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Extension Cord Voltage Drop Calculator
Voltage lost along a garden or grow-room extension cord, by gauge, length and amps.
Extension Cord Voltage Drop Calculator
Chlorova · updated for 2026
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Resistance figures are for copper at about 75°C and are rounded — aluminum cords and cheap copper-clad wire run 2× or worse, and a cord coiled in a reel adds inductive loss the flat-cable formula here does not count. Any drop over 5% deserves a heavier gauge, a shorter run, or a real circuit installed by an electrician.
The extension cord voltage drop calculator sizes the wire between a grow tent, pump or shop tool and the wall outlet — the piece nearly everyone gets wrong. Longer cords and thinner gauges both waste volts as heat along the cable, so the light or motor at the end sees less than 120 and works harder. Enter gauge, length and amp load, and it returns the drop in volts, as a percentage, and the watts actually cooked off inside the cord. That is the math that decides whether 100 feet of 14 AWG is safe or a slow fire.
How to Use the Voltage-Drop Calculator
- Choose the wire gauge printed on the cord's jacket — do not guess.
- Enter the total one-way cord length in feet, end to end.
- Set the amp draw of the tool or fixture at the far end.
- Use running amps for continuous loads, but check startup amps for motors.
- Read the voltage drop in volts and as a percentage of 120 V.
- Aim for under 3% for motors and electronics, under 5% for lights.
- If the drop runs hot, move to a heavier gauge or a shorter cord.
- Re-run the calculation before daisy-chaining two cords together.
The Voltage-Drop Formula, Explained
The load current goes out on one conductor and returns on the other, so the effective length is twice the cord. Voltage drop = 2 × feet × amps × ohms-per-1000-feet ÷ 1000. A 12 AWG cord runs about 0.64 ohms per thousand feet, which is why 50 feet at 5 amps drops only 0.32 V — but a 14 AWG at 100 feet and 10 amps drops 2 V, and cheap aluminum cords roughly double that again.
Why percentage matters more than volts: a 3-volt drop off a 120 V line is 2.5% and usually invisible, but the same 3 volts off a 24 V low-voltage system is 12.5% and dim. Motors care most, because a low-voltage motor draws extra amps to make the same torque and heats itself; electronics with switch-mode supplies barely notice until the drop is bad enough to make the supply buzz or brown out, and lighting sits in the middle of the sensitivity range.
Worked Example: Long Run to a Shed Pump
- You are feeding a 5 A shed pump from the house with a 100-foot cord.
- The cord you own is 14 AWG — the thin white appliance kind.
- Drop: 2 × 100 × 5 × 1.02 ÷ 1000 = 1.02 V, which is 0.85% of 120 V.
- That number looks fine, but the pump's starting amps are near 15 A.
- At 15 A the drop triples: 3.06 V, or 2.55%, right on the comfort edge.
- A 12 AWG replacement cuts the drop to 1.9 V even at locked-rotor, and 10 AWG to 1.2 V.
- Heat waste at 15 A on the 14 AWG runs about 46 W spread along the cord — enough to soften cheap insulation over years.
- The right answer is 10 AWG for the run, or a small dedicated circuit to the shed.
Cord Resistance by Wire Gauge
Approximate copper-conductor resistance in ohms per 1,000 feet at room temperature. Real cords vary with strand count and temperature, and aluminum or copper-clad cords of the same AWG run about 60% higher than the solid-copper figures shown here.
| Gauge | Ohms per 1000 ft | Typical use |
|---|---|---|
| 14 AWG | 1.02 | Short appliance cords, lamps |
| 12 AWG | 0.64 | Hardware-store 25–50 ft |
| 10 AWG | 0.40 | Heavy-duty outdoor, big tools |
| 8 AWG | 0.25 | Welder-grade, long sub-panel runs |
Key Terms
- AWG
- American Wire Gauge — smaller numbers mean thicker copper and lower resistance.
- Voltage drop
- The volts consumed as heat along the cord between outlet and load.
- Locked-rotor amps
- The starting surge a motor draws at zero speed, often 3–6× the running amps.
- Daisy chain
- Two or more cords plugged end-to-end, doubling the effective length and the drop.
