Voltage drop · NEC Chapter 9

Voltage Drop Calculator

Estimate single- and three-phase voltage drop in volts and percent using NEC Chapter 9 Table 8 conductor resistance. Enter the load, one-way run length, conductor size, and material to check your run against a voltage-drop limit before you pull wire.

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Single & three-phase
Instant volts + %

Voltage drop

NEC Chapter 9 · Table 8

Formula

Vd = k × R × I × L ÷ 1000

Field inputsVoltage drop

Conductor: Cu · Al
Phase: 1φ · 3φ
Output: Volts + %
Check: vs % limit

Compare any run against your drop limit before you pull wire.

Circuit inputs

Describe the run

Conductor material

Conductor size (AWG / kcmil)

Phase

Load current (amps)

One-way length (ft)

System voltage (V)

Max voltage drop (%)

Assumptions: uncoated conductor resistance from NEC Chapter 9, Table 8 at 75°C; one-way run length (the tool doubles it for single-phase); steady-state resistive load; power factor of 1.0. Real circuits with reactance, elevated temperature, or low power factor may drop more.

Estimated result

Voltage drop

Over 3.0%

Voltage drop

7.72 V

Voltage drop (percent): 3.22 %
Voltage at load: 232.28 V
Conductor resistance R (copper, #12): 1.93 Ω / 1000 ft
Phase multiplier (k): 2

Formula used

Vd = 2 × 1.93 × 20 × 100 ÷ 1000 = 7.72 V

Source: NEC Chapter 9, Table 8 (conductor DC resistance). Single-phase, 2-wire.

Safety & accuracy notice

This voltage drop calculator is a planning aid only. It is not engineering, a permit, or design approval, and it is not legal or code certification. Results are estimates that assume an ideal resistive load.

Always verify against the current adopted NEC (National Electrical Code), equipment listings and manufacturer instructions, and your local AHJ (Authority Having Jurisdiction — the inspector or department that enforces the code). Final circuit decisions should be reviewed and approved by a licensed electrician or professional engineer.

Voltage drop FAQ

What voltage drop is acceptable under the NEC?

The NEC recommends (in informational notes, not as a hard rule) a maximum of 3% voltage drop on a branch circuit or feeder, and no more than 5% total across both. Your AHJ or the equipment may require tighter limits. This tool defaults the comparison to 3%.

What formula does this calculator use?

Single-phase voltage drop is Vd = 2 × R × I × L ÷ 1000, and three-phase is Vd = √3 × R × I × L ÷ 1000, where R is the conductor resistance in ohms per 1000 ft (NEC Chapter 9, Table 8), I is the load in amps, and L is the one-way circuit length in feet.