Quick Answer: Use this page as the NEC Table 310.16 reference for copper and aluminum base ampacity rows and temperature columns. Match the conductor material, insulation, and terminal rating first; then move to the Wire Size Calculator for load-to-conductor sizing or to the Ampacity Calculator when ambient temperature, conductor count, or terminal limits must be combined into one answer.
NFPA 70, the National Electrical Code, is the authoritative U.S. electrical installation document. Use the NEC edition adopted by the AHJ for the project, then verify equipment markings and manufacturer instructions before final conductor selection. Reviewed 2026-08-04.
How to use Table 310.16 without misapplying it
Table 310.16 is the base-ampacity reference. It is not the whole conductor-sizing answer. Use the table in this order before treating a row as a final project value:
- Define the load path: branch circuit, feeder, service, motor, HVAC, EVSE, listed equipment, or dwelling service.
- Choose conductor material, insulation type, and the terminal temperature rating that governs the installation.
- Record ambient temperature, current-carrying conductor count, raceway/bundling conditions, and whether voltage drop matters for the run.
- Use the Wire Size Calculator for load-to-conductor sizing or the Ampacity Calculator when derating factors must be combined.
- Return to Table 310.16 only after the calculator path identifies which conductor material and temperature column control.
| Search task | Inputs to enter first | Calculator handoff |
|---|---|---|
| Wire size for a known load | load current, continuous-duty status, material, terminal rating, distance | Wire Size Calculator |
| Ampacity after derating | base conductor, insulation rating, ambient temperature, conductor count, terminal limit | Ampacity Calculator |
| Breaker and conductor coordination | calculated load, conductor ampacity, standard OCPD size, small-conductor rules | Breaker Sizing Calculator |
| Long feeder or branch-circuit run | current, voltage, phase, conductor material, length, voltage-drop target | Voltage Drop Calculator |
NEC Table 310.16 Reference After the Calculator
Copper Conductors - Insulated Conductors (0-2000V)
Based on ambient temperature of 30°C (86°F), not more than 3 current-carrying conductors in raceway:
| AWG/kcmil | 60°C (TW, UF) | 75°C (THW, THWN) | 90°C (THHN, XHHW) |
|---|---|---|---|
| 18 | — | — | 14A |
| 16 | — | — | 18A |
| 14 | 15A | 20A | 25A |
| 12 | 20A | 25A | 30A |
| 10 | 30A | 35A | 40A |
| 8 | 40A | 50A | 55A |
| 6 | 55A | 65A | 75A |
| 4 | 70A | 85A | 95A |
| 3 | 85A | 100A | 115A |
| 2 | 95A | 115A | 130A |
| 1 | 110A | 130A | 145A |
| 1/0 | 125A | 150A | 170A |
| 2/0 | 145A | 175A | 195A |
| 3/0 | 165A | 200A | 225A |
| 4/0 | 195A | 230A | 260A |
| 250 kcmil | 215A | 255A | 290A |
| 300 kcmil | 240A | 285A | 320A |
| 350 kcmil | 260A | 310A | 350A |
| 400 kcmil | 280A | 335A | 380A |
| 500 kcmil | 320A | 380A | 430A |
Aluminum Conductors - Insulated Conductors (0-2000V)
| AWG/kcmil | 60°C (TW, UF) | 75°C (THW, THWN) | 90°C (THHN, XHHW) |
|---|---|---|---|
| 12 | 15A | 20A | 25A |
| 10 | 25A | 30A | 35A |
| 8 | 35A | 40A | 45A |
| 6 | 40A | 50A | 55A |
| 4 | 55A | 65A | 75A |
| 3 | 65A | 75A | 85A |
| 2 | 75A | 90A | 100A |
| 1 | 85A | 100A | 115A |
| 1/0 | 100A | 120A | 135A |
| 2/0 | 115A | 135A | 150A |
| 3/0 | 130A | 155A | 175A |
| 4/0 | 150A | 180A | 205A |
| 250 kcmil | 170A | 205A | 230A |
| 300 kcmil | 190A | 230A | 260A |
| 350 kcmil | 210A | 250A | 280A |
| 400 kcmil | 225A | 270A | 305A |
| 500 kcmil | 260A | 310A | 350A |
Understanding Temperature Ratings
Why Temperature Columns Matter
| Column | Insulation Types | When to Use |
|---|---|---|
| 60°C | TW, UF | Residential terminals (most common) |
| 75°C | THW, THWN, XHHW | Commercial, industrial |
| 90°C | THHN, XHHW-2 | Derating calculations only |
Critical Rule: Use the LOWER of conductor insulation rating OR terminal rating.
