Section 300.10 requires that metal raceways be electrically continuous. This section also specifies that metal raceways be connected to boxes, fittings and enclosures in a manner that provides effective electrical continuity between the raceway and enclosure. Section 250.96 also requires the effective bonding together of all non-current carrying parts of an electrical system. Metal raceways are equipment grounding conductors and part of the ground-fault current path, regardless of whether a separate wire type equipment grounding conductor is installed. Therefore, as a general rule, metal raceways are required to meet the ground-fault current performance requirements in 250.4(A) & (B). In accordance with 300.10 it is necessary to ensure that the mechanical connection between a raceway and an enclosure provides an effective ground-fault current path. Where it is determined that the mechanical connection alone does not provide the required electrical connection, it is necessary to provide additional bonding means. Bonding-type bushings or locknuts can be used to comply with 300.10, 250.4(A) & (B) and 250.96(A). There are several sections in the Code that require specific methods such as bonding-type bushings or locknuts to provide the required bonding connection between a raceway and an enclosure.
The first section in which "bonding-type locknuts and bushings" are mentioned as a means to ensure bonding connections is 250.92(B). This section contains the requirements for bonding at electric services. Unlike feeder and branch circuit conductors, service conductors do not have overcurrent protection at their supply end. Where a ground-fault condition on the line side of the service overload device occurs, the bonding connections are essential to ensure some level of ground-fault protection at a point in the electrical system where the largest amount of ground fault current is available. The low-impedance paths for ground-fault current created through the bonding connections are generally the only ground-fault protection provided on the line side of the service equipment. Therefore this section does not recognize the use of double locknut, locknut and bushing, or fitting shoulder and locknut types of connections as reliable bonding connections, regardless of the system voltage or the type of knockout through which a raceway enters an enclosure. One of the methods specified in 250.92(B)(1) through (4) shall be used to ensure a reliable ground-fault current path. It is in 250.94(B)(4) that the use of bonding-type bushings or locknuts is recognized as a bonding means for services. The revised wording of this section for the 2002 NEC clarifies that bonding at services cannot be accomplished through the use of standard locknuts and bushings only.
As a general rule, on the load side of the service (feeders and branch circuits) and where the voltage to ground is 250 volts or less, there is no specific NEC requirement to provide bonding bushings or similar fittings unless it is determined that the metal raceway connection to an enclosure has been made in such a manner as to impede the electrical continuity of the ground-fault current path. Electrical raceways are always required to comply with the requirements of 300.10, 250.4(A) & (B) and 250.96(A) except as permitted for enclosures isolated from metal raceways per 250.96(B). In most cases reliable bonding connections can be assured through double locknut, locknut and bushing, or fitting and locknut types of connections. However there may be circumstances in which these types of connections do not ensure reliable bonding. Where unreliable bonding connections are determined to be the case, regardless of the system voltage, special bonding means such as bonding-type locknuts or bushings must be used to provide the required bonding connection.
Where the system voltage is over 250 volts to ground, 250.97 requires the use of one or more of the service bonding methods specified in 250.92(B)(2) through (4) to ensure bonding of metal raceways (and metal sheathed cables). Because of the higher voltage to ground, the ground-fault energy is of a higher magnitude in such systems, thus the need for a specific, more stringent bonding requirement. A separate equipment grounding conductor in the metal raceway or cable does not offset the requirement to provide the enhanced method of bonding. This is a requirement that covers the effective and reliable bonding of the metal raceway or cable armor to an enclosure. Unlike services, there is an exception to this requirement that permits the use of double locknut, and fitting and locknut, types of connections only where the raceway entry to the enclosure is not through concentric, eccentric or oversized knockouts. Until the 1996 NEC, all concentric and eccentric knockouts (in circuits over 250 volts to ground) required some additional bonding technique beyond the mechanical connection of the metal raceway or cable. Section 250.97 does permit raceway entries through concentric and eccentric knockouts without the use of additional bonding means where the concentric or eccentric knockouts have been specifically tested and listed for their bonding suitability in circuits over 250 volts to ground. The indication of this listing is found on the carton in which the boxes are shipped.
Another section that requires enhanced bonding methods is 250.100. This rule covers bonding in hazardous (classified) locations and applies to all electrical systems regardless of the voltage. This requirement specifies the methods for bonding at services per 250.92(B) as the means to ensure a reliable bonding connection. This requirement is based on the need to ensure arc-free ground-fault current return connections between raceways and enclosures in areas that have ignitable or potentially ignitable environments. This requirement is further supplemented by the special bonding requirements contained in 501.16(A), 502.16(A), 503.16(A) and 505.25(A).
