Last updated June 19, 2026 by our expert review team
What Size Wire Do I Need?
Match the wire to two limits: it must carry the current without overheating (ampacity), and keep voltage drop under about 3% over the run. For short runs, ampacity sets the size; for long runs, voltage drop usually wins and you step up a gauge or two.
Common residential branch circuits (copper):
| Breaker | Copper wire |
|---|---|
| 15 A | 14 AWG |
| 20 A | 12 AWG |
| 30 A | 10 AWG |
| 40 A | 8 AWG |
| 50 A | 6 AWG |
| 60 A | 4 AWG |
| 100 A | 3 AWG |
Ampacity minimums for 60°C NM-B cable (the familiar chart). THHN in conduit at 75°C carries more; long runs may still need one size up for voltage drop. Aluminum runs ~1–2 sizes bigger.
Expert Contributors
Ehsan Ghazanfari
Licensed Structural Engineer
FISE-certified structural engineer with 11+ years designing bridges, retaining walls, and foundations. MSc from Aalto University.
See full profileHawkin
Certified Cost & Estimating Professional
AACE-certified estimator working with 20+ insulation companies including the two largest franchises in America.
See full profileUpdated June 2026
How we verify our calculatorsMethodology
How the Wire Size Calculator Works
Two independent limits decide the gauge. Ampacity is how much current a conductor can carry without overheating — set by the NEC table for your wire material and the temperature rating of the terminals. Voltage drop is the voltage lost to resistance over the run; it grows with both current and distance.
The calculator finds the smallest conductor that clears the required ampacity, then checks its voltage drop over your run. If the drop exceeds your target, it upsizes until it fits — and recommends the larger of the two. See CDA ampacity guidance.
Formulas
Required ampacity = load A × 1.25 (continuous)
Voltage drop V = 2 × K × amps × length ÷ circular mils
Drop % = drop V ÷ system volts × 100
Wire = larger of the ampacity and voltage-drop result
Quick Reference
- 15 A
- 14 AWG Cu
- 20 A
- 12 AWG Cu
- 30 A
- 10 AWG Cu
- 40 A
- 8 AWG Cu
- 50 A
- 6 AWG Cu
- Drop target
- ≤ 3% branch
Pick your conductor
Copper or aluminum?
Material changes the gauge: aluminum carries less current per size, so it runs one to two gauges larger for the same load.
Copper
branch circuits
Choose it if: Outlet, lighting, and appliance circuits. Higher ampacity per gauge, easy terminations, the default for in-home wiring.
more amps per size
Aluminum
large feeders
Choose it if: Service entrances and long subpanel feeders where cost and weight matter. Size up ~1–2 gauges, use AL-rated lugs and antioxidant compound.
cheaper, lighter
NEC Ampacity Reference (310.16)
Allowable ampacity at the 60°C column — the basis for the familiar residential chart and for NM-B cable. Copper carries more current per gauge than aluminum, which is why aluminum feeders run a size or two larger:
| Wire size | Copper | Aluminum |
|---|---|---|
| 14 AWG | 15 A | — |
| 12 AWG | 20 A | 15 A |
| 10 AWG | 30 A | 25 A |
| 8 AWG | 40 A | 30 A |
| 6 AWG | 55 A | 40 A |
| 4 AWG | 70 A | 55 A |
| 2 AWG | 95 A | 75 A |
| 1/0 AWG | 125 A | 100 A |
| 2/0 AWG | 145 A | 115 A |
| 4/0 AWG | 195 A | 150 A |
NEC 310.16, 60°C column. 14/12/10 AWG are also capped to 15/20/30 A by the 240.4(D) small-conductor rule. Switch to 75°C in the calculator for THHN in conduit.
Why Long Runs Need Bigger Wire
Voltage drop is resistance × current over the full round-trip length. Double the distance and you double the drop — so a gauge that's fine for a 25-foot run can fall short at 120 feet, even though the load hasn't changed.
Excessive drop dims lights, overheats motor windings, and wastes energy as heat in the wire. Keeping a branch circuit under 3% (and the whole feeder-plus-branch path under 5%) is the standard target.
That's why detached garages, well pumps, and EV chargers far from the panel often need a conductor a size or two above the ampacity minimum.
Wire Sizing Examples
Kitchen Receptacle Circuit
20 A · 25 ft · 120VA standard 20-amp small-appliance circuit on a 25 ft homerun. Ampacity sets the size — 12 AWG copper, well under 3% drop.
Electric Range Circuit
50 A · 60 ft · 240VA 50-amp 240V range feed at 60 ft. 6 AWG copper carries the load with drop near 1% — the familiar 50A = 6 AWG pairing.
Detached Garage Well Pump
30 A · 150 ft · 240VA 30-amp pump 150 ft from the panel. Ampacity alone allows 10 AWG, but the long run pushes drop past 3% — so it's upsized to 8 AWG.
Avoid these
Wire Sizing Mistakes
Sizing by amps and ignoring distance
Ampacity is only half the job. A long run can fail the 3% voltage-drop target even when the gauge carries the current fine — always enter the real run length.
Using the 90°C ampacity column
NEC 110.14(C) limits you to the terminal's rating — usually 60°C or 75°C. The 90°C column is for derating math, not final sizing. Size from 75°C unless your equipment is rated higher.
Sizing aluminum like copper
Aluminum carries less current per gauge. A 50-amp copper run is 6 AWG, but aluminum needs 4 AWG — plus AL-rated terminals and antioxidant compound.
Assuming the ground matches the hots
The equipment grounding conductor is sized by the breaker per NEC 250.122, not the same as the hot conductor. And if you upsize the hots for voltage drop, the ground must grow proportionally.
Wire Size Calculator FAQs
What size wire do I need for a 50-amp circuit?
What size wire for a 30-amp and 40-amp circuit?
What size wire for a 100-amp subpanel?
When do I need to upsize wire for voltage drop?
Copper or aluminum wire — which should I use?
How does distance affect wire size?
Which temperature column should I use?
Does this size the ground wire too?
Know your load in watts? Convert it with the amp calculator first, then size the conduit with the conduit fill calculator.
Important Disclaimer
These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.