2008 NEC® Changes: Chapter 2 (Part 4)
NEC Digest®, February 2007 (necdigest®, National Electrical Code®, and NEC® are registered trademarks of the National Fire Protection Association, Inc., Quincy, MA 02169)
By James Stallcup, Sr. and Mark Ode
Editor's Note: James Stallcup Sr. and Mark Ode, two of the most knowledgeable and well-respected experts in the electrical industry, have teamed up to write a series of special Code Issues articles giving us the scoop on the most important upcoming changes for NEC 2008.
2008 NEC Products
In the fourth of a series discussing proposed changes to the 2008 National Electrical Code® (NEC®), the authors cover the remainder of the important changes to Chapter 2. To help simplify and clarify each change, words that have been deleted are shown with a strikethrough and words that have been added are underlined.
2005 NEC
240.92 Location in Circuit.
An overcurrent device shall be connected in each ungrounded circuit conductor as required in 240.92(A) through (D).
(A)Feeder and Branch-Circuit Conductors.Feeder and branch-circuit conductors shall be protected at the point the conductors receive their supply as permitted in 240.21 or as otherwise permitted in 240.92(B), (C), or (D).
(B)Transformer Secondary Conductors of Separately Derived Systems.Conductors shall be permitted to be connected to a transformer secondary of a separately derived system, without overcurrent protection at the connection, where the conditions of 240.92(B)(1), (B)(2), and (B)(3) are met.
2008 NEC
240.92 Location in Circuit.
An overcurrent device shall be connected in each ungrounded circuit conductor as required in 240.92(A) through (D).
(A)Feeder and Branch-Circuit Conductors.Feeder and branch-circuit conductors shall be protected at the point the conductors receive their supply as permitted in 240.21 or as otherwise permitted in 240.92(B), (C), (D), or (E).
(B)Feeder Taps.For feeder taps specified in 240.21(B)(2), (B)(3), and (B)(4), the tap conductors shall be permitted to be sized in accordance with Table 240.92(B).
Table 240.92(B) Tap Conductor Short-Circuit Current Ratings.
Tap conductors are considered to be protected under short-circuit conditions when their short-circuit temperature limit is not exceeded. Conductor heating under short-circuit conditions is determined by (1) or (2):
(1) Short-Circuit Formula for Copper Conductors
(2) (I2/A2)t = 0.0297 log10 [(T2 + 234)(T1< /SUB > + 234)]
(2) Short-Circuit Formula for Aluminum Conductors
(I2/A2)t = 0.0125 log10 [(T2 + 228)/(T1 + 228)]
where:
I = short-circuit current in amperes
A = conductor area in circular mils
t = time of short-circuit in seconds (for times less than or equal to 10 seconds)
T1 = initial conductor temperature in degrees Celsius.
T2 = final conductor temperature in degrees Celsius.
Copper conductor with paper, rubber, varnished cloth insulation, T2 = 200
Copper conductor with thermoplastic insulation, T2 = 150
Copper conductor with crosslinked polyethylene insulation, T2 = 250
Copper conductor with ethylene propylene rubber insulation, T2 = 250
Aluminum conductor with paper, rubber, varnished cloth insulation, T2 = 200
Aluminum conductor with thermoplastic insulation, T2 = 150
Aluminum conductor with cross-linked polyethylene insulation, T2 = 250
Aluminum conductor with ethylene propylene rubber insulation, T2 = 250
(C)Transformer Secondary Conductors of Separately Derived Systems.Conductors shall be permitted to be connected to a transformer secondary of a separately derived system, without overcurrent protection at the connection, where the conditions of 240.92(C)(1), (C)(2), and (C)(3) are met.
Author’s Comment: The panel action will increase the enforceability needed by the inspection community and limit the application of expanding feeder tap conductors for supervised industrial installations. This action recognizes the performance of the overcurrent device as a factor in determining the tap conductor size.
2005 NEC
240.21 Location in Circuit
(C)Transformer Secondary Conductors. Each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
FPN: For overcurrent protection requirements for transformers, see 450.3.
2008 NEC
240.21 Location in Circuit
(C)Transformer Secondary Conductors.A set of conductors feeding a single or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.
FPN: For overcurrent protection requirements for transformers, see 450.3.
Author’s Comment: The panel has added wording to clarify that more than one set of conductors often supply the same load when installed as parallel sets of conductors. The intent of the 2005 Code change was to clarify that these secondary conductor rules are not limited in their application to one set per transformer and intended to clarify that more than one set of conductors is permitted to be connected to a transformer secondary so long as the provisions of (C)(1) through (C)(6) are complied with.
2008 NEC
240.91 Protection of Conductors.
Conductors shall be protected in accordance with 240.4, unless otherwise permitted in 240.91(A).
(A)Devices Rated over 800 Amperes.Where the overcurrent device is rated over 800 amperes, the ampacity of the conductors it protects shall be equal to greater than 95% of the rating of the overcurrent device defined in 240.6, where the conductor is protected within recognized time vs. current limits for short circuit currents of up to 1000 seconds duration.
Author’s Comment: The proposal introduces the equivalent of a “next standard size” exception for large conductors in Supervised Industrial Installations. In these installations, conductors are protected against overload by load calculation and by monitoring, and against short circuit by selection of the overcurrent device as part of an overcurrent coordination study. Overcurrent protection of conductors in supervised industrial installations will be permitted to exceed the 800-ampere level rating permitted in 240.4(B), where the conductor is rated for 95% of the overcurrent device rating based on standard sizes in 240.6. The time duration of the short circuit current cannot exceed a 1000 second duration.
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) the 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.
Author’s Comment: Section 250.8 has been revised by deleting the specific prohibition of sheet metal screws since there are many other types of screws that are also 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 plus any self-tapping screw where at least two threads are formed are acceptable. It has also been changed into a list format for ease of use.
2008 NEC
250.14 Continuity of Grounded Conductors.The continuity of a grounded conductor shall not depend on a connection to a metallic enclosure, raceway, or cable armor.
Author’s Comment: This new section is intended to prohibit the connection of a grounded conductor to an equipment ground bar within service equipment or a separately derived system where the metal enclosure, raceway, or cable armor is used as a neutral path for current flow. A busbar or conductor must be used to make the connection between the neutral bar and the equipment ground bar, not the metal of the enclosure.
2005 NEC
250.28 Main Bonding Jumper and System Bonding Jumper.
(D)Size.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 1100 kcmil copper or 1750 kcmil aluminum, the bonding jumper shall have an area that is not less than 12 1/2 percent of the area of 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.
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) 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 1100 kcmil copper or 1750 kcmil aluminum, the bonding jumper shall have an area that is not less than 12 1/2 percent of the area of 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) 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) 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.
Author’s Comment: 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.
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Reprinted with permission from necdigest®, Copyright 2007, National Fire Protection Association, Quincy, MA 02169
necdigest®, National Electrical Code®, and NEC® are registered trademarks of the National Fire Protection Association, Inc., Quincy, MA 02169
