Voltage Drop Calculator
Calculate voltage drop over a wire run and check it against code limits.
About Voltage Drop Calculator
Voltage drop in a wire run is proportional to current, wire length and wire resistance. For single-phase, the factor of 2 accounts for the current flowing through both the hot conductor and the return (neutral) conductor. For three-phase, the factor is √3 because the three phases are offset and the net drop across a balanced load is smaller. The NEC (210.19) recommends limiting branch-circuit drop to 3% and total drop (feeder + branch) to 5%.
Formulas
Single-phase voltage drop
Vdrop = 2 × length(ft) × resistance(Ω/ft) × current(A)
Three-phase voltage drop
Vdrop = √3 × length(ft) × resistance(Ω/ft) × current(A)
Drop percentage
Drop % = (voltage drop ÷ source voltage) × 100
Worked example
With the default values below, Voltage Drop Calculator returns Voltage Drop of 7.7200. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Circuit Typeone of Single-phase (2-wire), Three-phase (3-wire)
- Single-phase (2-wire)
- Source Voltageminimum 1 V
- 120 V
- Load Currentminimum 0.01 A
- 20 A
- One-way Wire Run Lengthminimum 1 ft
- 100 ft
- Wire Gauge (AWG)one of #18 AWG, #16 AWG, #14 AWG, #12 AWG, #10 AWG, #8 AWG, #6 AWG, #4 AWG, #3 AWG, #2 AWG, #1 AWG, #1/0 AWG, #2/0 AWG, #3/0 AWG, #4/0 AWG, #250 AWG, #300 AWG, #350 AWG, #400 AWG, #500 AWG, #600 AWG, #750 AWG, #1000 AWG
- #12 AWG
- Conductor Materialone of Copper, Aluminum
- Copper
Results
- Voltage Drop
- 7.7200 V
- Drop %NEC recommends ≤ 3% for branch circuits.
- 6.43 %
- Voltage at Load
- 112.28 V
- NEC Status
- ⚠ EXCEEDS 5% total guideline (6.43%) — select a larger wire gauge.
- Wire Resistance
- 1.9300 Ω/1000 ft
Step by step
- 1Wire resistance
- 2Single-phase voltage drop= 7.7200 V
- 3Drop as percentage of source= 6.43%
- 4Voltage at load= 112.28 V
- 5NEC compliance= ⚠ EXCEEDS 5% total guideline (6.43%) — select a larger wire gauge.
Assumptions and limits
- The result is derived only from the 6 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What is the NEC 3% voltage drop rule?
NEC 210.19(A)(1) recommends (but does not mandate) that branch-circuit conductors be sized so voltage drop does not exceed 3%. The combined feeder and branch-circuit drop should not exceed 5%. High voltage drop wastes energy and can damage equipment.
Why does single-phase use 2× and three-phase use √3?
Single-phase uses 2× because current travels through both the hot and neutral conductors. Three-phase uses √3 ≈ 1.732 because the three phases are 120° apart; the geometric relationship reduces the effective round-trip distance.
What should I do if my drop exceeds 3%?
Select the next larger wire gauge (lower AWG number). Each gauge step up reduces resistance by about 20%. Alternatively, shorten the run by relocating the panel or adding a sub-panel closer to the load.
Is this for AC or DC circuits?
The calculation is valid for DC and resistive AC loads. For AC circuits with significant reactance (long runs of large conductors), the AC impedance from NEC Table 9 should be used instead of the DC resistance.