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Rigid nonmetallic conduit (RNC) installations

By: BridgeTower Media Newswires//August 24, 2023//

Mark Cook is an electrical education specialist and master electrician at Faith Technologies. He has been in the electrical industry since 1978 and owned an electrical contracting business from 1994 to 2015 in Arizona until his recent employment with Faith Technologies as a technical training consultant. He now provides CEU classes and exam prep class, as well as Arc Flash training, for Faith. He can be reached at [email protected].

Rigid nonmetallic conduit (RNC) installations

By: BridgeTower Media Newswires//August 24, 2023//

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PVC conduit or RNC is a versatile, relatively inexpensive, wiring method that is impervious to most all corrosive influences. Commonly, the conduit of choice for underground installations as well as environments with corrosive atmospheres like pool equipment rooms and battery storage. It never rusts, is light weight, nonconductive and easy to install on the job site.

However, this raceway option comes with pros and cons. Take a look and some changes to the 2023 National Electric Code and specifically Article 352 to become familiar with some key installation requirements that must be considered for your next RNC installation.

In Article 352 you will find Polyvinyl Chloride or PVC conduit comes in various sizes, lengths, and wall thicknesses. Schedule 40 is the most common, while Schedule 80 may be used to provide protection against physical damage due to the thicker wall. Type A and Type EB, PVC conduit, (A.K.A. DB-120) is Schedule 20, which has thinner walls and is typically used by the utility providers and intended for underground installations and concrete encased duct banks.

Although the external diameter may be the same for all of these types of RNC raceways, not all fittings are listed for interchangeable use. For example, the typical PVC fitting used for Sch. 40 may not be listed for Sch. 80 PVC conduit. Check with the manufacturer. I have found that Carlon® fittings are listed for both Sch. 40 and Sch. 80. Type EB cannot not be field bent with heat or a hot-box. Most all PVC conduits manufactured for electrical installations are rated for 90⁰C conductors and sunlight resistant, however all of these types of raceways will get a little sunburnt if exposed to direct sunlight for very long. I have found that paint helps protect that from happening. I know this might be controversial with some. Check with you AHJ for approval. Carlon® does provide guidance on applying water based or latex paint to RNC raceways. Painting conduit with solvent based coatings is not recommended. These coatings contain aromatic hydrocarbons which may diminish the physical properties of the PVC raceway.

352.10 has a long list of uses permitted for PVC conduit which is mostly inclusive except for where exposed to physical damage. Then Schedule 80 is permitted to provide protection. Do not assume that Sch. 80 may then fall under the same burial depths as rigid metallic conduit or IMC as permitted in 300.5. Lesser burial depths would only be permitted with at least 2” of concrete encasement or while the installation contains a 20-amp, GFCI protected circuit used for residential branch circuits. Long runs of RNC could result in the pulling rope cutting into the raceway at the bends during the installation of conductors and concrete encasement or GRC elbows can be used to mitigate that problem. As with all conduits, there is still a limit of 360 degrees of bends between pull points. 352.24(B)

352.12 has a list of uses not permitted which include theaters, and similar locations, hazardous (classified) locations, supports for luminaires and where the ambient temperature is in excess of 122⁰F. This rules out the deserts of Arizona during the summer. Sarcasm intended! Inversely, extreme cold temperatures can make the nonmetallic raceways brittle and more susceptible to damage from hard physical contact or from a harsh blow.

Supporting PVC is more stringent than metallic raceways because the conduit tends to droop or sag over time. You are required to follow the support tables in 352.30(B) which does not permit any size of RNC to be supported more than 8 feet apart. For example, trade size 2” PVC conduit must be supported at distances not greater than 5 feet. Ten feet or more may be permitted for most metallic raceways. Securing the PVC raceway is still 3 feet maximum from a fitting, box, or other conduit termination and the supports used must allow for lateral movement. This will account for expansion and contraction of the PVC due to temperature changes.

This brings me to the use of expansion fittings in accordance with new language found in 352.44 of the 2023 NEC.

PVC will expand and contract based on the difference of temperature within the installation environment. Little to no movement is considered for underground installations but as soon as the raceway emerges from the earth, expansion fittings must be installed as required in 352.44(B) and 300.5(J). If RNC is used for an exterior installation, expansion fittings must be installed to compensate for the predictive movement of a ¼” or more as per Table 352.44(A).

This Table can be used to calculate how much distance the raceway will move based on 100 feet of conduit and how temperature changes from season to season or location of installation. Expansion fittings must be installed as required by the manufacturer’s installation instructions.

Because the raceway is not conductive and part of the list approved as equipment grounding conductors located in 250.118, you must provide an EGC of the wire type sized in accordance with 250.122 and Table 250.122.

PVC conduit is permitted in sizes ½” up to 6” and can be made in 10-foot or 20-foot lengths.

Bends can be factory made or made in the field with heat using identified bending equipment. The bending radius shall not be less than shown in Table 2 of Chapter 9 and cannot exceed 360 degrees of bends between pull points.

The Code requires that all cut ends be trimmed inside and outside to remove rough edges. The use of PVC cutting tools make for a clean cut which makes this unnecessary unless a saw is used on larger sizes of conduit.

One more advantage to using RNC is there is no induced currents as when using a metallic raceway and allows for single conductors to be installed in each raceway or using Schedule 80 PVC for the protection of a single Grounding Electrode and not having to connect both ends of the raceway to the conductor. (See 300.20) PVC has less impedance on conductors when long runs are considered, and voltage drop is a concern.

Although, Rigid Nonmetallic Conduit may be the perfect choice for your next electrical installation, pay attention to its limits and disadvantages as well as the installation requirements to provide a fool-proof installation with little to no problems that may occur after long after the Green Tag. Follow the listing requirements (352.6) and use the conduit as intended and designed. Implement the use of expansion fittings where required and support raceways that meet the minimum support distances required in the Code or less. Your next installation will last longer and look better when using PVC conduit.


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