Calculate Wire Size
Enter your circuit parameters below to find the recommended AWG wire gauge. The calculator accounts for voltage drop limits and NEC ampacity ratings.
Wire Size Comparison
Voltage drop and power loss for nearby AWG sizes with your parameters:
| AWG | Diameter | Resistance (ohm/kft) | Voltage Drop | Drop % | Power Loss | Status |
|---|
AWG Wire Reference Table
Standard American Wire Gauge sizes with key specifications for copper and aluminum conductors at 75 degrees C (NEC Table 310.16).
| AWG | Diameter (in) | Area (kcmil) | Resistance Cu (ohm/kft) | Resistance Al (ohm/kft) | Ampacity Cu (75C) | Ampacity Al (75C) |
|---|---|---|---|---|---|---|
| 14 | 0.0641 | 4.11 | 3.14 | 5.17 | 15 | -- |
| 12 | 0.0808 | 6.53 | 1.98 | 3.25 | 20 | 15 |
| 10 | 0.1019 | 10.38 | 1.24 | 2.04 | 30 | 25 |
| 8 | 0.1285 | 16.51 | 0.778 | 1.28 | 40 | 35 |
| 6 | 0.1620 | 26.24 | 0.491 | 0.808 | 55 | 40 |
| 4 | 0.2043 | 41.74 | 0.308 | 0.508 | 70 | 55 |
| 3 | 0.2294 | 52.62 | 0.245 | 0.403 | 85 | 65 |
| 2 | 0.2576 | 66.36 | 0.194 | 0.319 | 95 | 75 |
| 1 | 0.2893 | 83.69 | 0.154 | 0.253 | 110 | 85 |
| 1/0 | 0.3249 | 105.6 | 0.122 | 0.201 | 125 | 100 |
| 2/0 | 0.3648 | 133.1 | 0.0967 | 0.159 | 145 | 115 |
| 3/0 | 0.4096 | 167.8 | 0.0766 | 0.126 | 165 | 130 |
| 4/0 | 0.4600 | 211.6 | 0.0608 | 0.100 | 195 | 150 |
| 250 | -- | 250 | 0.0515 | 0.0847 | 215 | 170 |
| 300 | -- | 300 | 0.0429 | 0.0707 | 240 | 190 |
| 350 | -- | 350 | 0.0367 | 0.0605 | 260 | 210 |
| 400 | -- | 400 | 0.0321 | 0.0529 | 280 | 225 |
| 500 | -- | 500 | 0.0258 | 0.0424 | 320 | 260 |
Resistance values are DC resistance at 75 degrees C. Ampacity ratings are for conductors in raceway or cable (NEC Table 310.16, 75C column). Actual installation conditions may require derating.
How Wire Sizing Works
Every electrical conductor has resistance. When current flows through this resistance, some voltage is lost as heat. This is called voltage drop. The key to proper wire sizing is selecting a conductor large enough to keep voltage drop within acceptable limits while also meeting NEC ampacity requirements for safety.
The Voltage Drop Formula
For single-phase circuits:
For three-phase circuits:
Where:
- VD = voltage drop (volts)
- L = one-way length of the wire run (feet)
- I = current in amps
- R = wire resistance per 1,000 feet (ohms/kft)
- 1.732 = square root of 3 (used for three-phase circuits)
The factor of 2 in the single-phase formula accounts for the complete circuit -- current travels out on the hot wire and returns on the neutral. For three-phase balanced loads, the 1.732 factor replaces the 2 because the three phases partially cancel each other's return current.
Why Wire Sizing Matters
- Safety: Undersized wire heats up, potentially causing insulation damage and fire
- Equipment protection: Motors and electronics may malfunction or be damaged by low voltage
- Energy efficiency: Larger wire means less power lost as heat in the conductors
- Code compliance: NEC requires both ampacity and voltage drop limits be met
- Cost optimization: Oversized wire wastes money; proper sizing balances safety with cost
NEC Voltage Drop Recommendations
The National Electrical Code (NEC) provides voltage drop guidance in informational notes:
| Circuit Type | Max Recommended Drop | At 120V | At 240V |
|---|---|---|---|
| Branch circuit | 3% | 3.6V | 7.2V |
| Feeder | 3% | 3.6V | 7.2V |
| Total (feeder + branch) | 5% | 6.0V | 12.0V |
While these are recommendations rather than strict requirements, following them ensures reliable operation. Sensitive equipment such as computer servers, variable-frequency drives, and medical devices may benefit from stricter limits (1-2%).
