Quick answer: enter the physical cord length once. This calculator doubles it once for the outgoing and returning current path, then uses a published copper resistance and your running current to estimate voltage lost and voltage delivered at the load. The result is arithmetic, not a code, safety, or equipment-suitability approval.
Extension Cord Voltage Drop Calculator
Model one temporary copper extension cord and one running load.
Runs locally in your browser. Entered values are not submitted to TPTK.
1. Enter the electrical values
Use the exact cord label, equipment nameplate/manual, and measured route.
Current mode: use running current from the connected equipment’s exact nameplate or manufacturer instructions. Starting/inrush current is not modeled.
2. Confirm the field decision gates
The calculator shows math but returns a no-go handoff when a gate is unresolved.
The model does not include plug/receptacle contact resistance, upstream branch-circuit drop, temperature change, flexible-strand variation, AC reactance, coiling, harmonics, duty cycle, startup/inrush, motor behavior, generator regulation, or equipment control electronics.
What The Two Result States Mean
The entered gates are confirmed
The tool reports the conductor-only arithmetic and records that each user-entered field gate was answered yes. It still does not approve safety, compliance, or equipment suitability.
One or more gates need resolution
The page may show the math, but it names the missing document, exceeded factory rating, prohibited arrangement, condition issue, or qualified-electrical question that stops the planning record.
No numeric output is shown
Fix missing, nonnumeric, or unsupported values first. The calculator will not leave a stale result visible.
The Voltage-Drop Formula And Units
The physical cord length is the one-way distance from source to load. Current travels through one conductor to the load and returns through another, so this model doubles the physical length exactly once.
Circuit length (ft) = 2 x physical one-way cord length (ft)Circuit resistance (ohms) = resistance (ohms/1,000 ft) x circuit length (ft) / 1,000Voltage drop (V) = running current (A) x circuit resistance (ohms)Estimated delivered voltage (V) = entered source voltage (V) - voltage drop (V)Voltage drop (%) = voltage drop (V) / entered source voltage (V) x 100Conductor loss (W) = running current (A) x running current (A) x circuit resistance (ohms)Watts mode current (A) = real input watts (W) / [source voltage (V) x power factor]
Published Copper Resistance Inputs
Southwire’s Power Cable Installation Guide publishes the following 25 C DC resistance values for uncoated concentric-lay stranded copper. The model uses only these values and does not infer ampacity from them.
| Gauge | Published basis | Resistance at 25 C |
|---|---|---|
| 14 AWG | Uncoated stranded copper, Class B/C | 2.63 ohms/1,000 ft |
| 12 AWG | Uncoated stranded copper, Class B/C | 1.66 ohms/1,000 ft |
| 10 AWG | Uncoated stranded copper, Class B/C | 1.04 ohms/1,000 ft |
Why no 16 or 18 AWG choice? The selected primary manufacturer table begins at 14 AWG. Keeping the selector narrower is more honest than mixing unsourced or differently defined resistance tables.
Manual No-JavaScript Worksheet
- Write the measured/documented source voltage and running amps. If you only have real watts, obtain the equipment’s power factor; do not assume it is 1.00.
- Write the physical one-way cord length and multiply it by two.
- Choose the published 14, 12, or 10 AWG resistance value above.
- Multiply resistance by circuit feet and divide by 1,000 to get circuit ohms.
- Multiply circuit ohms by running amps for voltage lost; subtract that from source voltage for estimated delivered voltage.
- Separately verify the exact cord factory rating, equipment manual, protection program, marking/location, inspection, route, employer/AHJ rules, and temporary-use boundary.
Reproducible Worked Examples
These examples test the arithmetic only. They do not recommend a gauge or authorize a load.
120 V, 100 ft, 12 A
Circuit length: 200 ft. Resistance: 0.332 ohm. Drop: 3.984 V. Estimated delivered voltage: 116.016 V. Conductor loss: 47.808 W.
120 V, 50 ft, 10 A
Circuit length: 100 ft. Resistance: 0.263 ohm. Drop: 2.630 V. Estimated delivered voltage: 117.370 V. Conductor loss: 26.300 W.
1,200 W, 120 V, PF 0.80
Derived current: 12.500 A. With 12 AWG at 100 ft: drop 4.150 V, estimated delivered voltage 115.850 V, conductor loss 51.875 W.
Voltage Drop Is Only One Part Of A Job-Site Cord Decision
Record the conductor estimate
Keep source voltage, physical length, gauge, resistance basis, running current, voltage lost, delivered voltage, and conductor loss together.
Check the real cord and work
Confirm GFCI or the applicable assured grounding program, exact marking/listing/location, inspection, route protection, factory rating, connected-equipment instructions, and employer/AHJ rules.
