Wiring Methods & Conductor Selection
Conductor temperature ratings, aluminum conductors, insulation types, ambient derating, conduit fill, burial depth, and expansion fittings.
- Select conductors based on temperature ratings and terminal limitations
- Explain aluminum conductor considerations for EVSE feeder circuits
- Apply ambient temperature derating factors to conductor ampacity
- Determine burial depth requirements for underground EVSE circuits
- Describe expansion fitting requirements for outdoor conduit installations
Lección 1
Conductor Temperature Ratings & Ampacity
Temperature Rating Basics
Every conductor has a temperature rating that indicates the maximum continuous operating temperature of its insulation. Common ratings include 60 degrees C, 75 degrees C, and 90 degrees C. Higher temperature ratings allow higher ampacity for a given wire size because the insulation can safely operate at higher temperatures.
For EVSE branch circuits, the 75 degrees C ampacity column from NEC Table 310.16 is generally used. This is because most equipment terminals (at the panel and at the EVSE) are rated for 75 degrees C. Even if the conductor insulation is rated for 90 degrees C, the ampacity must be based on the lowest-rated component in the circuit - typically the terminal.
Common Conductor Types
THHN/THWN-2: 90 degrees C dry, 75 degrees C wet
XHHW-2: 90 degrees C dry and wet
NM-B (Romex): 90 degrees C but limited by NEC 334.80
USE-2: Underground service entrance, 90 degrees C
Terminal Limitations
Most residential terminals: 75 degrees C rated
Ampacity column: Use 75 degrees C from Table 310.16
Exception: All 90 degrees C terminals allow 90 degrees C column
Always check: Equipment listing for terminal ratings
Ampacity Table Reference
| AWG (Copper, 75 degrees C) | Ampacity |
|---|---|
| 10 AWG | 35A |
| 8 AWG | 50A |
| 6 AWG | 65A |
| 4 AWG | 85A |
| 3 AWG | 100A |
| 2 AWG | 115A |
For a 40A continuous load (50A breaker), the minimum conductor is 8 AWG copper at 75 degrees C (rated for 50A).
Use the 75 degrees C ampacity column for EVSE branch circuit conductor sizing unless all terminals are rated for a higher temperature. The conductor temperature rating must match or exceed the terminal rating. For a 50A EVSE circuit, the minimum conductor is 8 AWG copper at 75 degrees C.