Understanding Voltage Drop: Why Wire Length Matters
How to calculate and limit voltage drop for reliable electrical circuits
Voltage drop is the reduction in voltage that occurs as current flows through the resistance of a wire. While a small drop is unavoidable, too much can cause lights to dim, motors to run hot, and sensitive equipment to malfunction. This guide explains how voltage drop works and how to keep it within safe limits.
What Causes Voltage Drop
All conductive materials have electrical resistance. As current flows through a wire, energy is lost as heat due to this resistance. The voltage available at the end of the wire is lower than at the source. The longer the wire, the higher its total resistance and the greater the voltage drop. Higher current also increases drop proportionally.
The Voltage Drop Formula
For a single-phase circuit, voltage drop (V) = 2 × L × I × ρ / A, where L is the one-way wire length in meters, I is the current in amps, ρ is the resistivity of the conductor (0.0172 Ω·mm²/m for copper), and A is the cross-sectional area in mm². The factor of 2 accounts for both the outgoing and return conductors. Voltage drop percentage = (VD / system voltage) × 100.
NEC Voltage Drop Limits
The NEC (National Electrical Code) recommends — but does not strictly require — that voltage drop not exceed 3% for branch circuits and 5% for the combined feeder and branch circuit. Exceeding these limits is not a code violation per se, but it can cause equipment problems and is considered poor practice. For sensitive equipment like computers or medical devices, keeping drop below 2% is prudent.
IEC and International Limits
IEC 60364-5-52 recommends a maximum 3% voltage drop for lighting circuits and 5% for other circuits. The UK's BS 7671 allows up to 3% for lighting and 5% for power, though 4% is the design target for combined runs. AS/NZS 3000 (Wiring Rules) allows up to 5% total from the point of supply to the point of use. These are design recommendations, not hard limits.
How to Reduce Voltage Drop
The most direct way to reduce voltage drop is to use a larger wire. Going from AWG 12 to AWG 10 roughly doubles the conductor area and halves the voltage drop. You can also reduce drop by increasing system voltage (240V circuits have one-quarter the drop of 120V for the same load), shortening wire runs, or splitting the load across multiple circuits. For long runs like outdoor lighting or subpanels, always calculate drop before selecting wire.
FAQ
How much voltage drop is acceptable?
NEC recommends 3% maximum for branch circuits and 5% combined. Most electricians target 2–3% as a practical design limit. For motor circuits, keep drop below 2% to avoid starting problems. For LED drivers and sensitive electronics, below 1.5% is better.
Does voltage drop matter for short runs?
For runs under 25 feet at normal residential currents, voltage drop is usually negligible and ampacity alone determines wire size. Voltage drop becomes a concern at runs over 50 feet, especially at higher currents.
Is voltage drop different for aluminum wire?
Yes. Aluminum has about 61% of the conductivity of copper (ρ ≈ 0.0282 Ω·mm²/m vs 0.0172 for copper). For the same cross-section, aluminum has about 64% more resistance than copper. When using aluminum, you must use a larger conductor — typically one or two AWG sizes larger — to achieve the same voltage drop as copper.