Wire Ampacity Calculator
NEC Table 310.16 — corrected ampacity with temperature derating, bundling, continuous load, and terminal rating limit. Covers AWG 14–4/0 and 250–2000 kcmil.
Wire Parameters
Corrected Ampacity
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How it works
This calculator applies NEC correction factors on top of the base ampacity from NEC Table 310.16 (conductors in raceway or cable, 60 Hz, 30°C ambient, up to 3 current-carrying conductors).
1 — Base Ampacity (Ibase)
Look up NEC Table 310.16 for your conductor size, material (Cu/Al), and temperature rating column (60/75/90°C). This calculator covers AWG 14–4/0 and 250–2000 kcmil.
2 — Temperature Correction (TCF)
NEC 310.15(B)(1):
At 30°C TCF = 1.000 (no change). Above 30°C it derates; below 30°C it upgrades (TCF > 1).
TCF = √((Tmax − Ta) / (Tmax − 30))At 30°C TCF = 1.000 (no change). Above 30°C it derates; below 30°C it upgrades (TCF > 1).
3 — Bundling Correction (BCF)
NEC Table 310.15(C)(1) — applies when >3 current-carrying conductors share a raceway:
4–6 → ×0.80 · 7–9 → ×0.70 · 10–20 → ×0.50
21–30 → ×0.45 · 31–40 → ×0.40 · 41+ → ×0.35
4–6 → ×0.80 · 7–9 → ×0.70 · 10–20 → ×0.50
21–30 → ×0.45 · 31–40 → ×0.40 · 41+ → ×0.35
4 — Terminal / Continuous Limits
NEC 110.14(C): most equipment terminals cap at 75°C regardless of conductor rating. NEC 210.20: continuous loads (≥3 hr) require the overcurrent device (and wire) to be rated at 125% of the continuous current.
Corrected Ampacity
Then apply terminal cap if set. For continuous load, divide corrected ampacity by 1.25 to get the usable design limit — or equivalently, the wire must have corrected ampacity ≥ 1.25 × Iload.
Icorr = Ibase × TCF × BCFThen apply terminal cap if set. For continuous load, divide corrected ampacity by 1.25 to get the usable design limit — or equivalently, the wire must have corrected ampacity ≥ 1.25 × Iload.
Which column to use?
Use the column that matches the lower of (a) the conductor's insulation rating and (b) the equipment terminal rating. For most receptacles and panels (75°C lugs), use the 75°C column. Use 90°C only when all connected equipment terminals are explicitly rated 90°C.
Worked example: 12 AWG Cu THHN in 45°C ambient attic, 5 conductors in conduit, 75°C terminal limit.
Base (75°C column) = 25 A. TCF = √((75−45)/(75−30)) = √(30/45) = 0.816. BCF = 0.80. Corrected = 25 × 0.816 × 0.80 = 16.3 A. (75°C terminal cap does not further reduce here.)
NM-B (Romex) special rule
NEC 334.80 limits NM-B ampacity to the 60°C column for sizes 14–10 AWG and the 75°C column for 8 AWG and larger — even though the individual conductors are rated 90°C. This is because the cable assembly traps heat. This calculator applies that restriction automatically.
Common Insulation Types
| Code | Max °C | Environment | Common use |
|---|---|---|---|
| THHN | 90°C | Dry only | Conduit wiring, panels, commercial |
| THWN-2 | 90°C | Wet or dry | Dual-rated with THHN on same spool (look for both marks) |
| XHHW-2 | 90°C | Wet or dry | Cross-linked PE insulation, heavy industrial |
| NM-B | 90°C conductor / 60°C assembly | Dry, concealed | Residential cable (Romex); always use 60/75°C col |
| USE-2 | 90°C | Underground, wet | Service entrance cable, direct burial |
| RHH / RHW-2 | 90°C | Wet or dry | High-temperature industrial; rubber insulation |
| THW | 75°C | Wet or dry | Older conduit installations |
| TW | 60°C | Wet or dry | Very old installations — minimum rating |
Frequently asked questions
What is wire ampacity and why does it matter?
