In this series, we’ve been looking at how the NEC affects your calculations. We started at the beginning and finished with Article 215 in our previous issue. That brings us to Article 220, which is all about calculations for services, feeders, and branch-circuits.
We won’t go through each requirement. But this overview will be helpful. The Article is divided into five Parts:
Part I. General. Features Table 220.3, Additional Load Calculation References.
Part II. Branch-Circuit Load Calculations. Features Table 220.12, General Lighting Loads by Occupancy.
Part III. Feeder and Service Load Calculations. Features five demand factor tables.
Part IV. Optional Feeder and Service Load Calculations. An alternative to Part III.
Part V. Farm Load Calculations.
For all circuits, use the nominal system voltages listed in 220.5.
Three key points for branch-circuits:
- Look for your specific occupancy [220.12] and specific load [220.14].
- If you don’t find your specific load in 220.14, calculate outlets based on 180 VA per outlet [220.14(L)].
- For nonlinear lighting loads (e.g., LEDs), base the load on the total amperage of the unit (e.g., ballast amps), not the total watts of the lamps [220.18(B)].
Three key points for feeders and services:
- The calculated load can’t be less than the sum of the branch loads [220.4.0].
- If two loads are unlikely to run simultaneously, you can use just the larger of the two in your calculations.
You can apply demand factors to reduce the calculated neutral load [220.61].