Aluminum wires are a cost-effective, lightweight alternative to copper — widely used in long-distance transmission lines, service entrance conductors, and large feeder circuits. But aluminum has roughly 61% the conductivity of copper, which means you always need a larger aluminum gauge to carry the same current.
Ampacity is the maximum continuous current (in amps) a wire can carry without exceeding its insulation’s temperature rating. Using the wrong wire size — or ignoring derating rules — can overheat insulation, cause breaker trips, or create a fire hazard.
⚠️ Safety note on older homes: Aluminum branch-circuit wiring installed before 1972 used an alloy prone to oxidation and loosening at connections, which has been linked to fire hazards. If your home has pre-1972 aluminum wiring on 15A or 20A branch circuits, have a licensed electrician inspect it. Post-1972 aluminum and modern AA-8000 series alloy wiring are considered safe when installed correctly.
Aluminum vs. Copper Wire Ampacity — Quick Comparison
Because aluminum conducts less efficiently than copper, you typically need to go one to two AWG sizes larger in aluminum to match a copper wire’s ampacity. Here’s a side-by-side reference at the common 75°C rating:
| Copper AWG | Copper Ampacity (75°C) | Equivalent Aluminum AWG | Aluminum Ampacity (75°C) |
|---|---|---|---|
| 14 AWG | 20 A | 12 AWG | 20 A |
| 12 AWG | 25 A | 10 AWG | 30 A |
| 10 AWG | 35 A | 8 AWG | 40 A |
| 8 AWG | 50 A | 6 AWG | 50 A |
| 6 AWG | 65 A | 4 AWG | 65 A |
| 4 AWG | 85 A | 2 AWG | 90 A |
| 2 AWG | 115 A | 1/0 AWG | 120 A |
| 1/0 AWG | 150 A | 2/0 AWG | 135 A |
| 2/0 AWG | 175 A | 3/0 AWG | 155 A |
| 3/0 AWG | 200 A | 4/0 AWG | 180 A |
Aluminum Wire Ampacity Chart (NEC — AWG Sizes)
The table below is based on NEC Table 310.16 for aluminum or copper-clad aluminum conductors in a raceway or cable, with no more than three current-carrying conductors, at an ambient temperature of 30°C (86°F).
| Aluminum AWG Size | Ampacity at 60°C (140°F) | Ampacity at 75°C (167°F) | Ampacity at 90°C (194°F) | Max Allowable Amps at 75°C * |
|---|---|---|---|---|
| 12 AWG | 20 A | 20 A | 25 A | 16 A |
| 10 AWG | 25 A | 30 A | 35 A | 24 A |
| 8 AWG | 30 A | 40 A | 45 A | 32 A |
| 6 AWG | 40 A | 50 A | 60 A | 40 A |
| 4 AWG | 55 A | 65 A | 75 A | 52 A |
| 3 AWG | 65 A | 75 A | 85 A | 60 A |
| 2 AWG | 75 A | 90 A | 100 A | 72 A |
| 1 AWG | 85 A | 100 A | 115 A | 80 A |
| 1/0 AWG | 100 A | 120 A | 135 A | 96 A |
| 2/0 AWG | 115 A | 135 A | 150 A | 108 A |
| 3/0 AWG | 130 A | 155 A | 175 A | 124 A |
| 4/0 AWG | 150 A | 180 A | 205 A | 144 A |
* The “Max Allowable Amps at 75°C” column reflects the 80% continuous load rule from NEC 210.19(A) and 215.2(A). For loads operating continuously (3 hours or more), the conductor must be sized at 125% of the load current — so multiply your load amps by 1.25 to find the minimum required wire ampacity, or conversely, multiply the wire’s rated ampacity by 0.80 to get the safe maximum for a continuous load.
