NEC recommends 3% max for branch circuits
Enter amperage and distance to see wire size results
Wire Sizing Reference Guide
AWG wire sizes, voltage drop formulas, and NEC code requirements for residential and commercial electrical circuits.
How to Choose the Right Wire Size
Wire sizing is determined by two independent requirements: ampacity and voltage drop. A wire must carry the load current without overheating, and it must deliver acceptable voltage to the end device. Both constraints must be satisfied, and the larger of the two resulting wire sizes wins.
- Ampacity requirements: The wire must be rated for at least the circuit breaker amperage. NEC 310.15 sets minimum ampacity based on conductor type, insulation rating, and installation conditions. Bundled or conduit-fill conductors may require derating.
- Voltage drop considerations: As current flows through a wire, resistance causes a voltage reduction. Long runs require larger wire to keep the drop within acceptable limits. NEC recommends 3% maximum for branch circuits and 5% total (feeder plus branch combined).
- NEC code requirements: Minimum wire sizes are set by the National Electrical Code. 14 AWG is the smallest allowed for 15A residential branch circuits, 12 AWG for 20A circuits. Local jurisdictions may adopt stricter standards.
When in doubt, go one size larger. The incremental cost of heavier wire is small compared to the cost of rewiring or dealing with equipment damage from chronic undervoltage.
Key Takeaways
- Size for ampacity first, then check voltage drop
- NEC 3% max voltage drop for branch circuits
- Local codes may require larger minimums than NEC
AWG Wire Size Chart
AWG (American Wire Gauge) numbers run backward - a lower number means a thicker, higher-capacity wire. Here are the most common sizes used in residential and light commercial work.
- 14 AWG (15A): Standard residential lighting and outlet circuits. Minimum for 15A breakers. Not suitable for appliances or long runs.
- 12 AWG (20A): Kitchen outlets, bathroom circuits, garage outlets. Required for any 20A circuit. The most common wire in residential work.
- 10 AWG (30A): Dryer circuits, water heaters, air conditioner disconnect feeds, and EV chargers. Handles significant loads at moderate distances.
- 8 AWG (40A): Range circuits, large HVAC equipment, and subpanel feeds in smaller structures.
- 6 AWG (50-60A): Main feeds for large appliances, hot tubs, and subpanel feeds. Stove and oven circuits typically use 6 AWG at 50A.
Aluminum wiring requires one size larger than copper for the same ampacity. Always use anti-oxidant compound on aluminum terminations and use listed lugs rated for aluminum.
Key Takeaways
- Lower AWG number = thicker wire = higher capacity
- Aluminum needs one size larger than copper for the same amps
- Use anti-oxidant compound on all aluminum terminations
Understanding Voltage Drop
Voltage drop is the reduction in voltage caused by conductor resistance. Every wire has resistance, and when current flows through resistance, voltage is consumed. The longer the run and the higher the current, the more voltage is lost before it reaches the load.
- Why it matters - equipment damage: Motors running on low voltage draw higher current to compensate, which causes overheating and premature failure. Sensitive electronics may malfunction or give inaccurate readings. LED drivers can flicker or fail early.
- Why it matters - code violations: NEC 210.19(A) informational note recommends branch circuit conductors be sized to limit voltage drop to 3%. Feeders should be sized to limit drop to 3% as well, with a combined total of 5% from service to outlet. Some jurisdictions make these recommendations mandatory.
- How distance affects wire sizing: Voltage drop is directly proportional to one-way run length. A circuit that requires 12 AWG for ampacity alone may need 10 AWG or 8 AWG once the run exceeds 75-100 feet. Sub-panels can help reduce voltage drop on long runs by bringing the power source closer to the loads.
The formula for single-phase voltage drop: VD = (2 x K x I x D) / cmil, where K is 12.9 for copper and 21.2 for aluminum, I is amps, D is one-way distance in feet, and cmil is circular mils of the conductor.
Key Takeaways
- NEC recommends 3% max for branch circuits, 5% total system
- Long runs often require upsizing beyond ampacity minimums
- Sub-panels bring power closer and reduce voltage drop on long runs
How to Use This Calculator
Enter your circuit amperage
Type the amperage draw of your circuit or appliance. This is the current the wire must carry continuously. Match this to your circuit breaker size for branch circuits.
Set voltage, distance, and material
Select your system voltage (120V for standard outlets, 240V for appliances). Enter the one-way distance from the panel to the load in feet. Choose copper or aluminum conductor.
