Commercial wiring methods: Back to basics

A working understanding of commonly used wiring methods is critical for electrical engineers who design power systems for buildings

By John Yoon October 26, 2021
Courtesy: McGuire Engineers


Learning Objectives

  • Review basic criteria for evaluating and selecting wiring methods.
  • Examine industry standards and physical characteristics for conductors, conduit raceways and busways that are commonly used in commercial construction.
  • Review standard labeling nomenclature used to identify conductors.
  • Understand relative advantages and disadvantages associated with conductors and conduit raceways.

The fundamental goal of electrical power distribution design is to safely transmit electricity to the various loads within a facility. There are multiple components that make up a typical electrical distribution system, but ultimately, the wiring methods interconnect everything. While they are frequently taken for granted, the wiring methods used in an electrical distribution system can have a significant impact on the system’s  overall performance and reliability.

To that end, all electrical engineers need a working understanding of commonly used wiring methods along with their relative advantages and disadvantages to help ensure that the most appropriate method is selected. 

Guiding principles 

The NFPA 70: National Electrical Code has numerous prescriptive requirements regarding wiring methods. However, there are multiple considerations beyond simple code requirements when selecting the most appropriate

 wiring method for a client. Among these are: 

  • Are initial, ongoing operation or potential replacement costs a greater concern for the client?  
  • How much power needs to be delivered? 
  • What type of conditions will it operate under (heat, moisture, potential for physical damage, etc.)? 
  • Is flexibility in accommodating future changes required? 
  • Does the local labor have the skills required to install a particular wiring method in a workmanlike manner? 

How each consideration factors into the final decision is at the discretion of the engineer. Ultimately, the best solution is the one that provides the best degree of safety while meeting the owner’s requirements.  

Standard conductor types: insulation

Conductors used in building constructions typically use one of two general insulation types, thermoplastic and thermoset. UL standards for these conductor types are; 

  • UL 83: Standard for Thermoplastic-Insulated Wires and Cables.  
  • UL 44: Standard for Thermoset-Insulated Wires and Cables.  

Conductors with thermoplastic insulation is usually less expensive, has slightly thinner insulation than comparable thermoset insulated conductors and is easier to manufacturer but the insulation will melt when exposed to heat.  

Thermoset insulation will not soften when exposed to heat, but will harden and age when overheated. Thermosetting insulation is also typically more flexible in cold ambient temperature. However, cables with thermoset insulation can be 5% to 20% more expensive. 

Figure 1: In this picture, the translucent nylon jacket can be seen on a THHN conductor. The nylon jacket provides additional protection for the underlying thermoplastic insulation and resistance to certain hydrocarbon fluids. Courtesy: McGuire Engineers

Figure 1: In this picture, the translucent nylon jacket can be seen on a THHN conductor. The nylon jacket provides additional protection for the underlying thermoplastic insulation and resistance to certain hydrocarbon fluids. Courtesy: McGuire Engineers

Conductor markings 

Per NEC and UL listing requirements, all conductors and cables are required to be marked at regular intervals along its length. These marking are intended to provide guidance regarding its physical characteristics and what applications it is rated for. However, the markings may seem like an alphabet jumble of letters and numbers to the untrained eye.  

Further confusing the issue is that many conductors carry multiple ratings to correspond to different applications. However, with a basic understanding of the abbreviations used in the labeling, general characteristics can be easily determined without any type of prior knowledge about that specific type of conductor.  

Per the UL listing requirements, markings on the conductor must identify the following information: 

Conductor material type 

Copper (Cu) or aluminum (Al)

Conductor size 

Sizes are given in American wire gauge (AWG) or thousand circular mils (KCMIL or MCM). Sometimes size in square millimeters is also included.

Insulation voltage rating  

Voltage rates for standard building conductors are typically 300, 600 or 1,000 volts.

