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You should size your compressor by how many people will be using air at one time and then add 25% to allow for duty cycle. In a typical auto shop (Not Body Shop), each person would use around 4-5 CFM. So, a 3 man shop would require a bare minimum of 12-15 CFM, but you would want to supply 16 - 20 CFM to allow for the maximum duty cycle of the air compressor (75% for most 2 Stage)
