Load Calculations and Plan Interpretation
The Load Calculations and Plan Interpretation domain tests Article 220 and the ability to read a panel schedule, one-line diagram, and floor plan. Expect to convert connected loads to demand loads, balance phases, and select service or feeder sizes.
Core rules
- Standard method: general lighting per 220.12, using 3 VA/ft² for dwellings and the Table 220.12 value for other occupancies. Receptacle outlets in non-dwellings use 180 VA per yoke per 220.14(I). Appliances use 220.53 at 75% when there are four or more; ranges use 220.55 and dryers use 220.54.
- Demand factors: Table 220.42 applies to lighting; 220.44 applies to receptacle loads with the first 10 kVA at 100% and the remainder at 50%; 220.61 covers neutral calculations.
- Optional method 220.82 for a dwelling uses 100% of the first 10 kVA plus 40% of the remainder and specific appliances. 220.84 applies to multifamily dwellings, and 220.87 applies to existing loads.
- Plan interpretation: identify voltage and phase, panel schedule circuits, connected versus demand load, continuous loads at 125%, and the service or feeder conductor and overcurrent device sizes.
Worked example
A 2000 ft² office has 7000 VA of general lighting at 3.5 VA/ft² and 20 receptacle outlets at 180 VA = 3600 VA. Lighting demand = 3000 + 4000 × 35% = 4400 VA. Receptacle demand = 3600 VA because it is within the first 10 kVA. Total = 8000 VA. At 208 V three-phase, current = 8000 / (1.732 × 208) = 22.2 A. A 100 A service is more than adequate, but verify the minimum service and add 25% of the largest motor if one is shown on the plan.
Traps
- Applying the lighting demand factor to receptacle loads.
- Using connected load without applying the correct demand factor.
- Double-counting general-use receptacles in a dwelling calculation.
- Omitting the 125% continuous-load factor.
- Reading a single-phase current for a three-phase panel without the √3 factor.