What is the Suitable Wire Size for 70A Breaker and Load?

Electrical Technology

How to Calculate the Correct Wire Size for a 70A Breaker and Load Circuits Based on NEC?

A 70-amp breaker are typically used for high-power devices like large HVAC systems, electric furnaces, and subpanels. These circuits must be carefully planned to ensure proper wire sizing for both safety and efficiency. In this article, we will explore how to select the appropriate wire size for a 70A breaker, based on the National Electrical Code (NEC).

The correct wire size for a 70-amp circuit is #4 AWG copper or #2 AWG aluminum, according to NEC tables. However, several factors influence wire size selection, including voltage drop, distance, insulation type, ambient temperature, and the type of conductor. Always consult a licensed electrician to verify compliance with the NEC and ensure the proper installation of your wiring system.

Related Post: What is the Right Wire Size for 60A Breaker and Outlet?

Factors Affecting Wire Size

When choosing the wire size for a 70-amp breaker and circuit, consider the following factors:

Voltage Drop: Longer wire runs experience greater voltage drop. If your wire run exceeds 50 feet, it’s advisable to increase the wire size to compensate for this drop and ensure efficient power delivery.
Temperature Rating: The ampacity of the wire depends on the insulation’s temperature rating. For example:
- At 60°C (140°F), a #4 AWG copper wire can carry 70 amps.
- At 75°C (167°F), the same wire can carry 85 amps.
Wire Insulation Type: Common wire types include THHN (Thermoplastic High Heat-resistant Nylon-coated) and THWN (Thermoplastic Heat and Water-resistant Nylon-coated), which provide high-temperature tolerance.

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What is the Right Wire Size for a 70A Circuit?

According to NEC Table 310.15(B)(16), the appropriate wire size for a 70-amp circuit is:

#4 AWG copper or
#2 AWG aluminum.

The material of the conductor and factors like temperature and voltage drop will affect the exact wire size. Solid copper wire is often preferred over aluminum due to its higher conductivity and lower resistance.

The following shows the ampacity of #4 AWG copper and #2 AWG aluminum wire at different ambient temperatures:

#4 AWG Copper

- 70 amps at 60°C (140°F)
- 85 amps at 75°C (167°F)
- 95 amps at 90°C (194°F)

#2 AWG Aluminum

- 75 amps at 60°C (140°F)
- 90 amps at 75°C (167°F)
- 100 amps at 90°C (194°F)

It clearly shows that a #4 gauge copper or #2 gauge aluminum conductors can safely handles the required amount of electric current in amperes for a 70-amp breaker or fuse.

These values are based on NEC Table 310.15(B)(16). Consider the involved distance and ambient temperature ratings into account while selecting the right wire size.

Related Post: What is the Correct Wire Size for 55A Breaker and Load?

Good to Know:

The correct wire size for a 70A breaker is #4 AWG copper or #2 AWG aluminum.
A 70A breaker is typically used for large, single appliances as a dedicated circuit (NEC 210.23).
120V circuits: Use 4-2 with ground wire for 70A breaker and load circuits.
240V circuits: Use 4-3 with ground wire for 70A breaker and load circuits.
240V with neutral: Use 4-4 with ground for 70A breaker and load circuits
You may use A 70A, 240V breaker as dedicated circuit for single unit only.
For long-distance runs (over 50 feet or 15.25 meters), consider upgrading to a larger gauge wire to compensate for voltage drop. The NEC recommends adding 20% more ampacity for every 100 feet (30.50 meters) of distance.

Selecting the Correct Wire Size for a 70A Breaker

Now, let’s break down how to choose the correct wire size for a 70A breaker and outlet for both continuous and non-continuous load circuits.

Continuous Load Circuit

For continuous loads (devices running for 3 hours or more), the NEC requires that the maximum current not exceed 80% of the breaker’s rating.

For a 70A breaker:

70A × 80% = 56A

This means the breaker should not be loaded with more than 56 amps continuously. If your device’s nameplate specifies a minimum circuit ampacity (MCA) of 56A, use the following formula to determine the appropriate breaker size:

56A × 125% = 70A

Thus, for a 70A breaker, a #4 AWG copper wire or #2 AWG aluminum wire is suitable, as it can carry the necessary current safely.

Non-Continuous Load Circuit

For non-continuous loads (such as general-purpose lighting or short-term use), the breaker can handle the full 70 amps without derating. In this case, both #4 AWG copper and #2 AWG aluminum are adequate wire sizes.

Good to Know: Always consider voltage drop in long run and ambient temperature when selecting the wire size.

How Many Amps Can a 70A Breaker Safely Handle?

