Wire Size Calculator - AWG Electrical Wire Sizing
Copper and aluminum conductor sizes for a 15–400 A breaker or fuse, from the 60 °C column of NEC Table 310.16, with a voltage-drop check.
Quick Reference: Common Wire Sizes
Residential Circuits:
- 15 A: 14 AWG copper
- 20 A: 12 AWG copper
- 30 A: 10 AWG copper
- 50 A: 6 AWG copper
Service/Feeders:
- 60 A Sub: 4 AWG copper
- 100 A Sub: 1 AWG copper
- 200 A Service: 2/0 copper or 4/0 aluminum (dwelling service, Table 310.12)
- Check voltage drop!
Important: These sizes are based on ampacity alone. Check voltage drop, and apply the correction for ambient temperature and the adjustment for more than three current-carrying conductors.
Wire Size Calculator
Calculate the correct AWG wire size for both copper and aluminum conductors
Input Parameters
Calculation Results
Wire Size Comparison for 20 A
Aluminum wire must be larger than copper for the same ampacity
Copper Wire
- Smallest copper size with a 60 °C ampacity of at least 20 A in NEC Table 310.16: 12 AWG (20 A).
- NEC 240.4(D) limits 12 AWG copper to 20 A overcurrent protection; 20 A is within that limit.
- Ground wire for a 20 A breaker or fuse, NEC Table 250.122 (copper column): 12 AWG.
- Voltage drop with 12 AWG copper at 20 A over 50 ft one way, single-phase 240 V (1.934 Ω per 1000 ft at 75 °C): 3.87 V, 1.61 %.
Aluminum Wire
- Smallest aluminum size with a 60 °C ampacity of at least 20 A in NEC Table 310.16: 10 AWG (25 A).
- NEC 240.4(D) limits 10 AWG aluminum to 25 A overcurrent protection; 20 A is within that limit.
- Ground wire for a 20 A breaker or fuse, NEC Table 250.122 (aluminum or copper-clad aluminum column): 10 AWG.
- Voltage drop with 10 AWG aluminum at 20 A over 50 ft one way, single-phase 240 V (2.001 Ω per 1000 ft at 75 °C): 4.00 V, 1.67 %.
Sized from the 60 °C column of NEC Table 310.16 (2023), limited by 240.4(D); ground wire from Table 250.122.
Assumes 30 °C (86 °F) ambient and no more than three current-carrying conductors together. Hotter locations or more conductors need the factors in Tables 310.15(B)(1)(1) and 310.15(C)(1): use the ampacity calculator.
A 120/240 V dwelling service or main power feeder is sized from NEC Table 310.12 instead: see the service entrance calculator.
Complete Guide to Wire Sizing
How Wire Sizing Works
Wire sizing involves selecting the correct conductor size based on several factors: current carrying capacity (ampacity), voltage drop over distance, temperature conditions, and installation method. The National Electrical Code (NEC) provides tables and rules to ensure safe and efficient electrical installations.
Key Factors in Wire Sizing
Ampacity: The maximum current a conductor can carry continuously without exceeding its temperature rating.
NEC Wire Sizing Rules
Table 310.16
Primary ampacity table for conductors rated 0-2000 V in raceway, cable, or earth. Most residential and commercial wiring references this table.
NEC 240.4(D)
Small conductor rule: 14 AWG copper limited to 15 A, 12 AWG copper to 20 A, 10 AWG copper to 30 A overcurrent protection.
NEC Table 310.16 - Wire Ampacity Ratings
| AWG | Copper (Amps) | Aluminum (Amps) | ||||
|---|---|---|---|---|---|---|
| 60 °C | 75 °C | 90 °C | 60 °C | 75 °C | 90 °C | |
| 14 | 15 | 20 | 25 | — | — | — |
| 12 | 20 | 25 | 30 | 15 | 20 | 25 |
| 10 | 30 | 35 | 40 | 25 | 30 | 35 |
| 8 | 40 | 50 | 55 | 35 | 40 | 45 |
| 6 | 55 | 65 | 75 | 40 | 50 | 55 |
| 4 | 70 | 85 | 95 | 55 | 65 | 75 |
| 3 | 85 | 100 | 115 | 65 | 75 | 85 |
| 2 | 95 | 115 | 130 | 75 | 90 | 100 |
| 1 | 110 | 130 | 145 | 85 | 100 | 115 |
| 1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| 2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| 3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| 4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
Ambient Temperature Correction
Ampacity is corrected when the ambient temperature differs from 30 °C (86 °F). Multiply by the factor for the conductor's temperature column.
