Which of the following wiring methods provides the best protection against physical damage?

Code Change Summary: Methods to secure and protect a grounding electrode conductor (GEC) have been revised.

The previous code language describing proper securing and protection from physical damage was a bit confusing and required a careful read in order to properly apply the code section.

The new 2017 revision to the section places all of the requirements into a list format and adds the suffix “XW” to the allowance to protect a GEC with type RTRC conduit. Now, if Type RTRC conduit is used to protect a GEC from physical damage, it must be Type RTRC-XW. The informational note in 355.10 makes it clear that Type RTRC-XW, is identified for areas of physical damage.

Also, a new requirement was added for GEC’s in contact with the earth to make it clear that even though the burial depth Table 300.5 does not apply to GEC’s, if subject to physical damage, a GEC in contact with the earth must now be buried or otherwise protected.

Below is a preview of Article 250. See the actual NEC text at NFPA.ORG for the complete code section. Once there, click on their link to free access to the 2017 NEC edition of NFPA 70.

2014 Code Language:

250.64(B) Securing and Protection Against Physical Damage.

Where exposed, a grounding electrode conductor or its enclosure shall be securely fastened to the surface on which it is carried. Grounding electrode conductors shall be permitted to be installed on or through framing members. A 4 AWG or larger copper or aluminum grounding electrode conductor shall be protected if exposed to physical damage. A 6 AWG grounding electrode conductor that is free from exposure to physical damage shall be permitted to be run along the surface of the building construction without metal covering or protection if it is securely fastened to the construction; otherwise, it shall be protected in rigid metal conduit RMC, intermediate metal conduit (IMC), rigid polyvinyl chloride conduit (PVC), reinforced thermosetting resin conduit (RTRC), electrical metallic tubing EMT, or cable armor. Grounding electrode conductors smaller than 6 AWG shall be protected in (RMC), IMC, PVC, RTRC, (EMT), or cable armor. Grounding electrode conductors and grounding electrode bonding jumpers shall not be required to comply with 300.5.

2017 Code Language:

250.64(B) Securing and Protection Against Physical Damage.

Where exposed, a grounding electrode conductor or its enclosure shall be securely fastened to the surface on which it is carried. Grounding electrode conductors shall be permitted to be installed on or through framing members.

(1) Not Exposed to Physical Damage. A 6 AWG or larger copper or aluminum grounding electrode conductor not exposed to physical damage shall be permitted to be run along the surface of the building construction without metal covering or protection.

(2) Exposed to Physical Damage. A 6 AWG or larger copper or aluminum grounding electrode conductor exposed to physical damage shall be protected in rigid metal conduit (RMC), intermediate metal conduit (IMC), rigid polyvinyl chloride conduit (PVC), reinforced thermosetting resin conduit Type XW (RTRC-XW), electrical metallic tubing (EMT), or cable armor.

(3) Smaller Than 6 AWG. Grounding electrode conductors smaller than 6 AWG shall be protected in RMC, IMC, PVC, RTRC-XW, EMT, or cable armor.

(4) In Contact with the Earth. Grounding electrode conductors and grounding electrode bonding jumpers in contact with the earth shall not be required to comply with 300.5, but shall be buried or otherwise protected if subject to physical damage.

For underground cables or raceways, ensure a minimum cover per Table 300.5. Cover is measured from the top of the underground cable or raceway to the surface of finished grade [Table 300.5, Note 1].

The interiors of enclosures or raceways installed underground are considered "wet locations." The associated cables and insulated conductors must be listed for use in wet locations per 310.8(C), and any connections or splices made in an underground location must be approved for wet locations [300.5(B)], as shown in Fig. 1.

Direct-buried conductors or cables can be spliced or tapped without a splice box [300.15(G)], if the splice or tap is made following 110.14(B).

When cables are run under a building, they must be installed in a raceway that extends past the outside walls of the building [300.5(C)].

What about moisture? If moisture could enter a raceway and contact energized live parts, a seal must be installed at one or both ends of the raceway. This is a common problem for equipment located downhill from the supply, or in underground equipment rooms. Section 230.8 provides information for service raceway seals. Section 300.7(A) addresses situations where a raceway travels through different temperature areas and must be sealed. This type of situation does not require an explosion-proof type of seal, products such as duct seal in a junction box should serve the purpose of stopping the circulation of moist air.

