Wire Ampacity Calculator determines allowable conductor current from wire sizes, material, insulation rating, ambient temperature, current-carrying conductors, and terminal limits.
How Wire Ampacity Is Calculated
The starting point is NEC Table 310.16, the standard ampacity table for insulated conductors rated up to 2000 volts. Every conductor material and size has three baseline ampacity values in that table — one for each insulation temperature class: 60°C, 75°C, and 90°C.
That table value only holds under two specific conditions: an ambient air temperature of 30°C (86°F), and no more than three current-carrying conductors bundled in the same raceway or cable. Change either condition and the ampacity has to be corrected.
$$ I_{corrected} = I_{table} \times C_a \times C_q $$
$I_{table}$ is the NEC Table 310.16 base ampacity for the chosen material, size, and insulation column. $C_a$ is the ambient temperature correction factor from NEC Table 310.15(B)(1) — it runs above 1.0 in cooler air and drops toward zero as ambient climbs, with the exact bracket depending on which insulation column you’re in.
$C_q$ is the adjustment factor from NEC Table 310.15(C)(1) for how many current-carrying conductors share the raceway: 100% for 1–3 conductors, 80% for 4–6, 70% for 7–9, 50% for 10–20, 45% for 21–30, 40% for 31–40, and 35% for 41 or more.
The corrected number isn’t automatically what you’re allowed to use, though. Three more code-based ceilings sit on top of it, and whichever value is lowest wins:
$$ I_{allowable} = \min(I_{corrected},\ I_{termination},\ I_{cable\ cap},\ I_{OCPD\ cap}) $$
$I_{termination}$ is the table ampacity at the temperature rating of the equipment terminals the conductor lands on — NEC 110.14(C) caps you at whatever the terminals are actually rated for, regardless of the conductor’s own insulation rating.
$I_{cable\ cap}$ only applies to NM-B and UF-B cable: NEC 334.80 and 340.80 fix their ampacity to the 60°C column no matter what the jacket says, even 90°C-rated NM-B.
General conductors in conduit, SE cable, and MC cable aren’t subject to this cap. $I_{OCPD\ cap}$ is the small-conductor overcurrent protection limit from NEC 240.4(D): 15 A for 14 AWG copper, 20 A for 12 AWG copper, 30 A for 10 AWG copper, and for aluminum, 15 A for 12 AWG and 25 A for 10 AWG. That cap exists specifically to stop ambient or bundling corrections from justifying an oversized breaker on small wire.
For parallel conductor sets, the per-conductor allowable ampacity gets multiplied by the number of sets:
$$ I_{total} = I_{allowable} \times N_{sets} $$
NEC 310.10(G) only permits paralleling on conductors 1/0 AWG and larger, and every set has to match in length, material, size, insulation, and termination.
Worked Example: 10 AWG Copper at 104°F in a Crowded Conduit
Take a 10 AWG copper conductor with 90°C-rated THHN insulation, run in conduit through an attic space where ambient hits 104°F (40°C). Five other current-carrying conductors share that same conduit, and the breaker it terminates on is rated for 75°C.
Table 310.16 lists 10 AWG copper at the 90°C column as $I_{table} = 40$ A. 40°C falls in the 36–40°C bracket of Table 310.15(B)(1), which corrects the 90°C column by $C_a = 0.91$:
$$ 40 \times 0.91 = 36.4 \text{ A} $$
Five current-carrying conductors falls in the “4 through 6” bracket of Table 310.15(C)(1), applying $C_q = 0.80$:
$$ 40 \times 0.91 \times 0.80 = 29.12 \text{ A} $$
That 29.12 A now has to be checked against the other two ceilings. The 75°C column for 10 AWG copper — the termination cap — is 35 A. There’s no cable-type cap, since this is a general conductor in conduit, not NM-B or UF-B. The small-conductor OCPD cap for 10 AWG copper is 30 A.
| Step | Calculation | Result |
|---|---|---|
| Table ampacity (90°C column) | — | 40.00 A |
| Ambient correction (36–40°C bracket) | 40 × 0.91 | 36.40 A |
| Conductor bundling adjustment (4–6 CCC bracket) | 40 × 0.91 × 0.80 | 29.12 A |
| 75°C termination cap | — | 35.00 A |
| Small-conductor OCPD cap (240.4(D)) | — | 30.00 A |
| Governing (lowest) value | min(29.12, 35.00, 30.00) | 29.12 A |
The corrected-and-adjusted figure of 29.12 A comes in under both the 35 A termination cap and the 30 A OCPD cap, so it governs. That conductor’s allowable ampacity is 29.12 A — nearly 11 A below its 40 A table rating, from heat and conduit fill alone.
What the Result Means
Four different limits can end up governing the final number, and which one it is tells you something different about the circuit:
When the corrected-and-adjusted ampacity governs, heat and conduit fill are the binding constraint — typical of hot attics, rooftop conduit runs, or raceways packed with more circuits than the table’s default three-conductor assumption.
