Wire Size Calculator
Calculate proper wire gauge for electrical circuits based on ampacity, voltage drop, and NEC requirements.
Circuit Information
Installation Details
Recommended Wire Size
Copper THHN
Circuit Current
Calculated load current
Wire Ampacity
Before derating
Voltage Drop
0.0V drop
Power Loss
100.0% efficient
Circuit Analysis
How it works
A wire size calculator picks the right conductor gauge for an electrical load. The wire must carry the current (ampacity) without overheating, and be thick enough that voltage drop over the run stays within limits — both push toward a thicker (lower-gauge) wire as amps and distance rise.
Gauge selection
Wire must satisfy: ampacity ≥ load current AND voltage drop ≤ ~3%
- ampacity
- max current a gauge can safely carry
- distance
- run length (longer needs thicker wire)
Worked example
- 15 A circuit
- Short run
- 14 AWG copper handles 15 A
- Check voltage drop is within 3%
14 AWG suffices for 15 A; a 20 A circuit needs 12 AWG.
Good to know
- Lower gauge number = thicker wire = more current capacity (12 AWG is thicker than 14 AWG).
- Long runs need an upsized wire to keep voltage drop under ~3%.
- Always follow the local electrical code and match the breaker to the wire's ampacity.
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Frequently Asked Questions
How do I calculate the correct wire size for my circuit?
Wire size depends on: 1) Current load (amps), 2) Circuit length (voltage drop), 3) Ambient temperature, 4) Number of conductors. Use NEC Table 310.15(B)(16) for ampacity. For runs over 100 feet, check voltage drop (should be < 3% for branch circuits, < 5% total).
What is AWG and how does wire sizing work?
AWG (American Wire Gauge) measures wire diameter - smaller numbers mean thicker wire. Common sizes: 14 AWG (15A circuits), 12 AWG (20A), 10 AWG (30A), 8 AWG (40A), 6 AWG (55A). Each gauge down (larger number) reduces area by ~20% and increases resistance.
What's the 80% rule for circuit breakers?
Continuous loads (3+ hours) must not exceed 80% of breaker rating per NEC 210.20(A). Example: 20A breaker × 0.8 = 16A max continuous load. This prevents overheating. Non-continuous loads can use 100% of breaker rating.
How does temperature affect wire ampacity?
NEC ampacity ratings assume 30°C (86°F) ambient. Higher temperatures require derating: 31-35°C use 91%, 36-40°C use 82%, 41-45°C use 71%. Example: 12 AWG rated 20A at 30°C only handles 16.4A at 40°C (20A × 0.82).
When do I need to upsize wire for voltage drop?
Long runs cause voltage drop. Calculate: VD = 2 × Length × Current × Resistance / 1000. Keep under 3% for branch circuits. Example: 100ft of 12 AWG at 20A = 4.8V drop on 120V (4%), requiring 10 AWG wire instead.
What wire types are best for different applications?
THHN/THWN: Most common building wire, dry/wet locations. NM-B (Romex): Residential branch circuits. UF-B: Direct burial. XHHW: Industrial, higher temperature. MC: Commercial armored cable. Always match wire type to application and local codes.