Voltage drop is the voltage a conductor loses over its length: Vd = 2 × K × I × L ÷ CM for single-phase (K = 12.9 copper / 21.2 aluminum, I = amps, L = one-way feet, CM = the wire's circular mils). The industry targets are 3% on a branch circuit and 5% total — but those are NEC recommendations (Informational Notes), not enforceable code. Drop matters on long runs and big loads: a detached garage, a well pump 300 feet out, an EV charger across the property. The fix is almost always to go up a wire size.
Worried about a long run? The voltage drop calculator computes the drop and percentage for your wire, load, and distance — free, no signup. This guide explains the formula, when drop actually matters, and why it's a recommendation rather than a hard rule.
🧮 The formula
Voltage drop uses the circular-mil (K-factor) method:
single-phase: Vd = 2 × K × I × L ÷ CM
three-phase: Vd = 1.732 × K × I × L ÷ CM
where K is the conductor's resistivity (12.9 for copper, 21.2 for aluminum at 75°C), I is the current in amps, L is the one-way run length in feet, and CM is the conductor's cross-section in circular mils (14 AWG = 4,110 CM; 12 AWG = 6,530 CM, and up from there). The 2× is because current travels out and back.
The takeaway from the math: drop grows with amps and distance, and shrinks as the wire gets fatter. Aluminum drops more than copper for the same size.
📏 The 3% / 5% targets
The widely cited guidance is no more than 3% drop on a branch circuit and 5% total from the service to the farthest outlet. Keep within that and equipment gets essentially its rated voltage.
These are recommendations, not code. The 3%/5% figures live in the Informational Notes to NEC 210.19 and 215.2 — and Informational Notes are explicitly non-enforceable (90.5(C)). So an inspector won't fail a circuit for 4% drop. They're still good engineering practice, especially for motors and heaters, which is why the calculator flags them as targets rather than pass/fail rules.
📍 When drop actually matters
On a normal 20–40 foot household branch circuit, voltage drop is negligible — the wire sized for ampacity is already fine. Drop becomes a real design factor when the run is long or the load is large, and especially when both:
- A subpanel in a detached garage 100+ feet away.
- A well pump hundreds of feet from the house.
- An EV charger or shop on a long run.
- Any motor — low voltage makes a motor draw more current and run hot.
Undervoltage dims lights, slows motors, and shortens equipment life, so on these runs it's worth checking even though it's not enforced.
🔧 The fix: go up a size
Because drop is inversely proportional to circular mils, the remedy is almost always to upsize the conductor. Jumping from 12 AWG to 10 AWG, or 10 to 8, cuts the drop substantially on a long run. It's cheaper than any alternative and doesn't change the breaker (the overcurrent device still matches the load, not the upsized wire).
Voltage drop is a separate question from ampacity: the wire size calculator picks the gauge that safely carries the current, and the voltage-drop check tells you whether that gauge also holds the voltage over your distance. On a long run, drop — not ampacity — often sets the wire size.
✅ The bottom line
Voltage drop grows with current and distance and shrinks with wire size. Aim for 3% on a branch and 5% total — as good practice, not code — and only sweat it on long runs and big loads, where drop rather than ampacity may govern the gauge. The fix is to go up a conductor size.
The 3%/5% figures are NEC Informational-Note recommendations, not enforceable requirements; confirm any actual code obligation and local amendments with your AHJ. Check your run with the voltage drop calculator.
Estimate your Voltage Drop materials
How much voltage do you lose over a wire run? Free NEC K-factor calculator for volts and % drop by wire size, length, and load — with the 3% check.
Estimate with the Voltage Drop Calculator →