Wire Size Calculator
Find the copper wire gauge (AWG) for a circuit from the current, wire length, voltage and voltage-drop limit, with an ampacity check and a full size comparison.
Results
| Item | Value |
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All copper sizes compared
| AWG | Ampacity | Voltage drop | Fits? |
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Quick Answer
Pick the smallest wire whose ampacity covers the load and whose voltage drop stays under your limit. Voltage drop = 2 × length × current × resistance per foot. A 30 A, 240 V circuit running 100 ft with a 3% drop limit and 60 °C wire needs 10 AWG copper, with a drop of about 5.99 V (2.50%).
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Table of Contents
- Wire Size Calculator
- Quick Answer
- Related Calculators
- How This Wire Size Calculator Works
- How to Use the Wire Size Calculator
- Core Formulas
- Key Terms
- Worked Example
- Real-Life Examples
- Ampacity vs Voltage Drop
- Copper Wire Sizes and Ampacity
- What Changes the Wire Size
- Common Mistakes to Avoid
- Frequently Asked Questions
- References & Sources
How This Wire Size Calculator Works
Enter the current the circuit will carry, the one-way distance from the panel to the load, the voltage and the most voltage drop you will accept. Choose the temperature rating and whether the load is continuous. The calculator checks every copper size from 14 AWG to 4/0 AWG and recommends the smallest one that can carry the current and keeps the voltage drop under your limit.
Quick explanation: a wire must pass two tests.
- Ampacity: it can carry the current without overheating.
- Voltage drop: it is thick enough that the voltage does not fall too much over the run.
- Smaller AWG number: means a thicker wire.
How to Use the Wire Size Calculator
- Enter the load current in amps.
- Enter the one-way wire length in feet.
- Enter the voltage and your voltage-drop limit.
- Choose the system, temperature rating and whether the load is continuous.
- Press Calculate and check the recommended size and the comparison table.
Core Formulas
- Single-phase or DC drop: VD = 2 × L × I × R
- Three-phase drop: VD = √3 × L × I × R
- Drop percent: VD ÷ V × 100
- Continuous load: size for 1.25 × the load current
L is the one-way length in feet, I is the current, and R is the wire's resistance per foot. The 2 in the single-phase formula counts the trip out and back.
Key Terms
- AWG: American Wire Gauge, the US system of wire sizes. Smaller numbers are thicker.
- Ampacity: the most current a wire can carry continuously without overheating.
- Voltage drop: the voltage lost along a wire because of its resistance.
- Continuous load: a load expected to run at full current for 3 hours or more.
- kcmil: thousand circular mils, a unit of wire area used for large sizes.
Worked Example
A 20 A, 120 V circuit runs 150 ft to a shed, with a 3% drop limit.
- Allowed drop = 3% × 120 V = 3.6 V
- 12 AWG: 2 × 150 × 20 × 0.001588 = 9.53 V, too much
- The smallest size that works is 6 AWG, with a drop of 2.37 V (1.98%)
Real-Life Examples
- A detached garage: a 30 A, 240 V feed at 100 ft needs 10 AWG at 60 °C.
- A shed at 150 ft: a 20 A, 120 V circuit needs 6 AWG to keep the drop near 3%.
- An EV charger: a continuous 50 A load at 240 V over 75 ft with 75 °C terminals needs 6 AWG.
- Low-voltage lighting: 12 V systems lose a large share to voltage drop, so wire size matters even more.
Ampacity vs Voltage Drop
- Ampacity: a safety limit, so the wire does not overheat.
- Voltage drop: a performance limit, so motors and lights get enough voltage.
- Short runs: ampacity usually decides. Long runs: voltage drop usually decides.
Copper Wire Sizes and Ampacity
Source: Wikipedia, American wire gauge, citing NEC 2014 Table 310.15(B)(16). Not more than three current-carrying conductors at 30 °C ambient.
| AWG | Resistance (Ω per 1,000 ft) | 60 °C | 75 °C | 90 °C |
|---|---|---|---|---|
| 14 | 2.525 | 15 A | 20 A | 25 A |
| 12 | 1.588 | 20 A | 25 A | 30 A |
| 10 | 0.9989 | 30 A | 35 A | 40 A |
| 8 | 0.6282 | 40 A | 50 A | 55 A |
| 6 | 0.3951 | 55 A | 65 A | 75 A |
| 4 | 0.2485 | 70 A | 85 A | 95 A |
| 3 | 0.197 | 85 A | 100 A | 115 A |
| 2 | 0.1563 | 95 A | 115 A | 130 A |
| 1 | 0.1239 | 110 A | 130 A | 145 A |
| 1/0 | 0.09827 | 125 A | 150 A | 170 A |
| 2/0 | 0.07793 | 145 A | 175 A | 195 A |
| 3/0 | 0.0618 | 165 A | 200 A | 225 A |
| 4/0 | 0.04901 | 195 A | 230 A | 260 A |
What Changes the Wire Size
- Distance: longer runs need thicker wire to limit voltage drop.
- Current: more amps need more copper.
- Voltage: the same drop in volts is a smaller percent at 240 V than at 120 V.
- Heat and bundling: hot spaces and many wires together lower the ampacity.
Common Mistakes to Avoid
- Using the round-trip length. Enter the one-way length. The formula doubles it.
- Ignoring voltage drop on long runs. Ampacity alone can leave a motor or light under-powered.
- Using a higher temperature column than the terminals allow. The lowest-rated part of the circuit sets the limit.
- Forgetting continuous loads. Chargers and heaters often run for hours.
- Using copper numbers for aluminum. Aluminum wire needs a larger size.
Frequently Asked Questions
Find the smallest wire whose ampacity covers the current and whose voltage drop stays under your limit.
Often 10 AWG copper for short runs. Longer runs may need thicker wire. For 100 ft at 240 V and 3% drop, this calculator gives 10 AWG.
Often 12 AWG copper for short runs, but long runs can need 10 AWG or larger.
Many designers aim for about 3% on a branch circuit. Check your local code and equipment needs.
Enter the one-way length. The calculator counts both wires.
The heat rating of the insulation and terminals. Use the lowest rating of any part of the circuit.
No. It uses copper values. Aluminum needs a larger size.
Resistance adds up with length, so more voltage is lost.
A load that runs at full current for 3 hours or more, such as an EV charger. Wire is sized for 125% of it.
Yes. The breaker must protect the wire. Codes limit the breaker size for small wires.
Yes, choose single-phase or DC. Low-voltage systems often need a lower drop limit.
No. It is a planning tool. Follow local code and have a licensed electrician check the work.
References & Sources
| Source | Used For | Link |
|---|---|---|
| Wikipedia, American wire gauge | Copper resistance and ampacity by AWG, citing NEC Table 310.15(B)(16). | https://en.wikipedia.org/wiki/American_wire_gauge |
| Wikipedia, Voltage drop | How resistance causes voltage drop in conductors. | https://en.wikipedia.org/wiki/Voltage_drop |