Insulation material type  

Commonly encountered insulation types are: 

T – Thermoplastic 

X – Cross-linked polymer insulation 

R – Rubber thermoset (usually synthetic rubber) 

S – Silicon insulation

Maximum temperature under which it can operate, typically 75°C or 90°C. 

H – 75°C dry temperature heat rating 

HH – 90° dry temperature high heat rating 

No letter corresponds to a 60°C heat rating 

Rated moisture resistance  

W (75°C rating): damp and wet locations 

W with a ‘2’ suffix at end (90°C rating): damp and wet locations 

Supplemental jacket materials 

N – Nylon 

Other environmental conditions that it is rated for use in: 

SR – Sunlight resistance and evaluated for direct exposure to ultraviolet radiation. 

DIR BUR – Evaluated for direct burial in earth. 

GR1: 60°C rating when exposed to gasoline and similar light petroleum solvents. “Gasoline and Oil Resistant I” labeling is also used. 

GR2: 75°C rating similar alternate nomenclature using a Roman numeral two (“Gasoline and Oil Resistant II”) instead is also used. 

As stated earlier, it is not unusual for certain conductors have multiple ratings. The associated labeling often includes markings, such as machine tool wire (MTW) and vertical wire flame test (VW-1), which do not correspond with the above abbreviations or fall neatly into ratings commonly encountered in NEC.  

To put this into context, UL has a separate set of standards for appliance wiring materials (AWM) where a different set of abbreviations is used. This separate set of standards is intended for the internal wiring of appliances and equipment. Typically, the internal wiring for a piece of equipment is not subject to NEC requirements, but is cover by UL standards, which requires that listed materials be used in the manufacturing of equipment. 

Figure 2: Electrical metallic tubing conduit is available with colored coatings to aid in easy visual identification. Courtesy: McGuire Engineers

Figure 2: Electrical metallic tubing conduit is available with colored coatings to aid in easy visual identification. Courtesy: McGuire Engineers

Aluminum versus copper

Aluminum has several advantages over copper — cost, endurance fatigue, etc. It also has significantly different electrical characteristics than copper. Aluminum has 61% of the conductivity of copper on a volume basis and 200% on weight basis. As such, aluminum wire will always be larger but still significantly lighter than copper for a given ampacity. Most aluminum conductors are compact stranded, which reduces diameter by 9% to 10% to slightly offset this size difference. However, aluminum wiring for a given ampacity will typically still be one to two standard trade sizes larger than a copper conductor of similar ampacity.  

Use of copper still significantly outweighs that of aluminum conductors in the building construction industry. To understand why, the history of the use of aluminum conductors needs to be examined. Aluminum wire usage exploded in the mid-1960s due to copper shortages and associated high copper commodity prices. However, this also corresponded with an increase in electrical-related residential fires that raised serious concerns regarding the use of aluminum conductors. UL eventually delisted the most common type of aluminum building wire in the early 1970s.  

The root cause of these fires was materials incompatibility. Metals will expand when heated and contract when cooled. The materials commonly used in electrical distribution equipment and conductors (steel, copper and aluminum) have significantly different coefficients of thermal expansion. Aluminum has the highest, while steel has the lowest. Steel screws became more common than brass screws on receptacles at about the same time in the 1960s when the use of aluminum wire expanded. The most cited investigations regarding the causes of fire in residential installations using aluminum wire focused on the mechanical interface between the wire and the devices and associated difference in rates of thermal expansion.  

Most aluminum wire of this era used an “alloy” called AA-1350. AA-1350 has a purity of 99.5% aluminum or more. Because of its high purity, it typically is not considered to be an alloy. Aluminum wire and steel terminals would expand and contract at different rates. When repeatedly thermally cycled, this difference caused significant issues.  

There is an additional material characteristic known as “creep,” which is a critical consideration when there is mismatch in thermal expansion characteristics. Creep is the tendency of a material to experience permanent plastic deformation when exposed to mechanical stresses. So, when an aluminum conductor repeatedly expands against a steel terminal, it will eventually permanently deform, create a gap between the conductor and terminal, which results in an unstable electrical connection with high resistance. When these material characteristics are combined with sloppy installation practices, it easy to understand how this became an issue. 

