What is CMG cable?
CMG is a U.S. National Electrical Code (NEC) fire rating for general-purpose communications cable. CMG and CM are the same tier, which is why lessons and spec sheets often write it as CM/CMG. You'll find the marking on the jackets of ordinary Cat5e and Cat6 cable and on many patch cables, usually with a PVC jacket.
General-purpose cable passes a basic vertical-tray flame test. That makes it suitable for horizontal runs on one floor in spaces that aren't used for air handling, such as through offices and in wiring closets.
Where CMG can and can't go
- Allowed: horizontal runs on a single floor in ordinary spaces, and patch cords in rooms and racks.
- Not for risers: vertical runs between floors need riser-rated (CMR) cable or better.
- Not for plenums: drop ceilings or raised floors used for return air need plenum-rated (CMP) cable.
Codes have some exceptions and local rules can be stricter, so check the building's requirements before choosing a lower rating.
CMG among the cable ratings
| Rating | Use | Can substitute for |
|---|---|---|
| CMP | Plenum (air-handling) spaces | CMR, CM/CMG, CMX |
| CMR | Risers between floors | CM/CMG, CMX |
| CM / CMG | General-purpose horizontal runs | CMX |
| CMX | Limited residential use | None |
General-purpose cable costs less than riser or plenum cable, which is why it's tempting to use it everywhere. Using it in a plenum or riser is a code violation and a fire risk.
Outside cabling, CMG also stands for Cloud Management Gateway in Microsoft Configuration Manager, a cloud service for managing clients over the internet; it has nothing to do with cable ratings.
For the CompTIA A+ exam
Core 1 (220-1201) objective 3.2 lists plenum-rated cable; general-purpose CM/CMG cable isn't named. It matters as the contrast: general-purpose cable is fine for ordinary horizontal runs, but a drop ceiling used for air return calls for plenum cable, and a run between floors calls for riser cable or better.