Wire Sizing for Amps — AWG to Ampacity Chart Reference
Wire sizing for amps follows a one-line lookup in NEC Table 310.16 — pick the smallest AWG whose 75 °C column ampacity meets or exceeds the load (after the 1.25× continuous-load factor). This page tabulates copper and aluminum ampacity from 14 AWG to 4/0, lists the breaker-to-wire-size ladder, covers the small-conductor rule (NEC §240.4(D)), and walks through a worked example for a 100 A residential service. Reviewed by a licensed PE.
Wire ampacity calculator
The wire-size calculator picks the smallest NEC-listed conductor that satisfies both ampacity (NEC 310.16) and voltage-drop limits in one pass. Enter voltage, load current, run length, conductor material, ambient temperature, and number of CCCs in the same conduit; the tool returns the right copper or aluminum AWG along with the IEC mm² equivalent for international projects.
→ Wire size calculator · → Amp to wire size chart · → Voltage drop calculator
Wire ampacity formulas
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- I_cont = continuous load (3+ hr at full value)
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- I_non-cont = non-continuous load
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- NEC §210.20(A) for branch circuits, §215.3 for feeders
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- I_table = NEC Table 310.16 base ampacity (75 °C column typical)
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- k_T = ambient-temperature correction factor
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- k_fill = conduit-fill factor for > 3 CCCs (1.0, 0.80, 0.70, 0.50…)
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- 14 AWG Cu — max 15 A breaker regardless of ampacity column
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- 12 AWG Cu — max 20 A; 12 AWG Al — max 15 A
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- 10 AWG Cu — max 30 A; 10 AWG Al — max 25 A
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- L = one-way length (m)
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- I = load current (A)
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- R_ac = AC resistance per km (Ω/km, NEC Chapter 9 Table 9)
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- Limit: V_d / V_source ≤ 3 % branch / 5 % combined
Standards governing wire ampacity
| Document | Scope |
|---|---|
| NFPA 70 (NEC) Article 310 | Conductor ampacity tables 310.15, 310.16, 310.17, 310.20 |
| NEC §240.4 / §240.4(D) | Conductor protection — small-conductor rule |
| NEC §210.20 / §215.3 | Continuous-load factor 1.25× for branch and feeder |
| UL 83 / UL 854 | THHN/THWN and SE service-entrance cable |
| UL 44 | Thermoset-insulated wire (RHH/RHW, XHHW-2) |
| IEEE Std 141 (Red Book) | Industrial cable selection methodology |
| IEC 60364-5-52 | International cable selection — current-carrying capacity |
| AS/NZS 3008.1.1 | Australian / New Zealand cable selection tables |
Reference: copper wire ampacity (NEC Table 310.16, 75 °C)
| AWG | 60 °C (A) | 75 °C (A) | 90 °C (A) | NEC §240.4(D) max breaker |
|---|---|---|---|---|
| 14 | 15 | 20 | 25 | 15 A |
| 12 | 20 | 25 | 30 | 20 A |
| 10 | 30 | 35 | 40 | 30 A |
| 8 | 40 | 50 | 55 | — |
| 6 | 55 | 65 | 75 | — |
| 4 | 70 | 85 | 95 | — |
| 3 | 85 | 100 | 110 | — |
| 2 | 95 | 115 | 130 | — |
| 1 | 110 | 130 | 150 | — |
| 1/0 | 125 | 150 | 170 | — |
| 2/0 | 145 | 175 | 195 | — |
| 3/0 | 165 | 200 | 225 | — |
| 4/0 | 195 | 230 | 260 | — |
| 250 kcmil | 215 | 255 | 290 | — |
| 500 kcmil | 320 | 380 | 430 | — |
Reference: aluminum wire ampacity (NEC Table 310.16, 75 °C)
| AWG | 60 °C (A) | 75 °C (A) | 90 °C (A) | NEC §240.4(D) max breaker |
|---|---|---|---|---|
| 12 | 15 | 20 | 25 | 15 A |
| 10 | 25 | 30 | 35 | 25 A |
| 8 | 30 | 40 | 45 | — |
| 6 | 40 | 50 | 60 | — |
| 4 | 55 | 65 | 75 | — |
| 3 | 65 | 75 | 85 | — |
| 2 | 75 | 90 | 100 | — |
| 1 | 85 | 100 | 115 | — |
| 1/0 | 100 | 120 | 135 | — |
| 2/0 | 115 | 135 | 150 | — |
| 3/0 | 130 | 155 | 175 | — |
| 4/0 | 150 | 180 | 205 | — |
| 250 kcmil | 170 | 205 | 230 | — |
| 500 kcmil | 260 | 310 | 350 | — |
- Compute the load amperes For lighting and outlets, sum the maximum simultaneous load. For motors, use NEC Table 430.250 (3-phase) or 430.248 (1-phase) FLA. For HVAC, use the nameplate MCA per NEC §440.4. For continuous loads (3+ hr at full value), multiply by 1.25.
