The process of performing load calculations requires you to dig into many different Sections of the NEC. For this reason, load calculations can easily result in code violations. But if you approach the calculations with a methodology, you can easily prevent those violations.
One step in a good methodology is to organize the calculations. Here’s a suggested approach for a facility:
- Review the distribution one-line diagram; identify every distribution point and load center.
- Identify the allowable demand factors for each known load using the relevant table(s) in Article 220. List the loads and group them according to demand factor.
- Identify which loads are continuous (those not identified default to noncontinuous) for purposes of applying the 125% rule.
- Determine if Table 220.12 applies (type of occupancy).
- Using Tables 220.1 and 220.3, list all applicable code Sections so you can refer to your list, not your memory.
Here are some things to remember when doing load calculations:
- You calculate loads for branch circuits, feeders, and services per the steps laid out in Article 220. There is no other acceptable method to determine or define a load.
- Generally, you install feeders to meet the calculated load (even if it’s larger than the installed load). The calculated load on a feeder is the sum of the branch circuit loads it supplies multiplied by applicable demand factors (see Table 220.55).
- Generally, you install branch circuits to supply the actual equipment connected on those circuits.