Ampacity is the maximum continuous current a conductor can carry without exceeding its insulation temperature rating under specified installation conditions. Exceeding ampacity causes the insulation to overheat and degrade — eventually causing a fire — even if the circuit breaker never trips (breakers protect against short circuits and severe overloads, not slow thermal damage). The NEC establishes minimum ampacity requirements so conductors are thermally safe before the protective device acts.
Why do I need to derate for ambient temperature?
NEC Table 310.16 assumes a 30°C (86°F) ambient. In hotter environments — attic spaces in summer routinely reach 50–60°C, boiler rooms and engine bays even more — the conductor's ability to shed heat is reduced, so the same current produces a temperature rise that now pushes the insulation past its rating. The correction factor
TCF = √((Tmax−Ta)/(Tmax−30)) accounts for this. At 40°C a 90°C-rated wire carries about 91% of its table value; at 50°C it drops to 82%; at 70°C only 58%. Note that below 30°C ambient the TCF exceeds 1.0, meaning the wire can technically carry more than the table value — a useful upgrade in cold climates.What counts as a "current-carrying conductor" for bundling derating?
Per NEC 310.15(C)(1), count all phase conductors and any neutral that carries net unbalanced current. Equipment grounding conductors (green or bare copper) are never counted. In a standard 120 V circuit (hot + neutral + ground), the hot and neutral both count if the neutral carries return current — that is 2 conductors, no derating needed. A balanced 3-phase 4-wire circuit's neutral carries near zero current and typically is not counted. However, if the load is non-linear (variable-frequency drives, electronic ballasts), the neutral can carry harmonic current exceeding the phase current and should be counted.
Can I use the 90°C ampacity column for THHN wire?
THHN conductors are rated 90°C, but NEC 110.14(C) requires that the actual current be limited by the lower of the conductor's rating and the terminal temperature rating of any connected device. Most residential and commercial equipment (breaker lugs, outlet wiring terminals, switch terminations) is rated only 75°C. So in practice you use the 75°C column as the final working limit, and reach for the 90°C column only when every piece of connected equipment is explicitly rated 90°C. The 90°C column is primarily useful for derating calculations: if conditions force a heavy derating (high ambient + bundling), starting from the higher 90°C base ampacity may let you remain within a safe limit after applying all factors.
What is a continuous load and when does the 125% rule apply?
NEC 210.20 defines a continuous load as one expected to remain on for three hours or more — lighting, HVAC, refrigeration equipment, and similar sustained loads. For continuous loads the NEC requires overcurrent devices and their conductors to be rated at 125% of the continuous current. Practically: if your load draws 16 A continuously, the circuit must be rated for 20 A (16 × 1.25), and the wire must have an ampacity of at least 20 A after all corrections. This calculator shows the effective design limit when the continuous checkbox is checked.
What does "AWG" mean and how does the numbering work?
AWG stands for American Wire Gauge. Counterintuitively, a lower gauge number means a thicker wire: 4/0 (000) is the largest AWG listed here at roughly 11.7 mm diameter, while 14 AWG is about 1.6 mm. Each decrease of 3 gauge numbers roughly doubles the cross-sectional area (and ampacity). The notation "4/0" means "0000" (four zeros), read as "four-aught." Sizes larger than 4/0 are measured in kcmil (thousands of circular mils): 250 kcmil is common for 200 A services, while 1000 kcmil is a very large feeder conductor roughly 29 mm in diameter.
When should I use aluminum conductors?
Aluminum (or copper-clad aluminum) conductors are common for services, feeders, and large branch circuits. They are significantly lighter and cheaper than copper for the same ampacity, which is why utilities and electrical contractors use them for 100 A and 200 A services. For the same ampacity rating, aluminum requires a larger AWG than copper (typically two sizes up: copper 4 AWG ≈ aluminum 2 AWG for 75°C ampacity). Important: aluminum requires CO/ALR rated devices for receptacles and switches (to handle aluminum's expansion characteristics), and anti-oxidant compound is recommended on all aluminum terminations. Small-gauge aluminum (14 or 12 AWG) for branch circuits is rare today due to past fire history with aluminum's propensity to loosen at connections.