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Voltage Drop Calculator

Calculate voltage drop for electrical wire runs. Check NEC compliance for copper and aluminum wiring.

Voltage Drop Worksheet
Wire material
Phase

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Understanding Voltage Drop

Voltage drop occurs when electrical current flows through a conductor. The wire's resistance causes some voltage to be lost as heat, reducing the voltage available at the load. Excessive voltage drop can cause equipment to malfunction or operate inefficiently.

Voltage Drop Formula

  • Single Phase: VD = 2 × L × R × I / 1000
  • Three Phase: VD = √3 × L × R × I / 1000

Where L = one-way length (ft), R = wire resistance (Ω/1000ft), I = current (amps).

NEC Voltage Drop Limits

  • Branch circuits: 3% maximum recommended
  • Feeder + branch combined: 5% maximum recommended
  • These are recommendations, not code requirements, but are considered best practice

How to Reduce Voltage Drop

  • Use a larger wire gauge: Going from 14 AWG to 12 AWG cuts resistance by ~37%. Each step up in wire size roughly halves the voltage drop
  • Shorten the wire run: Move the panel closer or use a sub-panel near the load
  • Use copper instead of aluminum: Copper has ~60% of aluminum's resistance per foot. Aluminum requires going 1-2 gauge sizes larger to match copper's performance
  • Increase source voltage: 240V circuits have half the current of 120V for the same wattage, dramatically reducing voltage drop

Common Wire Gauge Reference

  • 14 AWG: 15-amp circuits, lighting, most bedroom outlets
  • 12 AWG: 20-amp circuits, kitchen, bathroom, garage outlets
  • 10 AWG: 30-amp circuits, dryers, large window AC units
  • 8 AWG: 40-amp circuits, electric ranges, large appliances
  • 6 AWG: 50-60 amp circuits, hot tubs, sub-panels, EV chargers

For long runs (over 50 feet), you'll often need to go 1-2 gauge sizes larger than the minimum to stay within the 3% voltage drop recommendation. This is especially important for outbuildings, detached garages, and well pumps.

When Voltage Drop Matters Most

  • LED lighting: Even small voltage drops can cause flickering or dimming
  • Motors and compressors: Low voltage causes overheating, reduced efficiency, and shortened lifespan
  • EV chargers: Long runs to a garage or carport need careful wire sizing for the 40-50 amp draw
  • Well pumps: Often 200+ feet from the panel, making voltage drop a primary concern

Copper vs. Aluminum Wire

PropertyCopperAluminum
Resistivity1.68 × 10⁻⁸ Ω·m2.82 × 10⁻⁸ Ω·m
WeightHeavier~30% lighter
CostHigher~40% less
Typical useBranch circuits, interiorsService entrances, feeders

Aluminum requires anti-oxidant compound at connections and rated connectors (marked AL/CU). Using copper-only connectors with aluminum wire causes oxidation, loose connections, and fire risk. Most modern residential branch circuits use copper; aluminum is common for 100+ amp feeders and service entrance cables where cost savings are significant.

Wire Gauge and Resistance

American Wire Gauge (AWG) uses a counterintuitive numbering system — smaller numbers mean larger wire. Each step down by 3 gauge sizes (e.g., 14 to 11 AWG) roughly doubles the cross-sectional area and halves resistance. For long runs, upgrading wire gauge is the most practical way to reduce voltage drop. The cost of slightly larger wire is almost always less than the ongoing energy loss from an undersized conductor.

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Frequently Asked Questions

What is the maximum voltage drop allowed by the NEC?

The NEC recommends no more than 3 percent voltage drop on a branch circuit and no more than 5 percent in total for the feeder and branch circuit combined. On a 120 V circuit, 3 percent is 3.6 V, and on a 240 V circuit it is 7.2 V. Going past these figures makes motors run hotter and lights dim or flicker.

How do I calculate voltage drop for a 100-foot run?

A 100 foot single-phase run of 12 AWG copper carrying 20 A drops about 6.35 V, which is 5.3 percent on a 120 V circuit. The formula is voltage drop = 2 x length x resistance x current / 1000, using 1.588 ohms per 1000 feet for 12 AWG copper: 2 x 100 x 1.588 x 20 / 1000 = 6.35 V.

What size wire do I need for a 200-foot run?

A 200 foot run at 240 V carrying 60 A typically needs at least 4 AWG copper to stay within the 3 percent NEC recommendation. Longer runs, heavier loads and aluminum wire all raise the drop, because aluminum has higher resistance than copper at the same gauge. Recalculate with the actual load, distance, conductor and source voltage before buying wire.

Does a 240 V circuit reduce voltage drop?

Yes. For the same wattage, a 240 V circuit draws half the current of a 120 V circuit, and voltage drop is proportional to current, so the drop in volts is cut in half. As a percentage the gain is larger still, because half the volts are lost from a supply twice as high, so the percentage drop falls to one quarter.