Confirm phase and voltage drop limit
Select single-phase for residential circuits or three-phase for commercial work. The NEC recommends a maximum of 3% voltage drop for branch circuits. Adjust if your project requires a tighter limit.
Review the recommended AWG and results
The calculator shows the minimum AWG that satisfies both the ampacity and voltage drop requirements. You also get actual voltage drop percentage, voltage at the load, total wire length, and an estimated material cost.
Wire Size and Voltage Drop Formulas
Single Phase: VD = (2 x K x I x D) / cmil
Three Phase: VD = (1.732 x K x I x D) / cmil
K Factor: Copper = 12.9, Aluminum = 21.2
VD = Voltage Drop, I = Amps, D = One-way Distance (ft), cmil = Circular MilsWhere:
- VD
- = Voltage drop in volts
- K
- = Resistivity constant - 12.9 for copper, 21.2 for aluminum
- I
- = Current in amps (circuit load)
- D
- = One-way distance from panel to load in feet
- cmil
- = Circular mils - the cross-sectional area of the conductor
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Frequently Asked Questions
How do I calculate the correct wire size?
Wire sizing has two requirements: ampacity and voltage drop. First, the wire must be rated to carry the circuit amperage continuously. Second, the wire must be thick enough to keep voltage drop within acceptable limits over the run length. Use this calculator to check both at once. The formula for single-phase voltage drop is VD = (2 x K x I x D) / cmil, where K is 12.9 for copper and 21.2 for aluminum, I is current in amps, D is one-way distance in feet, and cmil is the circular mils of the conductor.
What does NEC say about voltage drop?
The National Electrical Code (NEC) 210.19(A) includes an informational note recommending that branch circuit conductors be sized so that the voltage drop does not exceed 3%. For feeders, a separate 3% recommendation applies (NEC 215.2(A)). The combined voltage drop from the service entrance to the furthest outlet should not exceed 5%. Some local jurisdictions adopt these recommendations as mandatory requirements, so always verify with your local AHJ (Authority Having Jurisdiction).
What is the difference between copper and aluminum wire?
Copper has better conductivity than aluminum, so copper wire carries more amps per gauge size. Aluminum wire requires roughly one AWG size larger to match the ampacity of copper. For example, 4 AWG copper carries 70A while 2 AWG aluminum is needed for the same load. Aluminum wire is significantly lighter and cheaper, making it common for large feeder runs and service entrance conductors. When using aluminum, always use anti-oxidant compound at all terminations and use lugs that are listed for aluminum conductors.
Why does distance matter for wire sizing?
Every wire has electrical resistance, and resistance causes voltage to drop as current flows through it. The longer the wire, the more resistance, and the greater the voltage lost before reaching the load. Voltage drop is directly proportional to run length - double the distance and the drop doubles. A 12 AWG wire that works fine for a 20A circuit 30 feet from the panel may produce unacceptable voltage drop at 100 feet. Long runs often require upsizing to 10 AWG or 8 AWG even though the ampacity requirement alone would allow 12 AWG.
What happens if I use wire that is too small?
Undersized wire creates two problems. Overheating: A wire carrying more current than its ampacity rating will overheat, damage insulation, and create a fire hazard. Circuit breakers protect against this, but the wire may still run hot even below the breaker trip point. Excessive voltage drop: Underdimensioned wire for a long run will cause motors to run hot and overheat, sensitive electronics to malfunction, and lights to dim under load. Always size wire to both the ampacity requirement and the voltage drop limit.
What wire size do I need for a 20-amp circuit?
A 20-amp branch circuit requires a minimum of 12 AWG copper or 12 AWG aluminum per NEC 210.19(A). However, if the run is longer than about 50-75 feet, you may need to upsize to 10 AWG to keep voltage drop within the 3% NEC recommendation. Use this calculator with your actual run length to confirm. Common 20-amp applications include kitchen outlet circuits, bathroom circuits, garage circuits, and some lighting circuits.
Can I use this calculator for three-phase circuits?
Yes. Select "Three Phase" in the Phase field and the calculator uses the three-phase voltage drop formula: VD = (1.732 x K x I x D) / cmil. The 1.732 factor (square root of 3) accounts for the phase relationship in a three-phase system. Three-phase circuits also require three conductors instead of two, so total wire length is calculated as distance x 3. Three-phase distribution is common in commercial buildings, industrial facilities, and larger HVAC equipment.
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