Overview
The voltage drop calculator determines the voltage loss across a conductor based on its material, size, length, and the load current. It checks the result against the NEC recommendation of 3% maximum for branch circuits and 5% maximum for the combined feeder and branch circuit.
If the selected conductor fails the voltage drop check, the calculator recommends the minimum wire size that meets the threshold.
Inputs
- Source voltage: 120V, 208V, 240V, 277V, 480V, or custom
- Phase: Single-phase or three-phase
- Conductor material: Copper (K=12.9) or aluminum (K=21.2)
- Conductor size: AWG 14 through 2000 kcmil
- One-way length: Distance from source to load in feet
- Load current: Circuit amperage in amps
- Circuit type: Branch circuit (3% limit) or feeder (2% limit, with 5% total check)
Calculation Method
The calculator uses the standard NEC voltage drop formula:
Single-phase: VD = (2 × K × I × L) / CM Three-phase: VD = (1.732 × K × I × L) / CM
Circular mil (CM) values are from NEC Chapter 9, Table 8. The K factor represents the DC resistance of the conductor material at 75°C per the NEC calculation method.
The output shows: voltage drop in volts, voltage drop as a percentage, pass/fail status, and the NEC reference (210.19(A) Informational Note No. 4).
NEC References
- NEC 210.19(A) Informational Note No. 4 — branch circuit voltage drop recommendation (3%)
- NEC 215.2(A)(4) Informational Note No. 2 — feeder voltage drop recommendation (3%, 5% total)
- NEC Chapter 9, Table 8 — conductor properties including circular mil area
- NEC Chapter 9, Table 9 — AC resistance and reactance for conductor impedance
Try it yourself
Calculate voltage drop for single-phase and three-phase circuits per NEC 2026 standards.
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