The aluminum conductor industry addressed these concerns by using a different type of aluminum alloy, AA-8000. This alloy has creep and elongation properties that are more like copper than the original AA-1350 alloys. As such, they are less likely to become loose in a terminal after repeated thermal cycling.  

Before the 2011 NEC, Section 310.14 specifically required AA-8000 aluminum be used in any aluminum building conductors. It is unclear why this requirement was removed from the NEC but assumed to be due the guiding principle within the NEC that all electrical equipment and materials should be appropriate listed by a certifying agency (i.e., UL). 

As before, the primary concern with using aluminum conductors is in how they are terminated. Where used for wiring devices, terminal lugs must be dual CO/ALR rated. Most mechanical screw-type lugs used in electrical distribution equipment have this dual rating. Connectors must be marked “AL” or AL9CU (90°C) or AL7CU (75°C). While the availability of listed connection methods reduces the uncertainty when an installation use aluminum conductors, preparation and mechanical execution of installation can still significantly impact performance. There is no substitution for poor workmanship. 

Figure 3: Cloth-insulated conductors are commonly found in older electrical installation (before 1960). Reuse and/or modification of existing cloth wiring is not recommended. Courtesy: McGuire Engineers

Figure 3: Cloth-insulated conductors are commonly found in older electrical installation (before 1960). Reuse and/or modification of existing cloth wiring is not recommended. Courtesy: McGuire Engineers

Stranded versus solid conductors

A common electrical specification calls for use of solid conductors in all branch circuit wiring for 10 AWG and smaller and stranded conductors in 8 AWG and larger. However, it is common for electrical contractors to attempt to substitute use of stranded for solid conductors. The question is why is there this conflict? To gain some perspective, let us look at the relative advantages of each. 

Stranded conductors are: 

  • More flexible and easier to pull through conduit. 
  • Less likely to break if subject to frequent flexing. 
  • Generally required for sizes 8 AWG and larger by NEC, either 310.106(c) or 310.3(c) depending on the edition of the code used. However, there are exceptions such as mineral-insulated (MI) cable will usually have solid conductors. 

Solid conductors are:  

  • Easier to terminate in a consistent manner for smaller trade sizes.  
  • Less likely to be damaged/frayed when terminating at wiring devices. As such, terminations are less sensitive to installation workmanship issues. 
  • Incrementally cheaper. 

From an engineer’s standpoint, ensuring a minimum level of quality in an installation is usually an overriding consideration. As such, most engineered designs attempt to minimize the number of variables associated with the installation of electrical equipment. By specifying certain types of materials and equipment, the engineer can force a contractor to use certain means and methods for installation. This can help ensure the consistency in the workmanship of an installation. 

However, in a competitive bidding environment, material costs are usually consistent between the various electrical contractors bidding on that project. Often, the ability of a contractor to perform the required work with lower labor costs than a competitor can mean the difference between getting and losing a project. As such, anything that can make an installation quicker and easier will usually be embraced by the contractor. Where an incidental increase in material costs can realize a significant labor savings, such as the ability more easily pull wire through conduit, a contractor’s resistance to using certain wiring methods is easily understandable. 

Figure 4: The conduit in this picture is covered with snow melting salt residue. While galvanization will improve the corrosion resistant of steel conduit, consideration has to be given to the conditions in which it will be installed. Alternate conduit types or protection methods may be recommended. Courtesy: McGuire Engineers

Figure 4: The conduit in this picture is covered with snow melting salt residue. While galvanization will improve the corrosion resistant of steel conduit, consideration has to be given to the conditions in which it will be installed. Alternate conduit types or protection methods may be recommended. Courtesy: McGuire Engineers

Obsolete conductor types

When modifying electrical distribution systems in older buildings, it is not uncommon to see obsolete wiring methods and materials that do not correspond to current standards. Attempting to interconnect new electrical distribution equipment with old wiring can cause significant safety issues. For the purposes of this article, let us examine two conductor types that are commonly encountered in older buildings, TW and cloth-insulated conductors. 

