Back to basics: Commercial building wiring methods
Electrical engineers and designers should have a practical understanding for the application of raceways, wiring, cabling and busways
- Review the different types of raceway and conductor methods.
- Understand the differences with manufactured cabling assemblies.
- Learn about the application of busways and bus ducts.
Electrical engineers and designers have different methods of serving loads within a building when it comes to the use of wiring, cabling and busway. The decision to specify one method over another or a combination of multiple methods can be driven by many factors including local jurisdictional requirements, new construction versus remodel, project cost, owner preference or standards, building height and/or type and future goals for the building.
In selecting various wiring, cabling or busway systems, the engineer or designer is typically faced with compromises in functionality, future flexibility and first cost, all of which should be considered before proceeding with the specifications for the project installation. The “when,” “where” and “why” for applying certain wiring, cabling and busway methods is based on the 2017 edition of NFPA 70: National Electrical Code.
Raceway and conductor methods
NFPA 70: National Electrical Code Chapter 3 covers several wiring methods, and this article will focus mainly on commercial construction applications of 600 volts or less. NEC Article 300 – General Requirements for Wiring Methods and Materials applies to all wiring installations unless modified by other articles in Chapter 3. NEC 300 covers requirements such as protection against physical damage, minimum cover for underground installations, securing and supporting, mechanical/electrical continuity and where boxes, conduit bodies or fittings are required.
NEC Article 310 – Conductors for General Wiring applies to the individual conductors used in the specified wiring methods and the requirements for their designations, insulation, marking, ampacity and allowable uses. Note that Articles 300 and 310 do not apply to integral or internal parts of listed equipment.
The raceway and conductor method consists of metallic or nonmetallic conduit or tubing with multiple insulated phase, neutral and possibly an equipment ground conductor depending on the applicable method and NEC requirements. Each raceway and conductor installation is field fabricated based on the design plans and specifications.
Most consultants typically specify conductors of the type THHN/THWN (thermoplastic high heat-resistant nylon jacket/thermoplastic heat and water-resistant nylon jacket) or XHHW (XLPE high heat-resistant and water-resistant) in copper or aluminum for these applications. Copper typically is specified for branch circuits and either copper or aluminum is specified for feeders.
Some of the common types of raceways used in commercial applications include:
- Intermediate metal conduit (type IMC) per NEC 342. IMC is a thin-wall version of RMC with a galvanized exterior and an interior corrosion resistant coating. It is typically specified as a lower-cost solution to RMC where it offers the same levels of protection.
- Rigid metal conduit (type RMC) per NEC 344. RMC (also known as RGS or GRC) is a thick wall galvanized steel conduit with a zinc coating on the exterior and interior of the raceway. RMC is typically specified where the maximum protection is required from physical damage.
- Flexible metal conduit (type FMC) per NEC 348. FMC (also called greenfield) is a conduit available in multiple types of metal and wall thicknesses. It’s typically specified for the final connections to motors, transformers or other vibrating or moving equipment.
- Liquidtight flexible metal conduit (type LFMC) per NEC 350. LFMC is a conduit similar to FMC, but with a polyvinyl chloride jacket. LFMC is typically specified for final connections to equipment in damp or wet locations.
- Rigid polyvinyl chloride conduit (type PVC) per NEC 352. PVC conduit is available in multiple types but most commonly specified as either Schedule 40 or Schedule 80, referencing the wall thickness. It generally is specified for underground applications directly buried or concrete encased and can also be used in some corrosive environment applications. Because this raceway is nonmetallic, it does not offer any grounding capabilities.
- Electrical metallic tubing (type EMT) per NEC 358. EMT is by far the most common raceway in commercial construction. EMT is a threadless thin-wall steel tubing with a galvanized exterior and a corrosion-resistant interior coating, which is also an aid in pulling conductors.
It should be noted that cable trays as covered in Article NEC 392 are not raceways. Cable trays are a structural system used to mechanically support and manage cables.
For reference, the NEC uses two terms regarding protection but the NEC does not define the terms “physical damage” or “severe physical damage,”, which is left to the authority having jurisdiction’s discretion. Physical damage might be limited to forcible human contact with tools or equipment whereas severe physical damage would entail mechanized or vehicular contact. The physical location of the installation will also factor into the determination.
