How to Size Electrical Wire Properly: NEC Ampacity & Voltage Drop
Complete practical guide to sizing copper and aluminum electrical wire using ampacity tables, temperature correction, conduit fill derating, and voltage drop formulas.
Step-by-Step Procedure
Determine the Continuous and Non-Continuous Load
Calculate total circuit load. If loads run for 3 hours or longer (continuous), multiply them by 125% per standard electrical codes.
Select Conductor Material & Insulation Type
Select Copper or Aluminum and identify insulation temperature rating (60°C, 75°C, or 90°C, e.g. THHN/THWN-2).
Apply Thermal Derating Factors
Derate standard table ampacity for ambient temperature (above 30°C) and conduit fill (more than 3 current-carrying conductors).
Verify Voltage Drop Over Length
Calculate the circuit voltage drop over the total distance. If drop exceeds 3%, upsize the conductor gauge until drop is within limits.
Match Conductor with Appropriate Circuit Breaker
Ensure the overcurrent protective device (breaker or fuse) does not exceed the conductor's rated ampacity (or NEC small conductor limits: 14 AWG max 15A, 12 AWG max 20A, 10 AWG max 30A for copper).
Worked Practical Example
50-Amp Electric Vehicle (EV) Charger Circuit
A 40A continuous Level 2 EV charging station installed 80 feet from the main breaker panel in conduit.
- •Continuous load: 40 A × 1.25 = 50 A minimum required ampacity
- •Conductor selection: 6 AWG THHN Copper (rated 65A @ 75°C terminals)
- •Overcurrent protection: 50A double-pole circuit breaker
- •Voltage Drop Check (240V, 40A, 80 ft, 6 AWG copper):
- •R_loop = 2 × (80 ft × 0.3951 Ω/1000ft) = 0.0632 Ω
- •ΔV = 40 A × 0.0632 Ω = 2.53 V (1.05% drop, well under the 3% limit)
Common Mistakes to Avoid
- ✕Using the 90°C column of NEC Table 310.16 for final breaker sizing when circuit breaker terminals are only rated for 75°C.
- ✕Overlooking voltage drop on long conduit runs despite meeting basic thermal ampacity.
Frequently Asked Questions
Can I use Aluminum wire for household branch circuits?
Modern electrical codes strictly prohibit aluminum wire for small 15A and 20A branch circuits due to thermal expansion and creep hazards. Aluminum is allowed for large feeders and service entrance cables when rated AA-8000 alloy and installed with listed terminations.