Voltage Drop Calculator | Free NEC-Compliant Wire Sizing Tool
NEC Voltage Drop Calculator
Calculate voltage drop for copper and aluminum conductors. Supports single-phase and three-phase circuits, 120V through 480V. Recommends minimum wire gauge per NEC guidelines.
All gauges at your settings
| AWG | Drop (V) | Drop % | At load (V) | Status |
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Understanding voltage drop
Voltage drop is the reduction in voltage along a conductor caused by the conductor's resistance. Every wire has resistance, and when current flows through that resistance, some voltage is "lost" as heat in the wire instead of reaching the load. The longer the wire and the higher the current, the more voltage is lost.
The voltage drop formula
Single-phase: Vdrop = 2 × L × R × I / 1000
Three-phase: Vdrop = 1.732 × L × R × I / 1000
Where L = one-way distance (feet), R = resistance per 1000 feet (ohms/kft from NEC Chapter 9 Table 8), I = current (amps).
NEC voltage drop recommendations
NEC 210.19(A) Informational Note No. 4 and NEC 215.2(A) Informational Note No. 2 recommend that branch circuit voltage drop not exceed 3% and that total voltage drop (feeder + branch) not exceed 5%. While these are recommendations and not requirements, exceeding them causes motors to overheat, lights to dim, and sensitive electronics to malfunction.
Copper vs aluminum
Aluminum has approximately 61% the conductivity of copper, meaning aluminum conductors must be oversized compared to copper for the same current and voltage drop. The typical rule of thumb is to go up two AWG sizes when switching from copper to aluminum (e.g., #10 copper ≈ #8 aluminum). However, aluminum is significantly lighter and cheaper, making it the standard for service entrance conductors and large feeders.
How to reduce voltage drop
Three options: increase wire gauge (lower resistance), increase system voltage (higher voltage means less current for the same wattage), or reduce distance (shorter run). For long runs like barn feeds, well pumps, or detached garages, upsizing wire is the most practical solution.
Shop wire and terminations
Conversions Tech stocks bulk wire, compression lugs, terminals, and insulated taps for every AWG size. Request a quote for project pricing.
