We’ve been walking through the NEC from page 1 to see which requirements affect circuit calculations and how they do that. By now, it should be apparent that these are mostly load calculations.
We left off with Article 210, in our previous issue. Does Article 215 affect your calculations? Only a little bit. Let’s see how it does that….
The first calculation requirement we come to is for minimum rating and size. When sizing conductors, you first must determine the load. If any loads on that feeder are continuous, the feeder must have an ampacity not less than the noncontinuous load plus 125% of the continuous load [215(A)(1)(a)].
But there’s a caveat. Before you can select the correct conductor size from the ampacity tables (to support the calculated load), you will have to do some additional calculations to apply adjustment factors. If it turns out that applying these adjustment factors results in a larger ampacity conductor, then you must use the higher ampacity rather than the one you calculated in the previous step [215(A)(1)(b)].
You should also calculate voltage drop. This isn’t an NEC requirement, but it is a good engineering practice for many reasons including lower energy bills and increased equipment life. The NEC provides recommended values (from other standards) in the Informational Note to 215.2. The values are not upper limits. A good financial case can be made for investing in larger conductors to get a voltage drop of 2% instead of 5%.