Electrical
Cable Ampacity Reference
Base ampacity tables for copper and aluminium conductors (NEC 310.15(B)(16) / IEC 60364-5-52), temperature correction factors, conduit fill derating, AWG-to-mm² size comparison and worked derating examples. Use alongside the Cable Ampacity Calculator.
Last updated: June 2026
Terms
- AWGAmerican Wire Gauge
- The American sizing system for wire thickness. Confusingly, a smaller AWG number means a thicker wire — AWG 10 is thicker (and carries more current) than AWG 14.
- IECInternational Electrotechnical Commission
- The international body that writes electrical safety standards used across most of the world outside North America. When a chart cites an IEC number, it's pointing to the official rulebook behind a calculation.
- NECNational Electrical Code
- The set of electrical safety rules used in the United States — the American equivalent of IEC standards used elsewhere.
Base Ampacity — Copper Conductors in Conduit
NEC Table 310.15(B)(16). Assumes 30 °C ambient, not more than 3 current-carrying conductors in a raceway.
| AWG / kcmil | Approx. mm² | 60 °C insulation (TW, UF) | 75 °C insulation (THWN, XHHW) | 90 °C insulation (THHN, XHHW-2) |
|---|---|---|---|---|
| 14 AWG | 2.1 | 15 A | 20 A | 25 A |
| 12 AWG | 3.3 | 20 A | 25 A | 30 A |
| 10 AWG | 5.3 | 30 A | 35 A | 40 A |
| 8 AWG | 8.4 | 40 A | 50 A | 55 A |
| 6 AWG | 13.3 | 55 A | 65 A | 75 A |
| 4 AWG | 21.2 | 70 A | 85 A | 95 A |
| 3 AWG | 26.7 | 85 A | 100 A | 110 A |
| 2 AWG | 33.6 | 95 A | 115 A | 130 A |
| 1 AWG | 42.4 | 110 A | 130 A | 150 A |
| 1/0 AWG | 53.5 | 125 A | 150 A | 170 A |
| 2/0 AWG | 67.4 | 145 A | 175 A | 195 A |
| 3/0 AWG | 85.0 | 165 A | 200 A | 225 A |
| 4/0 AWG | 107 | 195 A | 230 A | 260 A |
| 250 kcmil | 127 | 215 A | 255 A | 290 A |
| 300 kcmil | 152 | 240 A | 285 A | 320 A |
| 350 kcmil | 177 | 260 A | 310 A | 350 A |
| 400 kcmil | 203 | 280 A | 335 A | 380 A |
| 500 kcmil | 253 | 320 A | 380 A | 430 A |
| 600 kcmil | 304 | 350 A | 420 A | 475 A |
| 750 kcmil | 380 | 400 A | 475 A | 535 A |
Base Ampacity — Aluminium Conductors in Conduit
NEC Table 310.15(B)(16) — Aluminium / copper-clad aluminium. Same conditions as above.
| AWG / kcmil | 60 °C | 75 °C | 90 °C | Notes |
|---|---|---|---|---|
| 12 AWG | 15 A | 20 A | 25 A | Minimum NEC Al feeder size (generally) |
| 10 AWG | 25 A | 30 A | 35 A | |
| 8 AWG | 30 A | 40 A | 45 A | |
| 6 AWG | 40 A | 50 A | 55 A | |
| 4 AWG | 55 A | 65 A | 75 A | |
| 2 AWG | 75 A | 90 A | 100 A | |
| 1/0 AWG | 100 A | 120 A | 135 A | |
| 3/0 AWG | 130 A | 155 A | 175 A | |
| 4/0 AWG | 150 A | 180 A | 205 A | |
| 350 kcmil | 210 A | 250 A | 280 A | |
| 500 kcmil | 260 A | 310 A | 350 A | Common service entrance size |
Aluminium conductors are approximately 80 % of copper capacity at the same AWG size but are significantly cheaper and lighter at larger sizes. Use Al for feeders ≥ 1/0 AWG — avoid Al for branch circuits and connections to devices not rated for Al.
