Wire Sizing for Sub Panels
Running a sub panel to a garage, workshop, basement, addition, or outbuilding is one of the most common electrical projects that requires careful wire sizing. The feeder cable connecting the main panel to the sub panel must be large enough to handle the sub panel’s full amperage without overheating or causing excessive voltage drop. Here is how to determine the right wire size for your sub panel installation.
Standard Wire Sizes by Sub Panel Amperage
The following wire sizes are based on NEC Table 310.16 for 60 degrees Celsius rated copper and 75 degrees Celsius rated aluminum conductors at standard installation conditions. These sizes apply to typical residential runs up to approximately 50 to 75 feet.
For a 30-amp sub panel, use 10 AWG copper or 8 AWG aluminum. For a 40-amp sub panel, use 8 AWG copper or 6 AWG aluminum. For a 50-amp sub panel, use 6 AWG copper or 4 AWG aluminum. For a 60-amp sub panel, use 6 AWG copper or 4 AWG aluminum. For a 100-amp sub panel, use 3 AWG copper or 1 AWG aluminum. For a 125-amp sub panel, use 2 AWG copper or 1/0 AWG aluminum. For a 150-amp sub panel, use 1 AWG copper or 2/0 AWG aluminum. For a 200-amp sub panel, use 2/0 AWG copper or 4/0 AWG aluminum.
Copper vs. Aluminum for Sub Panel Feeds
For sub panel feeder circuits, aluminum wire is a common and cost-effective choice. The cost savings over copper are substantial, especially at larger gauges. A 100-amp feed to a detached garage using 1 AWG aluminum costs significantly less than 3 AWG copper for equivalent ampacity. Modern aluminum alloy conductors (AA-8000 series) are specifically designed for building wire and perform reliably when installed with proper anti-oxidant compound and correctly torqued connections. For shorter runs inside the home (such as a basement sub panel located near the main panel), copper offers easier handling and slightly smaller physical size.
Voltage Drop Calculations
For runs longer than 50 to 75 feet, voltage drop becomes a significant consideration. The NEC recommends total voltage drop from the service entrance to the farthest outlet not exceed 5%, with no more than 3% on any individual feeder or branch circuit. For a 100-amp sub panel feed at 240 volts, a run of 100 feet in 1 AWG aluminum produces a voltage drop of approximately 2.5%, which is acceptable. At 150 feet, the drop increases to approximately 3.8%, suggesting an upgrade to 1/0 AWG aluminum. For detached buildings more than 100 feet from the main panel, always run a voltage drop calculation to verify your wire size is adequate.
Number of Conductors
A sub panel feeder requires four conductors: two hot conductors (for 240V service), one neutral conductor, and one ground conductor. In a sub panel, the neutral and ground buses must be separated (not bonded together as they are in the main panel). The ground conductor can be sized according to NEC Table 250.122 based on the overcurrent device protecting the feeder. For a 100-amp feeder, the ground must be at least 8 AWG copper or 6 AWG aluminum.
Cable Types for Sub Panel Feeds
For indoor runs (basement to basement, same building), SER (Service Entrance Round) cable or individual THHN conductors in conduit are standard choices. For underground runs to detached buildings, UF-B cable rated for direct burial or individual THWN conductors in buried PVC conduit are required. Direct burial cable must be buried at least 24 inches deep (or 18 inches if protected by PVC conduit).
IB Lighting stocks SER cable, individual conductors, and UF-B cable in all sizes needed for sub panel installations. Call (800) 674-9019 for pricing and bulk availability, or visit our online shop.