What is the Correct Wire Size for a 40A Breaker and Load?

Electrical Technology

How to Determine the Suitable Wire Size for a 40A Breaker and Load Based on NEC?

Correct wire size selection for a 40A breaker and load circuit is crucial for the efficient, safe, and protected operation of both equipment and personnel. The wire and cable used in a 40-amp circuit must comply with the National Electrical Code (NEC), IEEE, IEC, or other applicable local standards.

A 40-amp breaker is a standard size available in 1-pole and 2-pole configurations for common 120V and 240V circuits. These breakers are used in high-power and heavy-duty circuits like EV charging, electric stoves and ranges, central air conditioning, or electric heating systems (HVAC). They protect cables rated at 40 amps or more and the circuit in case of overload or short circuit when the load circuit draws 40 amps or less during normal operation.

According to the NEC, the correct wire size for a 40-amp circuit is #8 AWG copper or #6 AWG aluminum. Keep in mind that the correct wire size depends on multiple factors such as distance, voltage drop, ambient temperature, type of cable and wire, type of insulation, etc. The following step-by-step guide will help you determine the suitable wire size for a 40-amp breaker and load circuits.

Related Post: What is the Suitable Wire Size for a 35A Breaker and Load?

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What is the Suitable Wire Size for a 40A Breaker?

Based on NEC Table 310.15(B)(16) (formerly Table 310.16), the standard wire size for a 40-ampere breaker is #8 AWG copper wire.

If you use aluminum instead of copper with a 40A breaker, you typically need #6 AWG aluminum wire due to aluminum’s lower conductivity and high resistance compared to copper.

Both #8 AWG copper and #6 AWG aluminum wires can handle:

40A at 60°C (140°F)
50A of current at 75°C (167°
55A at 90°C (194°F)

Due to the high power and amperage in the circuit, we recommend using #8 gauge solid copper wire instead of #6 gauge aluminum or stranded wire with 40A breakers and load circuits for safety, conductivity, and efficiency. Solid wire offers several advantages over stranded wire, which is why U.S. homes typically use solid rather than stranded wire.

Good to Know:

For a 40A circuit breaker, the correct wire size is #8 AWG for copper and #6 AWG for aluminum.
Use 14-2 with ground for 120V, 40-Amp breaker and load.
Use 14-3 with ground for 240V, 40-Amp breaker and load.
Use 14-4 with neutral and ground for 240V, 40-Amp breaker and load.
Longer runs (when the distance is more than 50 ft (15.25 meters) require an upgrade and larger wire gauge size to compensate for voltage drop.
According to the NEC – 310-16, add 20% of additional ampacity for every 100 feet (30.50 meters) of distance (for example between main panel and subpanel) to counter the voltage drop.
Related Post: What is the Right Wire Size for a 30A Breaker and Outlet?

How to Select the Right Wire Size for a 40A Breaker and Load?

Selecting the right wire size involves several factors i.e. conductor material (copper or aluminum), circuit length and run, ambient temperature, and installation environment (e.g., indoor, outdoor, conduit). The following example shows the correct wire selection for 40A continuous and non-continuous load circuits:

Example:

Suppose you have installed a 40A (either 1-pole or 2-pole) breaker in the main panel or load center, and you need to wire it with a maximum of 40A load circuit. Follow the fowling simple steps to find the suitable wire size for a 40-Amp breaker and load circuits for both continuous and non-continuous load circuits according to NEC guidelines.

Continuous Load Circuit

The NEC’s 125% rule states that the maximum overcurrent protection device (MOCP) should handle 125% of the continuous load. As a safety factor, no more than 80% of the continuous load should be connected to the Overcurrent Protection Device (OCPD).

40A × 80% = 32A

Based on this rule, The amperes of the load circuit should not cross the maximum limit of 32A. In other words, you may only wire a 32-Amp of continuous load (lasts for 3-4 hours simultaneously) circuit to a 40-Amp breaker.

In second scenario where the minimum circuit ampacity (MCA) i.e. load circuit’s amperes are 32A, you may determine the correct breaker size for this circuit:

32A × 125% = 40A

According to NEC Table 310.15(B)(16), the #8 AWG wire size can carry 40A at 60°C (140°F) and 50A at 75°C (167°F).

Non-continuous Load Circuit

For non-continuous loads (e.g., general lighting circuits), the conductor size should be no less than 100% of the load. The same applies to overcurrent protection devices (OCPDs).

A 40A breaker can handle a maximum of 40A non-continuous load circuits, such as outlets and lighting points, while considering ambient temperature rating (Refer to 110.14(C) and 310.15(B)(2)).

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How Many Amps Can a 40A Breaker Handle Safely?

A 40-amp breaker can safely handle a maximum of 40 amps of current. The NEC states that breakers should not exceed 80% of their rated capacity for continuous loads (lasting three hours or more). Therefore, a 40A breaker is suitable for a maximum of 32A continuous load circuits.

