Voltage Drop Calculator & Explorer

Voltage drop is one of the few NEC-adjacent calculations that requires engineering judgment rather than a simple table lookup — the code provides guidance but not a single mandatory limit, and the right answer depends on circuit length, load type, conductor material, and conduit type. This calculator solves the AC voltage drop equation using conductor resistance and reactance from NEC Chapter 9 tables, automatically applying temperature correction to resistance. Select system voltage (120, 208, 277, 480, or 600 V), single- or three-phase, conductor size, material (copper or aluminum), conduit type (PVC/non-magnetic, aluminum, or steel), circuit run length, load current, power factor (0.5–1.0), and temperature rating. The Live tab shows voltage drop percentage and voltage-at-load in real time with a simplified circuit view and a breakdown of what factors drive the result. The Explorer tab provides six parametric charts — drop vs. length, drop vs. load current, drop vs. power factor, drop vs. temperature, drop vs. conductor size, and parallel conductors effect — so you can isolate any variable. The Comparison tab places your design against three alternatives (one size up, next material, longer run) with copper-vs-aluminum charts at every size. The Scenario tab provides six quick presets — Short Feeder, Long Undersized, Improved, Aluminum Feeder, Three-Phase, and Motor Load — plus a Scenario Lab for guided exploration with takeaways. A Design Challenge tab generates voltage-drop sizing problems. Target drop percentage is a user-set slider (1–7%), with ampacity compliance checked alongside the voltage-drop result. NEC guidance references 210.19, 215.2, 310.10(H), and 310.15.

Enlarge

A live look at the app you'll launch — every control on the left drives the visualizations on the right in real time.

• 01Calculate AC voltage drop percentage using conductor resistance and reactance from NEC Chapter 9 tables, applying power factor and temperature correction
• 02Explain the NEC guidance on voltage drop under 210.19 and 215.2 Informational Notes and distinguish it from a mandatory code requirement
• 03Compare copper and aluminum conductors at the same AWG size and explain why aluminum carries approximately 60% more resistance per unit length than copper
• 04Predict how increasing circuit length, load current, and power factor each independently affect voltage drop magnitude using the Explorer parametric charts
• 05Select the minimum conductor size that satisfies both ampacity (NEC 310.15) and a user-defined voltage drop target (typically 3% for branch circuits, 5% combined)
• 06Evaluate a long-run motor circuit using the Motor Load preset and explain why motor starting voltage drop is a separate consideration from steady-state drop