AmpSizerWhat size wire do you need?
Wire size calculator
8 AWG copper
on a 50 A device · 10 AWG ground · 1/2" conduit at 31% fill
1.9% drop at 60 ft
- Ampacity wants8 AWG
- Voltage drop wants8 AWG
- Current the conductor has to carry
- 50 A
- Usable ampacity after derating
- 50 A (table 50)
- Longest run before the drop passes 3%
- 93 ft
- In metric sizes
- 8.4 mm²
— had the ampacity and 240.4(D) still rejected it, at 0 A.
Set the terminal rating to 60 °C on NM cable or older gear: the lowest-rated part of the path governs.
A sizing aid from the published NEC tables, not a design — have it signed off by a licensed electrician against the code your inspector enforces. Calculations follow the US NEC; outside the US, follow your local electrical code.
One circuit check a month
MonthlyOne thing a month worth measuring, testing or recalculating on a home electrical system — the part of sizing that happens after the install. No product pitches, and nothing that needs a live panel.
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What size wire do you need for a circuit?
The AmpSizer wire size for a 50 A circuit running 60 ft on 240 V is 8 AWG copper, on a 50 A breaker with a 10 AWG ground in half-inch EMT. Voltage drop over that run is 1.9%, and the same conductor stays under 3% out to 93 ft.
Two rules produce that answer. Ampacity says 8 AWG copper carries 50 A at a 75 °C terminal. Voltage drop over 60 ft agrees, so ampacity is what decided — and the calculator says so, with the size the other rule would have taken.
Past 93 ft the two rules part company: drop starts asking for 6 AWG while ampacity is still happy with 8. That crossover is the whole reason a wire size calculator exists, and it moves with the load, the voltage and the metal.
The chart below is ampacity only — no distance in it, because a chart cannot know your distance. Use it to sanity-check the calculator, and use the calculator when the run is long.
| Breaker | Copper | Aluminium | Copper ground | What it usually feeds |
|---|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | 14 AWG | Lighting and general receptacles |
| 20 A | 12 AWG | 12 AWG | 12 AWG | Kitchen, bathroom and garage receptacles |
| 30 A | 10 AWG | 10 AWG | 10 AWG | Dryer, small water heater, RV outlet |
| 40 A | 8 AWG | 8 AWG | 10 AWG | Range, 40 A EV charger |
| 50 A | 8 AWG | 6 AWG | 10 AWG | Range, welder, 50 A EV charger, hot tub |
| 60 A | 6 AWG | 4 AWG | 10 AWG | Sub panel, large hot tub, tankless heater |
| 70 A | 4 AWG | 3 AWG | 8 AWG | Sub panel, larger tankless heater |
| 100 A | 3 AWG | 1 AWG | 8 AWG | Sub panel, or a small service |
| 125 A | 1 AWG | 2/0 AWG | 6 AWG | Sub panel |
| 150 A | 1/0 AWG | 3/0 AWG | 6 AWG | Service, or a large sub panel |
| 200 A | 3/0 AWG | 250 kcmil | 6 AWG | Typical whole-house service |
Why two rules and not one
Ampacity is about heat. A conductor carrying current warms up, and the table is the current at which the insulation reaches its rated temperature under standard conditions. It has nothing to do with how far the wire goes.
Voltage drop is about distance. Every foot of copper has resistance, and resistance turns some of your voltage into heat along the way. It has nothing to do with whether the conductor is safe.
A wire has to pass both. On a 20 ft kitchen circuit, ampacity decides every time. On a 180 ft run out to a well pump, voltage drop decides and the ampacity answer is two sizes too small. The point where they swap is what the longest-run line in the results is for.