- Pick the conductor material and temperature column Copper is more conductive per cross-section; aluminum is cheaper for large sizes (≥ 4 AWG service entrances). Read NEC Table 310.16 in the 75 °C column unless the entire termination chain (lugs, breakers, splices) is rated 90 °C. The 60 °C column applies only to small conductors and old equipment.
- Find the smallest AWG meeting the ampacity demand For 50 A copper THWN-2 in raceway: read down the 75 °C column to find 50 A — that's 8 AWG. For aluminum at the same load: 6 AWG (50 A at 75 °C). For free air or single conductors in tray, read NEC Table 310.17 instead of 310.16.
- Apply ambient and conduit-fill corrections Ambient > 30 °C → multiply by Table 310.15(B)(1) factor. More than 3 current-carrying conductors in raceway → multiply by Table 310.15(C)(1) factor. The corrected ampacity must still meet or exceed the load demand.
- Verify voltage drop NEC Fine Print Note recommends voltage drop ≤ 3 % on a branch and ≤ 5 % combined. Use the voltage-drop calculator with conductor R from NEC Chapter 9 Table 9 or the simple R = ρL/A formula. Long runs often dictate up-sizing beyond the ampacity-driven choice.
Worked example — 100 A residential service entrance
A 100 A overhead service to a single-family home. Pick the conductor.
- Required ampacity: 100 A continuous; per NEC §215.3, 1.25 × 100 = 125 A.
- Aluminum SE cable, 75 °C: 1/0 AWG = 120 A — too small. Use 2/0 = 135 A. ✓
- Copper alternative: 3 AWG = 100 A (just meets 100 A demand without the 1.25× factor) — acceptable for service-entrance because NEC §310.12 allows the unadjusted ampacity for residential services up to 400 A.
- Final selection: 2 AWG aluminum SE-U cable (most common stocked for 100 A residential).
Comparison — copper vs. aluminum wire sizing
| Aspect | Copper | Aluminum |
|---|---|---|
| Conductivity | 5.96 × 10⁷ S/m | 3.77 × 10⁷ S/m |
| Per-A cross-section | 1.0× (baseline) | ~1.6× (one AWG larger) |
| Cost per ampere | 2–3× aluminum | Lowest |
| Weight per ampere | Heavier | ~50 % of copper for same A |
| Termination concerns | None special | Anti-oxidant paste, listed lugs (CO/ALR) |
| Best for | Branch circuits, motor leads, control wiring | Service-entrance, sub-panel feeders, large feeders > 4 AWG |
Variants and related queries
Gauge wire amp — common breaker / wire pairings
The dominant breaker-to-wire pairings: 15 A → 14 AWG Cu; 20 A → 12 AWG Cu; 30 A → 10 AWG Cu; 40 A → 8 AWG Cu; 50 A → 8 AWG Cu (90 °C col., 60-A range OK with 6 AWG); 60 A → 6 AWG Cu; 100 A → 3 AWG Cu / 1 AWG Al; 200 A → 3/0 Cu / 250 kcmil Al. The site amp-to-wire chart gives the full ladder.
Wire amp rating chart
Industry-standard wire amp rating charts come from NEC 310.16 (US), IEC 60364-5-52 (international), and AS/NZS 3008 (Australia/NZ). Each table differentiates by conductor material (Cu / Al), insulation temperature rating (60 / 75 / 90 °C in US; 70 / 90 °C in IEC), and installation method (raceway / direct burial / free air). Always cite which table the rating comes from in the design package.
