Wire Size Calculator — NEC Ampacity & Voltage Drop Guide
Enter the system voltage, load current, one-way circuit length, wire material, maximum allowed voltage drop, and conduit type to estimate the minimum wire size for your circuit.
What size wire is needed for a secondary distribution box
The appropriate wire size for a secondary distribution box depends on the expected load, distance from the main panel, and conductor material, with common residential subpanels using #4 AWG copper or #1/0 AWG aluminum for 100A service.Determining Wire SizeThe wire connecting the main panel to a secondary distribution box, also called a feeder conductor, must be sized to safely carry the anticipated load without overheating or excessive voltage drop. The process involves:Load Calculation: Determine the maximum current the subpanel will carry, considering both continuous and non-continuous loads. Continuous loads (lasting 3 hours or more) require the conductor to handle 125% of the load, while non-continuous loads are counted at 100% of their rating .Demand Factors: Apply demand factors to account for the fact that not all circuits operate at full capacity simultaneously. This ensures a realistic sizing rather than simply summing all breaker ratings .Voltage Drop Consideration: For longer distances, increase wire size to minimize voltage drop. For example, a 100A subpanel 80 feet from the main panel may require #2 AWG copper instead of #4 AWG to maintain voltage within safe limits .Conductor Material: Copper and aluminum have different ampacities. Aluminum requires a larger gauge than copper for the same current. For instance, a 100A subpanel may use #4 AWG copper or #1/0 AWG aluminum .Practical Examples100A Subpanel, 80 feet away: #2 AWG copper or #1/0 AWG aluminum to reduce voltage drop .125A Subpanel: Typically #1 AWG copper or 2/0 AWG aluminum.150A Subpanel: 1/0 AWG copper or 3/0 AWG aluminum.200A Subpanel: 2/0 AWG copper or 4/0 AWG aluminum. Always add a safety margin (e.g., 20%) if the subpanel will supply high-demand equipment like welders or compressors .Additional ConsiderationsEnsure proper grounding and bonding of the subpanel.Use breakers and protection devices rated for the feeder current.Follow NEC, IEC, or local electrical codes for compliance and safety .Label circuits and maintain neat cable management inside the distribution box. By carefully calculating load, considering distance, and selecting the correct conductor material, you can ensure the secondary distribution box operates safely and efficiently.
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