Can You Answer This Commercial Service Exam Question?
Refer to NFPA Table 220.12 which requires a minimum of 3.5 va/sq.ft. for the general lighting load for office buildings.
5,000 sq.ft. x 3.5 va/sq.ft. = 17,500 va total lighting load
Recognize that according to NFPA Article 100-Definitions the general lighting load in an office building is considered a “Continuous Load” because “the maximum current that supplies the load is expected to continue for 3 hours or more.”
Refer to NFPA 210.19(A)(1) which states that “where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the minimum branch-circuit conductor size shall have an allowable ampacity not less than the noncontinuous load plus 125 percent of the continuous load.”
Remember we not trying to determine the size of conductor needed but rather how many circuits are required. Since the reciprocal of 125% is 80% a circuit supplying a continuous load can only be loaded to 80% of its rating.
Determine the allowable capacity per circuit by multiplying the voltage of the circuit by the ampere rating and the 80 % circuit capacity limitation for continuous loads.
120 volts x 20 amperes x 80% = 1920 va capacity per circui
Divide total light load va by capacity per circuit va
= 17,500 va/1920 va
= 9.11 circuits or 10 circuits-final answer
(Since you cannot run a fractional portion of a circuit whenever this calculation results in a remainder always round up to the next whole number)