The sizes people actually ask about
| Circuit | Copper at 75 °C | Aluminium | Ground | Where drop takes over on 240 V |
|---|---|---|---|---|
| 15 A | 14 AWG | 12 AWG | 14 AWG | 76 ft |
| 20 A | 12 AWG | 12 AWG | 12 AWG | 91 ft |
| 30 A | 10 AWG | 10 AWG | 10 AWG | 97 ft |
| 50 A | 8 AWG | 6 AWG | 10 AWG | 93 ft |
| 60 A | 6 AWG | 4 AWG | 10 AWG | 122 ft |
| 100 A | 3 AWG | 1 AWG | 8 AWG | 147 ft |
The last column is the run at which the ampacity answer stops being enough at a 3% target, for copper. Halve those distances for a 120 V circuit — the volts lost are identical, the percentage is doubled.
The aluminium column is what Table 310.16 permits, not what anyone stocks: nobody sells 12 AWG aluminium branch-circuit cable, and most jurisdictions will not accept aluminium below 8 AWG at all. Aluminium starts making sense on feeders, where the size difference costs less than copper does.
Frequently asked questions
What size wire for 50 amps 100 feet away?
6 AWG copper at 240 V, not the 8 AWG that ampacity alone allows. At 100 ft, 8 AWG copper drops 3.2% and 6 AWG drops 2.0%, so voltage drop takes over as the rule that decides. On 120 V at the same distance the answer moves again to 4 AWG, because the same volts lost are twice the percentage of a smaller number.
Can I use aluminium for a sub panel feeder?
Yes, and for feeders it is normal — SER cable and URD are aluminium almost everywhere. Two things change. It is two sizes larger for the same current, so 100 A wants 1 AWG aluminium where 3 AWG copper would do. And the terminations need the right compound and torque: aluminium creeps under a screw, and a loose aluminium lug is the failure that gave the metal its reputation.
Does the wire size change if the run is in conduit outdoors?
It can, twice over. Conduit in direct sun on a roof is treated as a hotter ambient, and above 86 °F the correction factors in Table 310.15(B)(1) start eating into ampacity. Underground conduit is usually cooler and helps. Set the ambient field to what the conduit will actually see, not to the weather forecast.
Why does the calculator give a bigger wire than the chart in the store?
Because the chart is ampacity only. A wall chart cannot know your distance, and distance is what voltage drop is made of. On a short run the two agree exactly; on anything over about 75 ft they stop agreeing, and this tool shows both numbers so you can see which one moved.
Do I have to upsize the ground wire too?
Only when you upsized the circuit conductors for voltage drop, and then yes — 250.122(B) makes you increase the equipment grounding conductor in the same proportion. Going from 8 AWG to 6 AWG for drop is a 1.59 times increase in area, so a 10 AWG ground becomes 8 AWG. It is the step most people miss.
How we calculate this
2 × amps × R × one-way feet ÷ 1,000 for single-phase and DC, 1.732 × for
balanced three-phase, with R the direct-current resistance of stranded uncoated conductors from
Chapter 9, Table 8. The conductor returned is the larger of the two requirements. The device is the
next standard rating in 240.6(A), capped by the small-conductor ceiling in 240.4(D). The ground
comes from Table 250.122 and the raceway from Chapter 9, Table 4 at the 40% fill limit. All of it is
read from the same data files that draw the reference charts.Worked example
With Load current 50, One-way run 60, System voltage 240 V, Metal Copper, Terminal rating 75 °C — the usual breaker, Ambient 86, Current-carrying conductors in the raceway 3, this page works out 8 AWG copper. on a 50 A device · 10 AWG ground · 1/2" conduit at 31% fill
- Current the conductor has to carry50 A
- Usable ampacity after derating50 A (table 50 )
- Longest run before the drop passes 3%93 ft
- In metric sizes8.4 mm²
Sources:
Where wire sizing goes wrong
The terminal, not the wire, often sets the limit. THHN is a 90 °C conductor, but almost every breaker and lug is listed at 75 °C, and 110.14(C) makes you use the lower rating. The 90 °C column is for derating arithmetic, not for getting a smaller wire.
Distance is measured along the cable, not across the room. Up the wall, along the joists and back down is easily half again the straight line, and voltage drop is linear in that number.
A bigger wire is cheap; a second trip is not. The cost gap between two adjacent sizes on a 60 ft run is usually less than an hour of labour. If the answer sits right on a boundary, take the larger one and stop thinking about it.