Worked Examples
Example 1 -- Kitchen 20A Circuit
Parameters: 20 amps, 120V, 50 feet one-way, copper, single-phase, 3% max drop
- Allowable drop: 120V x 3% = 3.6V
- 12 AWG (R = 1.98 ohm/kft): VD = (2 x 50 x 20 x 1.98) / 1000 = 3.96V (3.3%) -- slightly over
- 10 AWG (R = 1.24 ohm/kft): VD = (2 x 50 x 20 x 1.24) / 1000 = 2.48V (2.1%) -- passes
- Recommendation: 10 AWG copper (12 AWG meets ampacity but exceeds voltage drop limit at this distance)
Example 2 -- Workshop 30A / 240V Circuit
Parameters: 30 amps, 240V, 100 feet one-way, copper, single-phase, 3% max drop
- Allowable drop: 240V x 3% = 7.2V
- 10 AWG (R = 1.24 ohm/kft): VD = (2 x 100 x 30 x 1.24) / 1000 = 7.44V (3.1%) -- slightly over
- 8 AWG (R = 0.778 ohm/kft): VD = (2 x 100 x 30 x 0.778) / 1000 = 4.67V (1.9%) -- passes
- Recommendation: 8 AWG copper
Example 3 -- Long Run 50A / 240V Sub-Panel Feed
Parameters: 50 amps, 240V, 150 feet one-way, copper, single-phase, 3% max drop
- Allowable drop: 240V x 3% = 7.2V
- 6 AWG (R = 0.491 ohm/kft): VD = (2 x 150 x 50 x 0.491) / 1000 = 7.37V (3.1%) -- slightly over
- 4 AWG (R = 0.308 ohm/kft): VD = (2 x 150 x 50 x 0.308) / 1000 = 4.62V (1.9%) -- passes
- Recommendation: 4 AWG copper
Copper vs. Aluminum Wire
Both copper and aluminum are used as electrical conductors. Here is how they compare:
| Property | Copper | Aluminum |
|---|---|---|
| Conductivity | 100% (reference) | 61% |
| Weight | Heavier | ~30% lighter |
| Cost per foot | Higher | ~40-50% less |
| Size needed (same amps) | Baseline | ~2 AWG sizes larger |
| Thermal expansion | Lower | Higher (needs anti-oxidant) |
| Common uses | Branch circuits, residential | Feeders, service entrances |
Aluminum wire requires special connectors rated for aluminum (marked AL-CU or CO/ALR) and anti-oxidant compound to prevent corrosion at connections. It should never be used with connectors rated only for copper.
Frequently Asked Questions
How do I calculate the right wire size for my electrical project?
You need four values: current (amps), system voltage, one-way wire run distance, and your acceptable voltage drop percentage (typically 3%). Enter these into the calculator above and it will recommend the smallest AWG wire that meets both your voltage drop limit and NEC ampacity requirements.
What is the difference between copper and aluminum wire?
Copper has about 61% better conductivity than aluminum, meaning aluminum wire must be approximately 2 AWG sizes larger than copper for the same current capacity. Copper is standard for branch circuits and residential wiring, while aluminum is commonly used for larger service feeders and utility connections due to its lower cost and lighter weight.
What is acceptable voltage drop for electrical wiring?
The NEC recommends a maximum 3% voltage drop for branch circuits and 5% total from the service panel to the farthest outlet. For a 120V circuit, 3% equals 3.6V. For sensitive electronics and critical equipment, keeping voltage drop under 2% is advisable.
What is the difference between single-phase and three-phase power?
Single-phase power uses two conductors (hot and neutral) and is standard in residential settings. Three-phase power uses three hot conductors offset by 120 degrees, delivering more efficient power for commercial and industrial use. The voltage drop formula uses a factor of 2 for single-phase and 1.732 (square root of 3) for three-phase.
What wire size do I need for a 30 amp circuit?
For a 30-amp circuit, NEC requires a minimum of 10 AWG copper or 8 AWG aluminum. However, for longer runs you may need larger wire to limit voltage drop. For example, a 30A circuit at 240V running 100 feet one-way needs 8 AWG copper to keep voltage drop under 3%.
Can I use a smaller wire for a short run?
Even on short runs, you must meet the NEC minimum ampacity for the circuit breaker size. For example, a 20A breaker always requires at least 12 AWG copper, regardless of distance. Wire sizing involves both an ampacity check and a voltage drop check -- the wire must pass both.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.
Safety Disclaimer
This calculator is an educational tool for estimating wire sizes. Actual electrical installations must comply with the National Electrical Code (NEC), local codes, and be performed or inspected by a licensed electrician. Factors such as ambient temperature, conduit fill, insulation type, and derating may require adjustments beyond what this calculator provides. Always consult a qualified professional for electrical work.
Privacy & Limitations
- All calculations run entirely in your browser -- nothing is sent to any server.
- Results are computed using standard formulas and should be verified for critical applications.
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Wire Size Calculator FAQ
How do I calculate the right wire size for my electrical project?
To calculate wire size, you need four values: current (amps), system voltage, one-way wire run distance, and acceptable voltage drop (typically 3%). The calculator uses the formula VD = (2 x L x I x R) / 1000 for single-phase circuits to find the voltage drop for each AWG size, then recommends the smallest wire that keeps voltage drop within your limit while meeting NEC ampacity requirements.
What is the difference between copper and aluminum wire?
Copper has about 61% better conductivity than aluminum, meaning aluminum wire must be approximately 2 AWG sizes larger than copper for the same current capacity. Copper is standard for branch circuits and residential wiring, while aluminum is common for larger service feeders and utility connections due to lower cost and lighter weight.
What is acceptable voltage drop for electrical wiring?
The NEC recommends a maximum 3% voltage drop for branch circuits and 5% total from the service panel to the farthest outlet. For sensitive electronics and critical equipment, keeping voltage drop under 2% is advisable. On a 120V circuit, 3% means only 3.6V of drop is acceptable.
What is the difference between single-phase and three-phase power?
Single-phase power uses two conductors (hot and neutral) and is standard in residential settings. Three-phase power uses three hot conductors with current offset by 120 degrees, providing more efficient power delivery for commercial and industrial applications. Three-phase circuits have a multiplier of 1.732 (square root of 3) instead of 2 in voltage drop calculations.
What wire size do I need for a 30 amp circuit?
For a 30-amp circuit, NEC requires a minimum of 10 AWG copper or 8 AWG aluminum. However, for longer runs you may need larger wire to limit voltage drop. For example, a 30A circuit at 240V running 100 feet one-way needs 8 AWG copper to keep voltage drop under 3%.
Does this calculator store my data?
No. All calculations run entirely in your browser. No data is sent to any server, and nothing is stored.