Stop when the model ends
Use the manufacturer, employer safety lead, AHJ, or a qualified electrician for motors/startup, unusual sources, permanent wiring, custom temporary power, damaged equipment, unclear markings, or any unresolved condition.
What This Calculator Does Not Model
- Temperature
- The resistance table is a 25 C reference. Actual conductor temperature can raise resistance, and ambient temperature is not the same as conductor temperature.
- Connections
- Plug blades, receptacles, cord connectors, contact wear, corrosion, and connection heating add losses not included in the conductor-only estimate.
- Upstream source
- The entered source voltage may already reflect branch-circuit drop or generator regulation. The calculator does not model the wiring before the cord.
- Starting loads
- Motors, compressors, pumps, welders, and electronically controlled tools can draw differently during startup and operation. Use exact manufacturer instructions and qualified review.
- AC behavior
- The model uses published resistance and omits reactance, harmonics, waveform quality, and constant-power feedback at the estimated delivered voltage.
- Ampacity and permission
- Resistance is not ampacity. Gauge, breaker size, source voltage, or a small calculated drop does not prove the cord is permitted, correctly rated, suitable, safe, or code compliant.
Method, Provenance, And Corrections
Model version: 1.0, published July 16, 2026. The calculation is deterministic: the same valid inputs produce the same output. No user-entered scenario is sent to TPTK, saved to an account, or included in analytics.
Fact versus editorial boundary: resistance values and formulas are attributed below. TPTK’s decision to return a no-go handoff for unresolved field gates is an editorial risk-control design, not a quoted regulation or universal legal rule.
Corrections: report a formula, source, or accessibility issue through the TPTK contact page. Material corrections will update the visible version note and modification date.
Primary Sources
- Southwire Power Cable Installation Guide, Chapter 2, Table 2-6: 25 C stranded-copper resistance values.
- Southwire Voltage Drop Fundamentals and Best Practices: single-phase/DC formula and one-way length factor.
- Southwire Re3 Voltage Drop Calculator: voltage-drop inputs and professional/AHJ responsibility boundary.
- OSHA 29 CFR 1926.404: construction GFCI and assured equipment grounding conductor program requirements.
- OSHA 29 CFR 1926.405: extension-cord construction use, three-wire, and hard/extra-hard usage boundaries.
- OSHA 29 CFR 1910.334: inspection, damaged-equipment removal, grounding, and handling.
- OSHA Flexible Cords: strain relief, damage, service marking, and wet-connector context.
- CPSC Extension Cords FAQ: factory assembly and observable safety characteristics.
- CPSC Household Extension Cords Can Cause Fires: rating, inspection, outdoor marking, alteration, and listing checks.
Choose The Next Power Planning Owner
This calculator is affiliate-free. Product selection and other electrical questions stay with their existing pages.
Compare job-site extension cords
Use the separate buyer guide for service marking, length, factory rating, product evidence, and purchase routes.
Open the professional power hub
Choose runtime, airline battery, solar compatibility, vehicle USB-C, or this cord-run calculation by question.
Estimate portable-station runtime
Use the separate watt-hour model when the question is battery runtime rather than cord voltage drop.
Frequently Asked Questions
Do I enter extension cord length one way or round trip?
Enter the physical one-way cord length from source to load. The calculator doubles that value once to represent the outgoing and returning current path. Do not double the length before entering it.
Does a low calculated voltage drop mean the extension cord setup is safe?
No. The number is a conductor-only estimate. It does not establish ampacity, condition, listing, location suitability, GFCI or assured-grounding compliance, equipment permission, code compliance, or field safety.
Why does watts mode require power factor?
For single-phase AC, real watts divided by volts alone gives current only when power factor is 1.00. The calculator requires the user to enter a known power factor and uses current equals watts divided by volts times power factor. If power factor is unknown, use documented running amps or stop for manufacturer guidance.
Why are only 14, 12, and 10 AWG copper available?
The selected primary Southwire table publishes the exact 25 C stranded-copper resistance values used here beginning at 14 AWG. The calculator stays within those supported inputs instead of mixing tables with different conductor definitions.
Does the calculator include plug and connector resistance?
No. It models conductor resistance only. Plug blades, receptacles, contacts, corrosion, wear, temperature, upstream wiring, and other connections can add loss.
Can this calculator approve an extension cord for a motor, compressor, pump, or welder?
No. Running-current arithmetic does not model startup or inrush behavior, duty cycle, waveform, controls, or manufacturer-specific limits. Use the exact equipment and cord instructions and escalate uncertain systems to a qualified electrician or other responsible authority.
Can I use two extension cords if I add their lengths?
This calculator is intentionally limited to one factory-assembled cord. It does not authorize daisy-chaining or series-connected cords. Use a correctly designed single run or obtain qualified temporary-power guidance.