The last section that specifically lists bonding-type locknuts or bushings as one means to ensure a reliable bonding connection is 517.19(D). This section covers the grounding of panelboards supplying branch circuits in the critical care area of a health care facility where the panelboard is supplied from a grounded distribution system and the feeder circuit is installed in metal raceways or Types MC or MI cable. A medical patient's increased sensitivity to electrical shock is the basis for this requirement, as well as the other special grounding and bonding requirements in Article 517. It is imperative to maintain low potential differences between exposed conductive surfaces that are subject to patient contact. This requirement for specific types of bonding connections provides for a low impedance ground-fault current connection between the wiring method and the panelboard enclosure.
The terms "grounding bushing" and "bonding bushing" are quite often used interchangeably in describing these devices. However, the guide card information contained in the Underwriters Laboratories General Information Directory (UL White Book) makes a distinction between the two terms. A bonding bushing provides a means to make a reliable connection between a metal conduit and an enclosure. This means is typically one or more setscrews. A bonding bushing does not have provisions for connecting a bonding wire. Grounding bushings, on the other hand, provide a means to make a bonding connection between a conduit and enclosure, and also have some mechanism for connecting a bonding wire. These devices can be used with or without a bonding or grounding wire connection. The use of a grounding or bonding wire with a grounding bushing depends on the bonding or grounding function being performed. Additionally, there are bonding and grounding locknuts that can be used to provide a bonding connection between a conduit and enclosure similar to bonding and grounding bushings. Where grounding and bonding locknuts are used, an additional fitting will be necessary to give the conductor abrasion protection per 300.4(F) for insulated conductors 4 AWG and larger, and per 342.46 and 344.46, for intermediate metal conduit and rigid metal conduit respectively.
The number of bonding bushings required for a given installation will depend on the bonding functions that have to be accomplished. For example, given an installation where rigid metal conduit is used to enclose the service-entrance conductors from a self-contained meter enclosure (socket) to the service panelboard, the use of a single bonding-type bushing or locknut on either end of the conduit complies with the requirement to bond all of the non-current carrying metal parts. The construction of the meter enclosure provides a direct connection between that enclosure and the grounded service conductor. That connection bonds the meter enclosure. At the panelboard, the required main bonding jumper connects the enclosure and all equipment grounding conductors to the grounded service conductor. That connection effectively bonds the panelboard enclosure. At this point, the raceway is all that remains to be bonded, and a single bonding connection to one end of the service raceway effectively bonds that raceway. Any of the means specified in 250.92(B) can be used to bond the service raceway and, as discussed, bonding-type bushings and locknuts can be used to meet this bonding requirement.
Now, consider a similar installation, except in this case the metering enclosure contains current and potential transformers used for instrument rated metering. Unlike the self-contained enclosure, the metering enclosure configuration may not directly bond the enclosure to the grounded service conductor. In this case, the installation of bonding-type locknuts or bushings at both ends of the conduit effectively bonds together the metering enclosure, the metal service raceway and the service equipment enclosure.
There are two considerations that impact the answer to this question. The first is whether Type USE cable can extend directly from the site service equipment to the panelboard inside the mobile or manufactured home. A cable marked only Type USE is defined in 338.2 as service-entrance cable that is identified for underground use, having a moisture resistant outer covering, but it is not required to have a flame retardant covering. Therefore these cables are not suitable for use within a building or structure unless they are also marked with one of the insulation types from Table 310.13 that is suitable for use within a building or structure. The conductors between the service overcurrent device located at the exterior site service equipment and the mobile or manufactured home panelboard are feeders per Article 100. Section 338.10(B) provides requirements for using Types SE and USE as a branch circuit or feeder wiring method. Cable that is marked only as Type USE is required to be installed outside of buildings per 338.10(B)(4)(b). The only above ground use permitted is for connection to terminations that are located on the outside of a building or structure.
In the first printing of the 2002 NEC and NEC Handbook, 338.10(B)(4)(b) contains text allowing 6 feet of Type USE cable inside a building or structure for termination purposes. This provision was accepted during the proposal phase of the 2002 NEC process, however, a subsequent comment resulted in a final action to return to the text contained in the 1999 NEC. This 6-foot allowance for termination purposes inside a building or structure is an error in the first printing of the 2002 NEC. It should be noted that Type USE cable is permitted to be installed within a building if the cable is also marked as one or more of the conductor types shown in Table 310.13 that are permitted within a building or structure. However, since Type USE cable is not required to have an overall outer sheath, it cannot be used as an open cable wiring method above ground or inside a building or structure.
This leads to a second consideration regarding the installation that has been described in this question. Where Type USE cable emerges above ground for termination purposes, 338.10(B)(4)(b) requires physical protection for the cable assembly per 300.5(D) and attaching the open Type USE cable to the understructure of the mobile or manufactured home does not provide the physical protection required by this section. Although the skirting that is often provided around the base of a mobile or manufactured home does create an enclosure around the conductors, the fact is, the inside of this space is accessible to personnel who may be working on one or more of the electrical, mechanical, and plumbing systems associated with the unit and the cable could be damaged as a result of this activity. Since the cable is exposed within this accessible space it is not suitably protected from physical damage in accordance with 300.5(D)
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