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The gust effect factor Cg used in the external pressure equation from NBC 2015 4.1.7.3.(1) is 2.0 for the main force resisting system and 2.5 for secondary structural members and cladding, as per 4.1.7.3.(8).
These references are to 4.1.7.1 when using NBC 2010.
The gust effect factor Cgi used in the internal pressure equation from 4.1.7.3.(3) is ordinarily 2.0, but for large structures having a single unpartitioned volume without numerous overhead doors and openings it is derived from the equation in 4.1.7.4. For such structures, you calculate the factor yourself and enter it in the Site Information dialog.
In NBC 2010, the factor 2.0 is given in 4.1.7.1.(6)(c) and the equation in NBC Commentary I 22.