- Copper-clad aluminum
- Cheap cord stock with an aluminum core; higher resistance than solid copper at the same gauge.
What This Means for Long Garden Runs
- Cord coiled on a reel is not the same as cord laid out: a fully-coiled 50-footer runs hotter than a straight one because the returning conductor's heat cannot escape, so uncoil before drawing full load.
- The gauge printed on the jacket is the only number to trust; a physical look at the cord is misleading because insulation thickness has nothing to do with copper size.
- Anything with a motor — pumps, tillers, trimmers — deserves one gauge heavier than the light-only math suggests, because starting amps turn a comfortable 2% drop into a brown-out at the shaft.
- For a permanent shed or hoop-house run, the correct answer is nearly always a small sub-panel fed with proper underground cable rather than a giant 10 AWG extension you have to store every winter.
- Cheap '12 AWG' cords are sometimes really 14 or even 16 with copper-clad aluminum — if the price looks too good for the length, upgrade once from a real supplier rather than buying twice.
Common Mistakes & Fixes
- Sizing the cord by the running amps alone and forgetting the 3–5× inrush a pump or tiller pulls at startup.
- Daisy-chaining two 25-footers and calling it a 50-foot run without rechecking the drop for the doubled length.
- Choosing 16 AWG 'appliance' cords for a 10 A tool because the tag matches the plug and the price is cheap.
- Assuming a heavy-looking black jacket means thick copper — insulation thickness has nothing to do with AWG.
- Leaving a cord coiled on a reel while pulling full amps, so heat builds up and the insulation softens.
When to Use This (and When Not To)
Use it whenever a grow light, pump, tiller or shop tool is run on more than about 25 feet of cord, or fed from a temporary extension for hours at a time. It is US 120 V math; 240 V and 24 V systems behave differently on percentage even though the volt-drop formula is the same. For a permanent wired run to a shed or greenhouse, use a licensed electrician and the appropriate feeders and disconnects rather than a heavier extension cord. And if the load is under 1 A, the drop is almost never a real issue.
Extension Cord Voltage Drop Calculator: Frequently Asked Questions
How long can a garden extension cord be before voltage drop matters?
Under 25 feet of 12 AWG at normal garden loads, drop is usually negligible. Push the same 12 AWG to 100 feet and any motor over 5 A starts losing real volts. The right answer for anything past 50 feet is a heavier gauge, a shorter route, or a permanent circuit.
Is a 14 AWG cord OK for a grow tent?
Only for a short run under 25 feet at low amperage. Anything longer or heavier and you lose enough voltage that LED drivers buzz, ballasts overheat, and inline fans pull extra current to compensate. Move up to 12 or 10 AWG for the length a real tent usually needs.
Does voltage drop actually waste electricity?
Yes, and it wastes it as heat inside the cord itself. The watts consumed along the copper are exactly what does not arrive at the tool, and a long undersized cord can quietly throw off 40–100 W of pure heat — enough to soften cheap insulation over a growing season.
Why does starting a pump cause more drop than running it?
Because motors pull 3–6× their running amps at locked-rotor before they come up to speed, and voltage drop scales linearly with amps. A 100 ft cord that looks fine at 5 A running suddenly drops 3 V at 15 A startup, which is exactly when the pump needs full voltage the most.
How much drop is acceptable?
Design guides generally cap branch-circuit voltage drop around 3%, with a hard upper bound of 5% total. Sensitive electronics and motors should target 3% or under; incandescent lighting is more tolerant. Anything past 5% is a signal the cord gauge or route needs to change.
Does a coiled reel change the math?
The DC resistance stays the same on paper, but a fully-coiled cord traps its own heat and can run dangerously hot under a heavy load — a real fire risk, not a voltage one. Always uncoil an extension cord before pulling anything close to its rated amperage through it.
Sources & Further Reading
- Voltage Drop in Branch Circuits and Feeders — National Electrical Installation Handbook
- Extension Cord Safety and Voltage Drop — U.S. Consumer Product Safety Commission
- Cord Sizing for Farm and Garden Loads — Purdue University Extension Agricultural Safety
Chlorova Gardening Team
Builds, checks and documents every tool we publish
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Checked against extension-service publications
Formula verified against published references · Updated