The importance of limiting voltage drop

You may not know that the National Electric Code doesn’t make complying with its voltage drop criteria mandatory. But there are still some important design factors that you need to keep in mind . . . .

The NEC doesn’t make complying with its voltage drop criteria a requirement, which means the NEC treats voltage drop more of a functional concern than a safety one. We know this because voltage drop discussions are limited to Informational Notes:  see Articles 210.19(A), 215.2(A)(1)(b), 215.2(A)(1), and 310.15(A)(1) of NEC 2014. There are exceptions, such as with sensitive electronic equipment [Article 647.4(D)] and fire pump motors [Article 695.6(B)(2)].

Those notes “are informational only and are not enforceable” (see Article 90.5(C) of NEC 2014).

Your primary goal as a design engineer is to ensure that equipment operates correctly. First of all, equipment is rated to be used within a certain voltage range, and it might not work properly if the voltage supplied is lower than the equipment’s rated range. A power supply that says it will operate within a range of 100-240 volts probably won’t perform very well, if at all, if it’s fed 78 volts.

But reduced voltage also tends to lower operating efficiency. When efficiencies go down, all sorts of things happen: operating current goes up; heat production goes up (if for no other reason than because of increased I2R losses); and starting currents go up.



Also, the effects of voltage drop can have non-linear consequences, meaning the problems they create are exaggerated compared to the relatively “minor” impact initially perceived. The NEC explains this problem succinctly: “An applied voltage of 10 percent below rating can result in a decrease in efficiency of substantially more than 10 percent — for example, fluorescent light output would be reduced by 30 percent. Induction motors would run hotter and produce less torque” (see Article 215.2(4) of the NEC 2011 Handbook).

There are safety implications associated with excessive voltage drop, if not as severe as others considered. Higher operating currents than designed will overheat the cables supplying the equipment. Their insulation will degrade faster than expected. Over time, this excessive heat can can damage neighboring cables by subjecting them to higher ambient temperatures than they were designed for. This unintended exposure to elevated temperatures decreases a cable’s lifespan.

Cables that degrade prematurely have disastrous consequences if they short out against conduits or equipment. If the cables aren’t expected to wear out for a given period of time, then preventative maintenance, which is put in place to pre-emptively catch these kinds of problems and detect imminent cable failure, may not be able to detect the premature degradation.