure at that point (19.1.6.3) | — |
| End point of the calculation | The "effective point" where the supply characteristics are known (28.2.1.6) | The control valve set, pressure at the 'C' gauge (3.10, 3.11, 4.4.4.3 g) |
NFPA 13 total inside and outside hose allowance and duration for hydraulically calculated systems (Table 19.2.3.1.2): LH 380 L/min (100 gpm) for 30 min; OH 950 L/min (250 gpm) for 60 or 90 min; EH 1900 L/min (500 gpm) for 90 or 120 min. Inside hose is 0, 190 or 380 L/min in every class.
Supply curve against demand
- NFPA 13: the calculation is presented on Q^1.85 semi-exponential graph paper showing the supply curve, sprinkler demand, hose demand where applicable, in-rack demand and the pressure added by a fire pump (28.4.2.3). The supply analysis lists static and residual pressure at the source, total demand and the pressure needed at that demand (28.4.2.4). Flow test data, where adjustment is needed for daily and seasonal fluctuation, is adjusted before comparison with the demand (A.4.2).
- EN 12845: the pump set is chosen to meet the pressure and flow demand of the system (10.7.3, Figure 7: most unfavourable area, design pump flow, Qmax and most favourable area). For fully calculated pipework the submission includes a supply graph showing available pressure at any flow up to the maximum flow demand, and the demand curve of the most unfavourable (and if required the most favourable) area for each installation (4.4.4.3 f–g). The supply is tested at least at the installation maximum demand flow (8.6.1).
Common assumptions that are not in the text
- Neither standard says "ESFR shall not be used on a grid". The only layout-dependent ESFR rule is the tree layout for glycol systems in EN 12845-2 6.5.
- NFPA 13 has no design velocity limit (28.2.1.4). The 3 m/s (10 ft/s) figure relates only to flushing flow (6.11.2.1.3).
- NFPA 13 gives no exception to the gridded dry pipe ban (8.2.1).
- EN 12845 has no positive sentence saying "pipe tables only on tree layouts"; it forces grids, loops and intermediate level HHS layouts into full calculation (13.1).
- Neither standard sets a minimum number of branch lines for a grid or a minimum main size for a loop.
Design checklist
- Fix the system type before the layout: under EN 12845 any dry, alternate or pre-action section must be a tree; under NFPA 13 dry and double interlock preaction systems cannot be gridded.
- Match the sprinkler to the system: ESFR and residential sprinklers on wet systems only unless listed otherwise; under EN no quick-response sprinklers on dry or Type A pre-action systems.
- Under EN 12845 fully calculate grids, loops and anything with in-rack sprinklers; pipe tables can only be chosen for tree layouts without in-rack sprinklers.
- Build the design area with its long side parallel to the branch lines (ranges): 1.2√A on every layout under NFPA, 1.2√A or 2√A on grids only under EN.
- On grids, prove the worst position by moving the area: at least two additional calculations under NFPA, one sprinkler pitch in each direction under EN.
- Under EN 12845 also calculate the most favourable area and check the pump, velocities, tank capacity and flow meter against Qmax. For ESFR and CMSA (EN 12845-2) this area is not required.
- Join in-rack demand at the right point and, under NFPA, add the hose allowance at the connection nearer the source.
Frequently Asked Questions
Can a dry pipe sprinkler system be gridded?
No. NFPA 13-2025 8.2.1 prohibits gridded dry pipe systems with no exception in the clause. TS EN 12845+A2 11.1.1 allows grids and loops in wet installations only, so dry, alternate and pre-action installations are tree layouts.
Does NFPA 13 prohibit loops on dry systems?
NFPA 13-2025 has no clause prohibiting a loop on a dry or preaction system; the prohibition covers grids only (8.2.1, 8.3.2.7). EN 12845 accepts loops in wet installations only (11.1.1).
What is the most favourable area and why is it calculated?
In EN 12845 it is the area of operation at which flow is greatest for a given pressure at the control valve set (3.10). The flow where its demand curve meets the supply curve is Qmax (3.42); in a fully calculated system the pump, velocities, tank capacity and flow meter are checked against it (Table 14, 13.2.3, 9.3.2.3, 8.5.1). NFPA 13 has no equivalent check.
Which layouts does the 1.2√A rule apply to?
In NFPA 13-2025 the side parallel to the branch lines is at least 1.2√A in every density/area design (28.2.4.2.1). In EN 12845 the rule is written for grids only (13.4.3.1 c), with 2√A under ridges or deep beams; in terminal and looped layouts the range defines the remote side.
Can EN 12845 pipe tables be used on any layout?
No. Gridded and looped layouts and layouts with intermediate level HHS sprinklers are always fully calculated (13.1), as are in-rack sprinklers and their associated ceiling sprinklers (7.2.3.4).
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Download MEP Calc on the App StoreNFPA 13, Standard for the Installation of Sprinkler Systems, 2025 · TS EN 12845+A2:2026 (EN 12845:2015+A2:2026) · TS EN 12845-2 (EN 12845-2:2024). Clause, table and figure numbers refer to these editions; the binding text is the standard itself and the decision of the authority having jurisdiction.