NFPA 502 Annex D smoke control critical velocity errata

NFPA 502 2026 Annex D Errata

Posted 25th June 2025

A tunnel industry colleague pointed out a typesetting problem in Equation D.2.4c in the 2026 NFPA 502 Annex D.  That prompted us to review the entire Annex D issued in the NFPA 502 2026 edition for further editorial errors that may have been introduced by the NFPA after the committee output was concluded during the Second Draft Meeting (Annex D resolved on 27^th September 2024).

Several editorial errors / discrepancies were identified.  Some are minor, but others add confusion or even make the meaning quite wrong.  They are listed below in order of appearance in the Annex D.  A PDF of the list of errors we noted can also be downloaded here.  The equations in the 2026 Annex D are sourced from the Fire Technology paper Beyer Stacey & Brenn (2024) which can be found on Stacey Agnew’s webpage or from Fire Technology.

Stacey Agnew has already provided this information to the relevant people in the NFPA.  With NFPA’s processes, it appears that the introduced editorial errors can only be fixed by a tentative interim amendment (TIA) and not simply by issuing errata.  The TIA process is predicted (by NFPA) to take several months and there is very little chance that the corrections are made to the official version before 2026.

Until the editorial errors in Annex D are corrected by NFPA, the information here serves the people in the industry who are trying to make sense of the 2026 NFPA 502 Annex D.

• Section D.1, one sentence misses “velocity” after “confinement” (see below)

• Section D.2 states that several equations have been developed “in this annex” but most of them have not been developed in this annex.  The addition of “in this annex” is not correct here (see below).

• The text in the Annex D about the critical velocity calculation (Section D.2.4) has been made incorrect on how the characteristic length for convection should be interpreted for design (see below).

• The text “that can be found in” in section D.2.4 regarding the set of simplified equation should have been deleted and replaced with a semicolon.  The current sentence does not appear correct (see below).

• The headings “Set of full equations” before equation [D.2.4a] and “Set of simplified equations” before “Design assumption that led to the simplified set of equations are listed in Table D.2.4(a)” were deleted.  Without those headings, the current text appears confusing as no clear distinction between the two set of equations is provided.

• The “x” in the equation D.2.4a is confusing and should not be there.

• Equation D.2.4c is wrong. The whole fraction after the “e” needs to be an exponent with a negative sign in front (see below).

• Section D.2.4 below the full set of equations, the sentence is confused as it is not clear what is meant with user.  The original agreed text says “For tunnel designs that rely on FFFS … it is recommended to use a validated CFD model…”.  Now it says “For tunnel designs that rely on FFFS … it is recommended that the user utilizes a validated CFD model…”  It is unclear what a “user” is in that context (see below)?

• The format of the correlations in the text above equation D.2.4e is highly inconsistent.  The parameters H and A are sometimes italic and sometimes not.  Before and after the symbol “>” and “<” there is sometimes a space, and sometimes there is no space.  It has not been made clear that “m” is a unit and not a parameter/variable.  Same applies to “ft”.  It is not apparent why the units “MW” and “Kelvin” do not need conversion to US units while “metre” to “feet” needed to be converted in the text?  The dot on “Q” is missing.  The original agreed text looks different (see below).

• The adjustments in Section D.2.5 do not appear to be appropriate/correct after the editorial edits, and made the sentence confused (see below).

• Table D.2.4(a) is wrong.  Fire intensity is “MW/m²“ and so the conversion needs “ft²“ in the denominator instead of just “ft”.  The value for radiative fraction in US units is still “0.2” and not “N/A”.  The specific heat capacity of 1007 J/kgK translates to 0.240518 Btu/lbR and not 240.518 Btu/lbR.  The variable “Ta” should be in the same line as the text “Ambient air temperature”.  The note below the table has inconsistent formatting and sometimes has a space around “>” and sometimes not.

• Some of the US units in Table D.2.4(c) are incorrect/inconsistent. It is indicated that gravitational acceleration in “m/s²” has no units in US, which is incorrect.  Mixing “Btu/hr” with “Btu/sec”, “lb/sec”, and “ft/sec” will produce wrong results.  All units used in the equations should be based either on “hr” or “sec”.  The conversion of the 3.66 m to US units was missed.

• Reference “[10] MTFVTP, “Memorial Tunnel Fire Ventilation Test Program – Comprehensive Test Report,” Bechtel/Parsons Brinckerhoff, Boston, 1995.” is not listed in the Annex D.