How to Size Electrical Conductors According to the NEC
Selecting the correct wire size for residential, commercial, or industrial branch circuits requires balancing two primary engineering criteria: thermal current capacity (ampacity) and permissible voltage drop.
1. Thermal Ampacity (NEC Table 310.16)
Under the National Electrical Code (NEC), conductor ampacity is governed by Table 310.16 (formerly Table 310.15(B)(16)). This rating defines the maximum continuous electrical current a conductor can carry before thermal degradation of its insulation occurs. Standard building wire (such as THHN, THWN-2, or XHHW-2) is typically rated at 90°C, but terminal rating limitations (NEC 110.14(C)) generally require calculations to be based on 75°C limits for equipment rated over 100A or with standard mechanical lugs.
| Conductor Size | Copper Ampacity (75°C) | Aluminum Ampacity (75°C) | Nominal Circular Mils |
|---|---|---|---|
| 14 AWG | 15 A* | — | 4,110 CM |
| 12 AWG | 20 A* | 15 A* | 6,530 CM |
| 10 AWG | 30 A* | 25 A* | 10,380 CM |
| 8 AWG | 50 A | 40 A | 16,510 CM |
| 6 AWG | 65 A | 50 A | 26,240 CM |
| 4 AWG | 85 A | 65 A | 41,740 CM |
| 2 AWG | 115 A | 90 A | 66,360 CM |
| 1/0 AWG | 150 A | 120 A | 105,600 CM |
| 2/0 AWG | 175 A | 135 A | 133,100 CM |
| 4/0 AWG | 230 A | 180 A | 211,600 CM |
*Note: Overcurrent protection limitations apply per NEC 240.4(D) for small conductors.
2. Managing Voltage Drop (NEC Informational Note 210.19(A))
While the National Electrical Code does not strictly mandate a maximum voltage drop in branch circuits as an enforceable code violation, NEC 210.19(A) Informational Note No. 4 strongly recommends that total voltage drop should not exceed 3% on the branch circuit, and 5% across the combined feeder and branch circuit. This ensures reasonable efficiency of operation and protects connected equipment from undervoltage strain.
Required CM = (2 × K × I × L) / VD_target
- K (Resistivity Constant): Represents conductor resistance per mil-foot at operating temperatures. We use standard engineering benchmarks of 12.9 for Copper and 21.2 for Aluminum.
- I (Current): Design circuit load in Amperes.
- L (Length): One-way distance from source to load in feet.
- CM (Circular Mils): Cross-sectional area of the conductor per NEC Chapter 9, Table 8.
Why Long Wire Runs Require Stepping Up Size
Even if an 8 AWG copper conductor satisfies the thermal 50-amp rating for a 240V level 2 EV charger, running that circuit 180 feet will induce a voltage drop exceeding 3.5%. WireSizerPro automatically detects when voltage drop surpasses engineering limits and increments the recommended conductor size to 6 AWG or 4 AWG to preserve circuit performance.