Code Calculations, Load Computations, and Plan/Print Reading
Calculation questions are procedural. Set out the load components in a table and step through them in the order the Code requires; most errors come from skipping a step or mixing up demand factors.
Load computation order
- General lighting load from floor area and the basic load value in the applicable table.
- Small-appliance and laundry branch circuits at their required ratings.
- Specific appliances: ranges, dryers, water heaters, space heating, air conditioning, motors.
- Apply demand factors to the applicable components-lighting demand, range and dryer demand, and heating versus cooling (use the larger, not both).
- Add the largest motor at 25 percent.
- Compare the result with the service size and apply the continuous-load factor; then check neutral and grounding conductor sizing and voltage drop.
Plan and print reading
Know the symbols for receptacles, switches, panels, junction boxes, and equipment; read the scale and the title block; locate the panel schedules, riser diagram, and specifications. Panel schedules give circuit numbers, breaker sizes, and connected loads; the riser diagram shows feeder routing and ratings; specifications tell you which code edition and which listed products apply.
Traps
Adding heating and cooling together, forgetting the 25 percent largest-motor adder, using connected load instead of demand load, misreading a panel schedule, and ignoring notes on the drawing that override the plan.
Worked example
A feeder supplies a 60 A continuous load and a 40 A non-continuous load: 60 × 1.25 + 40 = 115 A. Choose conductors and overcurrent protection for at least 115 A, then verify voltage drop at the actual run length.