Free Calculator
Wire Size Calculator
Find the right copper or aluminum AWG wire gauge by amps, voltage, and distance — sized for both ampacity and voltage drop, so your circuit is safe and code-aware before you pull wire.
See SubcontractorHub for ElectricalSize Your Wire in Seconds
Enter your load, voltage, run length, and conductor material to see the AWG required by ampacity and by voltage drop.
The continuous load the circuit must carry, in amps.
Distance from panel to load, one direction.
📋 Important: All calculator results are ballpark estimates
The figures shown are approximate estimates based on typical 75°C ampacity values and a standard voltage-drop formula, and should be used for general planning purposes only. They are not a substitute for the National Electrical Code, local amendments, or a professional electrical design. Actual conductor sizing will vary significantly based on ambient temperature, conduit fill, termination ratings, and continuous-load factors. Always confirm any wire and breaker sizing with a licensed electrician before purchasing, permitting, or installing.
How Wire Size Is Determined
Two constraints govern wire size: ampacity and voltage drop
Every conductor has to satisfy two independent limits. Ampacity is the maximum current a wire can carry without overheating — it comes straight from the NEC ampacity tables. Voltage drop is how much voltage the wire loses over its length; too much drop means dim lights, hot conductors, and equipment that runs poorly. The correct wire size is whichever of the two constraints demands the thicker conductor (the lower AWG number). The calculator above sizes for both and reports the larger.
| Copper AWG | Typical ampacity (75°C) |
|---|---|
| 14 AWG | 20 A |
| 12 AWG | 25 A |
| 10 AWG | 35 A |
| 8 AWG | 50 A |
| 6 AWG | 65 A |
| 4 AWG | 85 A |
Note: breakers and terminations are often rated at 60°C for smaller conductors, which can lower the usable ampacity. This is one of many reasons a licensed electrician makes the final call.
The 3% voltage drop rule of thumb
The NEC recommends keeping voltage drop to 3% or less on a branch circuit, and 5% or less across the feeder and branch combined. These are informational recommendations rather than hard mandates, but most electricians design to the 3% target. Voltage drop grows with the length of the run, so a load that only needs 12 AWG for ampacity might need 10 or 8 AWG once it's run 100+ feet. The calculator uses the classic formula VD = (phase factor × K × amps × one-way length) ÷ circular mils, where K is 12.9 for copper and 21.2 for aluminum.
Copper vs. aluminum conductors
Copper carries more current per gauge and has lower resistance, so copper wire can be smaller for the same job. Aluminum is cheaper per amp and lighter, which makes it popular for large feeders and service-entrance conductors, but its higher K value (21.2 vs. 12.9) and lower ampacity mean aluminum usually needs to be one to two gauge sizes larger than copper to do the same work. Aluminum also requires listed connectors and anti-oxidant compound at terminations. Whichever material you choose, run the numbers for that material specifically — the calculator switches its ampacity table and K value automatically. If you manage electrical crews, electrical contractor software can keep material takeoffs and job costing consistent across every estimate.
What this calculator does not do
This is a planning aid, not a code-compliance tool. Real-world conductor and overcurrent sizing depends on factors this calculator intentionally leaves out:
- Temperature correction for high ambient conditions
- Conduit-fill and bundling derating when multiple conductors share a raceway
- The 125% continuous-load factor for loads running 3+ hours
- Termination temperature ratings (60°C vs. 75°C vs. 90°C columns)
- Overcurrent (breaker/fuse) sizing and equipment grounding conductors
- Local code amendments that override the national baseline
Safety note: Undersized wire is a fire hazard. Always verify final sizing against the current NEC, your local amendments, and a licensed electrician before purchasing conductors, pulling permits, or making any connection.
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Book a Free DemoFrequently Asked Questions
How do I calculate the right wire size for a circuit?
Wire size is set by two constraints: ampacity (the conductor must safely carry the load current) and voltage drop (the wire must not lose too much voltage over its length). Size the conductor for ampacity first using the NEC ampacity tables, then check voltage drop for the run length. Whichever constraint requires the thicker (lower-gauge-number) wire governs. This calculator does both and reports the larger of the two.
What is the 3% voltage drop rule?
The NEC recommends (in informational notes, not as a hard requirement) keeping voltage drop to no more than 3% on a branch circuit and no more than 5% total across feeder plus branch circuit combined. Excessive voltage drop causes dim lights, sluggish motors, and wasted energy. Longer runs need larger conductors to stay under the 3% target.
Why does aluminum wire need to be larger than copper?
Aluminum has higher resistivity than copper — its voltage-drop constant (K ≈ 21.2) is larger than copper's (K ≈ 12.9), and its ampacity per gauge is lower. To carry the same current with the same voltage drop, aluminum conductors typically need to be one to two gauge sizes larger than copper. Aluminum is still common for larger feeders and service entrances because it costs less per amp.
Does wire length affect the size I need?
Yes. Voltage drop is proportional to the one-way length of the run. A short circuit may be governed entirely by ampacity, but as the run gets longer the voltage-drop constraint takes over and forces a larger conductor. This is why a 20-amp circuit run 150 feet often needs bigger wire than the same load run 25 feet.
Is this wire size calculator NEC-compliant?
This tool is a planning aid that uses typical 75°C ampacity values and a standard voltage-drop formula. It does not apply temperature correction, conduit-fill derating, continuous-load factors, or breaker sizing rules, and it is not a substitute for the National Electrical Code, local amendments, or a licensed electrician. Always confirm final conductor and overcurrent sizing with a qualified professional before installation.
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Get a Free DemoAll calculations are ballpark estimates based on typical 75°C ampacity values and a standard voltage-drop formula, and should be verified against the National Electrical Code and a licensed electrician in your area before making any purchasing or installation decisions.