40A × 80% = 32A

For short, non-continuous loads, the breaker can handle up to 40 amps safely.

Use a 40A breaker for a 32A continuous load circuit.
Use a 40A breaker for a 40A non-continuous load circuit.

These ratings comply with NEC Sections 210.19(A), 215.2, and 230.42(A) for continuous and non-continuous loads, and 110.14(C) for ambient temperature.

The above calculations are based on NEC Table 310.15(B)(16) and 240.4(A) through (G), illustrating that the 8 AWG copper wire size can carry 40A at 60°C (140°F) and 50A at 75°C (167°F) which is the suitable size to use with a 40-Amp breaker.

Related Post: What is the Suitable Wire Size for 20A Breaker and Outlet?

How Many Watts Can a 40A Breaker Hold?

The power capacity of a 40-amp breaker can be calculated using the following formula:

$Watts = Volts \times Amps$

120V Circuit

In a standard 1P, 120V circuit, the max power a 40A breaker can hold:

40 A × 120 V = 4,800 W

Applying the safety factor (80% Load):

40 A × 80% = 32 A

Hence, no more than 3,840 watts of load should be connected to a 40A breaker:

32A × 120V = 3,840 watts

It means, you may use 40A breaker with a 120V, 3.5kW to 3.8kW electric range or water heater element.

240V Circuit

A 2-P, 40A breaker in 240V circuit can hold the maximum power:

40 A × 240 V = 9,600 W

Applying the safety factor of 80% for continues load:

40 A × 80% = 32 A

Based on the calculation, you may connect up to 7,680 watts of load to a 2-pole, 240V, 40A breaker:

28A × 240V = 7,680 watts

For example, you may use a 40A breaker with a 240V, 7.5kW water heater element.

These calculations assume that the breaker is not continuously loaded to 100% of its capacity. For continuous loads, the safe wattage is 80% of these values:

120V Circuit (Continuous Load): 3,840 W
240V Circuit (Continuous Load): 7,680 W

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How Many Outlets Can be Put on a 40A Breaker?

It is a good practice to use a 40-amp breaker for a dedicated circuit for high-power appliances, like electric ranges, water heaters, or large air conditioning units, instead of standard electrical outlets. The National Electrical Code (NEC) prohibits the use of standard 15-amp or 20-amp outlets (receptacles) with a 40-amp breaker because it would be unsafe.

When multiple general-purpose outlets are used on the circuit, a 40-Amp circuit breaker can support 18 outlets.

The NEC recommends that a circuit should not be loaded to more than 80% of its capacity to allow for safety margins. For a 40A breaker, the maximum continuous load should be:

40A × 0.8 = 32A

Theoretically, If each outlet is assumed to draw 1.5A, you can estimate the number of outlets by dividing the maximum continuous load by the per-outlet load:

32 A ÷ 1.5 A ≈ 21 outlets

Note that you should use a 40A breaker with one device and associated outlet at a time. For standard 15-amp or 20-amp outlets, use a 15-amp or 20-amp breaker, respectively. If you need a 40-amp breaker for a circuit, it’s likely intended for a single, dedicated appliance with a matching 40-amp receptacle. For more details, Refer to NEC Table 210.21(B)(3) for receptacle ratings for various circuit sizes.

If you need to install outlets, you should calculate the total expected load and ensure that the breaker and wiring are appropriately sized to protect the circuit and prevent overloading.

Is it Allowed to Use a 50A Outlet on a 40A Breaker?

In some special case, it is allowed to use a 50-amp receptacle on a 40-amp breaker. The safest practice is to ensure that both the breaker and outlet are rated for the same amperage. In simple terms, you should use a 40-amp breaker with a 40-amp rated outlet. Same is the case for 50A Breaker. It is good to keep in mind that the opposite scenario is not permitted i.e. it is permitted to use a 50A outlet on a 40A breaker, but it is not allowed to use a 40A outlet on a 50A breaker.

However, since NEMA standards do not include 40-amp outlets, let’s explore how to handle this situation.

According to the National Electrical Code (NEC), the breaker must match the amperage rating of the outlet to protect the wiring and devices connected to the circuit.

NEC Article 210.21(B)(3) states that the receptacle (outlet) must be rated for the load it will carry, and the breaker must protect the wiring and devices from overload. Using a 50A outlet on a 40A breaker could result in the breaker tripping during an overload. As the outlet could draw more current than the breaker is rated for, in case of improper operation or faulty breaker, this may lead to create a potential fire hazard. Hence, For a 40A breaker, a 40A-rated outlet is typically recommended.
A 50A outlet is designed to handle up to maximum of 50 amps of current for continuous load and 40A for non-continues load circuits. Same is the case for 40A breaker e.g. it can be used for maximum of 40A of non-continuous load and 32A of continuous load. Now, If a 50A receptacle is used on a 40A breaker, it might not cause immediate issues, but it could potentially allow more current to flow through the outlet than it is rated for, leading to overheating the associated breaker, wire and cables, tripping the breaker and potential fire hazards.
Additionally, for fixed appliances or equipment, you should follow the 80% rule (NEC 210.19(A)(1)) for continues circuits. For example, you should only load the breaker to 80% of its capacity for continuous loads. This means you should use a 40A breaker with devices that draw no more than 32A continuously to avoid the unwanted tripping of the breaker or overheating the associated wires and cables.