Temperature Correction Factors
| Ambient Temperature | 60 °C Wire | 75 °C Wire | 90 °C Wire |
|---|---|---|---|
| 10 °C or less (50 °F or less) | 1.29 | 1.20 | 1.15 |
| 11–15 °C (51–59 °F) | 1.22 | 1.15 | 1.12 |
| 16–20 °C (60–68 °F) | 1.15 | 1.11 | 1.08 |
| 21–25 °C (69–77 °F) | 1.08 | 1.05 | 1.04 |
| 26–30 °C (78–86 °F) | 1.00 | 1.00 | 1.00 |
| 31–35 °C (87–95 °F) | 0.91 | 0.94 | 0.96 |
| 36–40 °C (96–104 °F) | 0.82 | 0.88 | 0.91 |
| 41–45 °C (105–113 °F) | 0.71 | 0.82 | 0.87 |
| 46–50 °C (114–122 °F) | 0.58 | 0.75 | 0.82 |
| 51–55 °C (123–131 °F) | 0.41 | 0.67 | 0.76 |
| 56–60 °C (132–140 °F) | — | 0.58 | 0.71 |
| 61–65 °C (141–149 °F) | — | 0.47 | 0.65 |
| 66–70 °C (150–158 °F) | — | 0.33 | 0.58 |
| 71–75 °C (159–167 °F) | — | — | 0.50 |
| 76–80 °C (168–176 °F) | — | — | 0.41 |
| 81–85 °C (177–185 °F) | — | — | 0.29 |
How to Apply Temperature Correction
Multiply the table ampacity by the correction factor.
Common Wire Sizing Mistakes to Avoid
Ignoring voltage drop for long runs
Consequence: Equipment malfunction, dimming lights, motor failure
Solution: Always calculate voltage drop
Mixing copper and aluminum connections
Consequence: Corrosion, high resistance, connection failure
Solution: Use proper connectors rated for both materials
Undersizing neutral in multi-wire circuits
Consequence: Neutral overload with non-linear loads
Solution: Size neutral for maximum unbalanced load
Forgetting derating for bundled conductors
Consequence: Overheating when multiple wires in conduit
Solution: Apply NEC Table 310.15(C)(1) adjustment factors
Special Wire Sizing Considerations
More Than Three Current-Carrying Conductors
With more than three current-carrying conductors together, multiply the ampacity by the adjustment factor in NEC Table 310.15(C)(1).
Professional Installation Tips
Planning Phase
- Calculate both ampacity and voltage drop
- Consider future expansion needs
- Check local code amendments
Best Practices
- Label wire runs clearly at both ends
- Use proper torque specifications
- Document calculations for inspector
Common Issues
- Undersized neutrals in MWBC
- Missing derating in hot attics
- Wrong insulation type for wet areas
- Mixing wire types in same circuit
Wire Sizing Frequently Asked Questions
Q: What size wire for 20 amp circuit?
A: A 20 amp circuit requires 12 AWG copper wire or 10 AWG aluminum wire per NEC Table 310.16.
Q: How do I calculate wire size for distance?
A: For distance calculations: 1) Start with ampacity-based size from NEC 310.16, 2) Calculate voltage drop using VD = (2 × L × I × R) ÷ 1000. Distance often requires larger wire than ampacity alone.
Q: Can I use aluminum wire instead of copper?
A: Yes. Aluminum carries less current than copper of the same size, so it often needs a larger one: for a 30 A breaker, the 60 °C column of NEC Table 310.16 gives 10 AWG copper and 8 AWG aluminum. The calculator shows both.
Q: How many amps can 12 gauge wire handle?
A: 12 AWG copper wire is rated for 25 amps at 75 °C per NEC Table 310.16, but must be protected at 20 amps maximum per NEC 240.4(D).
Q: What size ground wire do I need?
A: Equipment grounding conductor size is based on overcurrent device per NEC Table 250.122: 15 A circuits need 14 AWG, 20 A circuits need 12 AWG, 30-60 A need 10 AWG, 100 A needs 8 AWG, 200 A needs 6 AWG copper ground.
Q: What wire size for 200 amp service entrance?
A: 2/0 AWG copper or 4/0 aluminum (dwelling service, Table 310.12). Verify local requirements.
Q: Can I use 14 gauge wire on a 20 amp circuit?
A: No. NEC 240.4(D) limits 14 AWG copper to 15 A overcurrent protection, even though Table 310.16 lists 20 A for it in the 75 °C column.
Q: What affects wire ampacity ratings?
A: Wire ampacity is affected by: conductor material (copper vs aluminum), insulation temperature rating (60/75/90 °C), ambient temperature (derating required above 86 °F), number of conductors in raceway (derating for heat), and installation method.
Q: What's the difference between stranded and solid wire?
A: Stranded wire is more flexible and resistant to fatigue, ideal for vibration or frequent movement. Solid wire is easier to terminate, and preferred for permanent building wiring. Both have identical ampacity ratings.
Sources
- NFPA 70 (NEC) 2023, Table 310.16
- NFPA 70 (NEC), 240.4(D) — NFPA revision record SR-8377 and 2023 text (Leviton reprint)
- NFPA 70 (NEC) 2023, Table 240.6(A), 240.4(B) and 240.4(D) (NFPA 2023-cycle Second Draft; Eaton and Leviton 2023 reprints)
- NFPA, Using the Tables in Article 250 of the NEC (fact sheet, 2020), Tables 250.66 and 250.122; Table 250.122 also in NFPA 2026 revision record SR-8360
- NIST (NBS) Handbooks 100 and 109; 75 °C and aluminum values calculated
- NIST (NBS) Handbook 100, Copper Wire Tables, Table 5
- NFPA 70 (NEC) 2023, Tables 310.15(B)(1)(1) and 310.15(C)(1)
Retrieved .