Baseline Sizing Assumptions and Ampacity Data
Wire sizing is not a guessing game; it is a strict calculation based on the National Electrical Code (NEC). Before pulling any wire, you must establish the baseline assumptions for your installation. The direct answers provided above rely on the following specific parameters:
- Conductor Material: Copper (unless explicitly stated as aluminum)
- Insulation Type: THHN/THWN-2 (rated 90°C in dry, 75°C in wet)
- Termination Temperature: 75°C (Standard for modern breakers and panel lugs)
- Ambient Temperature: 30°C (86°F) or lower
- Raceway Type: EMT conduit, PVC, or NM-B cable with no more than 3 current-carrying conductors
To determine the minimum allowable wire size, we reference NEC Table 310.16. While THHN wire has a 90°C insulation rating, NEC 110.14(C) mandates that we must size the wire based on the lowest temperature rating of any connected component. Since standard panel lugs and breakers are rated for 75°C, we must use the 75°C column for our ampacity lookup.
| AWG Size | Insulation Type | Temp Column Used | Allowable Ampacity | Max Standard Breaker |
|---|---|---|---|---|
| 8 AWG | THHN/THWN-2 | 75°C | 50A | 50A |
| 6 AWG | THHN/THWN-2 | 75°C | 65A | 60A (Next standard size down) |
| 4 AWG | THHN/THWN-2 | 75°C | 85A | 80A |
| 3 AWG | THHN/THWN-2 | 75°C | 100A | 100A |
| 1 AWG | THHN/THWN-2 | 75°C | 130A | 125A |
Why We Don't Downsize: Terminal Ratings and Heat
A common mistake among DIYers is looking at the 90°C column of Table 310.16, seeing that 8 AWG THHN is rated for 55A, and assuming it can safely handle a 60A load if the breaker is swapped. This is a severe fire hazard and a direct code violation.
The restriction comes down to terminal heat dissipation. The brass or aluminum lugs inside your subpanel, as well as the internal contacts of the circuit breaker, are tested and listed only up to 75°C. If you push 60A through an 8 AWG wire, the wire insulation might survive, but the heat will transfer down the copper into the breaker lug. Over time, this localized heating degrades the lug's mechanical grip, increases resistance, and creates a thermal runaway loop that ends in a melted terminal or a panel fire.
By using 6 AWG copper (rated 65A at 75°C) on a 60A breaker, the wire operates well below its thermal limit, ensuring the terminations remain cool and the mechanical torque specs hold firm over decades of thermal cycling.
Variables That Force an Upsize: Distance, Bundling, and Material
The baseline sizes assume a short, uncongested run. Real-world jobsite conditions frequently force you to upsize your conductors. Here is the decision framework for when the baseline answer changes.
1. Voltage Drop Over Distance
The NEC recommends a maximum 3% voltage drop on feeders. Let's run a voltage drop check for a 100A subpanel located 100 feet from the main panel. Using the standard formula VD = (2 × K × I × L) / CM (where K=12.9 for copper, I=100A, L=100ft, and CM=52,620 for 3 AWG):
- At 100 feet: Voltage drop is 4.9V. On a 240V system, that is a 2.04% drop. Result: 3 AWG is acceptable.
- At 150 feet: Voltage jump to 7.35V (3.06%). Result: Exceeds 3%. You must upsize to 2 AWG (CM=66,360) to bring the drop back down to 2.4%.
Always use a dedicated voltage drop calculator from a major wire manufacturer to verify long runs before buying copper.
2. Conductor Bundling and Derating
If you are pulling multiple circuits through the same conduit to reach your subpanel, the wires heat each other up. NEC 310.15(C)(1) requires ampacity derating when you have more than three current-carrying conductors (CCCs) in a raceway.
- 4 to 6 CCCs: Multiply baseline ampacity by 80%
- 7 to 9 CCCs: Multiply baseline ampacity by 70%
- 10 to 20 CCCs: Multiply baseline ampacity by 50%
Note: The equipment grounding conductor does not count as a CCC. The neutral counts as a CCC in a subpanel feeder because it carries unbalanced return current.
3. Aluminum vs. Copper
Never present aluminum and copper interchangeably. Aluminum has a lower ampacity per AWG size and expands/contracts more under heat. If you switch from copper to aluminum for a 100A feeder, you must jump from 3 AWG to 1 AWG. Furthermore, aluminum requires an anti-oxidant compound (like Noalox) applied to the wire strands before torquing into the lugs to prevent galvanic corrosion and high-resistance faults.
When to Pull in the AHJ or an Engineer
While the rules above cover 90% of residential and light-commercial subpanel feeds, specific scenarios require a licensed professional engineer (PE) or explicit guidance from your local Authority Having Jurisdiction (AHJ):
- Continuous Loads: If the subpanel will supply a commercial shop where the calculated load will run continuously for 3 hours or more (e.g., a massive server rack or industrial kiln), NEC 210.20 requires the breaker and wire to be sized at 125% of the continuous load. A 80A continuous load requires a 100A breaker and wire rated for 100A continuous (which means 125A ampacity wire, pushing you to 1 AWG copper).
- High Ambient Temperatures: If your conduit runs through an attic that reaches 120°F (49°C) in the summer, you must apply ambient temperature correction factors from NEC Table 310.15(B)(1). A 3 AWG wire in a hot attic may only be legally allowed to carry 80A, forcing an upsize.
- Service Entrance vs. Feeder: If you are upgrading a main service disconnect rather than wiring a downstream subpanel, utility regulations and local service entrance codes override standard feeder rules. Always defer to the utility and AHJ for meter-main combinations.
Frequently Asked Questions About Wiring a Sub Panel
Can I use aluminum wire for wiring a sub panel to save money?
Yes, aluminum (specifically XHHW-2 or THWN-2 aluminum alloy) is perfectly legal and often used for feeders to save on material costs. However, you must upsize the AWG compared to copper. For a 100A subpanel, use 1 AWG aluminum instead of 3 AWG copper. You must also clean the wire strands with a wire brush, apply an approved anti-oxidant paste, and strictly adhere to the manufacturer's inch-pound torque specifications on the lugs, as aluminum is highly susceptible to cold-flow loosening over time.
Do I need a ground rod when wiring a sub panel to a detached garage?
Yes. When wiring a sub panel to a detached structure, NEC 250.32 requires a grounding electrode system (typically two 8-foot ground rods spaced 6 feet apart, or a single rod if the resistance to ground is proven to be 25 ohms or less). You must run a 4-wire feeder (two hots, one neutral, one separate equipment ground) from the main panel. The neutral and ground must remain strictly isolated in the detached subpanel; do not install the green bonding screw in the subpanel's neutral bar.
What size ground wire do I need when wiring a 100A sub panel?
According to NEC Table 250.122, the minimum size equipment grounding conductor for a 100A feeder is 8 AWG copper or 6 AWG aluminum. However, if you upsized your current-carrying conductors to compensate for voltage drop over a long distance, NEC 250.122(B) requires you to proportionately upsize the ground wire as well. If you bumped your hots from 3 AWG to 1 AWG for a 200-foot run, your ground wire must also be increased by the same circular-mil ratio.
