There were 3,688 proposals for changes to the National Electrical Code (NEC) and 2,349 comments processed by the National Fire Protection Association (NFPA) staff at its headquarters in Quincy, Mass. The following is part three in a series of significant changes for the 2008 NEC. Part 1 appeared in the July 2007 issue of ELECTRICAL CONTRACTOR, and part 2 appeared in the October 2007 issue. The 2008 NEC was formally adopted in August 2007, and new -copies were available as of September 2007.

The following are some of the most important changes. Strikethrough text shows deletions, and underlined text shows additions. Commentary denoted by red text also follows some changes for explanation.

2005 NEC—250.8

Connection of Grounding and Bonding Equipment. Grounding conductors and bonding jumpers shall be connected by exothermic welding, listed pressure connectors, listed clamps, or other listed means. Connection devices or fittings that depend solely on solder shall not be used. Sheet metal screws shall not be used to connect grounding conductors or connection devices to enclosures.

2008 NEC—250.8

Connection of Grounding and Bonding Equipment.

(A) Permitted Methods. Grounding conductors and bonding jumpers shall be connected by one of the following means:

(1) Listed pressure connectors

(2) Terminal bars

(3) Pressure connectors listed as grounding and bonding equipment

(4) Exothermic welding process

(5) Machine screw-type fasteners that engage not less than two threads or are secured with a nut

(6) Thread-forming machine screws that engage not less than two threads in the enclosure

(7) Connections that are part of a listed assembly

(8) Other listed means

(B) Methods Not Permitted. Connection devices or fittings that depend solely on solder shall not be used.

Section 250.8 has been revised by deleting the specific prohibition of sheet metal screws since there are many other types of screws that also are prohibited, such as drywall screws, self--tapping tech screws with less than two threads into an enclosure and similar screws that may not provide an acceptable ground return path. Connections that are part of a listed assembly, where part of the listing process, are acceptable since the grounding connections have been tested. Machine screws with at least two threads or secured with a nut are acceptable, and any self-tapping screw, where at least two threads are formed, are acceptable. Section 250.8 has been changed into a list format for ease of use.

2005 NEC—250.20

Alternating-Current Systems to Be Grounded.

(D) Separately Derived Systems. Separately derived systems, as covered in 250.20(A) or (B), shall be grounded as specified in 250.30.

FPN No. 1: An alternate AC power source such as an on-site generator is not a separately derived system if the neutral is solidly interconnected to a service-supplied system neutral.

2008 NEC—250.20

Alternating-Current Systems to Be Grounded.

(D) Separately Derived Systems. Separately derived systems, as covered in 250.20(A) or (B), shall be grounded as specified in 250.30(A). Where an alternate source such as an on-site generator is provided with transfer equipment that includes a grounded conductor that is not solidly interconnected to the service supplied grounded conductor, the alternate source (derived system) shall be grounded in accordance with 250.30(A).

FPN No. 1: An alternate AC power source such as an on-site generator is not a separately derived system if the grounded conductor neutral is solidly interconnected to a service-supplied system grounded conductor neutral. An example of such situations is where alternate source transfer equipment does not include a switching action in the grounded conductor and allows it to remain solidly connected to the service supplied grounded conductor when the alternate source is operational and supplying the load served.

The extra text was added to clarify that an alternate source with a transfer switch—where the grounded conductor is not solidly connected together with the normal power grounded conductor and is switched when the ungrounded conductors are switched—is a separately derived system and must be grounded based on 250.30.

2005 NEC—250.22

Circuits Not to Be Grounded.

The following circuits shall not be grounded:

(1) Circuits for electric cranes operating over combustible fibers in Class III locations, as provided in 503.155.

(2) Circuits in health care facilities as provided in 517.61 and 517.160.

(3) Circuits for equipment within electrolytic cell working zone as provided in Article 668.

(4) Secondary circuits of lighting systems as provided in 411.5(A)

2008 NEC—250.22

Circuits Not to Be Grounded.

The following circuits shall not be grounded:

(1) Circuits for electric cranes operating over combustible fibers in Class III locations, as provided in 503.155.

(2) Circuits in health care facilities as provided in 517.61 and 517.160.

(3) Circuits for equipment within electrolytic cell working zone as provided in Article 668.

(4) Secondary circuits of lighting systems as provided in 411.5(A).

(5) Secondary circuits of lighting systems as provided in 680.23(A)(2).

