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Voltage Drop Explained (and the 3% Rule)

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When current flows through a wire it loses some voltage to the resistance of the conductor. That loss — voltage drop — is unavoidable, but it can be managed by choosing a wire gauge large enough to keep the drop within an acceptable range. This guide explains what voltage drop is, what the widely-cited 3% and 5% guidelines mean, and why longer runs generally require heavier wire.

What Voltage Drop Actually Is

Think of a garden hose: the longer and narrower the hose, the more pressure is lost before water reaches the nozzle. Electrical conductors work the same way. Every foot of wire has a small amount of resistance, and as current flows through that resistance, some voltage is "used up" getting to the load instead of doing useful work there.

A motor at the end of a long, undersized run may receive only 108 V on a 120 V circuit — enough to run, but the motor must draw more current to compensate, which generates extra heat and shortens its life. Lights dim. Sensitive electronics may malfunction or display erratic behavior. The losses are not a safety hazard in most cases, but they reduce performance and increase energy costs over time.

The 3% and 5% Guidelines — Recommendation, Not a Hard Rule

The NEC (National Electrical Code, NFPA 70) addresses voltage drop in Informational Notes attached to sections 210.19(A) and 215.2(A)(4). Per NEC 90.5(C), Informational Notes are explanatory material only and are not enforceable requirements — so these are best-practice recommendations, not mandatory limits that will fail an inspection in most jurisdictions (as of 2026, per authoritative NEC commentary; local amendments may differ).

The guidance breaks down as follows:

Circuit segment NEC guideline Enforceable?
Branch circuit only (210.19 note) ≤ 3% Recommendation only
Feeder only (215.2 note) ≤ 3% Recommendation only
Combined feeder + branch circuit ≤ 5% Recommendation only
Sensitive electronics (NEC 647.4D) ≤ 1.5% branch / ≤ 2.5% combined Mandatory per NEC
Fire pump motors (NEC 695.7) ≤ 15% during motor start Mandatory per NEC

In practice, most electricians and engineers treat the 3%/5% thresholds as a design target because the consequences of exceeding them are real — even if not an automatic code violation. Check with your local Authority Having Jurisdiction (AHJ), as some states and municipalities have adopted amendments that make these limits enforceable.

The Voltage-Drop Formula (Conceptually)

The standard formula used for sizing conductors to a voltage-drop limit is based on conductor resistance and is published in NEC training materials and references such as Ugly's Electrical Reference:

Vd = phaseFactor × K × I × L / CM

  • Vd — voltage drop in volts
  • phaseFactor — 2 for single-phase (out and back), √3 for three-phase
  • K — resistivity constant for the conductor material (copper vs. aluminum)
  • I — design current in amperes
  • L — one-way run length in feet
  • CM — conductor area in circular mils (larger wire = more CM = less drop)

The percentage voltage drop is then (Vd / system voltage) × 100. You don't need to work through this math manually — use the wire size calculator to get a wire size that satisfies your target drop percentage automatically.

Why Long Runs Need Bigger Wire

The formula makes one thing clear: voltage drop scales directly with run length (L). Double the distance, double the drop — unless you also upsize the conductor. Below is an illustration of how drop grows with distance for a 20 A load on 12 AWG copper at 120 V (single-phase). These numbers are approximate and for illustration only; use the calculator for your actual circuit.

Approx. % drop 3% 25 ft 50 ft 75 ft 100 ft 150 ft 200 ft One-way run length — 12 AWG copper, 20 A, 120 V (illustrative) Within 3% guideline Approaching limit Exceeds 3% guideline

The takeaway: on a typical 20 A, 120 V branch circuit, 12 AWG copper generally stays within the 3% recommendation for runs under roughly 75–100 feet. Beyond that, upsizing to 10 AWG or larger is often warranted — but the exact threshold depends on your actual load, voltage, and phase. Let the calculator determine the right size for your specific numbers.

Common Situations Where Voltage Drop Matters Most

  • Detached garages and outbuildings — long feeder runs from the main panel
  • EV charger circuits — high continuous current over runs of 50+ feet
  • Solar and off-grid systems — DC wiring at low voltages amplifies percentage drop
  • Well pumps and irrigation motors — long underground runs with high starting current
  • Outdoor lighting circuits — daisy-chained loads at the far end of a long run
  • Sensitive audio/video or lab equipment — where even 2% drop may cause issues

Get the Right Wire Size for Your Run

Rather than working through the formula by hand, use the calculator on this site. Enter your load in amps, the one-way run length, conductor material, system voltage, phase, and your target voltage-drop percentage. The tool returns the minimum AWG that satisfies both ampacity and voltage-drop requirements, along with the actual calculated drop at that size.

Open the Wire Size Calculator →

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