Subpanel wiring size is the physical cross-sectional area (measured in AWG or kcmil) of the feeder conductors required to safely carry the maximum calculated load current from the main panel to a secondary panel without exceeding temperature limits or acceptable voltage drop. In a real installation, your chosen wire size dictates your minimum conduit diameter, the bending radius required at junction boxes, the torque settings on your panel lugs, and whether your 240V welder will run at full arc or stall out from voltage sag at the end of a long run. Homeowners and junior apprentices commonly confuse the feeder breaker size with the wire ampacity, or mistakenly assume the neutral and equipment grounding conductors must be the exact same gauge as the hot legs.
Where You Meet Subpanel Wiring Size in Practice
You will almost always encounter feeder sizing when powering a detached garage, a basement workshop, or an agricultural outbuilding. In 2026, the baseline for a detached garage subpanel has shifted heavily from 60 amps to 100 amps or even 125 amps. This is driven by the widespread adoption of Level 2 EV chargers (which pull 48 amps continuous) and the popularity of high-draw 240V workshop tools like plasma cutters and MIG welders.
The physical reality of subpanel wiring size is just as important as the electrical theory. Pulling four lengths of 4/0 AWG aluminum through 150 feet of 2-inch PVC conduit in the hot sun is a brutal, knuckle-busting workout. Conversely, pulling 3 AWG copper is physically easier but will cost you three to four times as much at the supply house. Sizing your wire correctly means balancing the upfront material cost against the physical labor of the pull and the long-term performance of your circuits.
The Math: A 100-Amp Feeder Numeric Example
Let's run the numbers for the most common modern scenario: a 100-amp subpanel in a detached garage, located 150 feet from the main service panel, operating on a 240V single-phase split system. We need to satisfy two rules: NEC ampacity tables and acceptable voltage drop (typically kept under 3% for feeders).
First, we look at base ampacity. According to Southwire's standard ampacity charts and NEC Article 310.16 (using the 75°C column, which is standard for most panel terminations), a 100A load requires a minimum of 3 AWG Copper or 1 AWG Aluminum. However, ampacity only tells us the wire won't melt; it doesn't account for voltage drop over distance.
To calculate voltage drop, we use the formula: VD = (2 × K × I × D) / CM
- K = 21.2 (resistivity constant for aluminum at 75°C)
- I = 80A (realistic continuous load, rather than the full 100A breaker rating)
- D = 150 feet (one-way distance)
- CM = 83,690 (circular mils for 1 AWG aluminum)
Plugging in the numbers: (2 × 21.2 × 80 × 150) / 83,690 = 6.08 Volts.
However, if you plan to add a second EV charger or a large air compressor later, bumping up to 1/0 AWG Aluminum (CM = 105,600) drops the voltage loss to just 2.0% and provides excellent headroom for future expansion, while still fitting comfortably in 1.5-inch PVC conduit.
Decision Tree: Picking Your Exact Wire Gauge and Material
Use this decision matrix to lock in your exact material list based on your target breaker size and the physical distance of your run. All sizes assume copper equipment grounding conductors and 75°C rated terminations.
| Target Breaker | Max Distance | Material | Hot & Neutral Size | Ground Size (Cu) | Min Conduit (PVC) |
|---|---|---|---|---|---|
| 60 Amp | 50 ft | Copper | 6 AWG THHN | 10 AWG Bare | 1 inch |
| 100 Amp | 100 ft | Aluminum | 1 AWG XHHW-2 | 8 AWG Bare | 1.25 inch |
| 100 Amp | 150 ft | Aluminum | 1/0 AWG XHHW-2 | 8 AWG Bare | 1.5 inch |
| 200 Amp | 150 ft | Aluminum | 2/0 AWG XHHW-2 | 6 AWG Bare | 2 inch |
Common Confusions: Neutral, Ground, and Breaker Sizing
The National Electrical Code (NEC) has specific rules for feeder conductors that frequently trip up DIYers who assume all four wires in a subpanel feed must be identical.
The Neutral Conductor: Under NEC 220.61, the neutral wire only needs to be sized for the maximum unbalanced load (the 120V portion of your panel), not the full 240V load. In a pure 240V workshop, the neutral could theoretically be much smaller. However, in practice, electricians almost always pull a neutral the exact same size as the hot legs (a 'full-size neutral') for subpanels. This prevents dangerous mistakes if the load balance changes later and allows the neutral to handle harmonic loads from modern electronics.
The Equipment Grounding Conductor (EGC): Ground wire sizing is dictated by NEC 250.122, and it is based entirely on the size of the feeder breaker, not the calculated load. A 100A breaker requires a minimum 8 AWG copper ground, regardless of whether you used 1 AWG or 1/0 AWG aluminum for your hots. Never downsize your ground to save money; it is the critical path for clearing a short circuit.
The 60°C vs 75°C vs 90°C Column: Even if your wire is rated for 90°C (like most modern THHN/XHHW-2), you must use the 75°C column for ampacity sizing because the lugs inside standard residential load centers are only rated for 75°C. Sizing based on the 90°C column is a severe code violation that can lead to melted lugs under continuous load.
Frequently Asked Questions
Can I use NM-B (Romex) for an outdoor subpanel feeder in conduit?
No. NM-B is strictly rated for dry, indoor locations. Once conduit runs outdoors or underground, it is considered a 'wet location' by the NEC, even if the conduit is sealed. Water will eventually condense inside outdoor conduit. You must use wet-rated conductors like THWN-2 or XHHW-2, or use a direct-burial cable like URD (Underground Residential Distribution) or SER (Service Entrance Cable) if routed properly.
Do I need a main breaker or exterior disconnect at the subpanel?
Yes. Recent iterations of the NEC (2020 and 2023) require an exterior disconnecting means for detached structures. You can achieve this by either installing a subpanel with a main breaker (which acts as the disconnect) or by installing a standalone 100A disconnect switch on the outside of the garage before the wire enters the building. Furthermore, the neutral and ground bars in the subpanel must remain isolated (separated); they are only bonded together at the main service disconnect.
What torque should I use on the subpanel lugs?
Always check the manufacturer's sticker inside the panel door. For 1/0 AWG aluminum on a standard 100A residential lug, the torque is typically between 40 and 50 in-lbs (inch-pounds, not foot-pounds). Use a calibrated torque screwdriver or torque wrench. Under-torqued aluminum lugs will loosen over time due to thermal expansion and cold flow, leading to high-resistance arcing and panel fires.
