Growth Spurt

For many years, Article 250, which covers grounding and bonding in the National Electrical Code (NEC), only contained two tables. Table 250.122 was used for sizing the equipment grounding conductors, based on the size of the overcurrent protective device in the circuit. Table 250.66 was used for sizing grounding electrode conductors and for sizing the grounded conductor, the system bonding jumper, the main bonding jumper, and supply-side bonding jumpers. However, a change in the 2014 NEC added another table to Article 250. 

This new table was developed and inserted into the 2014 NEC to apply specifically to the second function of Table 250.66 and to make Article 250 easier to use. “Grounded Conductor, Main Bonding Jumper, System Bonding Jumper, and Supply-Side Bonding Jumper of Alternating-­Current Systems” is located in 250.102(C)(1). Similar to Table 250.66, the sizes of these grounded or bonding conductors in new Table 250.102(C) (1) are based on the size of the largest ungrounded supply conductors or equivalent area for parallel conductors and start at 2 AWG or smaller copper conductors (1/0 or smaller aluminum conductors) with sizes up through 1,100 kcmil copper (1,750 kcmil aluminum). The sizes of the grounded or bonding jumpers start at 8 AWG copper (6 AWG aluminum) and go up to 2/0 copper (4/0 aluminum). Where the ungrounded conductors exceed 1,100 kcmil copper (1,750 kcmil aluminum), the table requires compliance with one of four notes located at the bottom of the new table. These notes are part of the mandatory requirement of the table. The notes provide additional sizes of grounded or bonding jumpers, where ungrounded conductors are larger than 1,100 kcmil copper (1,750 kcmil aluminum) and where the grounded or bonding jumpers must carry fault current. 

Note 1 of Table 250.102(C)(1) states where ungrounded conductors are larger than 1,100 kcmil copper (1,750 kcmil aluminum) the grounded conductor or bonding conductors shall have an area not less than 12½ percent of the area of the largest ungrounded supply conductor or equivalent area where parallel conductors are used. It also states that the grounded or bonding conductor is not required to be larger than the largest ungrounded conductors or set of ungrounded conductors where installed in parallel. 

Note 2 states that, if the ungrounded supply conductors are different materials from the bonding conductors (copper, aluminum or copper-clad aluminum), the minimum size of the grounded or bonding conductors must be based on the assumed use of ungrounded supply conductors of the same material. The ampacity of the grounded conductor or the bonding conductor must be equivalent or more than the installed ungrounded supply conductors. Assume, for example, that four 500 kcmil aluminum conductors are paralleled and the conductors used for bonding purposes are copper. Each aluminum set of conductors has an ampacity in the 75°C column of Table 310.15(B)(16) of 310 amperes (A). The smallest copper conductor that would match that ampacity is 350 kcmil copper at 310A, so four paralleled 350 kcmil copper conductors would equal 1,400 kcmil. The total of 1,400 kcmil times 12½ percent requirement (for more than 1,100 kcmil) equals 175 kcmil for the size bonding jumper for connecting all four rigid metal conduit raceways. Using Table 8 in Chapter 9 would require at least a 4/0 copper bonding conductor (a 3/0 has a kcmil value of 167.800 kcmil, and a 4/0 has 211.600 kcmil).

Since Table 250.102(C)(1) has been added to 250.102(C), the text in that section has been modified from the 2011 NEC by creating a new (1) that covers the size of the supply-side bonding jumpers for supply conductors installed in a single raceway or cable and a new (2) that covers the size for parallel conductors in two or more raceways. In each of these subsections, the supply-side bonding jumpers are sized based on Table 250.102(C)(1). Where parallel conductors are installed in two or more raceways, an individual supply-side bonding jumper can be installed from each raceway or cable and sized from Table 250.102(C)(1) based on the size of ungrounded conductors in each raceway or cable. A single supply-side bonding jumper can be installed from raceway or cable to raceway or cable whereby the supply-side bonding jumper would be sized based on the combined size of the largest set of parallel conductors.

New Table 250.102(C)(1) is used for sizing these conductors or bonding conductors in Article 250 where there isn’t overcurrent protective devices ahead of the conductors. As can be seen, even though there are now three tables in Article 250, not much has changed technically.

About the Author

Mark C. Ode

Fire/Life Safety Columnist and Code Contributor
Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and .

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