--- title: "Wire Size for a 100 Amp Subpanel: Copper, Aluminum | FigureYard" description: "Feeder wire for a 100 amp subpanel: 3 AWG copper or 1 AWG aluminum on a short run, larger for a detached garage or shop far from the house." url: https://figureyard.com/energy/wire-size-for-100-amp-subpanel/ image: https://figureyard.com/og/energy/wire-size-for-100-amp-subpanel.png --- [Electrical, Heating & Insulation](https://figureyard.com/energy/) # Wire Size for a 100 Amp Subpanel A 100 amp subpanel is fed with 3 AWG copper or 1 AWG aluminum, both rated for exactly one hundred amps in the code's higher temperature column. That column applies to THHN or THWN wires in conduit and to SER cable when the breaker and the panel lugs are rated for it, which they usually are. The smaller sizes you may see quoted for a 100 amp house service do not apply here: that allowance is only for the service that feeds a whole home. Reviewed by FigureYard editorial team · Updated October 13, 2026 · 6 sources *Interactive calculator on the page (inputs: The wire feeds; Load current; Circuit voltage; One-way distance; Conductor; Wire type; Runs for 3 hours or more at a time; Circuit; Allowed voltage drop (%)). The results below are for its default inputs.* **Wire size** **3 AWG** copper - Current the wire must carry: **100.0** A - Rated for: **100** A - Voltage drop: **1.23** % - Longest run within 3%: **147** ft Each copper size for this circuit | Size | Rated | Drop | Fits | | --- | --- | --- | --- | | 8 AWG | 50 A | 3.89% | Too small for the current | | 6 AWG | 65 A | 2.46% | Too small for the current | | 4 AWG | 85 A | 1.54% | Too small for the current | | 3 AWG | 100 A | 1.23% | Yes | | 2 AWG | 115 A | 0.97% | Yes | | 1 AWG | 130 A | 0.77% | Yes | The calculator is preset to a 100 amp feeder at 240 volts in conduit. Enter the distance to the subpanel; a detached garage or barn at the far end of the yard often needs the next size up for voltage drop. ## How to use this calculator 1. Keep the current at 100 amps, the rating of the feeder breaker in the main panel. 2. Choose copper or aluminum. Aluminum is by far the more common feeder at this size because it costs much less. 3. Enter the one-way distance from the main panel to the subpanel along the trench or the conduit route. 4. Open More options and set the allowed drop to 2 percent if you want to keep the feeder tight and leave room for the circuits beyond it. ## Worked examples ### A subpanel in an attached garage Load current: 100 A · Circuit voltage: 240 V · One-way distance: 40 ft · Wire type: THHN/THWN wires in conduit (75 °C) - Wire size: 3 AWG copper - Current the wire must carry: 100.0 A - Rated for: 100 A - Voltage drop: 0.82% - Longest run within 3%: 147 ft ### A detached workshop across the yard Load current: 100 A · Circuit voltage: 240 V · One-way distance: 150 ft · Conductor: Aluminum - Wire size: 1/0 AWG aluminum - Current the wire must carry: 100.0 A - Rated for: 120 A - Voltage drop: 2.51% - Longest run within 3%: 179 ft ### A basement subpanel for a finished room Load current: 100 A · Circuit voltage: 240 V · One-way distance: 25 ft · Conductor: Aluminum - Wire size: 1 AWG aluminum - Current the wire must carry: 100.0 A - Rated for: 100 A - Voltage drop: 0.53% - Longest run within 3%: 142 ft ## Feeder size for a 100 amp subpanel by one-way distance | One-way distance | Copper, THHN/THWN wires in conduit (75 °C) | Aluminum | | --- | --- | --- | | 25 ft | 3 AWG | 1 AWG | | 50 ft | 3 AWG | 1 AWG | | 100 ft | 3 AWG | 1 AWG | | 150 ft | 2 AWG | 1/0 AWG | | 200 ft | 1 AWG | 2/0 AWG | Each cell: the smallest conductor rated for a hundred amps in the higher temperature column that keeps the drop within the branch-circuit limit. ## Why a subpanel feeder is bigger than a service The code lets the service conductors of a single-family house carry only 83 percent of their