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What is the difference between NM cable and NMB cable?

Published in Electrical Cable Types 3 mins read

The primary difference between NM (Non-Metallic) cable and NMB cable lies in their temperature ratings, with NMB cable being the modern standard due to its higher heat resistance.

Understanding NM and NMB Cable

While "NM cable" is the general term for non-metallic sheathed cable used in residential and light commercial wiring, "NMB cable" represents the current, enhanced standard of this wiring type. Essentially, today's NM cable is NMB cable. The distinction is rooted in a significant technical improvement indicated by the "B" suffix.

What is NM Cable?

NM cable, short for Non-Metallic sheathed cable, is a type of electrical cable commonly used for indoor wiring in dry locations. It consists of two or more insulated conductors (hot and neutral) and a bare ground wire, all encased within a non-metallic (typically PVC) sheath. It is widely recognized and frequently referred to by the brand name Romex, which is a product name for this type of cable.

What is NMB Cable?

NMB cable is the modern designation for NM cable. The crucial difference, indicated by the "B" suffix, signifies that its conductors are designed for 90-degree Celsius ampacity ratings according to the National Electrical Code (NEC). This higher temperature rating means the cable's insulation can safely withstand hotter operating conditions compared to older, non-"B" rated NM cables.

Key Differences Summarized

Feature NM Cable (Older Designation) NMB Cable (Modern Standard)
Type General non-metallic sheathed cable Non-metallic sheathed cable with improved insulation
Suffix No "B" suffix "B" suffix
Temperature Rating Typically 60°C or 75°C for conductors 90°C for conductors, as per NEC ampacity ratings
Availability Largely phased out in new manufacturing Standard and almost exclusively manufactured today
Common Use Used in older installations Widely used for most residential and light commercial wiring

Practical Implications of the "B" Suffix

The 90°C rating of NMB cable offers several advantages and practical considerations for electrical installations:

  • Ampacity Calculations: While the conductors are rated for 90°C, the actual allowable ampacity (current-carrying capacity) for the circuit is often determined by the lower temperature rating of connected equipment, such as circuit breaker terminals or receptacle terminals (typically 60°C or 75°C). However, the 90°C rating is crucial when applying correction factors for ambient temperature or bundling multiple cables, as it provides a higher baseline from which to derate.
  • Heat Resistance: The improved insulation allows NMB cable to perform reliably in environments where temperatures might rise, such as within conduits, insulation, or tightly packed areas.
  • NEC Compliance: NMB cable meets current NEC requirements for non-metallic sheathed cable, ensuring that new installations adhere to safety and performance standards. For more details on electrical wiring standards, consult the National Electrical Code (NEC).
  • Versatility: NMB cable is suitable for use in dry locations and can be run through wood or metal framing members, making it a versatile choice for residential wiring, including branch circuits for outlets, switches, and lighting fixtures.

In essence, when discussing or purchasing "NM cable" for modern electrical projects, you will almost invariably be referring to and acquiring NMB cable, which has become the industry standard due to its enhanced thermal performance and compliance with contemporary electrical codes.