Keeping the Bad Outside: Grounding electrodes required for outdoor transformers

Unsplash / Sen Lee
Unsplash / Sen Lee
Published On
Jul 15, 2021

Section 90.2(B)(5) states that the National Electrical Code does not apply to installations that are under the exclusive control of the electric utility company. Has anyone ever questioned the validity of this requirement since utility distribution is outside the scope of the NEC ? Do municipal electrical inspectors actually look for this electrode connection to the grounded conductor within the utility company’s transformer?

What is the reason for this requirement in the first place, and why are we worried about supplying a grounding electrode within the utility company’s transformer? Shouldn’t the electric utility company be responsible for this connection?

I will attempt to answer these questions and provide an explanation of an expanded and similar version of this requirement where dealing with premises-owned outdoor transformers.

Where a utility-owned electrical transformer supplying the electrical service is located outside of the building, at least one additional grounding connection must be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building, as stated in 250.24(A)(2).

Most utility companies have specification manuals dealing with the requirements that apply to their particular applications and installations. These specifications often deal with the thickness and specific opening sizes, as well as the sizes and number of raceways to be installed, especially where the pad is a poured concrete pad or a preformed fiberglass pad for ground-mounted transformers. Utility companies will also frequently require a driven ground rod or other type of grounding electrode be supplied for the ground-mounted transformer or a wrapped electrical grounding electrode (commonly called a butt-wrapped electrode) on the bottom of a wooden power pole.

Where one is not provided by an electrical contractor doing the power installation at the facility, the utility company will often install its own grounding electrode. The purpose of the grounding electrode connected to the grounded conductor for the utility company transformer is the same reason found in 250.4(A)(1) for premises-owned transformers. The most important reason for the grounding electrode is to stabilize the voltage to earth during normal operation so that a zero reference is provided for the grounded neutral conductor. Without this stabilized reference to zero, voltage can fluctuate, causing major issues within the electrical system. The grounding electrode also limits the voltage imposed by lightning, unintentional contact with higher-voltage lines and line voltage surges, keeping them from entering into the buildings from outside power sources (keeping “bad stuff” outside). This applies to utility company power sources and premises power sources.

Section 250.30 in the 2011 NEC , covering grounding of separately derived alternating current systems, was modified by adding 250.30(C), which deals with outdoor-premises separately derived power sources. Basically, it states that, if the source of the separately derived system is located outside of the building or structure, a grounding electrode connection must be made at the source location (the outside transformer or power source) to one or more grounding electrodes in compliance with 250.50. This would apply to separately derived transformers, generators and other external sources of power, such as UPS modules, modular power systems and energy storage systems.

In addition, the installation must comply with all the requirements in 250.30(A) for grounded, separately derived systems and 250.30(B) for ungrounded, separately derived systems. For a grounded system, a system bonding jumper installed at the transformer with the grounding electrode connection on the secondary side of a transformer would then require the installation of PVC or another acceptable nonmetal raceway from the transformer to the building being supplied. In the disconnecting means at the building, the grounded conductor would then be isolated from the disconnecting means enclosure, would only carry neutral current and would not be a part of a ground fault current path. The supply-side bonding jumper would be connected to the equipment grounding bar in the disconnecting means and would be sized based on Table 250.102(C)(1) since there isn’t an overcurrent protective device.

Section 250.32(A) and (B), dealing with feeders to separate buildings, would require the building grounding electrode conductor be connected to the equipment grounding bar in the enclosure at the separate building, and not to the neutral as would be the case for a service.

Remember, you do not want the grounded conductor from the building feeder disconnecting means to be in parallel with the equipment grounding conductor or supply-side bonding jumper so that neutral current is also traveling on the equipment grounding conductor or the supply-side bonding jumper back to the transformer or other power source. It could cause major issues with your feeder and could be exceedingly dangerous.

About the Author

Mark C. Ode

Fire/Life Safety, Residential 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 Mark.C.Ode@ul.com.

Stay Informed Join our Newsletter

Having trouble finding time to sit down with the latest issue of
ELECTRICAL CONTRACTOR? Don't worry, we'll come to you.