Wire Ampacity Calculator for NEC Tables 310.16 and 310.17. It derates for ambient heat and wire count, applies the terminal rating, then gives the breaker size and checks the load.
Count only wires that carry current. Leave out grounds and a neutral that carries only unbalanced current.
Installation and Load Check
Parallel sets need 1/0 AWG or larger. Type XHHW-2 is exempt from the rooftop adder.
Step-by-Step Ampacity Formula
| Step | Value |
|---|---|
| Table 310.16, 90°C column | 75 A |
| Ambient 86°F, Table 310.15(B)(1)(1) | × 1.00 |
| 3 current-carrying, Table 310.15(C)(1) | × 1.00 |
| Derated wire ampacity | 75 A |
| 60°C terminal column, NEC 110.14(C) | 55 A |
| Allowable ampacity | 55 A |
Ampacity Chart for These Conditions
| Wire Size | Table | Derated | Final | Breaker |
|---|---|---|---|---|
| 18 AWG | 14 A | 14 A | 14 A | 7 A |
| 16 AWG | 18 A | 18 A | 18 A | 10 A |
| 14 AWG | 25 A | 25 A | 15 A | 15 A |
| 12 AWG | 30 A | 30 A | 20 A | 20 A |
| 10 AWG | 40 A | 40 A | 30 A | 30 A |
| 8 AWG | 55 A | 55 A | 40 A | 40 A |
| 6 AWG | 75 A | 75 A | 55 A | 60 A |
| 4 AWG | 95 A | 95 A | 70 A | 70 A |
| 3 AWG | 115 A | 115 A | 85 A | 90 A |
| 2 AWG | 130 A | 130 A | 95 A | 100 A |
| 1 AWG | 145 A | 145 A | 110 A | 110 A |
| 1/0 AWG | 170 A | 170 A | 150 A | 150 A |
| 2/0 AWG | 195 A | 195 A | 175 A | 175 A |
| 3/0 AWG | 225 A | 225 A | 200 A | 200 A |
| 4/0 AWG | 260 A | 260 A | 230 A | 250 A |
| 250 kcmil | 290 A | 290 A | 255 A | 300 A |
| 300 kcmil | 320 A | 320 A | 285 A | 300 A |
| 350 kcmil | 350 A | 350 A | 310 A | 350 A |
| 400 kcmil | 380 A | 380 A | 335 A | 350 A |
| 500 kcmil | 430 A | 430 A | 380 A | 400 A |
| 600 kcmil | 475 A | 475 A | 420 A | 450 A |
| 700 kcmil | 520 A | 520 A | 460 A | 500 A |
| 750 kcmil | 535 A | 535 A | 475 A | 500 A |
| 800 kcmil | 555 A | 555 A | 490 A | 500 A |
| 900 kcmil | 585 A | 585 A | 520 A | 600 A |
| 1000 kcmil | 615 A | 615 A | 545 A | 600 A |
| 1250 kcmil | 665 A | 665 A | 590 A | 600 A |
| 1500 kcmil | 705 A | 705 A | 625 A | 700 A |
| 1750 kcmil | 735 A | 735 A | 650 A | 700 A |
| 2000 kcmil | 750 A | 750 A | 665 A | 700 A |
Wire Ampacity Calculator: Three Limits, One Answer
Ask what 6 AWG copper THHN can carry and three different numbers are all defensible. The 90°C column of NEC Table 310.16 says 75 A, the 75°C column says 65 A and the 60°C column says 55 A. The Wire Ampacity Calculator shows which one applies to your installation and why.
On its default settings the answer is 55 A. The wire itself is good for 75 A, but the tool assumes 60°C terminals for wire this size, as NEC 110.14(C) does when equipment carries no marking. The second card reads Terminal Rating Sets the Limit, with 20 A lost to it and 65 A available on 75°C terminals.
The rest of the screen turns that ampacity into decisions. It names 60 A as the largest breaker allowed, 44 A as the most continuous load, and checks the 40 A load entered against both. A step table in the Wire Ampacity Calculator lists each factor in order, and a chart repeats the math for every wire size under the same conditions.
Wire Ampacity Formula
The wire ampacity formula in the NEC starts from a table value and multiplies it by two factors. One corrects for the air temperature around the wire, and the other adjusts for how many current-carrying wires share the raceway or cable.
The derated figure is not yet the answer. Wire maker Cerrowire describes the last step in the notes to its ampacity calculator. The final value is the lowest of the corrected and adjusted ampacity, the equipment temperature limit and any cable type limit.
The table values here match the manufacturer's ampacity chart. Copper 3/0 AWG is 200 A at 75°C, for example, and aluminum 3/0 AWG is 155 A. Table 310.16 assumes 86°F air and no more than three current-carrying wires, which is why both factors read "None" on the default result.
Wire Derating for Heat and Wire Count
Derating is where an electrical wire ampacity calculator earns its place, because the two factors stack. Take 12 AWG THHN with nine current-carrying wires in a conduit at 104°F. The 90°C table value of 30 A is multiplied by 0.91 for the heat and 0.70 for the wire count, which leaves 19.1 A.
The wire count factors run from 80% for four to six wires down to 35% for 41 or more. Count every conductor that carries current, including spares that will be energized later. Leave out equipment grounds and a neutral that carries only the unbalanced current of the other wires.
