For a 100A subpanel, install a 100A 2-pole breaker feeding #3 AWG copper THHN hots and neutral, plus a #8 AWG copper Equipment Grounding Conductor (EGC). Add a #6 AWG copper Grounding Electrode Conductor (GEC) to a ground rod. This assumes copper wire, 75°C terminals, and 30°C ambient.
The 100A Sub Panel Grounding Diagram & Core Assumptions
Before pulling any wire, you must understand the physical layout of a code-compliant subpanel. The most critical rule in the National Electrical Code (NEC) regarding subpanels is the strict separation of the neutral and ground currents after the main service disconnect.
In your sub panel grounding diagram, the physical layout requires four distinct feeder wires arriving from the main panel: two ungrounded conductors (hots), one grounded conductor (neutral), and one Equipment Grounding Conductor (EGC). Inside the subpanel, the neutral bar must be completely isolated from the metal enclosure. The green bonding screw or bonding strap that ships with the panel must be removed. The EGC terminates on a separate ground bar that is directly bonded to the metal enclosure. If the subpanel is in a detached structure, NEC 250.32 also requires a local grounding electrode (like a ground rod), connected to the ground bar via a Grounding Electrode Conductor (GEC).
- Material: Copper conductors (Aluminum requires different sizing, addressed below).
- Temperature Rating: 75°C column (standard for modern residential breakers and lugs).
- Insulation Type: THHN/THWN-2 (rated 90°C, but terminated at 75°C limits).
- Ambient Temperature: 30°C (86°F) or lower.
- Conduit: PVC Schedule 80 underground or EMT above ground, containing exactly 3 current-carrying conductors.
Feeder and Grounding Wire Sizing (NEC Tables)
Sizing a subpanel feeder isn't just about matching the breaker; it is about matching the terminal temperature ratings of the lugs. Most 100A breakers and panel lugs are rated for 75°C. While THHN wire is rated for 90°C in the conduit, NEC 110.14(C) forces us to use the 75°C column for termination sizing.
| Conductor Function | NEC Reference | AWG Size (Copper) | 75°C Ampacity / Basis |
|---|---|---|---|
| Hot 1 & Hot 2 (Ungrounded) | 310.16 / 240.4 | #3 AWG | 100A |
| Neutral (Grounded) | 220.61 / 310.16 | #3 AWG | 100A (Sized same as hots for standard feeders) |
| Equipment Ground (EGC) | 250.122 | #8 AWG | Sized by breaker rating (100A = #8 Cu) |
| Ground Rod Wire (GEC) | 250.66 | #6 AWG | Max size required for a ground rod electrode |
Why #3 AWG and Not One Smaller?
You might look at NEC Table 310.16 and see that #4 AWG THHN is rated for 85A in the 75°C column. Under NEC 240.4(B), you are sometimes allowed to round up to the next standard breaker size (100A) if the exact ampacity doesn't match a standard breaker. However, relying on this exception for a full 100A panel is a benchmark of poor bench and jobsite practice. If your subpanel draws a continuous load near 80A, the #4 AWG wire will operate near its thermal limit, and voltage drop will compound the heat. By stepping up to #3 AWG (rated exactly 100A at 75°C), you eliminate the rounding-up exception, ensure the terminals run cool, and build in a margin for voltage drop.
Voltage Drop Check at 100 Feet
Wire sizing isn't complete without a voltage drop check. The NEC recommends (via Informational Notes in Article 210 and 215) keeping feeder voltage drop under 3%. Let us check our #3 AWG copper pick at a standard detached garage distance of 100 feet.
The Parameters:
- Voltage: 240V (Line-to-Line)
- Distance (D): 100 feet (one way)
- Current (I): 80A (Assuming an 80A continuous max draw on a 100A panel)
- K Factor (Copper): 12.9 ohms-cmil/ft
- Circular Mils (CM) for #3 AWG: 52,620
The Calculation:
Voltage Drop = (2 × K × I × D) / CM
Voltage Drop = (2 × 12.9 × 80 × 100) / 52,620
Voltage Drop = 206,400 / 52,620 = 3.92 Volts
The Result:
Percentage Drop = (3.92V / 240V) × 100 = 1.63%.
This is well under the 3% recommended limit. Our #3 AWG copper pick is validated for a 100-foot run. If your run exceeds 140 feet, you must upsize to #2 AWG to maintain the 3% threshold.
Decision Tree: Adjusting for Aluminum, Length, and Bundling
The #3 AWG copper baseline assumes ideal conditions. Use this decision tree to adjust your materials list based on your specific jobsite realities. Do not mix aluminum and copper sizing logic; they have different ampacities and termination requirements.
| Jobsite Condition | Required Adjustment | Concrete Part / Size Pick |
|---|---|---|
| Default: Run is under 100ft, copper, 30°C ambient. | Use baseline sizing. | #3 AWG Cu (Hots/N), #8 AWG Cu (EGC) |
| Long Run: Distance is between 101ft and 160ft. | Upsize hots and neutral by one AWG step to mitigate voltage drop. | #2 AWG Cu (Hots/N), #8 AWG Cu (EGC) |
| Material Change: Using Aluminum (XHHW-2) to save cost on long runs. | Aluminum has lower ampacity. Must use 75°C column for Al. | #1 AWG Al (Hots/N), #6 AWG Al or #8 Cu (EGC) |
| Conduit Bundling: Pulling a second circuit (e.g., 20A EV charger) in the same conduit. | 4+ current-carrying conductors triggers NEC 310.15(C)(1) 80% derating. | #2 AWG Cu (Hots/N) minimum to survive derating. |
| High Ambient: Conduit runs across a sun-baked roof or >113°F attic. | Apply NEC Table 310.15(B)(1) temperature correction factors. | #2 AWG Cu minimum; consider #1 AWG if >140°F. |
When an Engineer or AHJ Must Confirm
While the specifications above cover 95% of residential detached garage, workshop, and barn subpanels, certain edge cases require a licensed professional engineer (PE) or explicit pre-approval from your local Authority Having Jurisdiction (AHJ). Do not proceed without professional review if:
- Soil Resistivity is Extreme: If a soil resistivity test shows values above 1,000 ohm-meters, a standard ground rod will not achieve the NEC 250.56 requirement of 25 ohms to ground. An engineer may need to specify a ground ring, chemical electrodes, or a Ufer (concrete-encased) ground.
- Feeders Exceed 400A: Once you move into 400A to 800A service feeds for large shops or multi-dwelling units, parallel conductor rules (NEC 310.10(H)) and complex magnetic field management in conduit require engineered drawings.
- Separate Structures with Livestock: If the subpanel feeds a barn or agricultural building, stray voltage from neutral-to-ground potential differences can be lethal to livestock. The AHJ may require an isolated ground bar setup or specific equipotential bonding plans that exceed standard residential NEC 250.32 rules.
- Utility Transformer Proximity: If your subpanel is fed from a new utility transformer drop rather than your home's main panel, the service entrance grounding rules (NEC Article 250 Part III) apply, which are vastly different from feeder rules.
For standard residential applications under 200A, stick strictly to the #3 AWG copper baseline, isolate that neutral bar, torque your lugs to the manufacturer's printed inch-pound specifications using a calibrated torque screwdriver, and call for your rough-in inspection before closing the panel dead front.
