› wire-size-underground-circuitSizing Electrical Wire for Underground Circuit Cable - The Spruce Sep 25, 2022 · Voltage drop increases as the load on a circuit increases, and overloading a circuit contributes to excessive voltage drop. In other words, if you put too many loads on one circuit and exceed the standard 80 percent safe capacity (1,440 watts for 15-amp circuits; 1,920 watts for 20-amp circuits), you'll add unnecessary voltage drop.