A contractor asked me to inspect the grounding and bonding of a service that he installed for a customer. He was told that the service had been grounded wrong and that it was dangerous to be powered up by the utility. Therefore, the owner would not pay the contractor for services rendered. However, the owner agreed to an inspection by a third party to verify the installation complied with the National Electrical Code (NEC). They agreed that I should perform an inspection and generate a report that they would review. They specified that the report had to strictly address the sizing of the conductors used for grounding and bonding the service equipment and whether they complied with the NEC.

Grounding electrode system
First, I inspected the grounding electrode system to determine if there was one electrode or multiple electrodes forming a system. I found that the building steel was connected to a concrete-encased electrode with a 2 AWG copper conductor. Section 250.52(A)(3) only requires a 4 AWG copper conductor. The owner was told that this was the wrong size and that it failed to meet the NEC.

Second, I checked how the structural steel was grounded and found that it was bonded to a ground ring with a 1/0 AWG copper conductor. The ground ring was buried about 3 feet into earth surrounding the commercial facility. This method of grounding complied with Section 250.52(A)(2). The metallic water piping was connected to a utility (water) pipe that was nonmetallic, but it met the NEC because it was bonded to the structural steel with a 1/0 AWG copper conductor; therefore, Section 250.104(A)(1) was satisfied.

Next, I checked the size of the service-entrance (SE) conductors to see if the grounding electrode conductor that was run to the structural steel from the panelboard was large enough to comply with Table 250.66. My evaluation found that the service equipment was fed with conductor sizes of three 350 kcmil THWN and one 2/0 AWG THWN. All service conductors were copper. Based on the size of these conductors, the grounding electrode conductor that was run from the panelboard and connected to the structural steel was 1/0 AWG copper and met the NEC. These SE conductors only required a 2 AWG copper grounding electrode conductor, so the 1/0 had more than enough capacity. By the NEC requirements, the concrete encased electrode needed a 4 AWG copper conductor as specified in Section 250.66(B). Just a 2 AWG copper was needed for the ground ring per sections 250.66(C), 250.52(A)(4) and 250.53(F). I noted in my report that the building structural steel was the termination point for the connection of the grounding electrode conductor and served as a bonding point for other routed electrode conductors. So far, the conductors used for grounding and bonding of the service were in compliance.

General practices ofgrounding and bonding
Section 250.4(A)(1) through (5) outlines the requirements for grounding and bonding electrical systems for safety. For example, Section 250.4(A)(1) covers the requirements for system grounding the utility transformer and the first disconnecting means at the service equipment in accordance with Table 250.66. Section 250.4(A)(2) covers the proper size equipment grounding conductor per Table 250.122. Sections 250.4(A)(3) and (4) specify the correct procedure for bonding the noncurrent-carrying metal parts together. Section 250.4(A)(5) requires that an effective ground-fault current path be designed so that fault current has an effective path to travel. The oversized conductors that the electrician installed for the grounding and bonding of the electrical system surely met the NEC requirements.

Ungrounded phase conductors
When calculating the phase-to-phase loads and adding this load to the phase-to-ground loads, the 350 kcmil, THWN copper conductors met the provisions of NEC 230.42(A)(1) and 220.61.

Grounded (neutral) conductor
I used sections 220.61 and 250.24(C)(1) to determine the size of the grounded neutral conductor. First, I calculated the neutral load per Section 220.61, and the calculation proved that the 2/0 AWG copper conductor was more than adequate for such load. Second, references to Section 250.24(C)(1) and Table 250.66 clarified that the 350 kcmil, copper SE conductors require only a 2 AWG copper conductor to serve as a return path for a ground-fault current condition. The 2/0 AWG grounded copper neutral conductor is more than adequate to handle the neutral load and serve as an effective ground-fault path for current if a ground fault accidentally occurred.

In conclusion, the oversizing of the grounding and bonding conductors did not, in any manner, violate the requirements outlined in the NEC for grounding and bonding an electrical service for safety. In my report, I explained it to both the contractor and facility owner by providing them with documentation.


STALLCUP is the CEO of Grayboy Inc., which develops and authors publications for the electrical industry and specializes in classroom training on the National Electrical Code and other standards, including those from OSHA. Contact him at 817.581.2206.