Cloth-insulated feeder wire was commonly used in buildings before the 1960s before the emergence of thermoplastic insulated conductors. The cloth insulation consisted of a woven cotton fabric impregnated with varnish, usually applied over rubberized insulating layer (see Figure 3). Due to organic nature of that insulation, oxidation and deterioration over time is to be expected. The cloth layer will often fray, become brittle and crack, thus compromising its ability to provide physical protection for the conductor within. Modification of cloth insulated wiring typically is not possible without causing further damage. In addition, the heat voltage ratings of these types of cloth insulated conductors typically is not defined. Reuse of this type of wiring is strongly discouraged. 

TW thermoplastic insulated conductors are typical of early 1960s to mid-1970s installations. While the longevity of the insulation is not suspect like cloth insulated wire, it still has limitations compared to modern THHN/THWN conductors. While TW typically has thicker insulation than THHN/THWN, it is only rated to 60°C. When interconnecting with modern conductors, the ampacity of those modern conductors is required by code to be derated to their 60°C value. The lack of the nylon jacket present in THHN/THWN also reduces TW conductors’ resistant to mechanical damage (see Figure 1).  

The nylon jacket usually provides oil and gasoline resistance (indicated by GR labeling on the insulation). If standard TW wire without that nylon jacket is exposed to petroleum solvents, it will need to be replaced. 

Metallic conduit 

Conduit is intended to function as a raceway and protect the conductors within it from damage. Metallic conduit is usually specified where severe physical damage is a concern. The primary question is what other type of damage could it be exposed to other than physical impact? The most common concern is corrosion damage due to environmental exposure. Corrosion is inevitable. The only question is how long it will take for that corrosion to impact the structural integrity of conduit and its ability to protect the conductors within it.  

However, corrosion can be controlled or slowed down. The exterior coating on steel conduit is usually zinc that is applied in a process called galvanizing. That zinc forms a sacrificial barrier that will slow the corrosion rate of the steel. The appearance of white rust as opposed to reddish-brown rust on the conduit indicates that the zinc coating is doing its job. A polymer top coating is often applied on top of the galvanized layer to provide additional protection. That topcoat can be tinted a specified color (red, blue orange, etc.) to aid in visual identification (see Figure 2).  

In extremely corrosive environments, the NEC requires that supplemental corrosion protection be required. Based on information from the American Galvanizers Association, galvanizing performs we when exposed to liquids between pH 4.0 (acidic, equivalent to vinegar) and 12.5 (basic, equivalent to ammonia). Outside of this range, supplemental protection is required. The supplemental protection could take the form of tape wraps, polyvinyl chloride coatings, epoxy or urethane paints, etc.  

Metallic conduit offers excellent fire resistance (steel melts at 2,800°F) and electromagnetic interference shielding properties compared to nonmetallic raceway systems. When properly installed, it also provides an effective ground fault path. 

There are three primary types of metallic conduit used in building construction: 

  • Electrical metallic tubing, governed by NEC Article 358 and UL 797: Electrical Metallic Tubing — Steel. 
  • Intermediate metal conduit, governed by NEC Article 342 and UL 1242: Standard for Electrical Intermediate Metal Conduit — Steel. 
  • Rigid metal conduit, governed by NEC Article 344 and UL 6: Standard for Electrical Metal Conduit Steel. 

The primary difference between these three types of conduits is thickness of the steel used in each. Being a simple commodity product, the overall cost of conduit is strongly influence by the amount of steel used to make it.  

