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Basic wire sizing guide for US 120 and 240 volts

American Wire Gauge (AWG) is a standardized system used for determining wire sizes. In this system, lower numbers represent larger wire gauges. Service cables, which are large insulated stranded copper cables, are used for various applications in households and industries. This article will discuss the safe capacities for various wire sizes, considerations for extending cable lengths, and selecting the appropriate wire gauge for your needs.

Safe Load Capacities​

The National Electric Code (NEC) recommends that for most household circuits (usually 15-30 amp), a safe capacity of 80% of the maximum rated load should not be exceeded. This is especially important for devices that require extended periods of operation, such as air conditioners or ballasts. For short periods (around an hour), it is safe to handle 95%-100% of the maximum rating.

Stranded Wires and Service Cables​

Insulated wires of #8 and larger are usually stranded, which makes them similar to service cables. For wire sizes larger than #8 or #6, service or "service type" cables are often required. These cables can be more expensive for long runs and may present challenges when connecting to the appropriate breaker for long distances. Using 240V for ballasts needing 2400+ watts of lighting power on a single circuit is a practical solution in such cases.

Cable Sizing Guidelines for 120V Circuits​

For 120V (US) circuits with 80% max load and a run of 50ft or less, consider the following:

  1. For each additional 50ft of cable/wire up to #8, upgrade to the next size. Consult your local codes if unsure about double and triple length runs.
  2. For each additional plug used in line with the 80% safe load, subtract 2% from the overall power usage.
  3. When using extension cords for continuous use at the MAX safe power usage, include their length in the total distance from the breaker box (+25ft and one gauge up).
  4. Ensure the correct gauge is used when making your own cord from materials like SJO cable. Some places may label wires based on size, not current.

Example​

For a 1000W light on a 12 amp circuit, use a 15 amp #12 extension cord no longer than 25ft.

Wire Sizes for Services​

The following are approximate wire sizes and their corresponding service capacities:

  • #4 (approx. 65-75A each) used for 100-115 amp service
  • #2 (approx. 90A each) used for 125-150 amp service
  • #1/0 (approx. 150A each) used for 200 amp service
  • #2/0 (approx. 175A each) typically for industrial or very long runs with a large load, 300-350 amp service
  • #0/3 (approx. 200A each) typically for industrial or very long runs with a large load, 400 amp service
Service cables are designed specifically for extra service lines and/or extra-long runs. 1/0 gauge is commonly sold connected as x/3 (retail), providing a path for both hots, neutral, and ground.

Conclusion​

This information serves as a general guide for selecting appropriate wire sizes and load capacities. When planning large-scale projects, it is essential to have a thorough understanding of codes and loads. Always consult local codes and professionals when in doubt.
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