Ambient correction works in both directions. A code column in EC&M notes that a 90°C conductor in 122°F air takes a factor of 0.82. Below 78°F the factor rises above 1, and the tool applies that gain as well.
Starting from the 90°C column is what makes THHN forgiving. Six current-carrying 12 AWG wires in one conduit derate to 24 A, which is still above the 20 A that 60°C terminals allow. The wire loses nothing in practice, and the result stays at 20 A.
Rooftop conduit in sunlight
Conduit lying on a sunlit roof runs far hotter than the air. The 2023 NEC adds 60°F to the outdoor temperature when a raceway or cable sits less than 3/4 in. above the roof, a distance the Steel Tube Institute notes was 7/8 in. in earlier editions. Selecting Under 3/4 in. Above Roof in the Wire Ampacity Calculator applies the adder.
On a 95°F day that makes the working temperature 155°F. A 10 AWG THHN wire drops from 40 A to 23.2 A, and the largest breaker becomes 25 A. Type XHHW-2 is exempt from the adder, so leave the rooftop setting off for it and the same wire holds its full 30 A terminal limit.
Terminal Ratings: 60°C, 75°C or 90°C
A wire is only as good as the lug it lands on. Under NEC 110.14(C), wire from 14 AWG to 1 AWG is treated as connected to 60°C terminals unless the equipment is marked for 75°C, and wire 1/0 AWG and larger uses the 75°C column. The Auto setting in the Wire Ampacity Calculator follows that rule.
Most of the gap between calculators comes from this one input. If the breaker and the device at the other end are both marked 75°C, set Terminal Rating to 75°C. The default 6 AWG result then rises from 55 A to 65 A, the breaker moves to 70 A and the continuous load limit becomes 52 A.
The 90°C column is rarely a final answer, since very little equipment is rated for 90°C terminations. Its job is to be the starting point for derating, which is why the tool keeps the table value and the final value on separate cards.
NM-B, SER and MC Cable Limits
Cable types carry a limit of their own. NM-B and UF-B contain 90°C conductors, yet their final ampacity is held to the 60°C column, so 6 AWG NM-B is a 55 A cable whatever the terminals say. SER, SEU and MC cable are held to the 75°C column.
The cable rule and derating interact the same way terminals do. Derating still starts from the 90°C value, so 12 AWG NM-B bundled with six current-carrying wires stays at 20 A. At ten wires or more the 50% factor takes it to 15 A, and the Wire Ampacity Calculator shows Derating Sets the Limit.
Breaker Size and Continuous Load
The third card of the Wire Ampacity Calculator gives the largest standard breaker or fuse for the wire. When the ampacity falls between standard sizes, NEC 240.4(B) allows the next size up for circuits of 800 A or less, which is how a 55 A wire gets a 60 A breaker. The card names the rule it used each time.
Small wire has fixed caps that override the table. Copper is limited to 15 A on 14 AWG, 20 A on 12 AWG and 30 A on 10 AWG, and aluminum to 15 A on 12 AWG and 25 A on 10 AWG. Rounding up is also off the table for circuits feeding more than one receptacle, and motor circuits follow their own article.
A continuous load is one that runs for three hours or more, and it may use only 80% of the breaker and wire rating. Set Load Type and enter the amps, and the fourth card shows the share of ampacity used and the room left. A wire that passes on ampacity can still fail on distance, which the Voltage Drop Calculator checks.
Parallel Wire Ampacity
Used as a parallel wire ampacity calculator, the tool multiplies the per-conductor result by the number of sets. Two sets of 500 kcmil copper THHN give 380 A each on 75°C terminals, 760 A in total, and an 800 A breaker. Paralleling is allowed only for 1/0 AWG and larger, and smaller sizes are rejected.
Three sets reach 1,140 A, and the breaker rule changes. Above 800 A the next size up is no longer allowed, so the tool steps down to a 1,000 A device. Sets that share one conduit also raise the wire count. Two three-phase sets are six current-carrying conductors, the 80% factor applies and the total falls to 688 A.
3 Phase Ampacity and Wire Count
Ampacity does not depend on phase or voltage, so there is no separate 3 phase ampacity calculator to reach for. What changes is the count. A three-phase circuit has three current-carrying conductors, and a balanced four-wire wye circuit still counts as three because its neutral carries only unbalanced current.
The neutral does count when most of the load is nonlinear, such as electronic drivers and power supplies, under NEC 310.15(E). Enter four wires in that case and the 80% factor applies. To work backward from a load to a conductor size, the Wire Size Calculator starts from amps instead of gauge.
Free Air, Busbar and Steel Wire
Single insulated wires in free air use NEC Table 310.17, where 6 AWG THHN is listed at 105 A. Selecting Single Wires in Free Air in the Wire Ampacity Calculator switches tables, but the terminal column still comes from Table 310.16. The default wire therefore lands back at 55 A, and the alert explains that the terminals are holding it back.
The tables behind this tool cover copper, aluminum and copper-clad aluminum building wire from 18 AWG to 2000 kcmil. They do not rate bare bars or steel conductors, so a busbar ampacity calculator or a steel wire ampacity calculator needs different data. Metric cable sizes can be matched to the nearest gauge with the AWG to mm² Calculator.