Aluminum Wire Ampacity Chart (NEC — Large Conductors in kcmil)
For service entrances, feeders, and commercial/industrial applications, conductors are sized in kcmil (thousands of circular mils) rather than AWG. These larger cables are the workhorses of 200A–800A+ services.
| Aluminum Size (kcmil) | Ampacity at 60°C | Ampacity at 75°C | Ampacity at 90°C |
|---|---|---|---|
| 250 kcmil | 170 A | 205 A | 230 A |
| 300 kcmil | 195 A | 230 A | 260 A |
| 350 kcmil | 210 A | 250 A | 280 A |
| 400 kcmil | 225 A | 270 A | 305 A |
| 500 kcmil | 260 A | 310 A | 350 A |
| 600 kcmil | 285 A | 340 A | 385 A |
| 700 kcmil | 310 A | 375 A | 420 A |
| 750 kcmil | 320 A | 385 A | 435 A |
| 800 kcmil | 330 A | 395 A | 445 A |
| 900 kcmil | 355 A | 425 A | 480 A |
| 1000 kcmil | 375 A | 445 A | 500 A |
| 1250 kcmil | 405 A | 485 A | 545 A |
| 1500 kcmil | 435 A | 520 A | 585 A |
| 1750 kcmil | 455 A | 545 A | 615 A |
| 2000 kcmil | 470 A | 560 A | 630 A |
How to Read the Aluminum Wire Ampacity Chart
Reading the chart correctly takes just three steps:
- Find your wire size — Locate the AWG or kcmil row that matches the wire you’re using or planning to install.
- Select the temperature column — Match the column to your wire’s insulation temperature rating. The most common ratings are:
- 60°C — Older thermoplastic insulation (TW, UF)
- 75°C — Most modern wiring in conduit (THWN, XHHW) — this is the most-used column
- 90°C — High-temp insulation (THHN, XHHW-2) — ampacity is higher, but NEC still limits terminations to 75°C in most cases
- Check continuous load limits — If the load runs for 3+ hours continuously (e.g., HVAC, water heaters), use the “Max Allowable Amps at 75°C” column, or multiply the rated ampacity by 0.80.
Practical example: You’re wiring a 40A circuit (e.g., an EV charger) using aluminum THWN-2 in conduit. Find 6 AWG in the 75°C column → 50 A rated. Since an EV charger is a continuous load, apply 80%: 50 × 0.80 = 40 A. A 6 AWG aluminum conductor is exactly right for this application.
Derating Aluminum Wire Ampacity
The ampacity values in the tables above assume standard conditions: no more than three current-carrying conductors, and an ambient temperature of 30°C (86°F). When real-world conditions differ, you must derate — reduce the ampacity accordingly to prevent overheating.
Derating for More Than Three Current-Carrying Conductors
When multiple wires share the same conduit or cable, they generate heat collectively, which raises the temperature around each conductor. NEC 310.15(C)(1) requires the following adjustment factors:
| Number of Current-Carrying Conductors | Adjustment Factor | Effect on Ampacity |
|---|---|---|
| 4 – 6 | 80% | Multiply rated ampacity × 0.80 |
| 7 – 9 | 70% | Multiply rated ampacity × 0.70 |
| 10 – 20 | 50% | Multiply rated ampacity × 0.50 |
| 21 – 30 | 45% | Multiply rated ampacity × 0.45 |
| 31 – 40 | 40% | Multiply rated ampacity × 0.40 |
| 41 and above | 35% | Multiply rated ampacity × 0.35 |
Example: You’re running six 10 AWG aluminum THHN wires in the same conduit. Rated ampacity at 90°C = 35 A. With 6 conductors, apply 80%: 35 × 0.80 = 28 A maximum safe current.
