Ampacity is the maximum continuous electrical current a specific wire gauge and insulation type can safely carry without exceeding its thermal limits.
When you ask which wire can carry a higher current, you are really asking about ampacity. This single metric dictates your breaker sizing, prevents insulation meltdown, and ensures your overcurrent protective device trips before the wire catches fire. Makers and DIYers commonly confuse ampacity (a strict thermal limit) with voltage drop (a performance metric over distance), and they mistakenly believe stranded wire carries more current than solid wire of the same AWG. In reality, solid wire has slightly more copper cross-sectional area than stranded wire of the same gauge, giving it a marginal thermal edge, though the NEC treats them identically for standard building wire.
The Physics and the Code: How We Measure Ampacity
Current carrying capacity is governed by three physical realities: cross-sectional area, material resistivity, and insulation thermal tolerance. Think of cross-sectional area like lanes on a highway; a wider road (lower AWG number) dissipates the heat generated by electron friction far more efficiently than a narrow one.
Material choice is the biggest differentiator. Copper is the baseline for residential wiring due to its high conductivity and thermal resilience. Aluminum, while significantly cheaper and lighter, has only about 61% the conductivity of copper by volume. This means an aluminum conductor must be physically larger—typically two AWG sizes up—to carry the exact same current as a copper conductor without overheating.
However, physics only gets you halfway there. The National Electrical Code (NEC) introduces the concept of temperature columns. Wire insulation like THHN is rated for 90°C, but the breakers and lugs you connect them to are usually only rated for 75°C or 60°C. The NEC mandates that your circuit ampacity is limited by the weakest link in the thermal chain, which is almost always the termination point.
Worked Numeric Example: Copper vs. Aluminum at 75°C
To see exactly which wire can carry a higher current, we look at the 75°C column of NEC Table 310.16, as this is the standard rating for most modern residential breakers and subpanel lugs. Notice how aluminum requires a larger physical diameter (lower AWG number) to match copper's ampacity.
| Wire Size (AWG/kcmil) | Copper Ampacity (75°C) | Aluminum Ampacity (75°C) | Typical Application |
|---|---|---|---|
| 8 AWG | 50A | 40A | Standard 40A/50A branch circuits |
| 4 AWG | 85A | 65A | 60A subpanel feeders, EV chargers |
| 2 AWG | 115A | 90A | 100A subpanel feeders |
| 1/0 AWG | 150A | 120A | 100A-125A heavy feeders |
Source: Adapted from the Cerro Wire Ampacity Chart and NEC Table 310.16.
Where You Meet This in Practice
You will face the copper vs. aluminum ampacity decision most often when running long feeder cables to detached garages, workshops, or subpanels. Because copper prices fluctuate wildly, a 200-foot run of 2 AWG copper can cost three to four times as much as 1/0 AWG aluminum. Aluminum is perfectly safe and code-compliant for feeders, provided you use the correct anti-oxidant paste (like Noalox) and torque the lugs to manufacturer specifications to prevent high-resistance connections.
SAFETY WARNING: Any work inside a panel or subpanel involves lethal mains voltage. Always de-energize the main breaker, lock or tag it out, and verify the bus bars are dead with a tested non-contact voltage meter and a multimeter before touching any conductors. Local code may require a licensed electrician for feeder installations.
When determining wire size for these high-load applications, follow this sequence:
- Calculate the Continuous Load: Identify the maximum expected amperage. If the load runs for 3 hours or more (like an EV charger or space heater), multiply the amperage by 1.25 (125%).
- Select the Breaker: Choose the next standard breaker size up from your calculated 125% load.
- Check Termination Ratings: Look at the breaker and panel lug markings. They will say 60°C or 75°C. This dictates which NEC column you must use.
- Pick the Wire: Using the correct temperature column, select a wire gauge whose ampacity meets or exceeds the breaker rating (or the 125% continuous load, whichever is higher).
Real-World Scenario Walkthrough: The Melted Subpanel Lug
Theory is clean; the jobsite is not. Here is a classic failure mode that illustrates what happens when you look at the wrong temperature column to figure out which wire can carry a higher current.
- The Setup: A DIYer is wiring a 60A subpanel to a detached workshop to run a large air compressor and a welder. To save money, they buy 6 AWG THHN copper wire. They look at the manufacturer ampacity chart and see that 6 AWG THHN is rated for 75A in the 90°C column. Since 75A is greater than their 60A breaker, they assume the wire is perfectly sized.
- The Numbers: The actual continuous load of the compressor and lighting is 52A. Under NEC 210.20(A), a 52A continuous load requires the circuit to be sized at 125%, which equals 65A. The wire they chose (6 AWG) has an ampacity of 65A in the 75°C column, but only 55A in the 60°C column.
- The Outcome: After two hours of running the compressor, the THHN insulation near the main breaker lug softens, chars, and emits a sharp burning plastic smell. The 60A breaker eventually trips on thermal overload, but the panel lug is permanently discolored and ruined.
- What Went Wrong: The builder fell victim to the 90°C trap. While THHN insulation can physically survive 90°C, NEC 110.14(C) dictates that you must size the wire based on the temperature rating of the terminations. Most standard 60A breakers are only rated for 75°C, and many older or budget panels are rated for 60°C. By using the 90°C column, the builder assumed the wire could carry 75A. In reality, the termination point was the bottleneck. The heat generated by the 52A continuous load couldn't dissipate fast enough through the 75°C-rated lug, causing a localized thermal failure long before the 60A breaker's magnetic trip engaged.
FAQ: Common Wire Sizing Confusions
Does stranded wire carry more current than solid wire?
No. For a given AWG size, solid and stranded wire have the same nominal cross-sectional area of conductive metal, and the NEC assigns them the exact same ampacity. In fact, because stranded wire has tiny air gaps between the individual strands, a solid wire of the same AWG actually contains a microscopic fraction more copper, making it marginally better at conducting heat and electricity. Stranded wire is chosen for flexibility and vibration resistance, not for higher current capacity.
Why do I need a larger wire for a long run if the ampacity is already high enough?
You are confusing ampacity with voltage drop. Ampacity is about preventing the wire from catching fire. Voltage drop is about ensuring your equipment actually receives enough voltage to operate efficiently. If you run a 10 AWG wire 200 feet to a 15A load, the wire will not melt (it is well within its 30A/75°C ampacity limit), but the voltage at the end of the run might drop below 110V, causing motors to overheat and draw even more current. For long runs, you must upsize the wire to mitigate resistance, even if the base ampacity requirement is met.
Can I mix copper and aluminum in the same circuit?
You should never splice copper and aluminum wire together using standard wire nuts or basic crimps. The two metals have different thermal expansion rates and create galvanic corrosion when they touch, leading to high-resistance joints that start fires. If you must transition between an aluminum feeder and copper branch circuits, do it at a properly rated terminal block or busbar inside a subpanel, using an anti-oxidant compound on the aluminum strands.
