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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
- MC metal-clad cable is available in aluminum and copper, while NM-B is exclusively copper.
- MC cable has metal armor, while Romex has a more delicate non-metallic sheathing. MC cable provides better mechanical protection due to its metal armor, making it suitable for environments where physical damage is a concern.
- MC cables contain THHN or THWN-2 conductors, which can operate at higher temperatures and are resistant to moisture. In contrast, NM-B cables do not have these properties, making them less suitable for high-temperature or wet environments. THHN or THWN-2 conductors also mean a higher current-carrying capacity for MC cable.
- Rated for a maximum operating temperature of 90°C, but the ampacity is based on 60°C. For MC metal-sheathed cable, THHN is rated for 90°C in dry locations; while THWN-2 rated for 90°C in wet or dry locations.
- 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-B is generally more flexible and easier to maneuver through walls and ceilings than the more rigid MC cable.
- NM cable should never be buried( NEC 2020 Article 334.12(B)(4)), while MC cable can be buried in conduit or even directly buried if specified by the manufacturer.
- MC cable has a built-in grounding feature with its metal sheath, while NM-B relies on a separate grounding conductor.
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 due to its non-metallic sheath. However, MC is also more often used indoors than outdoors.
- NM-B cables are commonly used in residential homes to wire interior circuits, while MC cables are used in commercial, residential, and industrial settings.
- 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.
- You can 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 be used as a service entrance cable instead of SER and SEU, whereas NM-B Romex does not fit this purpose.
- NM-B is banned from using near flammable gasses due to its non-metallic sheath.
- NM cable indoors is mostly installed in enclosed walls, ceilings, or floors, while type MC cable can be exposed without additional protection.
- MC is power, control, and lighting cable in feeders and branch circuits.
- MC is allowed for cable trays.
- MC cable is suitable for above-ground installations in garages, basements, and utility rooms where additional mechanical protection is beneficial. NM-B Cable should not be used in areas where it can be exposed to mechanical damage.
- MC cable can be used under raised floors in commercial settings.
- MC cable is advantageous for retrofit projects where existing walls and ceilings cannot be easily opened.
- MC cable is widely used in commercial and industrial settings due to its durability and ability to withstand harsher environments, whereas NM-B is primarily confined to residential applications.
- MC cable is suitable for temporary power installations at construction sites or events.
- MC cable is often used in residential garages where wiring needs to be exposed.
- MC cable can be safely used in basements and attics where additional protection from rodents.
- In areas with high foot traffic or where equipment might come into contact with wiring, MC cable provides additional safety and durability.
- Both MC and NM-B cables can be used in residential buildings for service entrance wiring, feeder circuits, and branch circuits. This includes wiring for general lighting, outlets, and appliances.
- Both cables can be used in dry, indoor locations such as walls, ceilings, and floors. This includes concealed and exposed wiring in residential settings.
- In areas that do not pose a risk of exposure to flammable gases, vapors, or explosive dust, both types of cables can be used safely.
Why Is NM-Romex Prohibited in Tall Buildings?
Before NEC 2002, NM-B Romex was prohibited in buildings that exceeded three stories, but that regulation has since been lifted. NM-B Romex is indeed prohibited from use in many commercial and industrial buildings, but the issue is not exactly just the height of the building:
- According to NEC Article 334.10, NM-B cable is allowed in one- and two-family dwellings and their attached or detached garages and storage buildings.
- It is also permitted in multi-family dwellings and other structures of Types III, IV, and V construction, provided the cables are concealed within walls, floors, or ceilings that offer a thermal barrier with at least a 15-minute finish rating.
- For buildings of Types I and II construction (non-combustible), NM-B cable can be used if installed within raceways. NM-B cable is permitted if it remains within the floor or dwelling unit where it originates, meaning it cannot leave that specific area. In other cases, NM-B Cable is banned from use in buildings of Types I and II construction.
- Additionally, it is against the code to use an NM (non-metallic sheathed) cable above a suspended ceiling in a non-dwelling unit.
- NM-B cable is not permitted in locations where it would be exposed to flammable vapors or gases. This eliminates the possibility of using this cable in many commercial and industrial buildings.
In summary, it is not that NM-B Romex is strictly prohibited within tall buildings; it is that there are particular limitations to using NM-B cable in commercial and residential buildings. Local regulations prohibit the use of NM-B in commercial buildings, such as the New York City Electrical Code and the Chicago Electrical Code.
Due to safety concerns, NM-B Romex is mostly good for combustible constructions. On the other hand, MC metal-clad cable is permitted for commercial and apartment buildings and is a common choice for such buildings.
At Nassau National Cable, MC cables and copper Romex wire are available at excellent rates.
