What Size Wire for 70-amp Sub Panel – Answered
When installing a subpanel, it is important to choose the right size wire to ensure safe and efficient operation. The appropriate wire size for a 70-amp sub panel largely depends on the distance the wire needs to run and the type of load it will be carrying.
This article aims to provide a comprehensive guide on determining the right wire size for a 70-amp subpanel installation, taking into account factors such as distance, voltage drop, and code requirements. So, let’s dive straight into the specifics.
Factors to Consider When Choosing Wire Size For Sub Panel
Below are the factors that will aid you in finding the right wire size for a 70-amp sub panel.
Types of Wire
You will need to choose the type of wire to use. Copper wire is the most common type of wire used for sub panels, as it is a good conductor of electricity and is relatively inexpensive. Aluminum wire can also be used, but it is less common and requires larger wire sizes due to its lower conductivity.
Distance and Voltage Drop
It is important to determine the distance between the main panel and the sub panel. This will help determine the voltage drop that may occur over the length of the wire. The National Electrical Code (NEC) recommends a maximum voltage drop of 3% for branch circuits, and a maximum of 5% for feeder circuits.
If the sub panel is located a significant distance from the main panel, voltage drop becomes a more critical factor. In such cases, it may be necessary to use a larger wire size to compensate for the voltage drop and maintain a safe and efficient electrical system. To calculate the drop, you can use an online voltage drop calculator or consult an electrician.
Wire Sizing Calculations
Let’s assume the total length of the cable run is 200 feet, and the ampacity for the main panel or subpanel is set at 70-amps. Considering that most American households utilize 240V appliances, we’ll use this voltage for our calculations.
The calculator suggests a 3-gauge copper wire when the voltage drop is 2.74%. If we reduce the voltage drop to exactly 2%, the recommended wire size may need to be upgraded to 1 AWG. Conversely, with a 5% voltage drop, which is still acceptable, the calculator now advises using the standard NEC recommendation of a 4-gauge copper wire for the same setup.
It’s crucial to note that these recommendations represent minimum requirements, and it is advisable not to go below them whenever possible. It is evident that not all scenarios align precisely with the standard 70-amp wire size chart, especially when factoring in voltage drop and cable length.
However, one could argue that unless you are constructing a subpanel for a detached garage that will link to the main panel or a similar project of comparable scale, there may be no need to deviate from the guidelines outlined in a standard wire gauge amp chart.
Installation Considerations
In addition to choosing the right wire size, it is important to follow all local codes and regulations when installing a subpanel. This may include obtaining permits, using the correct equipment, and having the installation inspected by a qualified electrician.
So, What Size Wire Is Needed for 70-amps?
For a 70-amp sub panel, the NEC recommends using a #4 copper wire for up to 100 feet, or a #2 copper wire for up to 150 feet. For aluminum wire, the recommended wire size is #2 for up to 100 feet, or #1/0 for up to 150 feet.
It is important to note that these wire sizes are based on the assumption of a single-phase circuit with a 240-volt load. If the sub panel will be used for a three-phase circuit, or if the load will be different, the wire size may need to be adjusted accordingly.
Frequently Asked Questions
Can I use a smaller wire size for a 70-amp sub panel if it’s a short distance from the main panel?
It’s essential to adhere to the recommended wire sizes to ensure safety and compliance with electrical codes, regardless of the distance.
Is it okay to use aluminum wire for a 70-amp sub panel?
Yes, aluminum wire can be used for a 70-amp sub panel, but it’s crucial to use the appropriate gauge and ensure proper installation to prevent issues such as oxidation.
Is 6 AWG good for 70-amps?
When using NM cable, the ampacity is limited to 60°C, resulting in a maximum of 55 amps for the next standard size being 60 amps. Therefore, the appropriateness of a #6 AWG feeder on a 70-amp OCPD depends on specific conditions and compliance with regulations.
Conclusion
Selecting the right wire size for a 70-amp sub-panel is crucial for a safe and effective electrical installation. Always refer to the NEC guidelines and consult with a qualified electrician to ensure compliance with local codes and standards. Thank you for reading. If you have any questions or need further clarification on sub-panel wiring or any related topic, feel free to ask. Your feedback is valuable to us!
Subscribe to our newsletter
& plug into
the world of circuits