Conductor Size
The National Electric Code (NEC) provides the minimum size requirements for conductors to prevent overheating and fire. The code allows the ampacityies for conductors to be determined from the tables or under engineering supervision. This calculator uses the tables only. To use the engineering supervision option please see -LINK to SERVICES PAGE-
A key component of the sizing process is determining the temperature rating of the conductor. The NEC provides three temperature ratings which are defined by the conductor type as seen in the table below:
| Conductor | Temperature | Type |
| Copper | 60°C (140°F) | TW, UF |
| Copper | 75°C (167°F) | RHW, THHW, THW, THWN, XHHW, XHWN, USE, ZW |
| Copper | 90°C (194°F) | TBS, SA, SIS, FEP, FEPB, MI, PFA, RHH, RHW-2, THHN, THHW, THW-2, THWN-2, USE-2, XHH, XHHW, XHHW-2, XHWN, XHWN-2, XHHN, Z, ZW-2 |
| Aluminum or Copper-Clad Aluminum | 60°C (140°F) | TW, UF |
| Aluminum or Copper-Clad Aluminum | 75°C (167°F) | RHW, THHW, THW, THWN, XHHW, XHWN, USE |
| Aluminum or Copper-Clad Aluminum | 90°C (194°F) | TBS, SA, SIS, THHN, THHW, THW-2, THWN-2, RHH, RHW-2, USE-2, XHH, XHHW, XHHW-2, XHWN, XHWN-2, XHHN |
The calculation is based on a maximum of three current-carrying conductors in raceway, cable or earth (directly buried). If more than three conductors are used than the calculation must be derated.
The National Electric Code (NEC) specifies the minimum size for cable tray systems which includes ladder, ventilated trough, ventilated channel, solid bottom and other similar structures. It’s important to note that cable trays are mechanical support systems and not raceways. Raceways are included in another section of the code.
Proper cable tray sizing is complicated and often existing cable trays are overloaded per the code. If you have questions or would like some engineering assistance to properly size your cable tray installation please contact us.