Quick answer
For 120 V, 15 A, 50 ft one-way, 12 AWG copper, the estimate is about 2.38 V drop or 1.99%, usually within a common 3% planning target.
Best for: comparing realistic scenarios before acting. Not for: final professional guidance, emergency decisions, or replacing product labels and local requirements.
How to use the calculator
- Enter the labeled inputs for this cable scenario.
- Adjust the optional assumptions shown in the form instead of relying on defaults blindly.
- Read the primary result, supporting result cards, and any warning notes together.
- Use share, copy, or print/PDF to save the exact scenario for later comparison.
What the results mean
The primary result is the main number to use first. Supporting cards show the inputs, ranges, assumptions, or follow-up checks that explain how to interpret it.
Decision notes for this calculator
Status: Safety-sensitive planning aid
The labeled inputs and visible assumptions shown in the form move the result most.
What to do next
Compare a conservative scenario with your real requirements before acting.
Save this scenario
Use Share to copy a clean URL with your inputs, Copy result for notes or email, and Print / PDF for a saved scenario. If the result looks wrong, report an issue with the Cable Calculator.
Formula / methodology
Voltage drop = current × conductor resistance × length × phase factor. Single-phase/DC pair uses a round-trip factor; three-phase uses 1.732.
Assumptions and limitations
- Inputs are assumed to be measured accurately and entered in the units shown.
- Rounding is intentional so the result is easier to use in real decisions.
- Vendor-specific behavior, local code, product labels, and regional pricing can override a generic calculator.
- Health, legal, tax, emergency, structural, chemical, and other high-risk decisions need qualified guidance beyond a web calculator.
Example calculation
Input: 120 V, 15 A, 50 ft, 12 AWG copper, single-phase. Output: about 2.38 V drop, 1.99% drop, and 35.7 W power loss.
Common mistakes
- Using one-way vs round-trip length incorrectly.
- Ignoring aluminum vs copper resistance.
- Treating a voltage-drop estimate as code approval.
- Forgetting temperature, conduit fill, breaker size, and local code.
FAQ
What formula does this page use?
Voltage drop = current × conductor resistance × length × phase factor. Single-phase/DC pair uses a round-trip factor; three-phase uses 1.732.
What changes the result the most?
Amperage, one-way run length, wire gauge, material, voltage, and phase factor drive voltage drop most; code requirements may be stricter than this planning target.
How should I use this result?
Use it as a planning estimate and compare important decisions with product documentation, labels, quotes, local code, or qualified guidance.
Why does this page show warnings?
Warnings call out assumptions that can materially change the result or create safety, cost, or reliability problems.
Can I share this scenario?
Yes. The share button copies a URL with the current inputs, while the main page remains the base calculator URL.
Sources and assumptions
References used for this calculator:
- Formula basis: Ohm's law voltage-drop planning using conductor resistance, one-way length, current, material, and phase factor.
- Limitations: Local code, breaker sizing, conduit fill, temperature derating, and installation method override this generic estimate.
- Method note date: June 24, 2026.
- Editorial scope: CalculatorShelf editorial math check; not a licensed electrical certification.
- When to contact a professional: Use a licensed electrician for permanent wiring, safety-critical circuits, or any installation subject to electrical code.
Method details
Updated: June 24, 2026. Calculation method: CABLE-1.0. Availability: planning aid.