Most residential equipment has 60°C terminals → Use 60°C column even with 90°C wire.
Terminal Temperature Limitations
| Equipment | Typical Terminal Rating |
|---|---|
| Residential panels | 60°C or 75°C |
| Residential breakers | 60°C or 75°C |
| Commercial equipment | 75°C |
| Industrial equipment | 75°C or 90°C |
Wire Size for Specific Amperages
Quick Lookup: What Size Wire for... (Copper, 60°C)
| Amperage Needed | Minimum Wire Size | Notes |
|---|---|---|
| 15A | 14 AWG | Lighting circuits |
| 20A | 12 AWG | Receptacles |
| 30A | 10 AWG | Dryers, small A/C |
| 40A | 8 AWG | Ranges, large A/C |
| 50A | 6 AWG | Ranges, EV chargers |
| 60A | 4 AWG (55A) or 6 AWG 75°C | Subpanels |
| 70A | 4 AWG | Service |
| 100A | 2 AWG (95A) or 1 AWG | 100A service |
| 125A | 1/0 AWG | 125A service |
| 150A | 2/0 AWG (145A) or 3/0 AWG | 150A service |
| 200A | 4/0 AWG (195A) or 250 kcmil | 200A service |
Quick Lookup: Aluminum Wire Sizes (75°C)
| Amperage | Aluminum Size | Copper Equivalent |
|---|---|---|
| 100A | 1/0 AWG | 2 AWG |
| 125A | 2/0 AWG (135A) | 1/0 AWG |
| 150A | 3/0 AWG (155A) | 2/0 AWG |
| 200A | 4/0 AWG (180A) | 4/0 AWG |
Residential Service Entrance Sizing
NEC 310.12 Service Conductor Sizing
| Service Size | Copper (60°C) | Copper (75°C) | Aluminum (75°C) |
|---|---|---|---|
| 100A | 2 AWG | 3 AWG | 1/0 AWG |
| 125A | 1/0 AWG | 1 AWG | 2/0 AWG |
| 150A | 3/0 AWG | 2/0 AWG | 3/0 AWG |
| 200A | 4/0 AWG | 4/0 AWG | 4/0 AWG |
| 225A | 250 kcmil | 250 kcmil | 250 kcmil |
| 400A | — | 500 kcmil | 500 kcmil |
Note: Service conductors may use 83% rule per NEC 310.12 for dwelling units.
Parallel Conductors
NEC 310.10(G) Parallel Conductor Requirements
When running conductors in parallel (1/0 AWG or larger):
| Requirement | Specification |
|---|---|
| Minimum size | 1/0 AWG |
| Same length | Required |
| Same material | Required (all copper or all aluminum) |
| Same circular mil area | Required |
| Same insulation type | Required |
| Same termination method | Required |
Parallel Ampacity Calculation:
Total Ampacity = Single Conductor Ampacity × Number of Conductors
Example: Two 4/0 AWG copper THHN in parallel:
Total = 260A × 2 = 520A
Common Insulation Types
Insulation Type Reference
| Type | Temperature | Description | Common Use |
|---|---|---|---|
| TW | 60°C | Thermoplastic, wet | General purpose, wet locations |
| THW | 75°C | Thermoplastic, heat-resistant, wet | General purpose |
| THWN | 75°C | THW with nylon jacket | Conduit, raceways |
| THHN | 90°C | Thermoplastic, high-heat, nylon | Dry locations |
| XHHW | 75°C wet, 90°C dry | Cross-linked polyethylene | Wet/dry locations |
| XHHW-2 | 90°C wet/dry | Enhanced XHHW | Industrial |
| USE | 75°C | Underground service entrance | Direct burial |
| UF | 60°C | Underground feeder | Direct burial |
Calculator Checks Instead of Static Answers
Use these scenarios as input prompts for the calculators rather than as standalone answers. The correct conductor depends on the adopted code path, terminal markings, insulation type, ambient temperature, conductor count, and voltage-drop needs.
Branch-circuit conductor check
Before using the table, collect:
- load current and continuous-duty status
- conductor material and insulation type
- equipment terminal temperature rating
- ambient temperature and conductor-count adjustment
- voltage-drop run length, if the run is long
Enter those values in the Wire Size Calculator, then compare the resulting conductor path with the Table 310.16 reference below.
