# Update of NFPA 502 Critical Velocity Calculator for 2020

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Created with software version: 2020.1

The NFPA 502 Standard for Road Tunnels, Bridges, and Other Limited Access Highways has been updated for 2020 and includes a new Annex D Critical Velocity Calculation. We have updated the Tunnel Critical Velocity Calculator to make this calculation.

A comparison of the 2020 calculation with the 2017 version is shown in Figure 1 below for a tunnel with a height of 5 m, width of 12 m, grade of -4%, ambient temperature 20 C, ambient density 1.1454 kg/m^3, and ambient heat capacity of 1.005 kJ/kg K. The comparison shows the critical velocity including the grade effect for a range of heat release rates from 5000 kW to 30000 kW.

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