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Making Load Calculations Easier

These tips will help you save time and reduce errors when you’re doing load calculations.

Lay it out, first. Beginning with the one-line, identify your load distribution points. When you’re done, you’ll have mapped out the service panels, feeder panels, and branch circuit panels. For each of these points, note which loads use which demand factors (you might want to organize this information in a spreadsheet). For example, lighting loads will use the demand factors shown in Table 220.42.

Evaluate all loads, as a discrete project step. Continuous loads must carry the 125% multiplier in your calculations. Ahead of actually doing the calculations, determine whether each load is continuous or noncontinuous. This way, you have the information on hand when doing the calculations. This eliminates wrong guesses made due to not wanting to stop to find the information.

Remember that feeders differ from branch circuits. The installation requirements are different (Article 215 versus Article 225). But so are the calculation requirements.

  • Feeder circuits. Generally, you size and install feeders to meet the calculated load (even if it exceeds the actual load). This involves adding up the branch circuit loads and applying demand factors that reduce the total calculated load.
  • Branch circuits. Generally, you size and install branch circuits based on actual equipment rated loads. Or, you calculate branch circuit loads per 220.12, 220.14, and 220.16.

Check your work. A great time-saver here is to compare your calculations to a relevant example in the NEC, Annex D.

Source: Mark Lamendola | Mindconnection