rating, because a whole home never draws its full service at once. A subpanel feeds part of the house or another building, so it gets no such allowance and is sized from the regular ampacity table: 3 AWG copper carries 100 amps and 1 AWG aluminum 100 amps. Compare the [100 amp service](https://figureyard.com/energy/wire-size-for-100-amp-service/) page, where the same rating takes smaller wire. ## Why NM-B cable does not appear here NM-B cable is rated from the lowest temperature column, where copper does not reach one hundred amps in the sizes this cable is made in. A 100 amp feeder is therefore run as single conductors in conduit, as SER cable inside the house, or as aluminum cable rated for burial to a detached building. Each needs two hots, a neutral and a ground, with the neutral and ground bars kept apart in the subpanel. ## Distance and a detached building A feeder is usually held to about 2 percent of drop, tighter than a branch circuit. Over a long trench that pushes the size up a step or two, and the table shows where. A detached building also needs its own grounding electrode and a disconnect. The breaker in the main panel stays at 100 amps when you upsize the wire. ## Common questions ### What size wire for a 100 amp subpanel? 3 AWG copper or 1 AWG aluminum, rated 100 amps and 100 amps in the higher temperature column, on a short run. A long run may need the next size up. ### Can I use 4 AWG copper for a 100 amp subpanel? No. 4 AWG copper is rated 85 amps. It appears for a 100 amp house service only because the service is sized for 83 percent of its rating, and a subpanel is not. ### What size aluminum wire for a 100 amp subpanel 150 feet away? Over that distance voltage drop usually sets the size rather than the current: enter it above to see the answer, typically one or two sizes larger than the short-run 1 AWG. ### Does a 100 amp subpanel need 4 wires? Yes: two hots, a neutral and an equipment ground. The neutral bar in the subpanel must not be bonded to the ground bar. ## Related calculators - [Wire Size Calculator](https://figureyard.com/energy/wire-size-calculator/) - [Voltage Drop Calculator](https://figureyard.com/energy/voltage-drop-calculator/) - [Wire Size for a 100 Amp Service](https://figureyard.com/energy/wire-size-for-100-amp-service/) - [What Size Wire for a 60 Amp Circuit](https://figureyard.com/energy/wire-size-for-60-amp/) - [Wire Size for a 200 Amp Service](https://figureyard.com/energy/wire-size-for-200-amp-service/) Reviewed by FigureYard editorial team on 2026-10-13. Formulas are tested against published values, and material figures come from the sources below. [How we build our calculators](https://figureyard.com/editorial-policy/). ## Sources - [Michigan State University Electrical Tech Note 227: Sizing Conductors to Minimize Voltage Drop (Table 227.1, dc resistance at 75 °C)](https://www.maec.msu.edu/download_file/view/147/303) - [Cerrowire Voltage Drop Table (rev. 09/2018)](https://www.cerrowire.com/wp-content/uploads/2018/09/Cerrowire_Voltage-Drop_210521.pdf) - [Electrical License Renewal (reproduces NEC 210.19(A)(1) text): 210.19(A)(1) Branch Circuits.](https://www.electricallicenserenewal.com/Electrical-Continuing-Education-Courses/NEC-Content.php?sectionID=818) - [Encore Wire (Prysmian): Wire Ampacity Size Table](https://www.encorewire.com/products/tools-and-resources/calculators/wire-size-table.html) - [City and County of Denver (Denver Residential Code 2021, adopting the 2021 IRC); viewer hosted by UpCodes: Chapter 36 Services: Denver Residential Code 2021 (based on the 2021 International Residential Code), Section E3603.1.1 Ungrounded Service Conductors](https://up.codes/viewer/denver/irc-2021/chapter/36/services) - [Cerrowire Ampacity Chart (ampacities based on the 2017 NEC Table 310.16)](https://www.cerrowire.com/wp-content/uploads/2021/04/Cerrowire_Ampacity_Chart_210405.pdf)