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Building Wire

MC vs. NM-B Romex In Residential Applications

MC Cable and NM-B Cable are some of the most common types of building wire for residential applications. This blog is designed to make it clear how their applications differ.

Short Summary of MC Metal Clad vs. NM-B Romex Differences 

  • While many building wires are available in both aluminum and copper options, MC is almost exclusively aluminum, and NM-B is copper.
  • MC cable has metal armor, while Romex has a more delicate non-metallic sheathing.
  • An MC Cable has THHN or THWN-2 conductors, which is not true for NM cables.
  • The armor of MC cable is a cheaper alternative to conduit, while NM-B can be used in conduit when subjected to physical damage.
  • Romex is easier to cut and work around, while MC is safer to install because of the armor.
  • NM cable is never buried, while MC cable can be buried in conduit or even directly buried if specified by the manufacturer.

Short Summary of MC Metal Clad vs. NM-B Romex Applications 

  • MC metal-clad cable is greenlit for wet applications with an appropriate jacket, while NM-B is banned from being used in wet locations.
  • MC metal-clad is allowed to be used both outdoors and indoors, while Romex is banned from outdoor use. However, MC is also more often used indoors.
  • NM is prohibited for hazardous locations, while MC-HL, a subtype of MC, is designed for hazardous areas. Note that regular MC should not be used in hazardous locations as it could become a subject to physical damage.
  • Use MC in corrosive conditions only if its armor is specifically corrosion-resistant. NM-B is not allowed in corrosive conditions. You can use building wires in conduit as a plausible alternative in these situations.
  • MC Cable is allowed to use as a service entrance cable instead of SER and SEU, whereas Romex is not.
  • NM-B is banned for use near flammable gasses.
  • NM cable indoors is mostly installed in enclosed walls, while type MC cable can be exposed.
  • MC is used as power, control, and lighting cable in feeders and branch circuits.
  • MC is allowed for cable trays.
  • NM-B is banned in buildings above three stories, while MC is allowed.

Why IS NM-Romex Prohibited in Tall Buildings?

NM-B Romex is the most common cable in private homes and short buildings; however, the National Electrical Code does not allow it for buildings that exceed three stories. For this reason, you will not find it in commercial housing and apartment buildings. 

The deeper reason behind this prohibition? Due to safety concerns, NM-B Romex is not suitable for fire-resistive and non-combustible buildings. It is only good for combustible constructions. The buildings above three stories are always of either fire-resistive or non-combustible type, hence the prohibition. There is commonly no way around this rule, but some counties may allow Romex in residential constructions. On the other hand, MC metal-clad cable is allowed for use in commercial and apartment buildings and is a common choice for such buildings.

At Nassau National cable, aluminum MC cables and copper romex wire are available at excellent rates.

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