The NEC provides a recommendation on voltage drop for branch circuits in Informational Note No. 4 following 210.19(A). A similar recommendation for feeders exists in Informational Note No. 2 following 210.2(A). Informational Note No. 1, following 310.15(A)(1), states that the ampacity tables don’t take voltage drop into consideration. And then it refers you to the two Informational Notes just mentioned.
There are two problems with these Informational Notes. The first problem is that these recommendations are not NEC requirements. They exist in the Informational Notes. Often during budget-cutting design discussions, the mentality is to do what the NEC requires and no more.
If you size conductors per the ampacity tables and aren’t required to size them any larger to accommodate voltage drop, your “by the ampacity tables” design will pass inspection. If you try to use larger conductors based on “you’ll save energy,” you might not be persuasive. These Informational Notes don’t require you to spend the money on “oversizing” conductors. So, the “thinking” goes, meet the NEC minimums and you’ve done a good job of engineering because you didn’t spend money unnecessarily.
The second problem is these Information Notes are intended to help you size conductors to “provide a reasonable efficiency of operation.” The numbers provided are 3% for any branch circuit and 5% for the combined voltage drop of feeder and branch-circuit conductors. That’s generally reasonable, but rarely optimal. In some environments, it’s not acceptable.
If we turn to Art. 647 [Sensitive Electronic Equipment], we see there is actually an NEC requirement to size conductors so as to limit voltage drop. But the limits aren’t 3% and 5% — they are half those values (i.e., 1.5% and 2.5%). Article 647 provides the requirements for “sensitive electronic equipment.” Limiting voltage drop to these values for other types of circuits will probably get you pretty close to optimal.
The limits in Art. 647 are good ones to use, if one project goal is to have an energy-efficient installation. You can point out that those values are in the NEC. Although they are for “sensitive electronic equipment,” you can easily argue that they make sense for other types of circuits where energy efficiency is a desired outcome.
There may be practical and cost considerations that make these values difficult to achieve throughout the entire distribution system. A couple of “tricks” can help you achieve something close, even if budget or other considerations get in the way. One trick is to make your home runs larger, but not upsize all of the conductors (larger conductors for the longer runs). Another is to distribute at a higher voltage for as long as you can (higher voltage means less drop in a given length).