You might have seen Tesla Level 2 charging stations where a 40A breaker protects a 50A outlet, allowing for a maximum charging current of 32A.

This demonstrates that you can use a 50A receptacle on a 40A breaker under certain conditions:

To wire a 50A receptacle and outlets (such as NEMA 14-50R or 6-50R) on a circuit with a 40A breaker, you must follow these conditions:
- Ensure that the 50A outlet does not draw more than 40 amps for non-continuous loads when using a 40A breaker.
- For continuous loads, the 50A outlet, when protected by a 40A breaker, must not draw more than 32A to comply with NEC rules for continuous load protection (80% rule).
The breaker is designed to protect the wiring and prevent overheating. Using a 50A receptacle on a 40A breaker could cause confusion if there’s an issue or if someone attempts to use a device that draws more than 40 amps. Thus, the circuit must be clearly labeled to indicate that the 50A outlet is protected by a 40A breaker. This labeling ensures that maintenance electricians or future owners are aware of the exact setup for safety and proper protection.

Suitable Types of Cables for a 40-Amp Breaker

For a 40-amp breaker and outlet, the types of cables used must be capable of safely carrying the current without overheating. The choice of cable material and type depends on the installation environment and specific application. Here are the different types of cables typically used:

Copper Wire:

NM-B (Non-Metallic Sheathed Cable): Suitable for indoor installations, such as wiring for electric ranges or water heaters, where the cable will run through walls, ceilings, or floors.
THHN/THWN (Thermoplastic High Heat-Resistant Nylon-Coated / Thermoplastic Heat and Water-Resistant Nylon-Coated): They are Used in commercial and industrial settings, often run through conduits. Common for air conditioners, electric vehicle chargers, and other high-power appliances.
UF-B (Underground Feeder Cable): Ideal for outdoor wiring, such as feeding power to a detached garage or an outdoor appliance. It’s suitable for underground installations without the need for a conduit.
SER (Service Entrance Cable): Often used to connect a subpanel or large appliance to the main electrical panel. They are suitable for indoors or outdoors, where protected by a conduit in outdoor installations.
MC Cable (Metal-Clad Cable): Suitable for environments where the cable might be exposed to physical damage. The metal-clad sheathing provides extra protection, making it ideal for high-traffic areas.

Aluminum Wire:

It is Used where lighter weight and cost are factors, such as in mobile homes or for certain high-amperage circuits. They Often used in industrial and commercial applications, particularly where high heat resistance is required.

XHHW (Cross-Linked Polyethylene High Heat-Resistant Wire): These kind of cables are used in industrial and commercial applications, particularly where high heat resistance is required

Applications of a 40-Amp Breaker and Outlet

A 40-amp breaker is generally used in circuits that require a higher current than typical household outlets. Common applications include:

Electric Ranges: Many residential electric ranges (stoves/ovens) require a 40-amp circuit.
Water Heaters: Larger electric water heaters might require a 40-amp circuit.
Air Conditioners: Some central air conditioning units or large window units may need a 40-amp breaker.
Electric Vehicle Chargers: Some Level 2 electric vehicle (EV) chargers are rated for 40 amps.
Large Appliances: Other heavy-duty appliances like some types of dryers, ovens, or machinery in workshops might require a 40-amp circuit.

Good to Know:

The ampere rating of a single receptacle installed on an individual branch circuit should not exceed that of the branch circuit.
The above calculations for breaker and wire sizes apply only to purely resistive load circuits, such as lighting.
For inductive loads (motors and HVAC), refer to NEC Article 440, particularly sections 440.22 and 440.32. If not sure, consult an HVACR contractor, instructor, and electrician.
A 40A branch circuit needs #8 AWG wire size for copper – NEC Table 210.24.(1).
A 40A outlet can be used for a 32A continuous load and a maximum 40A non-continuous load (210.19(A)).
It is against code to use a 50A outlet to draw 50A on a 40A or 45A breaker.
It is against code to use smaller gauge wire sizes (e.g., using 10, 12 AWG) instead of 8 AWG wire with a 40A breaker.
According to NEC 210.21(B)(3), it is permissible to use a 30A outlet on a 35A circuit if there are multiple receptacles on the circuit and only once used at a time.
Important Note: If you install a 40A outlet on a 40A circuit, it should only be used for single appliances as dedicated circuit with single outlet for a maximum of a 32A load. Drawing more than 32A simultaneously from a 40A breaker will overheat the circuit, potentially causing serious injury and fire hazards.