Electrical
Every wire gauge has three ampacity ratings, not one. Which of the three you are allowed to use has nothing to do with the wire and everything to do with what you screw it into. Get that backwards and you will confidently install a conductor the code says is too small.
Allowable ampacity in amperes for insulated conductors rated up to 2,000 V, not more than three current-carrying conductors in a raceway, cable or earth, based on a 30°C ambient. Aluminum has no NEC-rated size below 12 AWG, so those cells are genuinely empty rather than omitted.
| Copper | Aluminum | |||||
|---|---|---|---|---|---|---|
| AWG | 60°C | 75°C | 90°C | 60°C | 75°C | 90°C |
| 14 | 15 | 20 | 25 | — | — | — |
| 12 | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 | 40 | 50 | 55 | 35 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
To size a conductor for a specific load and run length rather than reading it off a table, use the wire gauge calculator, which checks ampacity and voltage drop together.
THHN is a 90°C insulation. Reading across to the 90°C column for 12 AWG copper gives 30 amps, and it is tempting to conclude that a #12 THHN conductor is good for 30 A. It is not, and the reason is NEC 110.14(C): a conductor must be sized at the temperature rating of the lowest-rated termination anywhere in the circuit.
Breakers, lugs and device terminals are typically listed for 60°C or 75°C. The wire may tolerate 90°C, but the screw it lands on does not, and heat travels. So for most circuits the 75°C column is your ceiling, and for a good deal of residential equipment it is the 60°C column. The 90°C rating is not decoration — it is the starting value for derating arithmetic, where you begin at 90°C, apply the adjustment and correction factors, and then confirm the result still clears the termination limit. You never simply install to it.
The three smallest common gauges are governed by NEC 240.4(D) as well as by the table. Whatever column your insulation qualifies for, overcurrent protection is capped at the 60°C value:
| Copper | 60°C | 75°C | 90°C | Max breaker |
|---|---|---|---|---|
| 14 AWG | 15 | 20 | 25 | 15 A |
| 12 AWG | 20 | 25 | 30 | 20 A |
| 10 AWG | 30 | 35 | 40 | 30 A |
This is why a 12 AWG branch circuit is a 20 amp circuit in every house in America even though the wire is rated 30 A at 90°C. The rule exists because small conductors have little thermal mass and terminate in devices that were never tested at those temperatures. There are narrow exceptions in 240.4(E) and (G) for specific applications, but for general branch circuits the cap is the cap.
The rule of thumb is "go up two sizes for aluminum." It is close enough to be useful and wrong often enough to be worth checking. At 75°C:
| Gauge | Copper | Aluminum | Al to match Cu |
|---|---|---|---|
| 6 AWG | 65 A | 50 A | 4 |
| 4 AWG | 85 A | 65 A | 2 |
| 2 AWG | 115 A | 90 A | 1/0 |
| 1/0 AWG | 150 A | 120 A | 3/0 |
| 4/0 AWG | 230 A | 180 A | > 4/0 |
Aluminum is still the right choice for many service and feeder runs — it costs less and weighs less, and the larger conductor is not usually a problem in a straight run. It becomes a problem in a conduit sized before anyone decided which metal was going in it, because the bigger conductor changes the conduit fill calculation too.
The chart's assumptions are stated in its own title, and both fail in ordinary work. More than three current-carrying conductors in a raceway triggers the adjustment factors of NEC 310.15(C)(1), which reduce ampacity in steps as the count rises. Neutrals sometimes count and sometimes do not; equipment grounding conductors never do.
Ambient temperature above 30°C triggers the correction factors of 310.15(B). An unconditioned attic in a hot climate is comfortably outside the table's basis, and rooftop conduit in sun is further outside it again. The two factors multiply — a full conduit in a hot attic can land a conductor well below what the chart suggests, and that combination is where undersized runs usually come from rather than from misreading a single number.
Ampacity answers one question: will this conductor overheat carrying this current. It says nothing about whether the voltage arriving at the far end is still usable. On a short run that does not matter. On a long one it decides the answer, and it is entirely possible for a conductor that passes ampacity comfortably to fail voltage drop badly.
That check needs conductor resistance and run length rather than a table lookup — the voltage drop formula walks through it by hand. For a specific load, run length and amperage, the wire size by amperage guide gives the direct answer for the common circuit sizes.
Grouped by manufacturer or publisher, then by document. Expand a group for the cited document, its verified-on date, and every value taken from it — all of it stays in the HTML either way.
1 value from this document
NFPA 70 (NEC), Table 310.16 (2023 ed.) — same conditions as the 60°C column · verified 2026-07-31
2 values from this document
NFPA 70 (NEC), Table 310.16 (2023 ed.) — same conditions as the 14 AWG entries · verified 2026-07-31
36 values from this document
1 value from this document
NFPA 70 (NEC), Table 310.16 (2023 ed.) — same conditions as the copper entries · verified 2026-07-31
34 values from this document
1 value from this document
Copper 12 AWG is rated 20 A at 60°C, 25 A at 75°C and 30 A at 90°C in NEC Table 310.16. In practice you protect it at 20 A: NEC 240.4(D) caps overcurrent protection for 12 AWG copper at that figure regardless of which temperature column its insulation qualifies for.
Copper 10 AWG shows 30 A at 60°C, 35 A at 75°C and 40 A at 90°C. NEC 240.4(D) limits it to 30 A of overcurrent protection, which is why a 10 AWG branch circuit is a 30 A circuit even when the wire is rated higher.
NEC 110.14(C) requires conductors to be sized at the temperature rating of the lowest-rated termination in the circuit. Most breakers and lugs are listed for 60°C or 75°C, so that is your column. The 90°C rating is used as the starting point for derating calculations, not for choosing the conductor.
Roughly two gauge sizes larger, though it varies. At 75°C, 65 A of copper 6 AWG is matched by 4 AWG aluminum, and aluminum 6 AWG carries only 50 A. Check the table rather than applying a fixed offset, because the gap is not constant across the range.
Yes. Table 310.16 assumes no more than three current-carrying conductors in a raceway at 30°C ambient. Beyond three, the adjustment factors in NEC 310.15(C)(1) reduce ampacity — a well-filled conduit can push a conductor below its table rating, which is why fill and ampacity have to be solved together.
No, and on long runs it is often not the binding one. Ampacity asks whether the conductor overheats. Voltage drop asks whether the load still receives usable voltage at the far end. A run long enough will need a larger conductor than ampacity alone requires, so check both before ordering wire.
