Even if you work with stamped drawings, you’ll eventually need to do commercial load calculations in the field or on a licensing exam. The NEC covers commercial calculations in Art. 220, but other articles also apply. For example, you must know the definitions in Art. 100, be familiar with what Art. 210 says about continuous loads, and understand the overcurrent protection requirements set forth in Art. 240.
Two items associated with this type of calculation repeatedly need clarification:
- VoltageThe voltage to use for your calculations depends on the system design voltage. Thus when you calculate branch-circuit, feeder, and service loads, you must use a nominal system voltage of 120V, 120/240V, 208Y/120V, 240V, 347V, 480Y/277V, 480V, 600Y/347V, or 600V unless otherwise specified (220.2) (Fig. 1 below).
- RoundingRefer to 200.2(B) to end the rounding mystery. When the ampere calculation exceeds a whole number by 0.5 or more, round up to the next whole number. If the extra is 0.49 or less, round down to the next whole number. For, example, round 29.5A up to 30A, but round 29.45A down to 29A.
Specific loads. Art. 220 doesn’t cover all specific loads. For example, you’ll find motors in Art. 430 and air conditioners in Art. 440. To know if you should look in another Article, use the NEC index.
Apogee consultancy Successfully done this job last 5 years. you can calculate your load via Apogee consulting.