What Does Voltage Drop Mean for Extension Cords?
Voltage drop is the voltage lost along a cord's length due to the conductor's own electrical resistance.
The formula is Vdrop = 2 × L × I × R — length (doubled for the hot-and-neutral round trip)
times current times the conductor's resistance per foot. The commonly-cited safe limit is
3% on a 120V circuit (a 3.6V drop), and it's why a cord's safe amperage shrinks as its
length grows, even though its gauge hasn't changed.
Why it matters more than most buyers expect
A cord's thermal ampacity rating (how much current it can carry without overheating) doesn't change with length — but voltage drop does, and it gets worse the longer the cord runs. On a long cord, voltage drop becomes the binding limit long before thermal overheating would be, which is why the Cord Gauge Ampacity Index reports a LOWER safe amperage at 100ft and 150ft than the same gauge's thermal cap would otherwise allow.
What happens when voltage drop is exceeded
Motors run hotter and deliver less torque, which can shorten their service life over repeated use; incandescent-style lighting dims; heating elements (space heaters) underperform their rated wattage; and some electronics can behave erratically under sustained low voltage. It is a performance and equipment-wear issue layered on top of the separate thermal overheating/fire-risk question that gauge and thermal ampacity address.
This page is general reference, not an electrical code determination.
Frequently Asked Questions
The loss of voltage along a cord's length caused by the conductor's own electrical resistance. The longer and thinner the cord, the more resistance, and the more voltage is lost before it reaches the tool or appliance at the far end.
Vdrop = 2 x L x I x R, where L is the one-way cord length in feet (doubled because current travels out on the hot conductor and back on the neutral), I is the current in amps, and R is the conductor's resistance in ohms per foot for that AWG gauge. See our Cord Gauge Ampacity Index for the full worked table.
3% is the commonly-cited recommended limit for branch circuits (a 3.6V drop on a 120V circuit). Some sources allow up to 5% for combined feeder-plus-branch circuits, but we round toward the more conservative 3% figure throughout this site.
Motors run hotter and weaker and can have a shortened service life; incandescent lighting dims; heating elements (space heaters) underperform; and in some equipment, control electronics can behave erratically. It's a performance and equipment-wear problem as much as a safety one, on top of the separate thermal/fire-risk question of an overloaded cord.