The correct wire size for a 70 amp sub panel is 4 AWG copper or 2 AWG aluminum, protected by a 70-amp double-pole breaker. This assumes standard 75°C terminations, 30°C ambient temperature, and no more than three current-carrying conductors in the raceway.
- Material: Copper (THHN/THWN-2) or Aluminum (XHHW-2 / SER)
- Temperature Column: 75°C (per NEC 110.14(C) for standard residential terminations)
- Ambient Temperature: 30°C (86°F) baseline
- Conduit Type: Single raceway (e.g., PVC or EMT) with a maximum of 3 current-carrying conductors
- System Voltage: 120/240V Single-Phase, 4-wire feed (2 Hots, 1 Neutral, 1 Ground)
Baseline Sizing and NEC Ampacity Requirements
When sizing feeders for a subpanel, you cannot simply match the wire's 90°C ampacity to the breaker size. The National Electrical Code (NEC) requires that the ampacity of the conductor be determined using the 75°C column of NEC Table 310.16 because the lugs inside standard residential panels and breakers are only rated for 75°C. Even if you use THHN wire (which has a 90°C insulation rating), the termination limit governs the final allowable ampacity.
Why 4 AWG copper and not one size smaller? A 6 AWG copper wire has an allowable ampacity of 65A in the 75°C column. Because 65A is less than the 70A breaker rating, it violates NEC 240.4(B) and creates a fire hazard at the termination lugs. You must step up to 4 AWG copper, which provides 85A of ampacity at 75°C, safely covering the 70A breaker requirement.
| Material | AWG Size | Insulation Type | 75°C Ampacity | 90°C Ampacity | Max Breaker |
|---|---|---|---|---|---|
| Copper | 6 AWG | THHN/THWN-2 | 65A | 75A | 60A (Too small for 70A) |
| Copper | 4 AWG | THHN/THWN-2 | 85A | 95A | 70A (Correct) |
| Aluminum | 4 AWG | XHHW-2 / SER | 65A | 75A | 60A (Too small for 70A) |
| Aluminum | 2 AWG | XHHW-2 / SER | 90A | 100A | 70A (Correct) |
Variables That Force You to Upsize the Wire
The baseline 4 AWG copper / 2 AWG aluminum answer assumes a relatively short run and standard installation conditions. Three primary variables will force you to upsize your conductors to prevent voltage drop, overheating, or code violations.
1. Voltage Drop Over Distance
While the NEC recommends a maximum 3% voltage drop for feeder circuits (which translates to 7.2V on a 240V system), it is an informational note, not a strict enforceable rule in all jurisdictions. However, exceeding it causes motors to overheat and electronics to brown out. Let's calculate the voltage drop for a 70A load on 4 AWG copper using the standard formula: VD = (2 × K × I × L) / CM.
- At 150 feet: Using K=12.9 (copper), I=70A, and CM=41,740 (for 4 AWG), the voltage drop is 6.49V (2.7%). This passes the 3% guideline.
- At 200 feet: The voltage drop increases to 8.65V (3.6%). This fails the 3% guideline. You must upsize to 3 AWG or 2 AWG copper to maintain acceptable voltage levels at the subpanel. Use a dedicated voltage drop calculator to verify your exact run length.
2. Conductor Bundling and Derating
If you are pulling multiple circuits through the same conduit, the heat generated by adjacent wires cannot dissipate. NEC Table 310.15(C)(1) requires ampacity derating when you have more than three current-carrying conductors in a single raceway. If you pull a second 240V circuit (adding two more hot wires) into the same conduit as your subpanel feeder, you now have four current-carrying conductors. This triggers an 80% derating factor. You must apply this 80% factor to the 90°C column of the wire (95A × 0.80 = 76A). While 76A still covers a 70A breaker, adding a fifth or sixth current-carrying conductor will drop the derated ampacity below 70A, forcing you to upsize to 3 AWG or 2 AWG copper.
3. High Ambient Temperatures
If your conduit runs through an attic that routinely exceeds 86°F (30°C) in the summer, you must apply ambient temperature correction factors from NEC Table 310.15(B)(1). For example, in an attic reaching 110°F (43°C), THHN wire requires an 87% correction factor. This reduces the 90°C ampacity of 4 AWG copper from 95A down to 82.6A. While still technically above 70A, it leaves very little margin for error, and many inspectors will require 3 AWG in high-heat environments.
Grounding, Neutral, and Conduit Fill Rules
Sizing the hot conductors is only half the job. A 70-amp subpanel requires a properly sized neutral, equipment grounding conductor (EGC), and an adequately sized raceway.
| Conductor Role | Copper Size | Aluminum Size | NEC Reference |
|---|---|---|---|
| Hot Legs (x2) | 4 AWG | 2 AWG | 310.16 |
| Neutral (x1) | 4 AWG | 2 AWG | 220.61 |
| Equipment Ground (x1) | 8 AWG | 6 AWG | 250.122 |
Conduit Fill Calculation: If you are pulling individual THHN/THWN-2 conductors through PVC conduit rather than using bundled SER cable, you must respect NEC Chapter 9 conduit fill limits. Pulling three 4 AWG wires and one 8 AWG ground requires a total cross-sectional area of roughly 0.284 square inches. A 1-inch Schedule 40 PVC conduit allows 0.333 square inches at 40% fill, which technically passes. However, pulling four large wires through 90-degree sweeps in a 1-inch pipe is physically exhausting and risks damaging the insulation. Best practice is to use 1.25-inch PVC to make the pull manageable and prevent wire scoring.
Crucial Subpanel Rule: Unlike a main panel, the neutral bar and ground bar in a subpanel must remain isolated. Do not install the green bonding screw or strap in the subpanel. The neutral carries return current, and bonding it to the ground at the subpanel creates parallel neutral paths, which can energize metal enclosures and cause shock hazards.
When to Call an Engineer or the AHJ
While the sizing above covers 95% of standard residential and light commercial 70-amp subpanel installations, specific scenarios require formal load calculations and approval from your local Authority Having Jurisdiction (AHJ) or a licensed electrical engineer.
- Continuous Loads Exceeding 80%: If the primary purpose of this subpanel is to feed a continuous load (defined by the NEC as a load expected to run for 3 hours or more, such as a heavy-duty EV charger or server rack), the 70A breaker must be derated to 80% (56A). If your calculated continuous load exceeds 56A, you must upsize the breaker to 90A or 100A, which subsequently requires 3 AWG or 1 AWG copper wire.
- Parallel Conductor Runs: If you are attempting to run two smaller wires in parallel to achieve 70A (e.g., two 8 AWG wires), stop. NEC 310.10(H) strictly prohibits paralleling conductors smaller than 1/0 AWG. You must use a single, properly sized conductor.
- Service Entrance Upgrades: If adding this 70A subpanel pushes your total calculated home load beyond your main service rating (e.g., upgrading from a 100A to a 200A main service), this involves meter base work and utility coordination. This is strictly outside DIY scope and requires a licensed electrical contractor.
Always verify your final wire and breaker selections against the specific temperature ratings printed on your subpanel's wiring diagram label. When in doubt, or if your local municipality has amendments to the NEC, consult your local electrical inspector before pulling the wire.
