National Electrical Code Top Ten Tips: Article 250 -- Grounding
by Mark Lamendola
Based on the 2023 NEC.
Please note, we do quote from copyrighted material. While the NFPA
does allow such quotes, it does so only for the purposes of education
regarding the National Electrical Code. This article is not a substitute
for the NEC.
These are the 10 Article 250 items we deem most important, based
on the pervasiveness of confusion and the potential costs of same.
- Figure 250.1 and Section 250.1 helps you overcome a common problem that leads to misappliction of Article 250. Most
folks are so overwhelmed by Article 250 that they immediately
get lost when confronted with it. However, it’s divided into 6
logical groupings of information and Section 250.1 tells you
what those are. Figure 250.1 shows you how they relate to each other.
Looking in the Table of Contents in the front of the NEC is also helpful, as you can get an idea of how each Article breaks down. You can also get an idea of how the different Articles relate to each other, since they tend to be grouped (take a look at the sequence of Article names headings in Chapter 3 for a really good example).
- Article 100 clarifies things by defining "Effective
ground-fault current path," Ground fault," and
"Ground-fault current path." These definitions, even if
understood, are not enough for proper application of grounding. You
also need to understand the other grounding-related definitions in NEC
Article 100. Those are bonding (and variations) and grounding (and
variations). It helps to remember that you ground on the supply side but bond on the load side. Bonding creates an equipotential plane, grounding does not. That's because you bond with metal and thus create a low-impedance path. Grounding connects to earth, which is a high-impedance path.
- Another source of panic and confusion when
dealing with 250 is that many other NEC Articles apply.
The discussions on this issue during the NEC 2002 revision process
were on how to address the concern that the NEC is "too
complicated" (as if electricity is simple?) and "all of
the related information should be in one place" (which would be fine
if every application were identical). The result was Table 250.3. It handles this
issue quite nicely, by providing an substantial cross-reference. In the 18 years between the 2002 and 2020 revisions, Table 250.3 has more than earned its spot in Article 250. Unfortunately, it was removed with the 2023 revision. Or was that unfortunate? With the 2023 revision, those variations in other Articles were replaced with references to Article 250. This is an even better solution than a table in Article 250 referring you to other Articles. BTW, that table was big enough that it took up a full page.
- Section 250.4 details the general requirements for
grounding and bonding. It begins by distinguishing between, and
giving requirements for, five categories of grounding: Electrical
system grounding, Grounding of electrical equipment, Bonding of
electrical equipment, Bonding of electrically conductive materials
and other equipment, and Effective ground-fault current path. It
also identifies and gives requirements for four categories of
- Section 250.6 addresses another fundamental concept of
grounding. That is, the prevention of "objectionable current
flow over the grounding conductors or grounding paths." It is worthwhile to read this slowly and deliberately, thinking of the reasons for each of the provisions here. If you have a solid understanding of basic circuit theory, this won't be a problem for you. Think especially of Kirchoff's Law of Parallel Circuits. Keep in mind that when two metallic objects are connected by zero resistance there is thus no voltage (potential) between them and if there's no potential then there can be no current flow between them. The resistance (or impedance) of a bonding jumper is so low that, for practical purposes, we can regard it as being zero. For further explanation of this topic, consider purchasing Mike Holt's Bonding and Grounding textbook (and maybe the video program, too).
- "A premises wiring system
supplied by a grounded ac service shall have a grounding electrode
conductor connected to the grounded service conductor, at each
service…." [250.24(A)]. Electrons are always trying to get back to the
source, not (as many wrongly believe) to ground. The rest of 250.24 details requirements for
doing this for different applications.
- You need a main bonding jumper [250.28]. This section has expanded over the past few Code cycles; read it carefully.
- Section 250.34 discusses portable and vehicle-mounted
generators. A good reference for understanding why these would
differ from stationary systems is IEEE-142.
- Section 250.52 gives the requirements for grounding
electrodes. This is a more complex topic than most people think.
IEEE-142 gives a thorough theoretical treatise of it. The NEC just
gives the minimal requirements for safety. Do not confuse grounding electrodes with equipment grounding conductors, either in form or in function.
- Section 250.58 instructs us to use "the same electrode
for grounding conductor enclosures and equipment in or on that same
building." That is beacuse the concept of "separate ground" is
nonsense. Two good sources for more information on this are Soares
Book on Grounding and IEEE-142.
We could easily address 10 more "top tips" for Article 250.
For example, Section V on Bonding has plenty of good information.
However, the purpose of this article is to cover some fundamentals in a quick
and easy-to-read manner.
To gain a solid understanding of 250, you need to set aside a specific amount of time each week—maybe
a 30-minute study session every other night, or maybe fifteen minutes at lunch
each day—and tackle one section at a time. Supplement that by reading Soares
Book on Grounding and IEEE-142.
Check out this grounding