Branch Circuits, Feeders, and Service Calculations
What this domain tests
This is the calculation heart of the exam. Candidates must be able to size branch circuits, feeders, and services using the standard method, and know when demand factors and optional methods apply. Expect questions that give you a load list and ask for a conductor size, an overcurrent device rating, or a service ampacity.
Concepts to master
- Branch circuit ratings and conductor sizing: general-purpose circuits, small-appliance and laundry circuits, and the rules for receptacle spacing in dwellings.
- Continuous vs. noncontinuous loads: continuous loads are taken at 125% for overcurrent sizing.
- Standard method service calculation: general lighting VA per square foot, small appliance and laundry branch circuits, appliance loads, dryer and range demand factors, and the largest-motor addition.
- Optional method: permitted for dwellings and some other occupancies; usually smaller but requires specific conditions.
- Demand factors: table-based, applied to ranges, dryers, and multiple appliance loads.
- Neutral calculation: the neutral load may be reduced under the unbalanced-load rules; this is a frequent trap.
Traps in questions
The most common error is forgetting the 125% continuous-load factor, or applying a demand factor that is not permitted for the occupancy. Another trap is using the optional method when the question's conditions do not allow it. Watch for questions that give you a load in kW and expect VA conversion, and for questions where the largest motor must be added at 25%.
Rule of thumb
For a dwelling service, the general lighting load is 3 VA per square foot; add 1,500 VA per small-appliance circuit and 1,500 VA for laundry, apply the table demand factor to the total, then add the largest appliance loads and the largest motor at 25%. Round conductor and device sizes up to the next standard rating.
Worked example
A 2,000 sq ft dwelling: lighting 6,000 VA + 4,500 VA small appliance/laundry = 10,500 VA; first 3,000 VA at 100% plus remainder at 35% gives roughly 5,625 VA. Add range and dryer demand loads, then the largest motor at 25%. Convert to amps at 240 V and select the next standard service size.