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I believe CBECC-COM uses EnergyPlus's "autosize" feature to size the baseline model, but I'm not completely sure. The ACM states that the Standard Design boiler is sized to be 25% larger than the peak loads, and chillers are sized to be 15% larger than peak loads. However, I have tried taking the baseline model IDF that CBECC-COM creates, set all available parameters to autosize, set global heating/cooling sizing factors to 1.25 and 1.15, respectively, and the sizing still comes out slightly different than what CBECC produces. So I'm not quite sure exactly how CBECC sizes the baseline model.

Whichever way it sizes, it definitely accounts for all internal/external loads, and does not use rules of thumb (i.e. 1 cfm/sf and 400 cfm/ton). The sizing accounts for all envelope components, such as wall u-factors, glass type and WWR. Regarding WWR, the prescriptive maximum is 40%, so if the proposed building has 50% WWR, the baseline model will only have 40%.