What is the Suitable Wire Size for 70A Breaker and Load?
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.
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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.
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
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- 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.
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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.
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Good to Know: Always consider voltage drop in long run and ambient temperature when selecting the wire size.
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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).
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
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.
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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
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Good to Know:
Resources:
- What is the Right Wire Size for 15A Breaker and Outlet?
- What is the Suitable Wire Size for 20A Breaker and Outlet?
- What is the Correct Wire Size for 25A Breaker and Load?
- What is the Right Wire Size for a 30A Breaker and Outlet?
- What is the Suitable Wire Size for a 35A Breaker and Load?
- What is the Right Wire Size for a 4.8kW, 240V Range: #10 or #12?
- How to Find the Right Wire Size for 100 Amp in AWG?
- How to Size a Load Center, Panelboards and Distribution Board?
- How to Determine the Number of Circuit Breakers in a Panelboard?
- How to Find the Proper Size of Circuit Breaker? Breaker Size Calculator & Examples
- How to Find The Suitable Size of Cable & Wire for Electrical Wiring Installation? (Metric & Imperial Systems)
- How to Find Voltage & Ampere Rating of Switch, Plug, Outlet & Receptacle
- American Wire Gauge “AWG” Chart – Wire Size & Ampacity Table
- American Wire Gauge “AWG” Calculator – AWG Size Chart & Table
- How to Wire 120V & 240V Main Panel? Breaker Box Installation
- How to Wire a Subpanel? Main Lug Installation for 120V/240V
- How to Wire a GFCI Circuit Breaker?
- How to wire a GFCI Outlet?
- How to Wire an Outlet Receptacle? Socket Outlet Wiring Diagrams
- Wire and Cable Size Calculator in AWG
- Electrical Wire and Cable Size Calculator (Copper & Aluminum)