When the termination rating governs, the conductor’s own insulation rating is going to waste: most breakers and panels rated 100 A or less are only listed for 60°C or 75°C terminations under NEC 110.14(C), so a 90°C conductor often can’t use its full 90°C number no matter how favorable the ambient and bundling conditions are.
When the 60°C cable-type cap governs, that’s unavoidable — NM-B and UF-B are locked to the 60°C column by 334.80 and 340.80 regardless of what temperature rating is printed on the jacket.
When the small-conductor OCPD cap governs, it’s acting as a floor: 14, 12, and 10 AWG copper (and 12 and 10 AWG aluminum) can’t be protected by a standard breaker larger than 240.4(D) allows, even if the derated math says otherwise.
If the calculator halts instead of returning a number, it’s one of two things: the material/size/insulation combination doesn’t exist in the table — 14 AWG aluminum isn’t listed at all — or the chosen insulation rating isn’t permitted at that ambient temperature, because the correction factor bottoms out at zero above a certain point for lower-rated insulation.
What Changes the Result
Ambient temperature moves the correction factor directly — colder than 30°C pushes it above 1.0, hotter pulls it down, and for 60°C-rated insulation the factor runs out entirely above 55°C ambient, meaning that insulation class isn’t permitted there at all.
Conductor bundling works in brackets, not a smooth curve: going from 3 current-carrying conductors to 4 drops the factor from 100% to 80% in one step, even though the conductor count barely changed.
Material matters on its own — aluminum ampacity runs noticeably below copper at the same AWG, roughly equivalent to stepping down a wire size.
Insulation rating only helps if the termination and cable type let you use it; a 90°C conductor landing on 60°C-rated terminals is still capped at the 60°C number for that final step. Parallel sets are the only way to add ampacity without upsizing a single conductor, but only for 1/0 AWG and larger, with every set required to match.
| Gauge | Copper OCPD Cap | Aluminum OCPD Cap |
|---|---|---|
| 14 AWG | 15 A | not listed — 14 AWG aluminum isn’t in the table |
| 12 AWG | 20 A | 15 A |
| 10 AWG | 30 A | 25 A |
Wire Ampacity FAQs
What’s the difference between a wire’s table ampacity and its allowable ampacity?
Table ampacity is the raw NEC Table 310.16 value for the conductor’s material, size, and insulation rating, valid only at 30°C ambient with three or fewer current-carrying conductors. Allowable ampacity is what’s left after ambient correction, bundling adjustment, and any termination, cable-type, or small-conductor caps are applied — it’s usually lower, sometimes significantly.
Why does a 90°C-rated wire sometimes only get a 60°C ampacity?
This happens with NM-B and UF-B cable specifically. NEC 334.80 and 340.80 fix their ampacity to the 60°C column regardless of insulation rating — the 90°C rating can still be used for the ambient and bundling correction math, but the final number is capped at 60°C.
Does the number of conductors in a conduit actually change how much current a wire can carry?
Yes. NEC Table 310.15(C)(1) reduces allowable ampacity as more current-carrying conductors share a raceway, since more conductors bundled together retain more heat. The reduction moves in brackets — 1 through 3 conductors gets no reduction, but 4 through 6 drops to 80% of table ampacity, 7 through 9 to 70%, and so on.
Is aluminum wire ampacity lower than copper of the same gauge?
Yes, consistently. At every gauge in Table 310.16, aluminum ampacity runs below copper — for example, 10 AWG copper at 90°C is rated 40 A, while 10 AWG aluminum at 90°C is rated 35 A. Aluminum also isn’t listed at 14 AWG at all; the smallest standard aluminum size is 12 AWG.
Can I use the 90°C ampacity if my breaker is only rated for 75°C terminations?
No. NEC 110.14(C) limits the ampacity to whatever temperature rating the equipment terminals are actually listed for, independent of the conductor’s own insulation rating. A 90°C conductor landing on 75°C terminals is capped at the 75°C column for that connection.
Why is there a maximum breaker size for small conductors even when the calculated ampacity is higher?
NEC 240.4(D) sets a hard overcurrent protection ceiling for 14, 12, and 10 AWG copper and 12 and 10 AWG aluminum — 15 A, 20 A, and 30 A for copper; 15 A and 25 A for aluminum. It exists so favorable ambient or bundling corrections can’t be used to justify an oversized breaker on wire that’s still physically small.
Can I run two smaller conductors in parallel instead of one larger conductor?
Only at 1/0 AWG and larger. NEC 310.10(G) permits parallel conductor sets starting at 1/0 AWG, provided every set matches in length, material, size, insulation, and termination — smaller gauges aren’t eligible for this treatment.
What ambient temperature does the base ampacity table assume?
30°C (86°F). NEC Table 310.15(B)(1) correction factors are built around that baseline — ambient temperatures below 30°C actually push the correction factor above 1.0, while anything above it reduces the allowable ampacity.
These figures are for planning and estimation purposes. Final wire sizing, breaker selection, and installation must be verified against your local adopted code and performed or inspected by a licensed electrician.