Quick start
- Enter the source voltage, such as 24 V, 120 V, 208 V, or 240 V.
- Enter the load current in amps for the device or circuit you are checking.
- Enter one-way length in feet from source to load, not round-trip distance.
- Choose copper AWG size and circuit type: single-phase/DC or balanced three-phase.
Best uses
Use this guide when you want to understand the estimate before checking equipment instructions, local code, and a qualified electrician or engineer.
- Estimate voltage drop for a branch circuit run.
- Compare common copper AWG wire sizes.
- Check percent voltage drop from source voltage.
- See load voltage after the estimated drop.
What this calculator is solving
The Voltage Drop Calculator estimates how much voltage may be lost in a simple copper AWG wire run. It is useful for early planning, homework, and sanity checks before a qualified electrical review.
Match each input label on the calculator to the source voltage, expected load current in amps, source-to-load one-way length in feet, copper AWG size, and single-phase/DC or balanced three-phase mode.
The formula in plain language
In plain language: Voltage drop = current x copper resistance per foot x one-way length x circuit factor. Single-phase/DC uses factor 2; balanced three-phase uses sqrt(3). The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
The calculator uses voltage drop = current x copper resistance per foot x one-way length x circuit factor. Single-phase/DC uses factor 2 for the out-and-back path. Balanced three-phase uses sqrt(3).
How to read the answer
Read volts dropped first, then percent drop, then load voltage. The percent drop is often the easiest warning sign because it scales the lost volts against the supply voltage.
- Voltage drop is the estimated volts lost in the conductor run.
- Percent drop compares that loss with the source voltage.
- Load voltage is the source voltage minus the estimated drop.
- Use the result as a planning signal before checking the allowed voltage range for the actual equipment.
Common mistakes to avoid
Most voltage-drop mistakes come from entering round-trip length, picking the wrong circuit type, forgetting that low-voltage systems are more sensitive, or treating one estimate as a final wiring decision.
- Do not enter round-trip length. The calculator already applies the circuit factor.
- Do not use copper AWG results for aluminum wire without a separate source-backed check.
- Do not treat a low percent drop as proof that breaker size, ampacity, temperature, insulation, terminals, or local code are acceptable.
- Ask a qualified electrician or engineer for real installations and safety decisions.
Quick 120 V branch example
Say you are checking a 120 V branch run with 15 A of load current, 75 ft of one-way distance, single-phase/DC mode, and 12 AWG copper. The calculator estimates 3.573 V of drop.
That is 2.98% of 120 V, so the load voltage estimate is 116.427 V. This does not approve the wire, but it tells you whether the run deserves a closer design check.
Why low-voltage runs can surprise you
The same volt loss matters more when the source voltage is small. A 24 V run with 5 A, 40 ft one-way distance, and 10 AWG copper estimates 0.3996 V of drop.
That leaves about 23.6004 V at the load, and the percent drop is 1.67%. The volt number looks small, but the percent is the part that tells you how much of the supply you lost.
- Compare percent drop, not only volts dropped.
- Check the device manual for the allowed input-voltage range.
- Use manufacturer data and qualified electrical guidance before choosing wire or protection.
What this guide does not decide
Voltage drop is only one part of wiring. Real work can also need ampacity, insulation rating, conductor material, temperature adjustment, raceway fill, terminals, continuous-load rules, breaker rules, equipment instructions, and local code.
Use this guide to understand the scale of the voltage loss. Use the calculator result as a question to investigate, not as a wiring permit or safety sign-off.
Research and references
These references support the voltage-drop concept, Ohm law background, units, and safety-limit wording. They do not replace equipment instructions, local code, or professional electrical review.
Worked examples for Voltage Drop Calculator
3.573 V drop, 2.98% drop, 116.427 V load
3.7692 V drop, 1.57% drop, 236.2308 V load
2.0533 V drop, 0.99% drop, 205.9467 V load
0.3996 V drop, 1.67% drop, 23.6004 V load
9.09 V drop, 7.58% drop, 110.91 V load
2.2478 V drop, 1.87% drop, 117.7523 V load
FAQ in plain language
When should I use the Voltage Drop Calculator?
Use it when your task matches one of these common needs: Estimate voltage drop for a branch circuit run. Compare common copper AWG wire sizes. It works best when you already know the measurements, amounts, units, or options the page asks for.
What is the Voltage Drop Calculator doing with my inputs?
In plain language: Voltage drop = current x copper resistance per foot x one-way length x circuit factor. Single-phase/DC uses factor 2; balanced three-phase uses sqrt(3). The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
What do the main Voltage Drop Calculator inputs mean?
Source voltage: the voltage at the supply side before the wire run loses voltage. Current amps: the load current used for this estimate, not a breaker-size decision. One-way length: the source-to-load distance in feet. The calculator applies the return-path or three-phase factor. Copper wire size: the AWG size used to look up approximate copper resistance per 1,000 feet. Circuit type: single-phase/DC or balanced three-phase, which changes the circuit factor.
How should I read the Voltage Drop Calculator answer?
Read the headline answer, then check the supporting lines and examples to understand how the calculator got there. If one input changes, rerun the tool and compare the new answer instead of guessing.
What should I double-check before trusting the answer?
Planning math only. It does not choose breaker or wire size and does not check ampacity, insulation, temperature, raceway fill, equipment instructions, or local code. Also check the unit, scale, mode, and result limit because small input changes can change the answer.
Why does circuit type change voltage drop?
A simple single-phase or DC run uses an out-and-back path, so the factor is 2. A balanced three-phase estimate uses sqrt(3). Real installations can need more detailed impedance and code checks.
Should I enter one-way length or round-trip length?
Enter one-way length from source to load. The calculator applies the circuit factor, so entering round-trip length would double-count the path for single-phase or DC runs.
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Privacy and copying results
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Use Copy answer when you want to save the inputs and result in notes, homework, a message, or a project list. Check the units, labels, and limits before copying.