Derating for Ambient Temperature
Ampacity tables assume an ambient temperature of 30°C (86°F). If your installation is in a hotter environment — such as an attic in summer, near industrial equipment, or in a hot climate — you must apply a correction factor from NEC Table 310.15(B)(1):
| Ambient Temp (°C) | Ambient Temp (°F) | Correction — 60°C wire | Correction — 75°C wire | Correction — 90°C wire |
|---|---|---|---|---|
| 10°C or below | 50°F or below | 1.29 | 1.20 | 1.15 |
| 11–15°C | 52–59°F | 1.22 | 1.15 | 1.12 |
| 16–20°C | 61–68°F | 1.15 | 1.11 | 1.08 |
| 21–25°C | 70–77°F | 1.08 | 1.05 | 1.04 |
| 26–30°C | 79–86°F | 1.00 | 1.00 | 1.00 |
| 31–35°C | 88–95°F | 0.91 | 0.94 | 0.96 |
| 36–40°C | 97–104°F | 0.82 | 0.88 | 0.91 |
| 41–45°C | 106–113°F | 0.71 | 0.82 | 0.87 |
| 46–50°C | 115–122°F | 0.58 | 0.75 | 0.82 |
| 51–55°C | 124–131°F | 0.41 | 0.67 | 0.76 |
| 56–60°C | 133–140°F | — | 0.58 | 0.71 |
| 61–70°C | 142–158°F | — | 0.33 | 0.58 |
| 71–80°C | 160–176°F | — | — | 0.41 |
Example: A 10 AWG aluminum THHN wire (90°C rated) is routed through an attic where summer temperatures reach 50°C. Rated ampacity = 35 A. Correction factor at 46–50°C for 90°C wire = 0.82. Derated ampacity: 35 × 0.82 = 28.7 A.
Applying Both Derating Factors Together
When both conditions apply (multiple conductors AND elevated ambient temperature), multiply both correction factors together before applying them to the rated ampacity:
Example: Six 8 AWG aluminum THHN wires (rated 45 A at 90°C) in a conduit running through a 40°C space. Apply conductor factor (0.80) and temperature factor (0.91): 45 × 0.80 × 0.91 = 32.8 A maximum safe current.
Which Aluminum Wire Size Do You Actually Need?
Here’s a practical quick-reference guide for the most common aluminum wire applications in residential and light commercial work (based on 75°C, standard conditions):
| Application | Typical Load | Recommended Aluminum AWG | Circuit Breaker Size |
|---|---|---|---|
| Small appliance branch circuit | 20 A | 12 AWG | 20 A |
| Electric dryer | 30 A | 10 AWG | 30 A |
| Range / oven | 40–50 A | 6 AWG | 50 A |
| EV charger (Level 2) | 40–48 A | 6 AWG | 50–60 A |
| AC / heat pump (large) | 40–60 A | 6–4 AWG | 50–60 A |
| Sub-panel feeder (100A) | 100 A | 1/0 AWG | 100 A |
| Sub-panel feeder (150A) | 150 A | 3/0 AWG | 150 A |
| Main service entrance (200A) | 200 A | 4/0 AWG or 250 kcmil | 200 A |
| Main service entrance (400A) | 400 A | 600 kcmil (parallel sets) | 400 A |
Always verify with your local electrical inspector and confirm NEC compliance — local amendments may apply.
Conclusion
Selecting the right aluminum wire size isn’t just about matching a number in a table — it’s about understanding the conditions your wire will actually operate in. Here’s the decision framework in a nutshell:
- Start with the ampacity table — find the wire size that meets or exceeds your load at the correct temperature rating.
- Apply the continuous load rule — if the load runs for 3+ hours, multiply your required ampacity by 1.25 (or use the “Max Allowable Amps” column).
- Derate for conductors — if more than 3 wires share a conduit, apply the conductor adjustment factor.
- Derate for temperature — if ambient temperature exceeds 30°C (86°F), apply the temperature correction factor.
- When in doubt, go up a size — upsizing a wire costs little compared to troubleshooting overheating or rewiring later.
Aluminum wiring is safe, code-compliant, and economical for the right applications — particularly feeders, service entrances, and runs over 100 feet where copper costs become significant. Always use anti-oxidant compound on aluminum connections, AL-rated terminals, and torque connections to specification.
Frequently Asked Questions
1. How do I choose the right aluminum wire size?
Start with the NEC ampacity table and find the wire that meets your load current at the appropriate insulation temperature rating (usually 75°C). Then check whether derating is needed for conductor count or ambient temperature. For continuous loads (running 3+ hours), size the wire at 125% of the load — or use the 80% column in the chart. If your run is long (over 100 feet), also calculate voltage drop, as aluminum’s higher resistance may require upsizing.
