Branch Circuits, Feeders, and Load Calculations
Branch circuits and feeders
Branch-circuit calculations begin with the load type: general lighting, small-appliance, laundry, receptacle, appliance, motor, or HVAC. Dwelling general lighting is typically 3 VA per square foot; small-appliance and laundry circuits are 1,500 VA each. Continuous loads are taken at 125% before adding to the feeder or service. Feeders add branch-circuit loads, apply demand factors only where permitted, and then size conductors and overcurrent devices.
Standard versus optional
Know both the standard and optional dwelling calculation methods. In the standard method, the first 3,000 VA of general lighting and receptacle load is at 100%, with the remainder at 35%; appliance and range demand factors may also apply. The optional method uses a different aggregation and often produces a smaller service. Commercial calculations may require 125% for continuous loads and 100% for noncontinuous loads.
Traps
Do not apply a demand factor before the 125% continuous-load multiplier. Keep VA and watts distinct when power factor is not unity. Watch for questions that mix 120 V and 240 V loads or omit the neutral calculation. A common error is sizing the feeder from the sum of breaker ratings rather than calculated load.
Worked example
A 2,000 ft² dwelling: general lighting 6,000 VA; small appliance 3,000 VA; laundry 1,500 VA; total 10,500 VA. Standard demand: 3,000 + (7,500 × 35%) = 5,625 VA. At 240 V, that is 23.4 A; the service and feeder must also carry the range, dryer, and HVAC loads. Rule of thumb: build the calculation in a table, load by load.