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By Mario Valdes,
Mike Holt Enterprises
INTRODUCTION
The purpose of this article is to provide the electrician with familiarity of the installation of non-metallic sheathed cable (Type NM) and construction type restrictions.
The building code limits construction types based on occupancy classification, building height and allowable area. Type NM cable consist of a PVC jacket that’s moisture resistant and flame-retardant, paper sheathing separator with 90°C rated conductors.
Why non-metallic sheath cable is not allowed in certain structures is a controversial topic; in simple terms you cannot have a combustible wiring method (Type NM cable) in non-combustible construction.
Let’s examine the construction types to further clarify uses permitted and uses not permitted for Type NM cable.
TYPE I: FIRE RESISTIVE CONSTRUCTION
Non-metallic sheathed cable is not permitted in a Type I construction building [334.12(A)(1)].
In Type I construction, the structural elements consist of non-combustible material. The exterior is usually concrete and the interior partitions typically consist of metal framing.
The buildings are allowed of unlimited heights if sprinkled in accordance with the building code.
- Examples of Type 1 construction include high rise buildings, parking garages, stadiums, and arenas.
TYPE II: NON COMBUSTIBULE CONSTRUCTION
Non-metallic sheathed cable is not permitted in a Type II construction building [334.12(A)(1)].
In Type II construction, the structural elements consist of non-combustible material with a lesser fire rating than Type 1 construction. The exterior is usually bolted or welded steel and the interior partitions typically consist of metal framing. The buildings have different height limitations depending on if sprinkled or non-sprinkled in accordance with the building code. Examples of Type II construction include big box supermarkets, strip malls, warehouses, and school.
- Examples of Type II construction include big box supermarkets, strip malls, warehouses, and schools.
TYPE III: ORDINARY CONSTRUCTION
Non-metallic sheathed cable is permitted in Type III construction buildings for one or two family dwellings, multifamily dwellings and other structures where concealed within a 15 min finish rating (1/2’’ drywall) [334.10(1),(2),(3)].
In Type III construction, the structural elements consist of non-combustible material for the exterior, usually concrete or block wall. The interior partitions are made of combustible materials, typically wood framing. Type III construction buildings are allowed up to 6 stories if sprinkled in accordance with the building code.
- Examples of Type III construction include houses, duplexes, apartment buildings and mid-rise condominiums, and commercial buildings.
TYPE IV: HEAVY TIMBER CONSTRUCTION
Non-metallic sheathed cable is permitted in Type IV construction buildings for one or two family dwellings, multifamily dwellings, and other structures where concealed within a 15 min finish rating (1/2’’ drywall) [334.10(1),(2),(3)].
In Type IV construction, the structural elements consist of consists of heavy timber. The exterior is non-combustible fire treaded heavy timber and the interior partitions are combustible heavy timber framing. Type IV construction buildings are allowed up to 6 stories if sprinkled in accordance with the building code.
- Examples of Type IV construction include old factory mills, worship churches and wood cabins.
TYPE V: COMBUSTIBULE CONTRUCTION
Non-metallic sheathed cable is permitted in Type V construction buildings for one or two family dwellings [334.10(1),(2),(3)].
In Type V construction the structural elements consist of combustible material. The exterior is usually wood panels and the interior partitions are typically wood framing.
Type V construction buildings are allowed up to 5 stories if sprinkled in accordance with the building code.
• Examples of Type V construction include single family homes and townhouses.
SUMMARY
Understanding construction types can really make a difference as it relates to the usage of Type NM cable, there is no doubt Type NM cable is the most cost-effective wiring method and could provide a huge difference in job cost.
If ever in a type I or II construction type building take a look at NEC 334.10(5) as it states NM-B cable is permitted where installed within raceways. As an electrician its really simple, the construction type is shown on approved plans building code compliance analysis located in the front sheet.
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