Feeder or subpanel conductor check
Before selecting a feeder conductor, collect:
- feeder calculated load
- service voltage and phase
- copper or aluminum basis
- terminal rating at both ends
- raceway/bundling and ambient conditions
- voltage-drop target for the route
Use the Ampacity Calculator when derating controls, and use the Voltage Drop Calculator when the distance controls.
Dwelling service conductor check
A dwelling service question needs the adopted NEC edition, utility/AHJ requirements, service size basis, conductor material, terminal markings, and any local worksheet requirement. Use the residential service and wire-size calculators first; use Table 310.16 only after the applicable dwelling-service rule is known.
Hot attic or derated conductor check
For high ambient temperature, rooftop exposure, or multiple current-carrying conductors, do not copy a base table value. Enter the insulation rating, base conductor, ambient temperature, conductor count, terminal rating, and rooftop/bundling condition in the Ampacity Calculator, then compare the adjusted result with the terminal limit.
Copper vs Aluminum Comparison
When to Use Each Material
| Factor | Copper | Aluminum |
|---|---|---|
| Ampacity | Higher per size | ~78% of copper |
| Cost | Higher | Lower (larger size offsets) |
| Weight | Heavier | Lighter |
| Size for same ampacity | Smaller | 1-2 sizes larger |
| Terminations | Standard | Requires AL-rated devices |
| Common use | Branch circuits | Service entrance, feeders |
Copper to Aluminum Equivalents
| Copper Size | Copper 75°C | Aluminum Equivalent | Aluminum 75°C |
|---|---|---|---|
| 4 AWG | 85A | 2 AWG | 90A |
| 2 AWG | 115A | 1/0 AWG | 120A |
| 1/0 AWG | 150A | 3/0 AWG | 155A |
| 4/0 AWG | 230A | 250 kcmil | 205A |
Common Mistakes to Avoid
| Mistake | Why It's Wrong | Correct Approach |
|---|---|---|
| Using 90°C ampacity at 60°C terminals | Exceeds terminal rating | Use terminal rating column |
| Ignoring aluminum requirements | Improper termination | Use AL-rated devices, anti-oxidant |
| Not checking terminal temperature | May overload connections | Verify breaker/equipment rating |
| Mixing copper and aluminum | Galvanic corrosion | Use proper bimetallic connectors |
Related Calculators
| Calculator | Use When... |
|---|---|
| Wire Size Calculator | Selecting conductor for load |
| Voltage Drop Calculator | Long distance runs |
| Conduit Fill Calculator | Determining conduit size |
| Breaker Sizing Calculator | Matching wire to protection |
Summary
Key Rules:
- Use 60°C column for most residential (terminal limitation)
- Small-conductor limits still need the applicable NEC path before a table row becomes a final breaker or conductor answer
- Aluminum is ~78% of copper ampacity (use 1-2 sizes larger)
- Check terminal rating - limits usable ampacity
- Service conductors may use 83% rule (NEC 310.12)
FAQ
What is wire ampacity?
Ampacity is the maximum current a conductor can carry continuously without exceeding its temperature rating. It's determined by conductor size, insulation type, installation conditions, and ambient temperature.
Why are there different temperature columns?
Different insulation types have different maximum operating temperatures. You must use the ampacity for the weakest point in the circuit - usually the terminal/connection temperature rating, not the wire's insulation rating.
Can I use aluminum wire for branch circuits?
Aluminum is generally not recommended for 15A and 20A branch circuits due to historical connection issues. It's commonly used for service entrance conductors and large feeders when properly terminated.
What size wire for 200 amp service?
Enter the service size, dwelling-service basis, conductor material, terminal markings, ambient conditions, and local code path in the calculator workflow. Then compare the selected path with the adopted NEC edition, utility requirements, and AHJ worksheet.
How do I account for ambient temperature?
Apply NEC temperature correction factors to the base ampacity selected from the correct insulation column, then compare the adjusted value with the governing terminal limit. Use the ampacity calculator when temperature and conductor-count adjustments both apply.
What size wire for 30 amps?
Use the wire-size calculator with load current, duty cycle, conductor material, terminal rating, ambient temperature, conductor count, and run length. After the calculator identifies the governing path, use Table 310.16 as the reference for the base ampacity column.
What size wire for 50 amps?
Enter the equipment type, continuous-load status, terminal rating, conductor material, installation conditions, and voltage-drop needs before selecting a conductor. Then verify the Table 310.16 row, OCPD coordination, and equipment listing together.