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For a branch circuit, the method is still to add up the calculated load and determine whether the circuit is big enough to add more load.
For feeders, however, it is also permitted to use the actual demand record, which can be found in one of two ways:Use information from an actual demand record to find the highest demand that occurred during a one-year period (this is typically available from a utility or from utility bills for commercial and industrial occupancies) Measure demand for a minimum of 30 days and increase that amount to account for any loads that might not have been on during that period.
Once that entire actual recorded load is determined, increase it by 25% and compare the size of the existing feeder or service conductors. The amount by which the existing capacity exceeds the actual demand load increased by 25% is considered to be spare capacity.
© Hughes Associates,Inc. 2012