Calculate wire voltage drop for single and three phase circuits using NEC Chapter 9 resistance data, with distance-vs-drop chart and 3%/5% compliance check.
Units:
Result
Voltage drop—
Drop %—
Voltage at load—
Power loss—
—Enter values to see compliance with NEC recommendations.
Voltage drop vs. distance
Voltage drop curve 3 % limit (branch) 5 % limit (feeder) Current distance
Quick presets
Formulas & data sources
Single phase (two conductors):
V_drop = 2 × I × L × R / 1000
Three phase (balanced):
V_drop = √3 × I × L × R × PF / 1000
Where I is current (A), L is one-way length (ft or m), R is conductor resistance (Ω per 1000 ft or Ω per km), PF is power factor.
Resistance values taken from NEC Chapter 9, Table 8 (stranded, uncoated) at 75 °C. Temperature correction:
R(T) = R75 × [1 + α × (T − 75)]
α = 0.00393 /°C (copper), 0.00403 /°C (aluminum). Power loss in conductors is computed from I² × R_total.
Frequently asked questions
Why does voltage drop matter?
Low voltage at the load shortens motor life, dims lighting, trips electronics and wastes energy as heat in the conductors. Limiting drop keeps equipment operating within its rated voltage window and reduces wire heating losses.
What are the NEC voltage drop limits?
NEC 210.19(A) Informational Note 4 recommends no more than 3 % drop on branch circuits, and NEC 215.2(A) Informational Note 2 recommends 5 % total (feeder + branch combined). These are guidelines for efficient operation, not enforced code, but inspectors expect them on design drawings.
How do I reduce voltage drop?
Use a larger conductor (lower AWG or larger mm²), shorten the run, raise the system voltage (240 V instead of 120 V halves the current for the same power), switch from aluminum to copper, or split the load across parallel circuits. Doubling the wire cross-section roughly halves the drop.
Why is three-phase more efficient for long runs?
Three-phase uses √3 (≈1.732) in the formula instead of 2, and the line current for the same power is lower by √3. The net effect is roughly half the voltage drop of an equivalent single-phase run at the same line-to-line voltage, which is why motors and commercial feeders are almost always three-phase.
Does temperature really change conductor resistance?
Yes. Copper resistance rises about 0.39 % per °C and aluminum about 0.40 % per °C. A wire running at 90 °C has roughly 6 % more resistance than the 75 °C table value, so a long hot run can fail the 3 % rule even if the cold calculation passes.
Do I use the one-way or round-trip distance?
Enter the one-way distance from source to load. The single-phase formula already multiplies by 2 to account for the return path through the neutral or second hot. The three-phase formula uses √3 because the return current is shared across the three conductors.
Values are indicative and based on NEC Chapter 9, Table 8 resistance data. Always verify with local codes and a qualified electrician before installing wire.
This voltage drop calculator computes line loss for copper and aluminum conductors in AWG or metric mm^2, for single-phase two-wire and balanced three-phase circuits. Enter system voltage (120, 230, 240, 277, 400, 480 V or custom), load current, one-way distance, wire size, conductor temperature rating (60/75/90 C) and power factor. The calculator returns voltage drop in volts, drop percentage, voltage at the load and power lost as heat, and flags compliance with NEC 210.19 (3%% branch) and NEC 215.2 (5%% feeder) informational limits. Example: 200 ft of 12 AWG copper carrying 20 A at 120 V single phase gives roughly 7.9 V drop or 6.6%% – over the 5%% recommendation, so upsize to 10 AWG. A distance-vs-drop chart shows how the result scales with run length, and presets cover typical residential, EV, RV, solar DC and three-phase motor feeds.