Corrosion protection

Your first line of defense against corrosion is to use materials suitable for the environment [300.6].

Ferrous materials must be protected against corrosion both inside and outside by a coating of listed corrosion-resistant material. For example, when conduit is threaded in the field, threads must be coated with an approved coating that is electrically conductive and corrosion-resistant.

Nonferrous metal raceways such as aluminum rigid metal conduit don't have to meet the provisions of 300.6(A), but have their own requirements in Sec. 300.6(B).

If ferrous parts are protected from corrosion solely by enamel, they must not be used outdoors or in wet locations [300.6(A)(1)].

Boxes or cabinets having a system of organic coatings marked "Raintight," "Rainproof," or "Outdoor Type," can be installed outdoors [300.6(A)(2)].

Ferrous metal equipment can be installed in areas subject to severe corrosive influences, such as in concrete or in direct contact with the earth, if the material is approved for the condition or provided with corrosion protection approved for the condition [300.6(A)(3)].

For example, galvanized steel EMT can be installed in concrete at grade level and in direct contact with the earth only if supplementary corrosion protection is provided (UL White Book, “Guide Information for Electrical Equipment,” www.ul.com/regulators/2008_WhiteBook.pdf). EMT can generally be installed in concrete, including slabs above grade level, without supplementary corrosion protection (Fig. 3).

Aluminum equipment can be installed in areas subject to corrosive influences, if provided with supplementary corrosion protection [300.6(B)].

Nonmetallic equipment can be installed in areas subject to severe corrosive influences, if the materials are identified for the condition and:

  • Where exposed to sunlight, the materials are listed or identified as sunlight-resistant [300.6(C)(1)].
  • Where subject to exposure to chemical corrosion, materials or coatings are either inherently resistant to chemicals based upon their listing, or are identified for use with the specific chemical [300.6(C)(2)].

If mounting metallic raceway in an indoor wet location, such as a washdown area, each box, fitting, raceway, and cable must have at least 1⁄4 in. of airspace between it and the wall or supporting surface [300.6(D)].

Raceways exposed to differences in temperature

Along its length, a raceway can be exposed to different temperatures. When the difference is significant, this can result in a problem with condensation inside the raceway. To solve this problem, the NEC requires sealing the raceway with a material approved by the AHJ that will prevent the circulation of warm air to a colder section of the raceway [300.7(A)]. You don't need an explosionproof seal for this purpose.

When raceways are installed in a location where the temperature is subject to changes, the result may be movement of the raceway due to thermal expansion and contraction. Expansion fittings must be installed when the movement is calculated to be ¼ in. or more between boxes or fittings on PVC installations [300.7(B) and 352.44].

Table 352.44 provides the expansion characteristics for PVC conduit. For metallic raceways, you can determine the expansion characteristics by multiplying the values from Table 352.44 by 0.20 (ferrous) or 0.40 (aluminum). Table 354.44 provides the expansion characteristics for reinforced thermosetting resin conduit (RTRC) [300.7(B) FPN].

Raceways can't be used for multitasking

Raceways are to be used only for electrical conductors and cables. Nonelectrical components, such as pipes or tubes for steam, water, or gas, must not be installed inside electrical raceways or cable trays [300.8].

Bonding path

Metal raceways, cables, boxes, fittings, cabinets, and enclosures for conductors must be metallically joined [300.10]. This forms a continuous, low-impedance fault current path capable of carrying any fault current likely to be imposed [110.10, 250.4(A)(3), and 250.122] [Fig. 4]. It also eliminates dangerous differences of potential.

Metal raceways and cable assemblies must be mechanically secured to boxes, fittings, cabinets, and other enclosures [300.10]. However, two exceptions apply:

  • Short lengths of metal raceways used for the support or protection of cables [300.10 Ex 1].
  • Equipment enclosures isolated per 250.96(B) [300.10 Ex 2].

This article has covered many of the essential requirements for the installation of all types of wiring methods. The next installment will include more information about the securing and supporting of wiring methods.