Subpanel wire, technically called a feeder, is the heavy-gauge cable that carries bulk power from your main service panel to a secondary distribution board, dictating the maximum total load the downstream circuits can safely draw without tripping the upstream breaker or melting the insulation. Unlike standard branch circuit wire (like 14 AWG or 12 AWG NM-B) that feeds individual outlets, subpanel wire changes the architectural capacity of your home's electrical system, acting as the main artery for an entire garage, workshop, or addition. What people most commonly confuse subpanel wire with is standard branch wiring, leading to dangerous mistakes regarding neutral-to-ground bonding and ampacity derating.
Sizing Subpanel Wire: Ampacity Baselines and the 75°C Rule
Before you can calculate voltage drop, you must establish the baseline wire size based on ampacity. The NFPA 70: National Electrical Code (NEC) Table 310.16 is your primary reference. However, the most common mistake DIYers make is looking at the 90°C column for THHN/THWN-2 wire. Because standard residential breakers and panel lugs are rated for 75°C terminations (per NEC 110.14(C)), you must use the 75°C column to determine your maximum allowable ampacity, regardless of the wire's higher insulation rating.
| Wire Size (AWG/kcmil) | Material | 75°C Ampacity | Max Standard Breaker | Typical Use Case |
|---|---|---|---|---|
| 8 AWG | Copper | 50A | 50A | Small shed, hot tub (short run) |
| 6 AWG | Copper | 65A | 60A | Standard 60A garage subpanel |
| 4 AWG | Aluminum | 65A | 60A | Budget 60A garage (short run) |
| 2 AWG | Aluminum | 90A | 90A | Large workshop, EV charger prep |
| 1/0 AWG | Aluminum | 120A | 100A | Standard 100A detached building |
| 2/0 AWG | Aluminum | 135A | 125A | Heavy-duty 125A feeder |
Note: Aluminum is the industry standard for feeders today due to cost and weight, provided you use AA-8000 series alloy wire and treat the terminations properly.
Worked Example: Sizing a 60A Garage Feeder for Voltage Drop
Ampacity tables only tell half the story. If your subpanel is located far from the main panel, the resistance of the wire will cause the voltage to sag by the time it reaches the destination. The NEC recommends keeping voltage drop under 3% for feeders (and 5% total combined feeder + branch). Let's run the math for a 60A subpanel in a detached garage, 120 feet away from the main panel.
Step 1: Baseline Ampacity Selection
Looking at the table above, a 60A breaker requires a minimum of 6 AWG Copper or 4 AWG Aluminum. Let's choose 4 AWG Aluminum SER (Service Entrance Rated) cable for cost efficiency.
Step 2: Calculate Voltage Drop
We use the single-phase voltage drop formula: VD = (2 × K × I × L) / CM
- K (Resistance constant for Aluminum at 75°C AC) = 21.2
- I (Current) = 60A
- L (One-way length) = 120 feet
- CM (Circular Mils for 4 AWG) = 41,740
VD = (2 × 21.2 × 60 × 120) / 41,740
VD = 305,280 / 41,740 = 7.31 Volts
Step 3: Evaluate the Result
On a 240V circuit, a 3% drop is 7.2V. Our calculated drop is 7.31V (3.04%). We are slightly over the 3% recommendation. While not an immediate fire hazard, it will cause lights to dim when large motors start and can shorten the lifespan of sensitive electronics.
Step 4: Upsize the Wire
To fix this, we upsize to 2 AWG Aluminum (CM = 66,360).
VD = 305,280 / 66,360 = 4.60 Volts (1.91%).
This is well within the 3% limit. Therefore, for a 120-foot run, you must pull 2 AWG Aluminum instead of the baseline 4 AWG.
Per NEC 250.122(B), if you upsize your ungrounded conductors (the hots) for voltage drop, you must proportionately upsize your Equipment Grounding Conductor (EGC). A standard 60A circuit requires an 8 AWG Aluminum ground. Because we upsized the hots by a ratio of roughly 1.6x (66,360 / 41,740), you should bump the ground wire up to 6 AWG Aluminum to maintain the same fault-current clearing capacity.
Where You Meet Subpanel Wire in Practice (and Common Confusions)
When you are actually pulling this wire and terminating it in the field, theoretical math meets physical reality. Here is where installers frequently make critical errors.
The 4-Wire Mandate vs. Old 3-Wire Feeds
Prior to the 2008 NEC, it was legal to run a 3-wire feeder to a detached building (two hots and one neutral, with the neutral bonded to the ground bar at the subpanel). Today, NEC 250.32 strictly requires a 4-wire feeder: two hots, one isolated neutral, and one dedicated equipment ground. The neutral and ground bars in a subpanel must never be bonded. If you bond them, normal neutral return current will travel back to the main panel through the ground wire, energizing the subpanel enclosure and creating a severe shock hazard.
Aluminum Oxidation and Torque Specs
Aluminum wire oxidizes rapidly when exposed to air, creating a highly resistive layer that generates heat at the breaker lug. When terminating aluminum subpanel wire:
- Strip the insulation and immediately brush the exposed wire with a wire brush.
- Coat the stripped ends with an antioxidant compound like Noalox or Ideal Noalox.
- Insert the wire into the lug and tighten it using a calibrated torque screwdriver to the exact inch-pound specification printed on the breaker label. Hand-tightening causes loose connections that arc and melt the panel.
Feeder vs. Branch Circuit Derating
Subpanel wire is a feeder, meaning it supplies other overcurrent devices. Unlike branch circuits that supply specific appliances, feeders are subject to different continuous load calculations. If your subpanel will serve continuous loads (like baseboard heaters or EV chargers running for 3+ hours), the upstream breaker must be sized at 125% of the continuous load. A 60A breaker can only safely supply 48A of continuous load.
Subpanel Wire FAQ: Grounding, Materials, and Installation
Should I use SER cable or individual THHN wires in conduit?
For indoor runs through finished walls or attics, 4-wire SER (Service Entrance Rated) cable is faster and cheaper because it doesn't require pulling through conduit. For outdoor runs, underground burials (using direct burial wire like USE-2 or UF), or long runs with multiple bends, pulling individual THHN/THWN-2 wires through PVC or EMT conduit is vastly easier and dissipates heat better.
Can I use copper wire lugs on an aluminum feeder?
Most modern main breakers and panel lugs are rated 'AL/CU', meaning they are tested and listed for both aluminum and copper. However, you must verify the marking on the specific breaker you are using. If it only says 'CU', you cannot terminate aluminum wire directly to it; you would need to use a polaris connector or a lug kit to transition to a short copper pigtail.
Does the subpanel need its own ground rod if it's in a detached building?
Yes. Even though you are pulling a dedicated 4-wire feeder with an equipment ground, NEC 250.32(A) requires a grounding electrode system (typically two 8-foot copper ground rods spaced 6 feet apart) at the detached building. The ground rods connect to the subpanel's ground bar, not the neutral bar. This protects the building from lightning strikes and utility line surges, while the 4th wire in your feeder handles fault clearing for the breaker.
Where can I verify my voltage drop calculations?
Manufacturer tools are excellent for double-checking field math. The Southwire Voltage Drop Calculator allows you to input exact wire types, conduit materials (which affects AC reactance), and ambient temperatures to get a highly precise drop percentage before you buy the wire.
