Shadow, Light, and Truth BY RICHARD CADENA Voltage drop in the real world: How different setups produce different results YOU’RE PROBABLY FAMILIAR with voltage drop and how to calculate it. The standard formulas for single-phase and 3-phase systems are: Single-phase: V drop = 2 K x Q x I x d ÷ CM 3-phase: V drop = 1.732 x K x Q x I x d ÷ CM where K is the resistivity of copper at 75˚C (167˚F), Q is an alternating current adjustment factor, I is the current in amps, d is the distance of the run of cable, and CM is the circular mil cross-sectional area of the conductors. Okay, now that we’ve lost most readers— those who see math and turn the page (Stephen Hawking once wrote, “Someone told me that each equation I included in the book would halve the sales.”)—let’s talk about how voltage drop works in the real world. K is a “variable constant” It took me a while to realize how the temperature of a conductor can affect voltage drop in the real world. I run workshops where we would set up circuits, calculate the voltage drop, measure the voltage drop in real life, and compare the results. Sometimes the calculation was a bullseye and sometimes it wasn’t even in the same zip code. I used to attribute this to the quality and the number of the connectors in series, but I was never quite sure. Ever curious, I started using the results of voltage drop measurements to work backwards and calculate the value of K. K is the resistivity of copper, and its value is given as 12.9 in the NFPA 70 (National SUMMER 2023 In the real world, the temperature of a conductor has a marked effect on the voltage drop. This infrared scan paints a heat map of the conductors, indicating which are higher in temperature relative to the others. 18 SUMMER 2023
Protocol Summer 2023: Page 18
