For electricians & homeowners
Wire Size Calculator
Every wire is sized by two rules — it has to carry the current without overheating, and stay thick enough to hold voltage over the run. This tool takes the larger of the two and gives you the right NEC 310.16 gauge, in copper or aluminum.
A wire size calculator built for electricians — clear enough for anyone.
Too thin for the current and a wire runs hot — the fire risk inside your wall. Size up and it runs cool. This tool finds the smallest safe gauge, and checks voltage drop on long runs.
NEC-based
Table 310.16 ampacity
Transparent
Every tool shows its formula
Free & private
No signup, runs in your browser
Wire Size Calculator
The smallest safe AWG for your load and run — sized to ampacity and voltage drop.
Enter your load and run to size the wire.
Two checks — the thicker wire wins
Compare sizes — tap to inspect (drop % · amps)
Method — how the size is chosen
mult = 2 (1∅) or 1.732 (3∅); K = 12.9 copper / 21.2 aluminum. Ampacity = smallest AWG whose NEC Table 310.16 rating (terminal-temp column, 110.14(C)) ≥ load, then the 240.4(D) small-conductor cap. The recommended size is the thicker of the ampacity and voltage-drop picks. Voltage drop (3%/5%) is an NEC recommendation, not a mandate.
How to use it
How to use the wire size calculator.
It takes four quick inputs — your load, the circuit's voltage and one-way length, and the conductor material and temperature rating. The tool then sizes the wire two ways, by ampacity and by voltage drop, and gives you the larger, safer AWG — showing exactly which one governed.
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01
Enter your load
Type the load in amps. Not sure of the amps? Open Advanced and enter watts and volts instead — the tool converts it for you.
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02
Set voltage, phase & run
Enter the system voltage (120, 240, 208, 480), pick 1- or 3-phase, and give the one-way length. Longer runs drive more voltage drop.
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03
Pick material & temperature
Choose copper or aluminum and the termination column (60, 75, or 90 °C) — usually 75 °C. Flag continuous loads to size at 125%.
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04
Read your size — and why
Get the recommended AWG instantly, plus whether ampacity or voltage drop governed — so you know what's driving the size, not just the number.
The core idea
What determines your wire size
Enter your circuit once — the calculator runs both checks and hands you the thicker, safer gauge.
Wire size is set by the larger of two requirements. Ampacity means the conductor must carry the current without overheating; voltage drop means it must stay thick enough to hold voltage over the run. On short runs ampacity usually governs — on long runs, voltage drop does.
Two rules size every wire — the thicker one wins
Ampacity
The conductor must carry the load without overheating.
↑ higher currentVoltage drop
The conductor must stay thick enough over the distance.
↑ longer runsWire size = the larger of ampacity size and voltage-drop size
The method
How wire size is calculated
There's no single wire size formula — the calculator runs two checks and keeps the larger result. Ampacity is a table lookup; voltage drop is a real equation. Whichever demands the thicker conductor is your answer.
Check 1 · Ampacity
Smallest AWG whose NEC 310.16 rating is ≥ your load at the terminal temperature — then capped by 240.4(D).
A table lookup — not a formula.
Check 2 · Voltage drop
Worked example
On a long run, voltage drop wins
Same circuit, worked end to end: 30 A over a 120 ft copper run at 120 V. Ampacity alone allows 10 AWG — but at that length its voltage drop hits 7.5%, past the 3% target. Sizing for voltage drop forces 6 AWG.
Check 1 · Ampacity
Load 30 A → need a conductor rated ≥ 30 A.
At 75 °C that's 10 AWG (30 A, per 240.4(D)).
Ampacity wants 10 AWG
Check 2 · Voltage drop
Allowed drop = 3% × 120 V = 3.6 V.
CM = 2 × 12.9 × 30 × 120 ÷ 3.6 = 25,800.
Smallest AWG ≥ 25,800 CM = 6 AWG (26,240).
Voltage drop wants 6 AWG
6 AWG — voltage-drop-limited. Both checks pass at 6 AWG; ampacity's 10 AWG would drop 7.5% over 120 ft, so the longer run is what drives the size up.
The code behind it
Ampacity & the NEC rules that size wire
Ampacity is the current a conductor carries continuously without overheating, from NEC Table 310.16. It's listed in three temperature columns — 60, 75, and 90 °C — but you rarely use the 90 °C number: your terminations set the column, and it's usually 75 °C.
| Copper | 60 °C | 75 °C | 90 °C |
|---|---|---|---|
| 14 AWG | 15 | 20 | 25 |
| 12 AWG | 20 | 25 | 30 |
| 10 AWG | 30 | 35 | 40 |
| 8 AWG | 40 | 50 | 55 |
| 6 AWG | 55 | 65 | 75 |
| 4 AWG | 70 | 85 | 95 |
| 2 AWG | 95 | 115 | 130 |
Amps at each column, copper. Aluminum runs lower — about two sizes larger for the same load.
Small-conductor cap
Max breaker is 15 A on 14 AWG, 20 A on 12, 30 A on 10 — regardless of higher ampacity. Why branch circuits are 14/15, 12/20, 10/30.
NM-B / Romex
Sized from the 60 °C column even though the wire is 90 °C-rated — so 12 AWG NM-B is 20 A, not 25 A.