According to the NEC, a breaker should not be loaded beyond 80% of its capacity for continuous loads. Therefore, a 70A breaker can handle:

56A continuous load circuits (70A × 80% = 56A)
70A non-continuous load circuits

This complies with NEC Sections 210.19(A), 215.2, and 230.42(A).

Good to know:

Use a 70A breaker for 56A continuous load circuits or maximum 70A non-continuous load circuits.
If the load circuit is greater than 60A, it will require a disconnect switch due to the absence of 70A rated outlets and switches. (NEC 625.23 – 2011).

How Many Watts Can a 70A Breaker Hold?

To calculate the wattage capacity of a 70A breaker, use the formula:

Watts = Volts × Amps

120V Circuit:

A 1-P, 70A breaker on a 120V circuit can hold:

70A × 120V = 8,400W
- Continuous load: 70A × 80% = 56A
- 56A × 120V = 6,720W

Good to know: You may use a 70A breaker as a sub-panel to use more 15A, 20A,40A, etc. outlets for 120V circuits. According to the code, a 70A breaker for single unit should be wired with a disconnect switch.

240V Circuit:

A 2-P, 70A breaker in a 240V circuit holds:

70A × 240V = 16,800W
- Continuous load: 70A × 80% = 56A
- 56A × 240V = 13,440W

You can connect safely up to 13,440 watts of load on a two pole, 70A breaker in a 240V circuit. You may use a single load up to 70A or multiple load circuits where the total amperes doesn’t exceed 70A.

Good to know: A 70A breaker can handle a maximum of 8,400W and safely 6,720W in a 120V circuit, while it can handle a maximum of 16,800W and safely 13,440W in a 240V circuit.

Related Post: What is the Right Wire Size for 45A Breaker and Load?

How Many Outlets Can Be Put on a 70A Breaker?

A 70A breaker is typically designed for a dedicated circuit serving a single high-power appliance. It is not recommended to wire multiple outlets to a 70A breaker, as this could overload the circuit. NEC Table 210.21(B)(3) restricts the number of outlets based on branch circuit size. A 70A breaker should only serve one high-power device.

However, it is permissible to use a 70A breaker as a sub-panel and use 15/20/30 amp breakers with associated and rated outlets as branch circuits.

Suitable Cable Types for a 70A Breaker

When selecting a cable for a 70-amp breaker, choose the appropriate insulation and wire type:

Copper Wire: Use #4 AWG with THHN or THWN insulation for durability and high-temperature performance.
Aluminum Wire: Use #2 AWG with insulation rated for higher temperatures, such as THHN or XHHW.
USE (Underground Service Entrance Cable): Designed for direct burial, this cable is used for outdoor applications where moisture resistance is important.
MC (Metal-Clad Cable): MC cables inside metal sheath are used in industrial and commercial applications
Type SE Cable: SE (Service Entrance Cable) are used for feeding power to residential and small scale commercial applications.

Applications of 70A Breakers

Common uses for 70-amp breakers include:

Large HVAC systems (commercial refrigeration and air-conditioning)
Electric furnaces and water heaters
Subpanels in garages or workshops
electric welding machines
Heavy-duty electric stoves or ovens
Commercial kitchen equipment and devices.
Inverters and Data Centers
large power lighting in stadiums, theater etc.
Inductive loads such as motors, compressors, pumps etc.
High-power EV chargers and stations

Good to Know:

Ensure the ampere rating of a receptacle does not exceed that of the branch circuit.
The calculations for breaker and wire sizes above apply only to purely resistive load circuits, such as lighting.
For inductive loads like motors or HVAC systems, refer to NEC Article 440, especially sections 440.22 and 440.32. If unsure, consult an HVACR contractor or licensed electrician.
A 70A branch circuit requires #4 AWG copper wire or #2 AWG aluminum wire, according to NEC Table 310.15(B)(16) (formerly 310.16) and section 210.24.
A 70A breaker can support a 56A continuous load and a 70A non-continuous load, per NEC sections 210.19(A), 215.2, and 230.42(A).
It’s against code to use smaller wire gauges, like #6 or #8 AWG, instead of #4 AWG with a 70A breaker.
According to NEC 210.21(B)(3), it’s permissible to use two 40A or 50A outlets on a 70A circuit if there are multiple receptacles and only one is used at a time. The total load must not exceed 70A, such as when breakers feed branch circuits in a subpanel.
When installing a 70A circuit, dedicate it to a single appliance with a maximum load of 56A. Drawing more than 56A continuously will overheat the circuit and could cause serious injury or fire hazards.
Always consult a licensed electrician to ensure compliance with NEC standards and to verify the correct wire size for your specific installation.