National Electrical Code Top Ten Tips: Article 250 -- Grounding
by Mark Lamendola
Based on the 2017 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.
- 250.1 helps you overcome a very common problem. Most
folks are so overwhelmed by 250 that they immediately
get lost when confronted with it. However, it’s divided into 6
logical groupings of information and 250.1 tells you
what those are. See also Figure 250.1.
Looking in the Table of Contents in the front of the NEC is also helpful, as you can get an idea of how Articles break down. You can also get an idea of how they 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).
- 250.2 clarifies things by defining "Effective
round-fault current path," Ground fault," and
"Ground-fault current path." These definitions, if
understood, are not enough for proper application of grounding. You
also need to understand the grounding-related definitions in NEC
Article 100. Those are bonding (and variations) and grounding (and
- 250.3. 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
was 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). NEC Table 250.3 handles this
issue quite nicely, by providing an substantial cross-reference.
- 250.4. This 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
ungrounded systems. Figure 250.4 is a great visual for seeing which
Parts of 250 apply to various aspects of grounding.
- 250.6 addresses another fundamental concept of
grounding. That is, the prevention of "objectionable current
flow over the grounding conductors or grounding paths."
- 250.24(A) says, "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…." 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.
- 250.28. You need a main bonding jumper. "For a
grounded system, an unspliced main bonding jumper shall be used to
connect the equipment grounding conductor(s) and the
service-disconnect enclosure to the grounded conductor of the system
within the enclosure for each service disconnect. There are two
exceptions to this, but in no case can you use the earth as your
bonding jumper—its resistance is simply too many orders of
magnitude too high.
- 250.34 discusses portable and vehicle-mounted
generators. A good reference for understanding why these would
differ from stationary systems is IEEE-142.
- 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.
- 250.58 instructs us to use "the same electrode
for grounding conductor enclosures and equipment in or on that same
building." The concept of "separate ground" is
nonsense. Two good sources for more information on this are Soares
Book on Groundingand 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 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