Extension Cord Voltage Drop Calculator

How extension cord voltage drop is calculated

Single-phase voltage drop on a cord follows the same relationship as any single-phase circuit:

K is 12.9 for copper at 75°C, I is the load current, L is the one-way cord length, and CM is the conductor's circular mils. The leading 2 accounts for the round trip. Down the hot and back on the neutral. Circular mils for the common cord gauges (NEC Chapter 9, Table 8): 16 AWG = 2,580; 14 AWG = 4,110; 12 AWG = 6,530; 10 AWG = 10,380. Cords are copper, so aluminum's K-factor doesn't enter into it. At these small sizes AC and DC resistance are effectively equal. Skin effect doesn't matter until conductors get much larger. This calculator assumes a unity power factor (purely resistive load), the conservative assumption for a general-purpose tool. A lagging power factor lowers the true drop, so results here read as an upper bound rather than an exact figure.

Worked example

A 12A tool on a 50 ft, 14 AWG cord at 120V:

At 3.14% this is just past the 3% guideline. Swapping to a 12 AWG cord on the same run drops it to 2.37 V (1.98%), and the voltage at the tool climbs to about 117.6 V. This is the typical jobsite call: a 14 AWG cord is fine for short pulls but goes marginal on a 50 ft run with a real load.

Common mistakes

• Sizing by the cord's amp rating alone. A cord stamped 13A carries that current, but the rating says nothing about how much voltage it loses over 50 or 100 feet.
• Reading gauge backward. 16 AWG is thinner than 12 AWG. A higher number means more drop, not less.
• Daisy-chaining without adding lengths. Two 50 ft cords drop voltage like one 100 ft run, plus extra resistance at every connection.
• Ignoring motor startup. Motor-driven tools pull well above running current on inrush; a cord that's fine at running load can sag enough to stall or overheat the motor on startup.
• Mishandling distance. Enter the one-way length. The calculator already doubles it for the return conductor.

Frequently asked questions

What gauge extension cord do I need for power tools?

It depends on the tool's current draw and the cord length together, not the tool alone. A 12A tool on a 50 ft cord needs 12 AWG to stay near 3% drop; 14 AWG on that run hits about 3.1%, and 16 AWG is well past it. Field rule of thumb: 16 AWG for light loads under ~25 ft, 14 AWG for medium loads to ~50 ft, 12 AWG for heavy loads or runs past 50 ft, and 10 AWG for high-current tools on long runs. Run your exact load and length above to confirm.

How long can an extension cord be before voltage drop matters?

Voltage drop scales linearly with length, so doubling the run doubles the drop. For a 12A load, 14 AWG crosses 3% around 48 ft, 12 AWG holds to about 76 ft, and 10 AWG to roughly 120 ft. Total length is what counts. Two 50 ft cords behave like one 100 ft run. Size for the full distance from the receptacle to the tool.

Will a long extension cord damage power tools?

Low voltage at the tool can. Motors compensate for reduced voltage by drawing more current, which raises winding temperature and shortens motor life. Showing up as overheating, lost torque, and nuisance trips. Drops past roughly 5% are where motor tools suffer noticeably. Resistive loads like heaters and lights mostly just run weaker. If a saw bogs down or a compressor struggles to start at the end of a long cord, voltage drop is the usual cause.

Is a 16 AWG extension cord enough for a circular saw?

Usually not, except on a short run. A typical 15A circular saw on a 25 ft 16 AWG cord drops over 6%. Enough to cut power and stress the motor on startup. 16 AWG suits drills, lights, and small loads under ~25 ft. For saws, grinders, and other high-draw tools, use 12 AWG, or 10 AWG on longer runs. Check the saw's nameplate amps against the length.

Does extension cord voltage drop fall under the NEC?

Not directly. The NEC's 3% voltage-drop figure lives in the informational notes to NEC 210.19 and 215.2 and applies to permanently installed branch circuits and feeders. Not portable cords. Flexible cord is addressed elsewhere in the code for ampacity and use, but no NEC table sizes a cord for voltage drop. The 3% threshold here is borrowed as a sensible guideline because the physics are identical; it just isn't a code requirement for cords.

Can I daisy-chain extension cords?

Electrically, the drops add up. Two 50 ft cords equal one 100 ft run, and each connection adds resistance and a failure point. If you must chain them, put the heaviest gauge nearest the tool and size for the total length. A single correctly sized cord always beats several light ones strung together.

Seasonal safety guides

Plugging in holiday decorations rather than tools? These homeowner guides use the same cord and load principles.

