Calculating Wire Size for a 240V Circuit

Most household appliances run fine on 120V, but high-draw loads like clothes dryers, electric ranges, water heaters, and central air conditioners need 240V circuits with appropriately sized conductors. Getting the wire gauge right matters for both safety and performance. An undersized wire overheats; an oversized one costs more than necessary.

How 240V Circuits Differ from 120V

A 240V circuit uses two hot conductors, each carrying 120V relative to neutral, along with a ground and sometimes a neutral. The total voltage across the two hots is 240V. Because power equals voltage times current, a 240V circuit delivers twice the power of a 120V circuit at the same amperage. That's why a dryer or range can run on 30 or 50 amps rather than needing 60+ amps at 120V.

Wire sizing for 240V follows the same ampacity principles as any other circuit. You look at the load's rated current, apply the continuous-load rule if necessary, then pick a conductor that meets or exceeds that adjusted figure. For a full walkthrough of the underlying method, see our guide on how to size a cable step by step.

Common 240V Loads and Their Typical Amperage

Different 240V appliances draw very different amounts of current. Here's what you'll typically encounter:

Clothes Dryers (30 A)

Most residential electric dryers are rated at 5,000 to 5,500 watts. At 240V that works out to roughly 23 amps of actual draw, but the NEC and appliance manufacturers call for a 30 A circuit with 10 AWG copper wire. The extra headroom accounts for motor startup surges and heating element cycling.

Electric Ranges and Cooktops (40 A or 50 A)

Ranges are more variable. A basic four-burner electric range might list 8,000 watts, but a large slide-in range with a high-output burner can exceed 12,000 watts at peak. Most residential installations use a 50 A circuit with 6 AWG copper. Smaller cooktop-only units sometimes get by on 40 A with 8 AWG, but a 50 A circuit gives you flexibility for a future upgrade.

Electric Water Heaters (30 A)

A standard 4,500-watt, 240V water heater draws about 18.75 amps continuously. Because water heaters run continuously (the thermostat cycles, but the element is on the whole time it's active), that qualifies as a continuous load. More on that below. A 30 A circuit with 10 AWG copper is the standard choice.

Central Air Conditioners (varies, often 30 to 60 A)

HVAC circuits vary widely. A small 2-ton unit might need a 30 A circuit; a 5-ton unit could require 60 A or more. Always check the unit's nameplate for the Minimum Circuit Ampacity (MCA) and Maximum Overcurrent Protection (MOP) ratings. Those two numbers tell you exactly what the manufacturer requires.

The 80% Rule for Continuous Loads

A load is considered continuous if it runs for three hours or longer without interruption. Water heaters, EV chargers, and some HVAC equipment fall into this category. For continuous loads, the NEC requires the circuit to be sized at 125% of the load's rated amperage, or equivalently, the breaker should not be loaded beyond 80% of its rating.

Practically: if your continuous load draws 24 amps, you need a circuit rated for at least 30 amps (24 × 1.25 = 30). A dryer or range, by contrast, cycles on and off frequently and is treated as a noncontinuous load, so you size to the rated current without the 125% multiplier.

For a deeper look at how ampacity ratings work and what affects them, the ampacity explained article covers temperature ratings, conduit fill, and derating factors that apply to any conductor.

Choosing Wire by Breaker Size

The most straightforward approach for standard 240V residential circuits: pick the wire gauge that matches the breaker size. This table covers copper conductors in typical residential conditions (cable or conduit, 60°C or 75°C rated, ambient temperature around 30°C).

If you're using aluminum wire, bump up two sizes: where copper calls for 6 AWG, aluminum needs 4 AWG. Aluminum is common in larger feeders but rarely used for branch circuits in modern residential work. For a complete sizing chart across all gauges and materials, the AWG wire size chart has the full reference.

Worked Examples

Dryer Circuit (30 A)

A residential dryer has a nameplate rating of 5,400 watts at 240V. Rated current: 5,400 / 240 = 22.5 amps. This is a noncontinuous load, so no 125% multiplier applies. A 30 A breaker is the standard for dryers, and 10 AWG copper satisfies the 30 A rating with adequate headroom. You'll run a 10/3 cable (two hots, a neutral for the 120V controls, and a ground) from the panel to a 4-prong dryer outlet.

Range Circuit (50 A)

A slide-in electric range is listed at 11,400 watts. Rated current: 11,400 / 240 = 47.5 amps. Again noncontinuous, so you round up to the next standard breaker: 50 A. That calls for 6 AWG copper. You'll run 6/3 cable (two hots, neutral, ground) to a 4-prong range outlet. Some older installations used a 3-prong outlet without a separate ground; current code requires the 4-prong configuration for new installations.

Always verify your specific run against how to size a cable step by step, which accounts for run length and voltage drop, especially for long runs over 50 feet.

Voltage Drop on Long Runs

For most 240V circuits inside a home, voltage drop is a minor concern. A dryer 20 feet from the panel on 10 AWG copper loses well under 1% of voltage. But a detached garage, a pool equipment panel, or an HVAC unit at the far end of a house might involve 75 to 100 feet of wire. At that length, voltage drop can reach 3 to 5% on a heavily loaded circuit, which reduces efficiency and can stress motors.

The general guideline is to keep voltage drop under 3% for branch circuits. If your run is long, size up one gauge. A 50 A range circuit running 80 feet might warrant 4 AWG instead of 6 AWG to stay inside that threshold. The same logic applies to EV chargers; see wire size for an EV charger for that specific use case.

Frequently Asked Questions

What wire size do I need for a 240V 30 amp dryer circuit?

Use 10 AWG copper wire with a 30 A double-pole breaker. Run a 10/3 cable with ground from the panel to a 4-prong NEMA 14-30 outlet. That's the standard residential dryer circuit.

Can I use 8 AWG wire on a 50 amp breaker?

No. An 8 AWG copper conductor is rated for 40 amps in typical residential conditions. Pairing it with a 50 A breaker means the wire could carry more current than it's rated for before the breaker trips. Use 6 AWG copper for a 50 A circuit.

Do I need a neutral wire for a 240V circuit?

It depends on the appliance. A pure 240V load like a well pump or HVAC compressor only needs two hots and a ground; no neutral is required. An appliance with 120V controls, like a dryer or range, also uses a neutral to power electronics, timers, and displays. Check the appliance's wiring diagram or nameplate to confirm.

How does the 80% rule affect my breaker choice?

For continuous loads, multiply the load's running current by 1.25 to get the minimum breaker size. A 24-amp continuous load needs at least a 30 A breaker. For noncontinuous loads like dryers and ranges, you size directly to the rated current and pick the nearest standard breaker at or above that figure.

The information on this site is for educational reference only. Always verify circuit sizing against the current edition of the NEC and consult a licensed electrician before performing electrical work.