What gauge wire for 80 amp
For an 80 A circuit, 4 AWG copper (85 A at 75 °C) or 2 AWG aluminum (90 A at 75 °C). At the standard 1.25× continuous-load factor, the required ampacity is 100 A, which would push the copper choice up to 3 AWG (100 A); for non-continuous loads (such as cooking ranges), 4 AWG suffices.
Wire ampacity calculator and 4 wire ampacity
"4 wire ampacity" usually refers to the NEC Table 310.15(C)(1) adjustment for > 3 CCCs in conduit: 4 conductors → 0.80 factor (20 % derate); 7–9 → 0.70; 10–20 → 0.50. The neutral counts toward CCC count if it carries unbalanced current (e.g. on 4-wire wye lighting circuits with substantial harmonic content). Equipment grounding conductors do NOT count.
Frequently asked questions
- What size wire for a 100 amp service?
- For a 100 A residential service: 3 AWG copper or 1 AWG aluminum (both rated 100 A at 75 °C in NEC Table 310.16). For aluminum service-entrance cable (SE-U or SE-R), 1 AWG is the standard. For underground 100 A feeder (USE-2 / SER), use 2/0 AWG aluminum or 1/0 aluminum if the temperature column allows. Most utility supply tables list 1/0 aluminum for 100 A overhead and 2/0 for underground.
- What gauge wire for 100 amp sub-panel?
- Same as above — 3 AWG copper or 1 AWG aluminum for the feeder between the main panel and the sub-panel. Add 0.5 in voltage-drop margin per 50 ft of run if voltage drop matters for connected appliances. Most residential 100 A sub-panel installs in detached buildings use 4-conductor SER 1/0-1/0-1/0-#2 aluminum (3 phases + neutral, with separate equipment grounding conductor in PVC conduit).
- What size aluminum wire for a 100 amp service?
- 1 AWG aluminum or 1/0 aluminum, depending on the temperature rating of the conductor and terminations. NEC Table 310.16 75 °C column: 1 AWG aluminum is rated 100 A; 1/0 aluminum is rated 120 A. Most utilities and service-entrance suppliers stock 1/0 aluminum SE cable for 100 A residential services because it has a small built-in margin.
- What size wire for a 30 amp circuit?
- 10 AWG copper (rated 30 A at 60/75 °C in NEC Table 310.16) is the standard choice for a 30 A circuit. NEC §240.4(D) "small conductor rule" caps 10 AWG at 30 A regardless of temperature column. For 30 A aluminum: 8 AWG. Common applications include electric dryers (NEMA 14-30), tankless water heaters, and small EV chargers.
- What gauge wire for a 20 amp circuit?
- 12 AWG copper for 20 A (rated 25 A at 75 °C, capped at 20 A by NEC §240.4(D) small-conductor rule). 10 AWG aluminum for 20 A in aluminum-only installations. Used for kitchen and bathroom outlet branches, garage circuits, and any general-purpose branch where appliances may approach the breaker limit.
- What size wire for a 60 amp breaker?
- 6 AWG copper (rated 65 A at 75 °C in NEC Table 310.16) or 4 AWG aluminum (75 A). The 65 A copper ampacity exceeds the 60 A breaker rating — required by NEC §240.4. Common uses: 50 A EV charger sub-panel feeders, hot tub sub-panels, mid-size A/C condensers, range / dryer combinations.
Historic source — the AWG system
The AWG ladder uses a geometric progression — each step decreases the diameter by a constant factor (≈ 1.123) — so going six AWG sizes apart halves the cross-sectional area and doubles the resistance. This is why every electrician memorises the 6-AWG / 2× rule of thumb.
Related calculators and references
Sources and further reading
- NFPA 70 — NEC, Article 310, §240.4, §210.20, §215.3, §310.12 (2023 edition).
- UL 83 — Thermoplastic-Insulated Wires and Cables.
- UL 854 — Service-Entrance Cables.
- IEEE Std 141 — Recommended Practice for Electric Power Distribution for Industrial Plants (Red Book).
- IEC 60364-5-52 — current-carrying capacity of cables.
- AS/NZS 3008.1.1 — Australian / New Zealand cable selection tables.