Extension Cord Codes – What Those Markings Really Mean
The Jackets of extension cords are covered with imprints or embossing indicating how and where they can appropriately be used.
It is essential to choose the correct cable jacket if you plan to use the extension cord in a demanding environment.
Here’s the key to some of that code as outlined in the National Electrical Code.
Extension Cord Key
| S | Hard Service Flexible Cord |
| SJ | Junior Hard Service Flexible Cord |
| E | Thermoplastic Elastomer Insulation |
| T | Thermoplastic Insulation |
| O | Jacket is Oil Resistant |
| OO | Jacket & Conductors are Oil Resistant |
| R | Thermoplastic Rubber Insulation |
| X | Cross-linked Synthetic Polymer Insulation |
| HH | High Temperature |
| W | Moisture Resistant |
| N | Nylon Jacket |