Three codes, three pressurisation philosophies

Indian high-rise pressurisation is technically governed by NBC 2016 Part 4 §6. NFPA 92 and BS EN 12101-6 are referenced when the AHJ accepts performance-based design or the project’s insurance carrier mandates them. The three differ on a fundamental modelling assumption.

NBC 2016 Pt 4 §6.1.4 prescribes a fixed pressure differential of 50 Pa across stair doors with all-doors-closed, and a velocity criterion of 1 m/s across one open door. Airflow falls out by simple area × velocity.

NFPA 92 §4.4 requires actual calculation: leakage area of every stair door + every shaft + every smoke barrier penetration, summed and solved as a network with the design pressure objective. Always lower flow than NBC, because real buildings leak less than NBC’s worst-case assumption.

BS EN 12101-6 defines five classes (A through F) by door-opening scenario. Class D — single-door-open at fire floor + escape route operational — is the European default for high-rise residential and commercial. Class flow per scenario falls between NBC and NFPA 92 in most cases.

A worked 80 m residential tower in Mumbai

20-storey residential tower, single stair, fire-fighter lobby on every floor, smoke-vented lobby on alternate floors per NBC. Pressure differential 50 Pa across stair doors; lobby door 0.8 × 2.1 m at 1 m/s.

NFPA 92 saves about 12 % on installed fan kW. Capex saving on a 20-storey project: ~₹6-8 lakh on fan + VFD + control. The trade-off — NFPA 92 design depends on leakage assumptions that must be verified at site by smoke-door-leakage testing per NFPA 105. If that test fails, the design fails. NBC does not require this test, which is why most Indian high-rises default to NBC despite the capex penalty.

ASET/RSET — the smoke-control endpoint nobody measures

NFPA 92 introduces the ASET (Available Safe Egress Time) versus RSET (Required Safe Egress Time) calculation. ASET = how long the smoke layer stays above the head-height plane and CO concentration stays below ISO 13571 limits. RSET = how long it takes the slowest occupant to reach a refuge area. ASET > RSET is the safety criterion.

On the same 80 m tower above, ASET (from a CFD model in PyroSim with a 1.5 MW design fire) comes out to 580 s for the fire floor lobby. RSET (using SFPE Handbook 5th edition movement-speed correlations for residential) is 380 s for the typical floor. ASET/RSET ratio = 1.53 — comfortable safety margin.

For comparison, the same building under NBC’s prescriptive 50 Pa pressurisation produces an ASET of 720 s — longer, because NBC’s flow is conservative. Both pass. But only the NFPA 92 path produces a defensible number for an insurance-required performance-based fire-safety case (FSC).

When to use which

For everyday Indian high-rise — NBC 2016 Part 4 §6 prescriptive. AHJ accepts; insurance accepts; no testing required at site.

For complex geometries (atrium, mall, mixed-use, sky bridge, transfer floor) — NFPA 92 performance-based with CFD modelling. Always validate ASET > 1.5 × RSET to keep margin.

For European-flagged commercial real estate (DLF Cyber Hub style with global tenants) — BS EN 12101-6 with the smoke-management plan integrated into the Class designation. Class D is the typical target.