Advantages to raceways, conductors:
- Once installed, changes in circuit configuration are possible to an extent depending on the raceway type and size, such as changes in conductor size or conductor quantity.
- Types IMC or RMC can be installed in locations subject to severe physical damage.
- Type EMT can be installed in locations subject to physical damage.
- Types IMC, RMC, LFMC or EMT can be installed in dry, damp or wet locations (note NEC 358.10 for EMT was updated for 2017 with additional permitted uses).
- Raceways can be used more easily during remodel work by allowing the conductors to be changed out or within multitenant spaces and shell buildings where landlords provide raceways for tenants’ future use.
- Exposed installations of EMT, IMC or RMC are more appealing where craftsmanship is shown.
- Raceways can be abandoned where the conductors are removed from the installation.
- Underground installations with PVC are inexpensive and very common
Disadvantages of raceways, conductors:
- Installed costs are usually more expensive due to labor needed for initial raceway installation and then return labor to install conductors.
- Limited in the number of bends before a junction box or pull-box is required.
- Some types of conduit cannot be installed in locations subject to physical damage such as FMC, LFMC or PVC.
- Type PVC cannot be used for health care applications with patient care.
- Type PVC is listed as sunlight resistant and can be used exposed outdoors, however be aware that “resistant” does not mean sun-proof. Some environments will degrade the life of the PVC conduit where exposed to severe sun and heat. If the temperature variance exceeds 25°F expansion joints will be necessary. In extreme cold environments, PVC conduit will become very brittle.
- Type PVC is combustible and cannot be installed within a return air plenum found in Type I and II buildings as defined by the International Building Code for building construction types.
- Type PVC limits the installed conductor’s insulation temperature rating to that of the listing of the PVC conduit. Adjust the rated operating ampacity of the conductor accordingly.
- RMC and IMC typically use threaded connectors and elbows, which are more costly and time-consuming to install. RMC and IMC can be field threaded and bent with specialized tools and equipment.
- Requires more thought and coordination during installation versus cabling systems due to more rigid routing requirements.
- Engineer or designer is required to spend more thought/time on raceway sizing and potential derate impacts due to current carrying conductor quantities in a raceway versus a pre-manufactured cabling system.
Manufactured cable assembly methods
As mentioned above, NEC Articles 300 and 310 also apply to manufactured cable assemblies. Manufactured cable assemblies consist of multiple insulated phase and neutral conductors with an insulated and/or bare equipment grounding conductor(s) wrapped and enclosed within either a metallic or nonmetallic sheath. Manufactured cable assemblies are available as standard product offerings from the manufacturer or as custom assemblies with specified conductor configurations, which may come with extended lead times and minimum order lengths. Consultants will find that these cable assemblies generally come standard with THHN/THWN or XHHW-2 (copper or aluminum) conductors in most cases.
Some of the common types of manufactured cable assemblies used in commercial applications include:
- Armored cable (type AC) per NEC 320. AC is a set of conductors assembled within a metallic sheath of either steel or aluminum armor where the sheath is also used as a ground path. It typically is specified for branch circuit connections between luminaires and power receptacles.
- Metal-clad cable (type MC) per NEC 330 is one of the more commonly used cable types in commercial construction. MC cable is a set of conductors assembled within a metallic sheath of either steel or aluminum interlocking armor where the sheath is not used as a ground path. MC cable is also available with an outer PVC jacket for wet locations or direct burial. It’s usually specified for branch circuit connections between luminaires and power receptacles, and also is common for distribution feeders to branch circuit panelboards.
- Mineral-insulated, metal-sheathed cable (type MI) per NEC 332. MI cable is generally used for specialized applications such as compliance with NEC 700 emergency systems. MI cable is a seamless copper sheath containing copper conductors within a magnesium oxide insulation.
- Nonmetallic sheathed cable (types NM, NMC, NMS) per NEC 334. NM cable (also called Romex) is a set of conductors assembled within a nonmetallic sheath with PVC insulation and a nylon jacket, usually with a bare copper grounding conductor. This is generally used for lighting, switches and receptacle branch circuits in light commercial and residential construction.
