Conductor Sizing & Ampacity
Wire ampacity from Table 310.16, overcurrent protection limits, temperature ratings, derating factors, continuous loads, and parallel conductors.
- Read ampacity values from NEC Table 310.16 for common conductor sizes
- State the overcurrent protection limits for #14, #12, and #10 AWG conductors
- Apply ampacity derating factors for multiple conductors in a raceway
- Calculate temperature correction factors for elevated ambient temperatures
- Explain the 125% continuous load factor and parallel conductor rules
Lección 1
NEC Table 310.16 - Conductor Ampacity & Temperature Ratings
The Most Important Table in the NEC
NEC Table 310.16 is the single most referenced table on the journeyman exam. It provides the allowable ampacity of insulated conductors based on conductor size, insulation type, and temperature rating. Every electrician must be able to look up ampacity values quickly and accurately.
The table is organized by three temperature columns:
| Column | Temperature | Common Insulation Types |
|---|---|---|
| 60 degrees C | 140 degrees F | TW, UF |
| 75 degrees C | 167 degrees F | THW, THWN, XHHW |
| 90 degrees C | 194 degrees F | THHN, THWN-2, XHHW-2 |
Key Ampacity Values to Memorize
These are the most commonly tested values from Table 310.16 for copper conductors:
| Wire Size | 60 degrees C | 75 degrees C | 90 degrees C |
|---|---|---|---|
| #14 AWG | 15A | 20A | 25A |
| #12 AWG | 20A | 25A | 30A |
| #10 AWG | 30A | 35A | 40A |
| #8 AWG | 40A | 50A | 55A |
| #6 AWG | 55A | 65A | 75A |
| #4 AWG | 70A | 85A | 95A |
| #3 AWG | 85A | 100A | 115A |
| #2 AWG | 95A | 115A | 130A |
| #1 AWG | 110A | 130A | 145A |
| 1/0 AWG | 125A | 150A | 170A |
| 2/0 AWG | 145A | 175A | 195A |
Important Distinction
The 90 degrees C column of Table 310.16 shows #12 AWG THHN at 30 amps. However, you cannot protect #12 AWG at 30 amps because NEC 240.4(D) limits it to 20 amps. The 90 degrees C column is used for derating purposes only - the final ampacity after derating cannot exceed what the termination temperature allows.
Per NEC Table 310.16, #12 AWG copper THHN/THWN-2 at 90 degrees C has an ampacity of 30 amps - but this is for derating calculations only. The actual overcurrent protection is limited by NEC 240.4(D) and termination temperature ratings.