EMT is thinner walled, lighter in weight and less expensive than both IMC and RMC. While NEC permits EMT use in wet locations, in direct contact with earth or encased in concrete, many jurisdictions have local amendments to the NEC that forbid these applications. It is allowed for use “where subject to physical damage,” it is not allowed where “subject to severe physical damage.” While used in multiple locations, these terms are not defined anywhere in the NEC.  

EMT is easier to bend than IMC or RMC but cannot be threaded due to the reduced wall thickness. As such only compression and setscrew fitting can be used when interconnecting or terminating EMT. 

IMC was developed in the mid-1970s as a cheaper alternative to RMC. Although it provides similar mechanical protection, IMC is thinner walled and lighter in weight than RMC — roughly about 1/3 lighter. IMC is approved for use in any area where RMC is allowed and can be threaded and installed with similar methods. Threaded fittings couplings and connectors are interchangeable between IMC and RMC. Because of their superior mechanical protection properties, IMC and RMC are commonly specified for incoming utility services and large electrical feeders. 

Use of RMC predates both EMT and IMC. It has its origins in the early days of electricity when steel gas piping was repurposed for use as an electrical raceway system. RMC has the thicker walls than both IMC and EMT. The thickness, in addition to facilitate the cutting of threads for fittings, also provides superior physical protection. However, the increased use of steel for any given trade size also means that RMC is more expensive than both EMT and IMC.  

Rigid PVC conduit  

The applicable codes and standards for PVC are NEC Article 352 and UL 651: Standard for Schedule 40, 80, Type EB and A Rigid PVC Conduit and Fittings. 

PVC has several characteristics that make it suitable for use in electrical conduits. It is corrosion resistant, generally flame retardant and nonmagnetic. Probably the greatest consideration is that it is dramatically cheaper that metallic conduit for larger trade sizes — often less than a quarter of the cost of RMC in a 4-inch trade size. PVC conduit is commonly used underground, where it is subject to corrosive influences and in wet locations. It can be used exposed where appropriately listed for the application and in concealed location where approved by the authority having jurisdiction.  

While commonly used in site work outside of a building, its use within a building is limited due to certain undesirable characteristics. While the high chlorine content of PVC reduces its ignitability compared to other types of plastic, unlike metallic conduit, it is still capable of burning. PVC has an ignition temperature of 734°F. Compare this to the 2,600°F melting point of steel. It is not allowed for use in plenums because it does not meet the 25/50 smoke/flame spread rule as defined by ASTM E84: Standard Test Method for Surface Burning Characteristics of Building Materials. When PVC does burn, it results in significant amounts of smoke along with the formation of acidic gases and highly toxic dioxins. 

Even in locations where it would not create a significant life-safety hazard if ignited, there are other important design considerations associated with PVC conduit. These typically revolve around ensuring the structural integrity of the PVC. PVC becomes stiffer as temperature decreases and impact resistance will decrease during cold weather. This brittleness can lead to cracking if subject to physical damage during cold weather. PVC will also plastically deform when exposed to high temperature. Per NEC section 352.12(D), it is not allowed to be used where subject to ambient temperatures greater than 50°C (122°F). 

PVC also has a coefficient of thermal expansion roughly five times greater than steel, which can lead to significant issues. A 100-foot run of PVC will change 4.06 inches in length if the temperature change is 100°F (see NEC Table 352.44). This temperature swing is not uncommon in regions with four distinct seasons. Exposure to direct sunlight can also easily increase the temperature of the material by 30°F. If these characteristics are not properly addressed, the change in length could result in conduit bowing and/or broken coupling and fittings. As such, NEC section 352.44 requires that expansion coupling be provided if the length changes is anticipated to be 0.25 inch or greater. Most expansion coupling can typically accommodate 4 to 8 inches of travel length. 

In the second part of this article, more contemporary wiring methods, metal-clad cable and busways will be examined. 

Author Bio: John Yoon, PE, LEED AP ID+C; is lead electrical engineer at McGuire Engineers, Chicago. He is a member of the Consulting-Specifying Engineer editorial advisory board.