• Conductor Material: Copper (unless explicitly noted as Aluminum)
• Insulation Type: THHN/THWN-2 (90°C rated, but terminated in 75°C equipment)
• Temperature Column: NEC Table 310.16, 75°C column
• Ambient Temperature: 30°C (86°F)
• Installation Method: Single raceway (PVC or EMT), no more than 3 current-carrying conductors
Always verify local AHJ requirements, as regional amendments to the NFPA 70 (National Electrical Code) may override baseline tables.
The Baseline Sizing Matrix for Subpanel Feeders
When wiring an electrical sub panel, the breaker size and wire gauge must be matched to the calculated load of the destination building or room, while respecting the thermal limits of the panel lugs. Most modern residential load centers and subpanels feature lugs rated for 75°C. Even though THHN wire is rated for 90°C, NEC 110.14(C) requires you to size the wire based on the lowest temperature rating of any connected component. Therefore, we use the 75°C column of NEC Table 310.16.
| Subpanel Rating | Max Continuous Load (80%) | Breaker Size (2-Pole) | Copper AWG (75°C) | Aluminum AWG (75°C) |
|---|---|---|---|---|
| 60 Amp | 48 Amps | 60A | #6 AWG | #4 AWG |
| 100 Amp | 80 Amps | 100A | #3 AWG | #1 AWG |
| 125 Amp | 100 Amps | 125A | #1 AWG | 1/0 AWG |
| 200 Amp | 160 Amps | 200A | 2/0 AWG | 4/0 AWG |
Why This Size and Not One Smaller?
A common mistake is attempting to use #4 AWG copper on a 100A breaker because #4 AWG is rated for 85A in the 75°C column. Under NEC 240.4(B), you are only permitted to round up to the next standard breaker size if the ampacity does not correspond to a standard overcurrent device rating and the circuit is not a continuous load. A subpanel feeder serving a workshop with lighting, heating, or compressors is routinely classified as a continuous load. Furthermore, 100A is a standard breaker size. Therefore, #4 AWG (85A) cannot be protected by a 100A breaker. You must step up to #3 AWG, which provides exactly 100A of ampacity at 75°C.
Variables That Force You to Upsize Your Feeder
The baseline matrix above assumes ideal conditions. In real-world jobsite scenarios, three primary variables will force you to abandon the baseline chart and upsize your conductors.
1. Voltage Drop Over Distance
The NEC recommends a maximum voltage drop of 3% for feeders (Informational Note to NEC 210.19(A)). Let us run a voltage drop check for our 100A (#3 AWG Copper) feeder at a distance of 100 feet, assuming an 80A continuous load at 240V.
Formula: VD = (2 × K × I × L) / CM
• K (Copper) = 12.9
• I (Current) = 80A
• L (Length) = 100 ft
• CM (Circular Mils for #3 AWG) = 52,620
Result: VD = (2 × 12.9 × 80 × 100) / 52,620 = 3.92 Volts.
Percentage: (3.92 / 240) × 100 = 1.6%. This is well under the 3% limit.
The Edge Case: If that same subpanel is located in a detached barn 200 feet away, the voltage drop doubles to 7.84V (3.2%). At 200 feet, #3 AWG fails the 3% recommendation. You must upsize to #2 AWG copper (CM = 66,360) to bring the drop back down to 2.5%.
2. Conductor Bundling and Derating
If you are pulling two separate subpanel feeders through the same conduit, or sharing a conduit with other multi-wire branch circuits, you must apply NEC Table 310.15(B)(3)(a) adjustment factors. While the neutral of a single-phase 120/240V feeder does not count as a current-carrying conductor (CCC), pulling two separate 120/240V feeders gives you four CCCs (the two ungrounded "hot" conductors from each feeder).
Four CCCs require an 80% derating factor. If your load requires 100A of capacity, you must divide by 0.8, meaning your wire must have a baseline ampacity of 125A before derating. #3 AWG (100A) is no longer sufficient; you must upsize to #1 AWG copper (130A at 75°C) to safely carry the load without melting the insulation.
3. Switching to Aluminum Conductors
Copper prices frequently drive DIYers and contractors toward aluminum feeders. Aluminum is perfectly legal and safe for subpanel feeders, provided you follow strict termination rules. Because aluminum expands and contracts more than copper under thermal cycling, it is prone to loosening at the lugs, which causes arcing and fires. If you use aluminum, you must:
1. Use the 75°C column (never the 90°C column for terminations).
2. Apply an anti-oxidant compound (like Noalox) to the stripped conductor ends.
3. Torque the lugs to the manufacturer’s exact inch-pound specification using a calibrated torque screwdriver or wrench.
When to Stop and Call the AHJ or an Engineer
While the EC&M National Electrical Code resources provide excellent guidance for standard residential feeders, certain scenarios cross the line from DIY wiring into engineered territory. You must involve your local Authority Having Jurisdiction (AHJ) or a licensed professional engineer when:
- Sizing Service Entrance Conductors: The rules for the wires coming from the utility meter to your main panel (NEC Article 230) differ significantly from feeder rules (NEC Article 215). Service conductors have specific physical damage and clearance requirements.
- Underground Thermal Resistivity: If you are burying a high-capacity feeder (over 200A) in poor soil conditions (like dry sand or heavy clay), standard underground ampacity tables may not apply. NEC 310.15(C) requires engineered thermal calculations to prevent the cable from cooking itself in the trench.
- Complex Derating Scenarios: If your conduit runs across a roof exposed to direct sunlight, you must add temperature offsets (NEC Table 310.15(B)(3)(c)). A conduit 12 inches above a roof in Phoenix, AZ, can easily see internal temperatures exceeding 140°F, severely slashing your wire's ampacity.
Frequently Asked Questions About Wiring Electrical Sub Panels
What size wire do I need for wiring a 60 amp electrical sub panel?
For a 60A subpanel feeder, you need a 60A 2-pole breaker and #6 AWG copper or #4 AWG aluminum THHN/THWN-2 wire. This assumes standard 75°C rated lugs and a run under 100 feet. If you are using NM-B (Romex) cable instead of THHN in conduit, you must use #4 AWG copper NM-B, because NM-B is strictly limited to the 60°C column of NEC Table 310.16, where #6 AWG is only rated for 55A.
Does wiring an electrical sub panel require a separate ground rod?
It depends entirely on the location of the subpanel. If the subpanel is installed in the same building as the main panel, you do not need a ground rod; you simply run a 4-wire feeder (2 hots, 1 neutral, 1 equipment ground) and keep the neutral and ground buses isolated. However, if you are wiring an electrical sub panel in a detached building (like a shed or detached garage), NEC 250.32 mandates a local grounding electrode system (typically two ground rods spaced 6 feet apart) bonded to the subpanel's ground bus, in addition to the equipment grounding conductor run with the feeder.
Can I use aluminum wire when wiring an electrical sub panel to save money?
Yes, aluminum is highly cost-effective for feeders 100A and above. For a 100A feeder, #1 AWG aluminum is significantly cheaper than #3 AWG copper. However, you must never mix aluminum and copper without proper bi-metallic lugs, and you must never use aluminum wire on terminals explicitly marked "Cu Only" (common on older or cheap breakers). Always verify the breaker and panel lug markings for "AL" or "CU/AL" ratings before purchasing your wire.
