40 Amp Wire Size: What Wire Size Is Needed For A 40 Amp Breaker/Fuse

40 Amp wires are commonly used in homes, RVs, boats, off-the-grid, and similar electric installations.

When calculating the required wire thickness, there are a few important rules that must be taken into consideration. Also, with such currents, one should never go cheap, because, sooner or later, thin and weak wires will cause plenty of issues.

Published: June 15, 2022.

AWG Wire Size Chart

The following AWG wire size chart lists some of the most common wire thicknesses with wire Ampacities (wire current carrying abilities) and wire dimensions, given at a certain temperature:

As one can see, the Ampacity of wires is given at three different temperatures (60°C/140°F, 75°C/167°F, and 90°C/194°F), but for most installations, 60°C/140°F is the maximum allowed wire surface temperature.

Also, if the value is not given for a required current, one has to look for a wire with stronger Ampacity.

For example, if we are going to look for 40 Amp wire size using default Ampacities, then:

- 60°C/140°F: 8 gauge wire (AWG 8) features an Ampacity of 40A,

- 75°C/167°F: 8 gauge wire (AWG 8) features an Ampacity of 50A,

- 90°C/194°F: 10 gauge wire (AWG 10) features an Ampacity of 40A.

However, before making any conclusions, there are a few more rules that must be taken into account.

80% Rule

The 80% Rule increases the safety of the electric installation - basically, when looking for a correct 40 Amps wire gauge, the current is increased using the 80% Rule. For example, we are NOT looking for a wire that features Ampacity of at least 40 Amps, but for a wire featuring Ampacity of:

I (A) = 40 Amps / 0.8 = 50 Amps

So, instead of looking for a 40 Amps wire gauge, we are practically looking for a 50 Amps gauge:

- 60°C/140°F: 6 gauge wire (AWG 6) features an Ampacity of 55A,

- 75°C/167°F: 8 gauge wire (AWG 8) features an Ampacity of 50A,

- 90°C/194°F: 8 gauge wire (AWG 8) features an Ampacity of 55A.

Since most electric wire standards apply for 60°/140°F temperature, we can safely say that for a short 40 Amp wire, one should use a 6 gauge wire (AWG 6).

Note: as one can see, one can use AWG 8 wire instead of AWG 6 wire for a 40 Amp wire, but without any safety margin. Not to mention the increased energy losses with the increase in wire length...

40 Amps Wire Length

In order to compensate for increased energy losses due to the wire length, Ampacity is increased by 10% for every 50 feet of the wire.

For example, when calculating the required Ampacity for the 50 feet, 100 feet, 150 feet and 200 feet long wires, we can use value of 50 Amps (obtained after applying the "80% Rule"):

- 50 feet wire: Ampacity = 50 Amps * 1.1 = 55 Amps

- 100 feet wire: Ampacity = 50 Amps * 1.2 = 60 Amps

- 150 feet wire: Ampacity = 50 Amps * 1.3 = 65 Amps

- 200 feet wire: Ampacity = 50 Amps * 1.4 = 70 Amps

Now, we have to check the required AWG value for given wire lengths, depending on the wire surface temperature - values are given in the following chart:

Note: rules, regulations, and laws differ from country to country - if unsure about local laws, contact a local certified electrician or company and ask them for advice, or even better, pay them to do the job for You. And to sign whatever they did with your electric installation!

40 Amps Wire Thickness - AWG 6 (6 Gauge Wire)

As one can see from the chart, for installations that feature a maximum wire temperature of 60°C/140°F, and for wire lengths up to 50 feet, one can use AWG 6 (6 Gauge Wire).

The physical dimensions of such wire are:

- diameter: 4.1154 mm, 0.1620 inches,

- cross-section area: 13.3016 mm², 0.0206 inches².

Default Ampacity values for 6 gauge wire are:

- 60°C/140°: 55 Amps,

- 75°C/167°F: 65 Amps,

- 90°C/194°F: 75 Amps.

Again, these are default values, without the 80% Rule (a very important safety rule) and without thickness correction due to the wire length.

Long Story Short: if You are looking for a "shorter" 40 Amp wire, go for 6 gauge wire (AWG 6).