Quick start
- Enter source voltage before the wire run loses voltage.
- Enter expected load current in amps and the source-to-load one-way length in feet.
- Enter the maximum voltage-drop percentage, such as 3%.
- Choose single/DC or balanced three-phase circuit type so the voltage-drop factor matches the run.
Best uses
Use this guide when you want to understand why the calculator picked a copper AWG size and what still needs review before a real installation.
- Estimate copper AWG size for a voltage-drop target.
- Compare branch-circuit and longer-run examples.
- See estimated voltage drop and load voltage together.
- Learn why current and length affect conductor choice.
What this calculator is solving
The Wire Size Calculator tests common copper AWG sizes and returns the first size that stays within the voltage-drop percentage you choose. It is useful for early planning and comparison before a qualified electrical-code review.
Match each input label on the calculator to source voltage, expected load current in amps, source-to-load one-way length in feet, maximum voltage-drop percentage, and single/DC or balanced three-phase mode.
The formula in plain language
In plain language: The calculator tests common copper AWG sizes from 14 AWG through 4/0 and returns the first size whose estimated voltage drop is within the selected percentage. Single/DC uses the out-and-back path; balanced three-phase uses the square-root-of-3 factor. The examples on the page are there so you can compare your inputs with a worked example before copying the answer.
The calculator tests supported copper AWG sizes from 14 AWG through 4/0. For each size, it estimates voltage drop from current, copper resistance per foot, one-way length, and circuit factor. Single/DC uses the out-and-back factor of 2; balanced three-phase uses sqrt(3).
How to read the answer
Read the answer as the first supported copper AWG size that meets the voltage-drop percentage you entered. Then check the supporting voltage-drop, percent-drop, and load-voltage lines before treating the result as useful.
- The main answer is the first supported copper AWG size that meets the voltage-drop target.
- Estimated drop and percent drop show why that size was selected.
- Load voltage shows the source voltage after the estimated drop.
- If the answer is near your limit, rerun with a tighter drop percentage or review a larger conductor with a qualified person.
Common mistakes to avoid
The big mistake is treating a voltage-drop estimate like final electrical approval. Real conductor sizing also needs ampacity, breaker size, insulation rating, terminals, raceway fill, temperature correction, conductor material, equipment instructions, and local code.
- Do not treat voltage drop as the only wire-sizing rule.
- Do not enter round-trip length when the calculator asks for one-way length.
- Do not use copper AWG results for aluminum conductors without a separate source-backed calculation.
- Do not ignore ampacity, breaker size, insulation, raceway fill, temperature, terminals, equipment instructions, and local code.
- Do not use this as a substitute for a licensed electrician.
Why one-way length matters
The calculator asks for the physical distance from the source to the load. For a simple single-phase or DC run, it applies the out-and-back factor internally, so entering the round-trip length would double-count the distance.
For example, a 120 V, 15 A, 75 ft run with a 3% target returns 12 AWG with about 3.573 V of drop, 2.9775% drop, and about 116.427 V at the load.
Two examples that show why voltage changes the answer
A 240 V, 30 A, 100 ft single/DC run with a 3% target returns 10 AWG, about 5.994 V of drop, 2.4975% drop, and about 234.006 V at the load.
A 24 V, 5 A, 40 ft single/DC run with the same 3% target returns 12 AWG, about 0.6352 V of drop, 2.6466666667% drop, and about 23.3648 V at the load. Lower voltage makes each lost volt count more as a percentage.
Single/DC versus balanced three-phase
Single/DC mode uses the out-and-back factor of 2. Balanced three-phase mode uses sqrt(3), so it can return a different drop estimate from the same length and current.
For a 208 V, 20 A, 150 ft balanced three-phase run with a 3% target, this calculator returns 10 AWG with about 5.1909562703 V of drop, 2.495652053% drop, and about 202.80904373 V at the load.
What still needs a real electrical check
Voltage drop is only one part of wire sizing. A real installation needs ampacity checks, breaker sizing, conductor insulation rating, terminal temperature limits, raceway fill, ambient temperature correction, conductor material, equipment instructions, and local electrical code.
Use this calculator for planning and comparison. Use the result as a question to bring into a code-aware review, not as permission to install a conductor.
Research and references
These references help check the measurements, units, limits, or safety notes used in this guide.
Worked examples for Wire Size Calculator
12 AWG; 3.573 V drop, 2.9775% drop, 116.427 V load
10 AWG; 5.994 V drop, 2.4975% drop, 234.006 V load
10 AWG; 5.1909562703 V drop, 2.495652053% drop, 202.80904373 V load
12 AWG; 0.6352 V drop, 2.6466666667% drop, 23.3648 V load
10 AWG; 3.5964 V drop, 2.997% drop, 116.4036 V load
6 AWG; 1.26432 V drop, 2.634% drop, 46.73568 V load
FAQ in plain language
When should I use the Wire Size Calculator?
Use it when your task matches one of these common needs: Estimate copper AWG size for a voltage-drop target. Compare branch-circuit and longer-run examples. It works best when you already know the measurements, amounts, units, or options the page asks for.
What is the Wire Size Calculator doing with my inputs?
In plain language: The calculator tests common copper AWG sizes from 14 AWG through 4/0 and returns the first size whose estimated voltage drop is within the selected percentage. Single/DC uses the out-and-back path; balanced three-phase uses the square-root-of-3 factor. 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 Wire Size Calculator inputs mean?
Source voltage: the voltage at the supply side before the wire run loses voltage. Current amps: the expected load current used for the voltage-drop estimate. One-way length: the source-to-load distance in feet, not the full out-and-back loop length. Max voltage drop: the percentage limit the calculator tries to stay under, such as 3%. Circuit type: single/DC uses an out-and-back factor of 2; balanced three-phase uses sqrt(3).
How should I read the Wire Size 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?
This is voltage-drop planning math for copper conductors only. It does not approve wire size or check ampacity, breaker size, insulation rating, terminals, raceway fill, temperature correction, aluminum conductors, equipment instructions, or local electrical code. Also check the unit, scale, mode, and result limit because small input changes can change the answer.
Is this the same as an electrical code wire-size chart?
No. This estimates voltage drop only. Real wire sizing also needs ampacity, conductor insulation, raceway fill, terminals, temperature, material, and local code rules.
Why can a long run need a larger wire?
Longer wire has more resistance. More resistance causes more voltage drop, so increasing wire size can reduce the estimated voltage lost along the run.
Related tools
- Wire Resistance CalculatorEstimate copper wire resistance from AWG size, length, and conductor count.
- Voltage Drop CalculatorEstimate voltage drop from current, wire length, voltage, phase, and copper AWG size.
- Electricity CalculatorEstimate electricity use and cost from watts, hours, days, and rate per kWh.
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