Voltage Drop Calculator
Calculate voltage drop over wire distance for single-phase and three-phase circuits. Find minimum wire gauge to meet NEC 3% recommendation, estimate power loss, and annual energy cost of voltage drop.
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Voltage Drop
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Voltage Drop % —
NEC Compliance (< 3%) —
Voltage at Load —
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ft
Voltage Drop
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Voltage Drop % —
Voltage at Load —
NEC 3% Check —
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°F
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Voltage Drop
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Voltage Drop % —
Voltage at Load —
NEC 210.19 Compliance —
Power Loss in Wire —
Annual Energy Loss —
Annual Cost of Voltage Drop —
Temperature Correction Note —
How to Use This Calculator
- Select your system voltage (120V for most residential outlets, 240V for appliances).
- Enter the load current in amps (from the device nameplate or breaker size).
- Enter the one-way distance from the panel to the load in feet.
- Select the wire gauge (AWG) you plan to use.
- See voltage drop in volts and percent, and whether it meets NEC recommendations.
- Use Three Phase tab for 3-phase circuits (commercial/industrial).
- Use Minimum Wire Size tab to find the smallest acceptable wire gauge for your run.
Formula
Single Phase: VD = I × R × 2 × Distance ÷ 1000 (R in Ω/1000ft)
Three Phase: VD = √3 × I × R × Distance ÷ 1000
Voltage Drop % = (VD ÷ Source Voltage) × 100
Example
120V circuit, 20A load, 100ft run, #12 AWG copper (1.98 Ω/1000ft). Total length = 200ft. R = 1.98 × 200÷1000 = 0.396 Ω. VD = 20 × 0.396 = 7.92V. Drop % = 7.92÷120 = 6.6% — exceeds 3%. Use #10 AWG (1.24 Ω/1000ft) → VD = 4.96V = 4.1%. Use #8 AWG for 2.6%.
Frequently Asked Questions
- NEC 210.19(A) recommends voltage drop not exceed 3% for branch circuits and 5% for the combined feeder and branch circuit. This is a recommendation, not a hard code requirement for most applications, but it's the industry standard for acceptable power quality.
- Single Phase: VD = 2 × K × I × D / CM. Or using resistance: VD = I × R, where R = conductor resistance (Ω/1000ft) × total length ÷ 1000. Three phase: VD = √3 × I × R × D. K = 12.9 for copper, 21.2 for aluminum at 75°C.
- Use this calculator to find the minimum wire size for your run. As a quick rule: upgrade one gauge size for every 50–75 feet beyond 100 feet on a 120V circuit. For a 200-foot run with a 20A load on 120V, #10 AWG is often needed where #12 would suffice at 100 feet.
- Yes — aluminum has about 61% more electrical resistance than copper of the same gauge. For aluminum wire, multiply resistance values by 1.61. To match copper performance, aluminum must be two wire sizes larger: #12 copper ≈ #8 aluminum in resistance.
- Three-phase voltage drop uses the formula: VD = √3 × I × R × one-way-distance. The √3 factor (≈1.732) accounts for the phase relationship. Three-phase systems are more efficient for long runs because they deliver more power with less voltage drop than single-phase systems of the same wire size.