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Wire size calculator / 240 V wire size

Wire size for a 240 V circuit

Ampacity is measured in amperes. Voltage-drop percentage depends on source voltage. This page separates those two checks for a hypothetical 240 V load.

A 240 V circuit does not get a new ampacity table. NEC Table 310.16 lists copper and aluminum by size and temperature, for not more than three current-carrying conductors at 30°C ambient. Voltage is not one of those columns. What voltage changes is the meaning of a volt of drop: percent drop is the volt drop divided by the source voltage.

The general NEC informational notes still talk about 3 percent on a branch or feeder and 5 percent combined. Those notes are not mandatory rules everywhere. Local and energy codes can impose their own limits, and this calculator does not include upstream feeder drop. The prefilled form uses the same 3 percent assumption as the home calculator until you change it.

10 AWG stranded copper resistance is 1.24 Ω/kft at 75°C. At 30 A and 100 ft one way, the single-phase Table 8 model (2 × I × R × L / 1,000, reactance omitted) gives 7.44 V (3.10%) on 240 V and 7.44 V (6.20%) on 120 V. The volt drop matches, so the 120 V percentage is twice the 240 V percentage in this example. Displayed volts are rounded to two decimals. This is the drop of that gauge, not a finding that the gauge is large enough.

What the calculator does with three different inputs

Each row is the home calculator’s 30 A example, or that example with the length or the voltage changed. Insulation, terminals, ambient, and conductor count stay on the preset. The estimate is whatever single conductor passes both checks. It is not an appliance installation.

Hypothetical 30 A examples. Not an appliance, range, or dryer design. Estimates use the calculator’s Table 8 model at 75°C conductor temperature.
Example Assumptions Copper estimate Aluminum estimate
30 A example at the preset length 240 V single-phase, 30 A noncontinuous, 40 ft one way, 3% drop limit, 75°C insulation, 60°C terminals, 30°C ambient, 2 current-carrying conductors, small-conductor check on, Table 8 resistance only 10 AWG (Ampacity governs) 8 AWG (Ampacity governs)
30 A at 100 ft 240 V single-phase, 30 A noncontinuous, 100 ft one way, 3% drop limit, 75°C insulation, 60°C terminals, 30°C ambient, 2 current-carrying conductors, small-conductor check on, Table 8 resistance only 8 AWG (Voltage drop governs) 6 AWG (Voltage drop governs)
30 A at 120 V and 100 ft 120 V single-phase, 30 A noncontinuous, 100 ft one way, 3% drop limit, 75°C insulation, 60°C terminals, 30°C ambient, 2 current-carrying conductors, small-conductor check on, Table 8 resistance only 6 AWG (Voltage drop governs) 4 AWG (Voltage drop governs)

30 A example at the preset length, copper. Voltage drop would allow 12 AWG. 12 AWG exceeds the NEC 240.4(D) small-conductor breaker limit for this load, so 10 AWG is required. Ampacity governs.

30 A example at the preset length, aluminum. Voltage drop would allow 10 AWG. 10 AWG exceeds the NEC 240.4(D) small-conductor breaker limit for this load, so 8 AWG is required. Ampacity governs.

30 A at 100 ft, copper. Ampacity would allow 10 AWG. Voltage drop requires 8 AWG, so voltage drop governs.

30 A at 100 ft, aluminum. Ampacity would allow 8 AWG. Voltage drop requires 6 AWG, so voltage drop governs.

30 A at 120 V and 100 ft, copper. Ampacity would allow 10 AWG. Voltage drop requires 6 AWG, so voltage drop governs.

30 A at 120 V and 100 ft, aluminum. Ampacity would allow 8 AWG. Voltage drop requires 4 AWG, so voltage drop governs.

Planning estimate only. Consult local code and a qualified electrician before buying wire or energizing a circuit. Presets are hypothetical loads; this tool does not design motor, PV, battery, appliance, or EV installations.

Sources for these cells: existing Table 310.16 numerical cells in NFPA’s public development record, pp. 142–143 (proposed changes are not treated as adopted code) and NFPA 70 (2017), Chapter 9 Table 8, printed p. 689, in a third-party-hosted copy. The same publication supplies 110.14 (terminals and aluminum), 210.19/215.2 (continuous loads), 240.4(D) (small conductors), 250.24/250.32 (grounding), and 625.41/625.42 (EV loads). Schneider Electric’s voltage-drop formulas support the arithmetic; Table 8 AC results here omit reactance and assume unity power factor. Resistance and geometry are construction- and temperature-specific. Adopted editions and local amendments can differ.

Related guides

Open the wire size calculator · Ampacity chart · Voltage drop calculator · Printable chart

Run the 240 V example

The form starts from this page’s example. The address stays on this guide until you edit a field. After an edit, a shared link includes those settings in the URL, and opening that link sends the URL to the host.

Circuit

Headline metal

Both metals are compared. Verify equipment compatibility; some equipment permits copper only.

System

General NEC notes suggest 3% on a branch or feeder and 5% combined. Special equipment and local codes may require other limits; upstream drop is not included.

Estimated size

Enter the load current and the one-way distance. The size appears here, with the limit that set it.

Temperature and bundling

These conditions affect the table ampacity used for the comparison.

Conductor insulation

Use the rating marked for wet or dry conditions. THHN is 90°C dry; XHHW is 90°C dry or 75°C wet. For NM-B use 90°C insulation and a 60°C cap below.

Terminal / cable ampacity cap

Use the lowest applicable rating at either end, including cable limits. The general unmarked default is 60°C for equipment rated ≤100 A or marked for 14–1 AWG. Confirm equipment markings before selecting 75°C; NM-B remains capped at 60°C.

Equipment grounding conductors are excluded. Verify neutrals, harmonic loads, and all other current-carrying conductors in the raceway or cable; the automatic count does not check these.

AC impedance (Chapter 9 Table 9)

At power factor 1, the reactance term is zero, but Table 9 AC resistance still applies. Unlisted sizes have no Table 9 result.

Planning estimate only. Consult local code and a qualified electrician before buying wire or energizing a circuit. Presets are hypothetical loads; this tool does not design motor, PV, battery, appliance, or EV installations.

Questions

Does 240 V let me use a smaller gauge than 120 V?

At the same current, length, and installation conditions, a smaller size can result if voltage drop was the limit. Table 310.16 has no voltage column, so the ampacity cells do not change when only the source changes from 120 V to 240 V. In the 10 AWG example at 30 A and 100 ft, the Table 8 model gives 7.44 V at both voltages. 6.20% of 120 V is twice 3.10% of 240 V in this example, because the volt drop matches. A short run can still be limited by ampacity or by the small-conductor ceiling. Planning estimate only. Consult local code and a qualified electrician.

Is a 240 V circuit always a 30 A circuit?

No. 30 A is one hypothetical input on this page, taken from the calculator’s 30 A example. A 240 V circuit can be any load the equipment and the adopted code allow. Enter the actual current. This page does not design appliances, ranges, or dryers.

Which metal should a 240 V circuit use?

The calculator compares copper and aluminum table values. The equipment listing decides which metal is allowed. Some equipment permits copper only. Torque instructions still apply. Higher aluminum resistance means the same label is not the same voltage drop.