• Why Do My Halloween Lights Trip the Arc-Fault (AFCI) Breaker?: why an AFCI reacts to the arc signature rather than the load, and how to separate LED noise from a real fault.
• Are 2-Prong Extension Cords Safe for Outdoor Halloween Decorations?: why an outdoor cord needs a third prong and a weather rating, and why the GFCI does not replace either.
• Why Do My Halloween Lights Flicker?: telling voltage sag on a shared cord apart from a loose connection, and the different fix each needs.
• Can You Plug Halloween Decorations Into a Garage Outlet?: budgeting the amps on a circuit that already feeds the freezer and the door opener.
• How Many Amps Does a Halloween Fog Machine Use?: about 3 to 10 amps by wattage, why the heater cycling hides the draw, and how to size the cord and circuit for it.
• Can You Plug a Fog Machine Into a Power Strip?: why a 10-amp fogger overwhelms a 15-amp strip, the daisy-chain trap, and the safer outdoor setup.
• Can You Run Halloween Decorations on a Portable Power Station?: matching the display to the battery's continuous, surge, and watt-hour ratings, and why a fogger is a poor fit.
• Can You Use a Cheater Plug for Halloween Decorations?: why a three-to-two adapter defeats the ground it seems to add, and why a GFCI is what protects you outside.
• How Many Amps Can a 16-Gauge Extension Cord Handle?: plan on about 10 amps, and why a long run maxes out on voltage drop before it ever reaches that limit.
• Can You Connect Two Different Gauge Extension Cords?: why the thinner cord caps the whole run, and how the two lengths stack for voltage drop.
• Can You Staple or Nail Down a Halloween Extension Cord?: why a staple or nail damages the insulation, and how to hang a cord without one.
• Can You Plug Halloween Lights Into a Light Socket?: the 660-watt adapter limit, and why the porch socket is still the wrong outlet.
• Can You Cover an Extension Cord With Leaves or Mulch?: why hiding a cord under leaves traps heat and hides damage, and what to do instead.
• Do Outdoor Outlets Need a Weatherproof Cover for Halloween Decorations?: the in-use bubble cover an unattended display needs, plus the GFCI beside it.
• What Size Generator Do I Need for Halloween Decorations?: sizing a portable generator to the display, and the carbon-monoxide rule that comes first.
• How Many Amps Does a Halloween LED Spotlight Use?: a fraction of an amp per light; the circuit total and cord are the real limits.
• How Many Watts Does a Halloween Projector Use?: reading the label, and why a garden projector and a video projector are worlds apart.
• How Many Amps Does a Halloween Inflatable Use?: the blower's running draw, how many fit on one circuit, and the startup surge.
• How Many Amps Does a Halloween Animatronic Use?: why one prop is trivial, and how a yard full of them sums up.
• How Many Amps Does a 12-Foot Skeleton Use?: the giant prop draws almost nothing; the cord and circuit total are what to size.
• How Many Amps Does a Halloween Strobe Light Use?: a fraction of an amp on its own, and how it counts against the circuit and cord.
• How to Keep Outdoor Cord Connections Dry: elevate, drip-loop, and cover the plug-to-plug junction.
• Can Halloween Lights Cause a Fire?: the overload, damage, and contact paths behind a display fire.
• Is It Safe to Run an Extension Cord Through a Window or Door?: why a pinched cord and a skipped GFCI make this a bad idea.
• Are LED Halloween Lights Cheaper to Run Than Incandescent?: the amp and cost math, and how many more strings LED lets you run.
• Can Halloween Lights Get Rained On?: protecting outdoor cords and connections from the weather.
• Why Is My Extension Cord Hot With Halloween Lights?: what a warm cord is telling you and when to unplug it.
• Can You Plug Halloween Lights Into a Power Strip Outside?: why an indoor strip is the wrong tool for the yard, and what to use instead.
• Can You Use an Indoor Extension Cord Outside for Halloween?: why the jacket, not the length, is what makes a cord outdoor-safe.
• How Many Halloween Light Strings Can You Connect End to End?: the fused-plug and wire limit on chaining light sets together.
• How Long Can an Extension Cord Be for Outdoor Halloween Decorations?: length limits by gauge before voltage drop bites.
• Why Does My Halloween Inflatable Keep Tripping the Breaker?: telling an overload apart from a ground fault.
• What Gauge Extension Cord for Outdoor Halloween Decorations?: matching cord gauge to the load and the run length.
• Can You Plug Extension Cords Together for Halloween Lights?: why chaining cords adds voltage drop and risk.
• How Many Amps of Halloween Decorations on One Extension Cord?: adding up the load so you do not overload the circuit.
• Do Outdoor Halloween Decorations Need a GFCI Outlet?: the shock-protection rule for anything plugged in outside.
• Will a Halloween Fog Machine Trip a Breaker?: why a fogger is a heavy load and how to fit it on the circuit.
• How Many Halloween Decorations Can I Run on One Outlet?: adding up the whole display against the circuit limit.
• Can I Leave Halloween Decorations Plugged In Overnight?: why a timer, not an all-night run, is the safe way to power a display.
• What Amp Rating Do I Need on an Outdoor Timer or Smart Plug?: sizing the timer so the switch itself is not overloaded.
• Why Are My Halloween Lights Dim at the End of the String?: the voltage-drop math behind a fading run and how to fix it.
• Can You Run an Extension Cord Across a Driveway for Halloween?: routing a long outdoor run safely and sizing it for the distance.
• Why Do My Halloween Lights Keep Blowing Fuses?: telling an overload apart from a faulted string, and fixing the right one.
• Can I Put a Bigger Fuse in My Halloween Lights?: why upsizing the plug fuse is the one fix you must never use.
• Why Does My Outdoor GFCI Keep Tripping With Halloween Decorations?: why water, not a bad outlet, is almost always behind the trips.
• Do Halloween Decorations Need a Dedicated Circuit?: adding up the display in amps to see whether one circuit is enough.
• Can You Bury an Extension Cord for Halloween?: why a cord can't go underground and how to route a long yard run instead.
• Can You Tape Down an Extension Cord for Halloween?: why a full tape wrap traps heat, and the safe way to flatten a cord across a path.
• Can You Plug Halloween Decorations Into an Outdoor Outlet Splitter?: why an extra outlet is not extra power, and the one circuit every plug still shares.
• How Much Does It Cost to Run Halloween Decorations?: turning the total watts into a dollar figure, and the same number into a safety check.

Related calculators

• Voltage Drop Calculator: For permanently installed branch circuits and feeders.
• Wire Size Calculator: Size a conductor from scratch for a load and distance.
• Wire Ampacity Calculator: Conductor ampacity with temperature and conduit-fill derating.