Derating
More than 3 current-carrying conductors bundled, or high ambient heat, lowers ampacity — upsize to compensate.
Continuous loads
Size the conductor and breaker for 125% of any load running 3+ hours continuously.
Ground wire
Upsize the equipment grounding conductor proportionally when you upsize conductors for voltage drop.
Levers
What pushes wire size up — and how to bring it down
Four things drive wire size up: more current, longer runs, aluminum instead of copper, and lower voltage. The biggest lever to bring it back down is voltage — running 240 V instead of 120 halves the current and cuts voltage drop to a quarter.
What pushes it up
- More currentHigher load needs more ampacity.↑
- Longer runVoltage drop grows with every foot.↑
- AluminumAbout two sizes larger than copper.↑
- Lower voltageSame watts pulls more amps.↑
How to bring it down
- 1Raise the voltage240 V halves the current, quarters the drop.
- 2Shorten the runRelocate the panel or a subpanel closer.
- 3Use copperAbout two sizes smaller than aluminum.
- 4Split the loadSeparate circuits each carry less.
The biggest lever · voltage
Double the voltage, quarter the drop
Voltage drop falls with the square of the voltage — so the same load on 240 V often drops the wire two full sizes versus 120 V.
Same 3,600 W load · relative voltage drop
Also in play
High ambient heat or more than 3 bundled conductors derate ampacity — upsize.
Loads running 3+ hours are continuous — size at 125%.
3% is the usual aim; 5% total is the NEC ceiling.
3-phase drops less — the multiplier is ×1.732, not ×2.
Reference
Wire size chart — what size wire for the amps
Pick a load current, material and temperature column to get the wire size by ampacity (NEC 310.16, with the 240.4(D) cap applied). On long runs, check the calculator above — voltage drop can push the size larger.
Minimum wire size
30 A · copper
75 °C column · 240.4(D) applied · by ampacity
Ampacity only. A longer run may need a larger conductor for voltage drop — size the full circuit in the calculator above.
| Load (amps) | Copper wire size | Aluminum wire size |
|---|---|---|
| 15 A | 14 AWG | 12 AWG |
| 20 A | 12 AWG | 10 AWG |
| 30 A | 10 AWG | 8 AWG |
| 40 A | 8 AWG | 8 AWG |
| 50 A | 8 AWG | 6 AWG |
| 60 A | 6 AWG | 4 AWG |
| 70 A | 4 AWG | 3 AWG |
| 80 A | 4 AWG | 2 AWG |
| 90 A | 3 AWG | 2 AWG |
| 100 A | 3 AWG | 1 AWG |
| 110 A | 2 AWG | 1/0 AWG |
| 125 A | 1 AWG | 2/0 AWG |
| 150 A | 1/0 AWG | 3/0 AWG |
| 175 A | 2/0 AWG | 4/0 AWG |
| 200 A | 3/0 AWG | 250 kcmil |
What size wire do I need for a 50 amp circuit?
For a 50 A circuit the common choice is 6 AWG copper or 4 AWG aluminum. (8 AWG copper is rated 50 A at 75 °C, but continuous loads like EV chargers, and runs over about 100 ft, push most 50 A circuits up to 6 AWG.)
What about 30, 100, or 200 amps?
At the 75 °C copper column: 30 A → 10 AWG, 60 A → 6 AWG, 100 A → 3 AWG. A 200 A dwelling service is commonly 2/0 copper or 4/0 aluminum under the NEC 83% rule. Aluminum runs about two sizes larger throughout.
How much bigger does aluminum need to be?
About two AWG sizes larger than copper for the same load — aluminum carries roughly 61% of copper's current. So where copper needs 6 AWG, aluminum needs 4 AWG.
What is ampacity?
Ampacity is the current a conductor can carry continuously without passing its temperature rating (NEC Table 310.16). It's one of the two things that set wire size — the other is voltage drop.
Do I use the 60, 75, or 90 °C column?
Almost always the 75 °C column — your terminations set it (110.14(C)). The 90 °C column is only a starting point for derating, and NM-B/Romex is sized from the 60 °C column.
When does voltage drop change the wire size?
On long runs. A rough trigger: when the one-way length in feet exceeds the system-voltage number, voltage drop often forces a larger wire than ampacity alone — the calculator checks both and keeps the larger.
Do I upsize for a continuous load?
Yes. For loads running 3+ hours (EV chargers, heaters), size the conductor and breaker for 125% of the load (NEC 210.19(A)). A 40 A continuous load needs a 50 A circuit.
Is it OK to use a bigger wire than required?
Yes — you can always go larger, never smaller. Oversizing costs more and can crowd terminals, but it's safe and lowers voltage drop. Undersizing is a fire risk.
Keep going
Related electrical calculators
Sizing the conductor is one step in the circuit. These pick up where it leaves off — check the run, protect it, and fit it in the raceway — and run on the same NEC-based engine.
How we keep this accurate
These calculators follow the National Electrical Code (NEC 2023): ampacity from Table 310.16, the 240.4(D) small-conductor rule, 110.14(C) terminations, and the 3% branch / 5% total voltage-drop guidance (210.19 & 215.2 informational notes). Results are for planning and estimating. Code adoption, local amendments, and field conditions vary — verify with your AHJ and a licensed electrician before installation.