Robert asks us: I purchased your handbook about a month ago and I am half way thru reading it. My question, how can I explain in an everyday way to the field electrician (inspectors) that #6 is not the catch all Grounding electrode conductor? I also highly debate this with the staff engineer. I understand that 250.66 A is misapplied. Can you provide some guidance to help explain this to everyone. I appreciate your input . Also , let me make it clear that we are in Afghanistan and that the 2008 and 2011 NEC codes are the current standards . Thank you
There is a “catch-all” earthing electrode conductor (GEC) according to Table 250.66 of the National Electrical Code (NEC), and that is a 3/0 AWG copper conductor (this is true for the 2005, 2008, 2011, and 2014 versions of the Code). Some would argue that a 250 MCM aluminum conductor could be added to the list, however the NEC prohibits use of aluminum within 18 inches of the earth in 250.52(B)(1), 250.64(A), and 680.21(A)(1), making aluminum conductors only a partial option at best for the GEC.
According to Table 250.66, a #6 copper GEC is only good for electrical services of 150 amps and less.
Keep in mind that the GEC is the conductor that is used to bond your electrical service enclosure to the earthing electrodes. There are also requirements for the equipment earthing conductor (EGC) that can be found in Table 250.122. The EGC is the earth wire that is routed with each circuit (from the circuit breaker to the load), often referred to as the third wire earth. According to Table 250.122, a #6 EGC is acceptable for up to a 200 amp circuit.
So in short, a #6 copper conductor may be used as a GEC for a 150 amp or less service, or it may be used as an EGC for a 200 amp or less circuit.
Also for the record, you generally see a 4/0 AWG copper conductor used as the “catch-all” earth wire for power systems, as this meets the National Electrical Code 3/0 AWG requirement, IEEE, ANSI, MILSPEC, and many other industry standards. If you are bonding communication equipment with a 200-amp or less service, the “catch-all” earthing conductor becomes a #2 tinned solid copper conductor, due to a number of industry standards (Motorola’s R56, ATT-TP-76416, ANSI/TIA/EIA-J-STD-607-A, and numerous others) regulating the earthing of data communication systems.
Of note, is that in NEC 250.52(A)(7) it lists a maximum GEC requirement for driven earth rods as a #6 AWG. What is not clearly spelled out in the NEC, is that you still must meet the circular mils requirements for your service in Table 250.66. We cover this issue fairly extensively starting on Page 208 of the book. This is really a week-spot in the Code and it has not been well addressed. If you look at the illustration on Page 209, you can see that the service requires a properly sized GEC to the earth bar, and then each of the five (5) different electrodes only needs the maximum listed GEC. However, if you add up the circular mils of each of the five (5) GEC’s individually, you will see that it meets the requirements of Table 250.66.
I hope this helps and we hope you enjoy the book!
BTW, we would love to see some pictures of you and your staff with the book in Afghanistan! Many of our team members are former military themselves, and we all wish to thank you for your service. Be safe.
The Engineering Team at E&S Earthing Solutions
Photo credit: RevDesign