2. What is derating, and when is it necessary?
Derating means reducing a wire’s usable ampacity below its listed rating to account for heat-generating conditions. It’s required when more than three current-carrying conductors share a conduit (they heat each other up) or when the ambient temperature exceeds 30°C (86°F). Failing to derate in these situations can cause insulation damage, nuisance tripping, or — in severe cases — a fire.
3. Why does the 90°C column show higher ampacity, but you still use the 75°C limit?
The 90°C rating is the insulation’s maximum temperature, not the design operating point. NEC 110.14(C) requires that terminations (breakers, lugs, connectors) in most equipment be rated for 75°C. Running a 90°C wire into a 75°C-rated breaker means the termination limits the system — so you still use the 75°C ampacity value for circuit design, even with THHN or XHHW-2 cable. The 90°C column is mainly useful after applying derating factors.
4. Is aluminum wire safe for home wiring?
Modern aluminum wiring — using AA-8000 series alloy (post-1972) in sizes 8 AWG and larger — is safe when installed with AL-rated devices, proper connectors, anti-oxidant compound, and correct torque on terminals. The safety concerns primarily relate to older, small-gauge (12 or 14 AWG) aluminum branch circuit wiring from the 1960s–early 1970s. That older wiring used a different alloy that expanded and contracted more, loosening connections over time. If your home has this era of wiring, consult a licensed electrician.
5. How does insulation type affect ampacity?
Insulation type determines the maximum temperature the wire can safely operate at. A THWN wire is rated to 75°C, while THHN is rated to 90°C. Higher-rated insulation allows more current to flow before the temperature limit is reached. However, as noted above, NEC terminal limits mean you often can’t fully exploit the 90°C rating in practice — but it gives you more headroom after derating.
6. Can I use aluminum wire for 15A and 20A branch circuits?
Technically yes — NEC allows aluminum in 12 AWG for 20A circuits — but it’s generally not recommended for small branch circuits. The majority of electrical fires linked to aluminum wiring involve small-gauge branch circuits where receptacles, switches, and outlets weren’t rated for aluminum. Most electricians use copper for 15A and 20A circuits and reserve aluminum for feeders (30A and above) and service entrances where the economics strongly favor it.
7. What happens if I exceed the recommended ampacity?
Exceeding ampacity causes the wire to generate more heat than its insulation can handle. In the short term, the breaker should trip — but oversized or faulty breakers may not. Over time, repeated overloading degrades insulation, can cause arcing at loose connections, and is a leading cause of electrical fires. Always size wire correctly and never bypass overcurrent protection.
8. Can I bury aluminum wire directly without conduit?
Yes, with conditions. USE-2 or direct-burial-rated aluminum cable can be buried directly at the required depth (typically 24 inches for direct burial, 18 inches in conduit per NEC Table 300.5). For service laterals, direct-burial aluminum triplex or SER cable is very common. Always follow NEC Table 300.5 burial depths and local utility requirements. Use conduit where the cable could be disturbed or where local code requires it.
9. What is the difference between AWG and kcmil?
AWG (American Wire Gauge) is used for wire sizes from 40 AWG (very thin) down to 4/0 AWG (very thick). Counterintuitively, lower AWG numbers mean thicker wire. Beyond 4/0 AWG, conductors are measured in kcmil (thousands of circular mils), which directly represents the cross-sectional area. So 250 kcmil is larger than 4/0 AWG, and 2000 kcmil is the largest common size in the NEC tables.
10. What other factors impact wire selection beyond ampacity?
Several other factors matter in real installations: voltage drop (long runs require upsizing to keep voltage within 3–5% of supply voltage); short-circuit rating (the wire must withstand fault currents until the breaker clears); conduit fill (NEC limits how many wires fit in a given conduit); termination compatibility (always use AL-rated lugs and devices for aluminum); and local code amendments (some jurisdictions have stricter requirements than NEC minimums). When in doubt, consult a licensed electrician or your local Authority Having Jurisdiction (AHJ).
