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What Size Wire for 40 Amps?

For a non-continuous 40 amp load with the displayed 75 C termination defaults, start with 8 AWG copper or 8 AWG aluminum. This is the first appliance-band page after the 30 A small-conductor ceiling.
Load
40 A
Distance
0 ft entered
Band
Appliance / mid branch
Defaults
Non-continuous, 75 C lugs, 86 F, 240 V single-phase
Conductor count
3 current-carrying

Computed Starting Point

The table uses common defaults. Change the calculator inputs for the actual installation.

MaterialComputed sizeOCPDUsable ampacityGoverning constraintDefault check
Copper8 AWG40 A50 ATermination rated 75 CPasses defaults
Aluminum8 AWG40 A40 ATermination rated 75 CPasses defaults
What Size Wire for 40 Amps? under common non-continuous defaults: THHN or XHHW-2 insulation, 75 C terminations, 86 F ambient, 240 V single-phase, no more than three current-carrying conductors, and 0 ft route length entered.

What 40 A ChangesAt 40 A, both copper and aluminum show 8 AWG under the default short-run assumptions, but they do not have the same usable ampacity. This page is the appliance-band entry point, not a generic permission to reuse every 30 A wiring habit.

Run the wire size calculator

Adjacent Amp Ratings

See whether stepping one standard size up or down forces a conductor jump under the same defaults.

LoadCopper sizeCopper OCPDAluminum sizeAluminum OCPD
30 A10 AWG30 A8 AWG30 A
40 A (this page)8 AWG40 A8 AWG40 A
45 A8 AWG45 A6 AWG45 A
Copper and aluminum starting sizes for 40 A and the neighboring published amp pages, under the same defaults as the result above.

A plateau means neighboring amp ratings share a conductor size under these defaults. A jump means a standard breaker change often needs a larger wire; confirm with your real ambient, length and terminations.

40 A Starts the Appliance Band

Appliance / mid branch context for a 40 A circuit.

A 40 A wire-size search commonly comes from a cooktop, range circuit, water heater, 32 A EVSE setting, listed equipment, or a dedicated shop load. The question is usually whether the job has moved beyond the familiar 30 A small-conductor answer.

Under the displayed 75 C termination defaults, 40 A moves copper from 10 AWG at 30 A to 8 AWG. Aluminum is also 8 AWG here, but it has no margin above the displayed 40 A usable ampacity and can move larger on 60 C equipment.

What Changes This Answer

Checks that can change the starting conductor under the displayed defaults.

  • Separate 30 A from 40 A

    The adjacent 30 A page uses 10 AWG copper under these defaults. A 40 A circuit moves copper to 8 AWG.

  • Watch the 45 A aluminum jump

    Copper stays 8 AWG at 45 A under these defaults, but aluminum moves from 8 AWG to 6 AWG.

  • Read the equipment nameplate

    Ranges, cooktops, heaters and listed equipment may specify minimum circuit ampacity and maximum overcurrent protection.

  • Check continuous-load cases

    A 32 A EVSE setting can map to a 40 A circuit after the 125 percent step. A true 40 A continuous load is a different input and should be rerun with continuous load enabled.

  • Confirm 75 C for 8 AWG aluminum

    The aluminum row relies on the displayed 75 C termination default. Equipment limited to 60 C can force a larger aluminum conductor.

40 Amp Wire Size FAQs

What size wire for 40 amps?
Under the non-continuous 75 C termination defaults on this page, start with 8 AWG copper or 8 AWG aluminum. Equipment limited to 60 C can force larger aluminum.
Why is 40 A different from 30 A?
The 30 A page uses 10 AWG copper under the same defaults. At 40 A, copper moves to 8 AWG.
Is 8 AWG aluminum the same as 8 AWG copper here?
They share the same size label under the displayed 75 C defaults, but their usable ampacity margins differ. Aluminum terminations must be listed, and 60 C equipment can change the aluminum answer.
When should I use the 45 A page instead?
Use the 45 A page when the equipment or breaker requirement is actually 45 A. Under these defaults, aluminum changes at that step.