Wire Ampacity & Derating Calculator
Start from the NEC Table 310.16 75°C ampacity for a copper or aluminum conductor, then apply the ambient-temperature correction and the more-than-three-conductor adjustment to get the derated ampacity. Useful for hot ambient runs and conduits with more than three current-carrying conductors.
Inputs
Derate chain
base = min(derated, termination column). NEC Table 310.16 · 310.15(B)(1) · 310.15(C)(1) · 110.14(C).
How this calculator works
The NEC ampacity tables assume a specific installation: 30°C (86°F) ambient and no more than three current-carrying conductors in the raceway or cable. Leave those conditions and the table value must be corrected. The calculator runs the standard derate chain:
derated ampacity = base (Table 310.16, 75°C) × temp factor (Table 310.15(B)(1)) × CCC factor (Table 310.15(C)(1))
The base ampacity comes from the 75°C column of Table 310.16 — the column that matches standard 75°C terminations per 110.14(C). The temperature factor corrects for ambient above or below 30°C (it can be greater than 1.0 in cool spaces), and the conductor-count adjustment kicks in whenever more than three current-carrying conductors share a raceway or cable. The two factors multiply — a hot roof and a crowded conduit stack. The result is also capped at the base 75°C value as the termination limit, so a favorable correction never rates the circuit above what its lugs can accept.
Worked example
8 AWG copper THWN in a conduit with six current-carrying conductors, run through a 45°C (113°F) mechanical space:
- Base ampacity, Table 310.16, 8 AWG copper at 75°C: 50 A.
- Ambient correction for the 41–45°C band, 75°C column: × 0.82.
- Adjustment for 4–6 current-carrying conductors: × 0.80.
- 50 × 0.82 × 0.80 = 32.8 → derated ampacity 32 A (the calculator floors to the whole ampere).
Enter the same inputs above (8 AWG copper, 45°C ambient, 6 conductors) to reproduce this result — the wire that carries 50 A on the bench is a 32 A conductor in that installation.
Reference: allowable ampacity (75°C column)
Excerpt from NEC Table 310.16 — the 75°C column the calculator starts from, read from the same verified data module. 30°C ambient, not more than three current-carrying conductors.
| Size | Copper — 75°C | Aluminum — 75°C |
|---|---|---|
| 14 AWG | 20 A | — |
| 12 AWG | 25 A | 20 A |
| 10 AWG | 35 A | 30 A |
| 8 AWG | 50 A | 40 A |
| 6 AWG | 65 A | 50 A |
| 4 AWG | 85 A | 65 A |
| 2 AWG | 115 A | 90 A |
| 1/0 AWG | 150 A | 120 A |
| 4/0 AWG | 230 A | 180 A |
| 250 kcmil | 255 A | 205 A |
| 500 kcmil | 380 A | 310 A |
Adjustment factors for more than three current-carrying conductors, Table 310.15(C)(1) — the same function the calculator applies:
| Current-carrying conductors | Factor |
|---|---|
| 1–3 | 1.00 |
| 4–6 | 0.80 |
| 7–9 | 0.70 |
| 10–20 | 0.50 |
| 21–30 | 0.45 |
| 31–40 | 0.40 |
| 41 and above | 0.35 |
Frequently asked questions
What size wire do I need for 100 amps?
From the NEC Table 310.16 75°C column, 3 AWG copper or 1 AWG aluminum carries 100 A at standard conditions. For the main power feeder or service to a dwelling, NEC 310.12 permits sizing at 83% of the service rating, which is how 4 AWG copper ends up on many 100 A residential services. High ambient temperature or more than three current-carrying conductors in the raceway can force a size up — run the derate before settling on a size, and verify against the code edition adopted in your jurisdiction.
Can I use the 90°C ampacity column for THHN wire?
Only as a starting point for derating — almost never as the final answer. NEC 110.14(C) limits the circuit to the temperature rating of its terminations, and breakers and lugs are typically rated 75°C (60°C on some small or older equipment), so the finished circuit can’t exceed the 75°C column even though THHN insulation is rated 90°C. This calculator currently starts the derate from the 75°C column and caps the result at the 75°C termination limit, which is conservative; the 90°C-start option is pending.
When does wire ampacity have to be derated?
Two triggers: ambient temperature different from the table’s 30°C (86°F) base, corrected per NEC 310.15(B)(1), and more than three current-carrying conductors in a raceway or cable, adjusted per 310.15(C)(1) — four to six conductors take an 80% factor, seven to nine take 70%, and it keeps falling from there. The two factors multiply, which is exactly what this calculator does. Rooftop conduit, attic runs, and bundled home-runs are the installs that most often get caught.
Does the neutral count as a current-carrying conductor?
Usually not. A neutral that carries only the unbalanced current of a multiwire branch circuit is not counted for the adjustment factor. It does count in a 2-wire circuit, when it serves two phases of a three-phase 4-wire wye system, or when the load is mostly electronic or other nonlinear equipment, because harmonic currents keep the neutral loaded even when the phases balance. Equipment grounding conductors never count.
Why is 12 AWG copper limited to a 20 amp breaker when the table says 25 amps?
NEC 240.4(D), the small-conductor rule: regardless of table ampacity, overcurrent protection is capped at 15 A for 14 AWG copper, 20 A for 12 AWG, and 30 A for 10 AWG, with lower caps for aluminum. The higher table value still matters — it’s the number you derate from, so 12 AWG can absorb some derating and keep its 20 A breaker. The calculator flags the 240.4(D) cap whenever a small conductor is selected. A few applications, such as motor branch circuits, have their own rules that allow larger protection on small wire.
Is the 80% breaker loading rule actually in the code?
Not as a blanket rule. The NEC requires conductors and overcurrent devices to be sized at 125% of continuous loads — loads expected to run for three hours or more (210.19, 210.20) — which works out to the familiar 80% when you invert it. Noncontinuous loads can use the full rating, and 100%-rated breaker assemblies are an exception. Treat 80% as field shorthand for the continuous-load rule, not as a standalone code section.
Method: base ampacity from NEC Table 310.16 (legacy 310.15(B)(16)), 75°C column, multiplied by the ambient-temperature correction factor (Table 310.15(B)(1)) and the conductor-count adjustment factor (Table 310.15(C)(1), legacy 310.15(B)(3)(a)), then capped at the 75°C termination limit per 110.14(C). The 60°C and 90°C columns are not yet applied — conservative for 90°C-insulated wire, whose derate may start from the higher column. Verify against the governing edition of the NEC and any local amendments for your jurisdiction.
Just need the base numbers? See the wire ampacity chart (Cu & Al at 60/75/90°C, 14 AWG to 1000 kcmil) and the full correction & adjustment factors chart. Pair with the Voltage Drop Calculator to size the run, the Conduit Fill Calculator for the raceway, and the ground wire size chart for the EGC.