The new (5) provides a reference to low-voltage (15-volt or less) transformers for underwater lighting in permanent swimming pools that also are not permitted to have a grounded secondary for the transformer.

2008 NEC—250.28

Main Bonding Jumper and System Bonding Jumper.

(D) Size. Main bonding jumpers and system bonding jumpers shall be sized in accordance with 250.28(D)(1) through (D)(3).

(1) General. Main bonding jumpers and system bonding jumpers shall not be smaller than the sizes shown in Table 250.66. Where the supply conductors are larger than 1,100 kcmil copper or 1,750 kcmil aluminum, the bonding jumper shall have an area that is not less than 12½ percent of the area so the largest phase conductor except that, where the phase conductors and the bonding jumper are of different materials (copper or aluminum), the minimum size of the bonding jumper shall be based on the assumed use of phase conductors of the same material as the bonding jumper and with an ampacity equivalent to that of the installed phase conductors.

(2) Main Bonding Jumper for Service with More Than One Enclosure. Where a service consists of more than a single enclosure as permitted in 230.71(A), the main bonding jumper for each enclosure shall be sized in accordance with 250.28(D)(1) based on the largest ungrounded service conductor serving that enclosure.

(3) Separately Derived System With More Than One Enclosure. Where a separately derived system supplies more than a single enclosure, the system bonding jumper for each enclosure shall be sized in accordance with 250.28(D)(1) based on the largest ungrounded feeder conductor serving that enclosure or a single system bonding jumper shall be installed at the source and sized in accordance with 250.28(D)(1) based on the equivalent size of the largest supply conductor determined by the largest sum of the areas of the corresponding conductors of each set.

New (2) and (3) provide direction on how to size main bonding jumpers for services with more than one enclosure and to size system bonding jumpers for separately derived systems with more than one enclosure. 250.28(D)(1) would apply to either services or separately derived systems with only one enclosure. The comment has added titles to the expanded text on main and system bonding jumpers.

2005 NEC—250.30

Grounding Separately Derived

Alternating-Current Systems.

(A) Grounded Systems. A separately derived AC system that is grounded shall comply with 250.30(A)(1) through (A)(8). A grounded conductor shall not be made to any grounded circuit conductor on the load side of the point of grounding of the separately derived system except as otherwise permitted in this article.

2008 NEC—250.30

Grounding Separately Derived

Alternating-Current Systems.

(A) Grounded Systems. A separately derived AC system that is grounded shall comply with 250.30(A)(1) through (A)(8). Except as otherwise permitted in this article, a grounded conductor shall not be connected to normally non-current carrying metal parts of equipment to equipment grounding conductors, or be reconnected to ground on the load side of the point of grounding of a separately derived system.

These changes clarify the current requirement in more prescriptive language. This proposed revision adds more specific restrictions of the grounded conductor connections to any ground connection on the load side of the service disconnect.

2005 NEC—250.30

Grounding Separately Derived

Alternating-Current Systems.

(A) Grounded Systems.

(4) Grounding Electrode Conductor, Multiple Separately Derived Systems. Where more than one separately derived system is installed, it shall be permissible to connect a tap from each separately derived system common grounding electrode conductor. Each tap conductor shall connect the grounded conductor of separately derived system to the common grounding electrode conductor. The grounding electrode conductors and taps shall comply with 250.31(A)(4)(a) through (A)(4)(c).

2008 NEC—250.30

Grounding Separately Derived

Alternating-Current Systems.

(A) Grounded Systems.

(4) Grounding Electrode Conductor, Multiple Separately Derived Systems. Where more than one separately derived system is installed, it shall be permissible to connect a tap from each separately derived system to a common grounding electrode conductor. Each tap conductor shall connect the grounded conductor of the separately derived system to the common grounding electrode conductor. The grounding electrode conductors and taps shall comply with 250.31(A)(4)(a) through (A)(4)(c). This connection shall be made at the same point on the separately derived system where the system bonding jumper is installed.

This final sentence was added to make it clear that the grounding electrode conductor connection must be made at the same point on the separately derived system where the system bonding jumper is installed, either within the separately derived system or within the first disconnecting means on the secondary side of the separately derived system, where provided. This sentence is similar to the one in 250.30(A)(3) for a single separately derived system.    

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at mark.c.ode@us.ul.com. JAMES G. STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the NEC and OSHA, as well as other standards. Contact him at 817.581.2206. JAMES W. STALLCUP is vice president and senior editor at Grayboy.