Branch Circuits, Feeders, and Service Calculations
What you must understand
This domain tests standard load-calculation procedure: general lighting load by occupancy type (typically 3 VA per square foot for dwellings), small-appliance and laundry branch circuits, receptacle and appliance demand factors, and the application of demand factors to ranges, dryers, and heating versus cooling loads. Know how to size a service based on calculated load, how to apply the 125 percent continuous-load multiplier, and how to add the largest motor at 25 percent. Understand the difference between branch-circuit, feeder, and service conductors.
Traps
Questions often tempt you to add heating and cooling together; only the larger of the two is used. Another common trap is forgetting that the general lighting load is continuous and requires the 125 percent factor when sizing the service or feeder. Receptacle outlet counts per branch circuit and the 80 percent rule for continuous loads appear frequently. Watch for whether a range or dryer is being calculated by the demand table versus nameplate rating.
Rule of thumb and worked example
A useful rule of thumb: a 3 VA per square foot dwelling general lighting load for 2,000 square feet is 6,000 VA; add 1,500 VA per small-appliance circuit and 1,500 VA for laundry. Example: 2,000 sq ft × 3 VA = 6,000 VA; two small-appliance circuits = 3,000 VA; laundry = 1,500 VA. Subtotal 10,500 VA. Applying the first 3,000 VA at 100 percent and the remainder at 35 percent gives 3,000 + (7,500 × 0.35) = 3,000 + 2,625 = 5,625 VA before adding fixed appliances and the largest motor at 25 percent.