- Service-entrance cable (types SE, USE) per NEC 338. SE cable is a set of conductors assembled within a nonmetallic sheath with cable reinforcement and an outer PVC jacket. Generally, it’s used for service drops to a meter pedestal or as a distribution feeder to a branch circuit panelboard.
- Underground feeder and branch-circuit cable (type UF). UF cable is a set of conductors assembled within a nonmetallic sheath with PVC insulation and a nylon jacket contained within a gray PVC outer jacket. UF is generally used for outdoor direct burial feeders and branch circuits to detached garages and lighting fixtures.
Advantages to cable assemblies:
- Installed cost generally lower, mainly due to labor not needed twice to install conduit and then wire.
- Install time is faster due to less labor and more flexibility on cable routing with the exception being MI cable, which requires special installation methods.
- Not limited in the number of bends during installation.
- Factory-tested assembly.
- Types AC and MC can be used where flexibility is necessary for equipment connections.
- Type MI cable is fire resistant (a special type MC cable is also available that is fire resistant).
- Type MI cable is typically installed in free air, which allows higher ampacity ratings with equally sized conductor compared to other types (verify with the manufacturer’s listing and ratings).
- Type NM cable is likely the most inexpensive method available (verify with the local jurisdiction for the application as some AHJs do not allow this method along with types SE and UF).
Disadvantages of cable assemblies:
- Securing and supporting required at a shorter spacing.
- Once installed, changes in circuit configuration are not possible.
- Cannot be installed where subject to physical damage.
- Types AC or MC cable must be installed in dry locations (unless provided with a listed jacket).
- A special type AC or MC cable is required for health care applications with patient care (verify with the local jurisdiction as some AHJs do not allow AC or MC for patient care areas).
- Exposed installations are not appealing although when installed in cable trays it is beneficial.
- Some jurisdictions will not allow cable assemblies to be abandoned within building construction.
- Type NM cable is limited by building type and occupancy (and is generally regarded as a residential or light commercial installation).
- Types NM, SE and UF are combustible; note some jurisdictions will not accept their use.
- Contractor is required to have on hand or order for the exact circuit configuration for use on a project. Alternatively, common raceway and conductor sizes can be on hand and used in multiple configurations providing greater application flexibility.
- Parallel sets and transformer secondaries require yet another cable type to be ordered or on hand due to larger ground size requirements.
- More planning required during cabling procurement.
Another type of wiring method includes a busway, which is sometimes called bus duct. By definition, a busway is a metal enclosed raceway with factory-mounted busbars. All types of busways fall under the scope of NEC Article 368, which includes service-entrance, feeder and branch-circuit busways. Generally, busway comes in either feeder style or plug-in style configurations with either insulated or bare conductors.
Advantages to busways:
- More compact in physical size than multiple conduits for the same ampacity feeder.
- Available in many ampere ratings from 60 to 4,000 amps.
- Plug-in style busway offers flexibility to add and change load takeoffs.
- Total voltage drop with bus duct application is less impactful. Be aware of extremely long runs of feeder bus duct where one is looking to maintain 2% voltage drop.
Disadvantages of busways:
- Installed cost potentially higher, especially if many elbows and offsets are needed.
- Susceptible to failure due to water leaks within the building (note some jurisdictions require sprinkler-proof plug-in busway, which has an ingress protection rating of IP54).
- Cannot be installed where subject to severe physical damage.
- The available short-circuit current on the bus duct could be very high. Be aware of the short-circuit current rating when using plug-in circuit breaker modules.
- 100% rated circuit breaker plug-in modules may be not available.
- Limited space for bus plugs in vertical applications depending on equipment room layout and position of bus duct.
These wiring methods are only a part of all the potential listed methods available on the market for use on a project. Although these are the wiring methods you will most likely encounter on a commercial project, be aware that due to other circumstances another wiring method may be more applicable. Always check the code and with your AHJ for the “uses permitted” and “uses not permitted” for each wiring method application.