When it comes to grounding and bonding, understanding the terminology is crucial for correct applications of the requirements in Article 250. I recommend opening your 2017 through 2026 Code® book, take a green colored highlighter, review Article 100, highlighting every term that is followed by (CMP-5). Those are all the important terms that must be understood when navigating Article 250 for grounding and bonding requirements.
While working on a jobsite, someone might ask you to pull the “ground wire." What color wire would you pull? The Green wire! That wire is called something, and it’s not the “ground." You need to know what to call it. It is called an equipment grounding conductor or an EGC. You must know what each component or conductor of the effective ground-fault current path is called and what is the purpose of it. The EGC is just one of three essential parts of the effective ground-fault current path:
Equipment Grounding Conductors
Main Bonding Jumper or System Bonding Jumper
Grounded (neutral) Conductor
The use of the earth shall not be considered part of the effective ground-fault current path! See 250.4(A)(5)
Three Tables should be remembered to be able to verify sizing and applications to grounding and bonding of electrical systems. Since the 2014 NEC®, a new Table was added which, I feel, brought ease of use and clarity to the applications of the old, existing Table 250.66.
I will try to simplify using and applying these Tables in this month’s column.
First, let’s explore Table 250.122 and application of the equipment grounding conductor (EGC) and proper sizing of that wire. If the feeder or branch circuit is protected by a fuse or circuit breaker, it must be provided with an EGC. In 250.118, you can review the list of qualifying Equipment Grounding Conductors. The EGC may be a metallic raceway or listed cable armor. It could be metallic enclosures, cable trays or wireways, or a separate wire. The wire could be bare, covered or insulated. For Healthcare facilities, in accordance with 517.13(A), you might have both a qualifying raceway and a wire-type EGC. The size of a wire-type EGC will be found in 250.122 and Table 250.122. You need to understand and highlight the words in the heading “NOT EXCEEDING”. This will help with correct application of the Table. For example:
A 100-amp overcurrent protective device (OCPD) must have an EGC not smaller than 8 AWG. But if you go up to a 125-amp breaker you would need to increase the EGC up to a 6 AWG. Then the 6 AWG would be permitted for every standard size OCPD up to 200 amps. Larger than 200 amps and up to 300 amps, a 4 AWG would be required.
Next is Table 250.66 which is the requirements for the grounding electrode conductors. If you are sizing a wire that connects the electrical system to the earth by the use of a grounding electrode, Table 250.66 is the place. The earth makes a great ground. The problem is the earth does not have terminals to attach wires to. We use the qualifying grounding electrodes as described in 250.52. These would include ground rods, Ufer grounds, water pipe and ground rings. When applying the rules of 250.66, you need to pick off the low hanging fruit first. Ther are three options that should be considered first.
1. 250.66(A) Ground Rods – is not required to be larger than 6 AWG.
2. 250.66(B) Concrete Encased (Ufer) – is not required to be larger than 4 AWG.
3. 250.66(C) Ground Rings – is not required to be larger than the conductor used for the ground ring which is typically not less than 2 AWG.
If the grounding electrode is not one of those described above then you end up using sizes in accordance with Table 250.66
Finally, Table 250.102(C)(1) which is everything else, including the grounded (neutral) conductor. The calculated neutral conductor size must be compared to the minimum size, required in Table 250.102(C)(1). Without a properly sized grounded conductor, you have a compromised effective ground-fault current path. This Table looks and acts like 250.66 except when you get into sizing grounded and bonding jumpers based on the ungrounded conductors being sized at over 1100 kcmil. Table 250.66 stops at 3/0 AWG. A 3/0 AWG, copper wire is the largest wire required to connect to the earth as per Code®. But 250.102(C)(1) requires the user to do the math and size those conductors at 12.5 percent of the total kcmil or paralleled equivalent. That rule can be found in Note 1
As you look at both Tables it shows a row for “2/0 or 3/0 AWG” and one for “over 3/0 AWG”. Below that, you can see another row for “350 kcmil through 600 kcmil” and then another row for “Over 600 kcmil." The use of the words, “over 3/0, 350, and 600 kcmil," is not by accident. It references the paralleled conductors that are not always calculated in an even number of kcmils. If you take two, 2/0 AWG, it doesn’t add up to 4/0 AWG. Two 2/0 AWG equals 133,100 Circular Mils (CM) each, for a total of 266,200 Cir. Mils. That is “Over 3/0 AWG through 350 kcmil." Calculate four, paralleled 3/0 AWG. What is the total kcmil? Chapter 9 -Table 8, helps us do the math by adding the circular mil of each 3/0 AWG which is 167,800 circular mils each. A “mil” equals 1/1000 of an inch. Four of these wires equates to 671,200 CM which is “Over 600” kcmil.
To summarize:
√ A green wire protected by a fuse or breaker – Table 250.122.
√ A wire connected to the earth – Table 250.66.
√ Everything else – Table 250.102(C)(1).
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