Temperature Correction Factors
NEC Table 310.15(B)(2)(a). Multiply base ampacity by the correction factor for the actual ambient temperature. Ambient temperature reference = 30 °C.
| Ambient temp. (°C) | 60 °C insulation CF | 75 °C insulation CF | 90 °C insulation CF |
|---|---|---|---|
| ≤ 10 °C | 1.22 | 1.20 | 1.29 |
| 11 – 15 °C | 1.17 | 1.14 | 1.22 |
| 16 – 20 °C | 1.12 | 1.09 | 1.15 |
| 21 – 25 °C | 1.06 | 1.05 | 1.08 |
| 26 – 30 °C | 1.00 | 1.00 | 1.00 |
| 31 – 35 °C | 0.94 | 0.94 | 0.91 |
| 36 – 40 °C | 0.87 | 0.88 | 0.82 |
| 41 – 45 °C | 0.79 | 0.82 | 0.71 |
| 46 – 50 °C | 0.71 | 0.75 | 0.58 |
| 51 – 55 °C | 0.61 | 0.67 | 0.41 |
| 56 – 60 °C | 0.50 | 0.58 | 0.00 — not allowed |
Conduit Fill Derating Factors
NEC 310.15(C)(1). Applies when more than 3 current-carrying conductors are in a single raceway. Count only current-carrying conductors — not equipment grounding conductors or neutral conductors of balanced 3-phase systems.
| Number of current-carrying conductors | Derating factor | Design impact |
|---|---|---|
| 1 – 3 | 1.00 | No derating required |
| 4 – 6 | 0.80 | Upsize conductor by 1–2 AWG or split conduits |
| 7 – 9 | 0.70 | Consider 2 separate conduits |
| 10 – 20 | 0.50 | Significant upsize needed; multiple conduits preferred |
| 21 – 30 | 0.45 | Multiple conduits strongly recommended |
| 31 – 40 | 0.40 | High conductor count — thermal analysis required |
| 41+ | 0.35 | Engineering review required; conductor bunching undesirable |
Worked Example — Derating Calculation
Circuit: 10 AWG copper, THWN (75 °C), in conduit with 6 current-carrying conductors, ambient temperature 40 °C.
Step 1 — Base ampacity: 10 AWG Cu @ 75 °C = 35 A (NEC Table 310.15(B)(16))
Step 2 — Temperature correction: 40 °C ambient, 75 °C insulation → CF = 0.88
Step 3 — Conduit fill factor: 6 conductors → CF = 0.80
Step 4 — Derated ampacity: 35 × 0.88 × 0.80 = 24.6 A
Step 5 — Design load check: 125 % of continuous load must not exceed 24.6 A → maximum continuous load = 24.6 ÷ 1.25 = 19.7 A
Recommended breaker: 20 A (next standard size at or below derated ampacity)
Frequently Asked Questions
Why does the NEC limit terminal ratings to 75 °C even with 90 °C cable?
NEC 110.14(C) requires that connections be made at the lowest temperature rating of any connected component. Most lugs, terminals and breaker connections in panels and devices are rated only to 75 °C. So even if you install 90 °C THHN cable, you must use the 75 °C ampacity column when the terminations are not rated above 75 °C — which is the typical case for circuits ≤ 100 A. The 90 °C column can be used when both the cable and all terminations are rated 90 °C, or when applying derating factors to allow use of a larger base ampacity before derating.
Does the neutral conductor count for fill derating?
Only current-carrying conductors count. A neutral conductor of a 3-phase balanced circuit (where harmonic current is negligible) does not count. However, the neutral of a single-phase 2-wire or 2-phase system is current-carrying and must be counted. For circuits supplying non-linear loads (computer power supplies, VFDs), the neutral can carry significant harmonic current and should be treated as current-carrying per NEC 310.15(E).
When should I use aluminium conductors?
Aluminium is cost-effective for service entrances, feeders and branch feeder panels at 1/0 AWG or larger. It is lighter than copper by about two thirds, which matters for long vertical runs. Never use aluminium for branch circuits to standard devices — twist connectors and outlets are not rated for aluminium and will cause loose connections over time. Use anti-oxidant compound on all aluminium-to-copper terminations and ensure lugs are listed for aluminium.
What does IEC 60364-5-52 say compared to NEC?
IEC 60364-5-52 (Installation Methods B1/B2/C/E/F/G) provides similar tables in mm² for European and international projects. The derating philosophy is the same — ambient temperature correction and grouping (bundling) derating — but the installation method classifications differ. IEC method B2 (cables in conduit) is broadly equivalent to NEC in-conduit installations. IEC grouping factors are in Table B.52.17 and range from 0.80 for 2 circuits to 0.44 for 12 circuits.
Can I combine temperature and fill derating factors?
Yes — the two factors are multiplied together, as shown in the worked example above. This is explicitly required by NEC 310.15(C)(1) and IEC 60364-5-52 Clause 523. The combined derating can be severe: 6 conductors in a conduit at 40 °C ambient gives 0.80 × 0.88 = 0.70 combined factor — meaning you need a conductor with at least 43 % more base ampacity than the load to end up with a code-compliant derated value.