The inspector had been called in on the case to see if an
explanation could be given as to what had happened. He was
very puzzled as he closely examined the rigid nonmetallic
conduit installation. There were several broken straps as well
as a broken solvent weld where the rigid nonmetallic conduit
entered a junction box. Who had caused such damage to the
installation? What had they used to pry apart the solvent weld
without leaving any marks? The broken straps didn’t appear
to have been hit with a hammer or other heavy object. What
happened here? How could this case be explained?
The explanation is simple. What we have here is a case of
movement. Rigid nonmetallic conduit (RNC) has found wide
acceptance in modern electrical installations. There are
actually different types of RNC available. The type we are
probably most familiar with is polyvinyl chloride. The
expansion and contraction characteristics for each of these
types is different. For example, polyvinyl chloride conduit
and fiberglass reinforced epoxy conduit are both rigid
nonmetallic conduits. However, each of them has its own
characteristics including different coefficients of linear
expansion. This means they expand and contract as a result of
temperature changes at different rates. Although the Code®
requirements are the same for each type of rigid nonmetallic
conduit, due to the differing coefficients of linear
expansion, the results of the calculation made to determine
the amount of expansion and contraction will vary. For the
sake of this article we will be making our calculations based
on the coefficient of linear expansion of rigid nonmetallic
conduit made of polyvinyl chloride (PVC).
Rigid nonmetallic conduit first appeared in the 1962 NEC as a new Article 347. At the time, it was somewhat
revolutionary as no other nonmetallic raceways were permitted
by the Code. Rigid nonmetallic conduit laid the
necessary ground work to allow the later acceptance of such
raceways as electrical nonmetallic tubing, nonmetallic
wireways, and surface nonmetallic raceways.
The 1996 NEC Article 347 differs only slightly in
requirements from the 1962 version. One of those differences
occurs in Section 347-9. The 1996 NEC not only requires
consideration be given to expansion and contraction, as was
required by the 1962 version, but also provides a specific
beginning threshold at which fittings are to be utilized to
provide for such expansion and contraction. It states,
"Expansion fittings for rigid nonmetallic conduit shall
be provided to compensate for thermal expansion and
contraction where the length change, in accordance with the
tables in Chapter 9, is expected to be 0.25 in. (6.36 mm) or
greater in a straight run between securely mounted items such
as boxes, cabinets, elbows, or other conduit
terminations."
Expansion and contraction of RNC equal to or exceeding 0.25
inch may result in damage to the raceway, fittings, supports,
and boxes. Such movement can pull the raceway out of a fitting
or box and break straps, boxes, and fittings. Because of the
potential for damage to items associated with the installation
of RNC as well as the RNC, it is necessary to consider whether
or not expansion or contraction will meet the stated threshold
thereby requiring the installation of fittings to compensate
for the movement of the RNC. This article is intended to
provide some guidance on the application of the requirements
in Section 347-9 as they relate to expansion and contraction
of RNC.
There are two requirements in 347-9 which need to be
considered in determining whether or not an expansion fitting
is required. The first of these requirements is the amount of
expansion and contraction of the RNC. The amount of expansion
and contraction is a property of the material itself. For
instance, polyvinyl chloride will have the same amount of
expansion or contraction for each degree of temperature
change. The same is true of rigid steel conduit, the air we
breathe, the water we drink, and the brick we build our homes
out of. Each of them will have a certain amount of movement
for each degree of temperature change. As the temperature
increases expansion takes place and as temperature decreases
contraction occurs.
The second requirement which needs to be considered when
determining the need for an expansion fitting is whether or
not the RNC is installed between securely mounted items. As
temperatures increase and decrease the RNC is caused to expand
and contract. This movement is restricted by securely mounted
items. This results in stress being applied to straps,
fittings, termination points, solvent welds, etc. If enough
stress is applied, something has to give.
The wording found in Section 347-9 requires compliance with
the Section when RNC is installed "between securely
mounted items such as boxes, cabinets, elbows, or other
conduit terminations." Figure
A shows an overhead drop to a building which utilizes an
RNC installation with a weather head. Because the weather head
is not "securely mounted" to the building, an
expansion fitting is not required. The RNC will freely expand
and contract away from or toward the securely mounted meter
base. It is important to provide enough room for expansion to
occur. If the weatherhead were located beneath an overhang or
other obstruction which would not allow the RNC to move, it is
possible that as the RNC bowed due to the pressure applied
from expanding against a fixed item, such as a roof overhang,
a strap, a coupling, or other item would break.
Figure B shows a safety switch and a junction box, both of which are
securely mounted. If the amount of expansion or contraction
equals or exceeds the threshold of 0.25 inch, movement of the
RNC installed between these two items may result in damage to
the raceway, straps, fittings, terminations, etc.
Figure C illustrates a run of RNC between a panel and a wireway with an
elbow between them. The language in 347-9 requires
consideration be given to expansion and contraction
"between securely mounted items such as boxes, cabinets,
elbows or other conduit terminations." In this
installation, we need to consider the run of RNC on each side
of the elbow separately to determine the need for an expansion
fitting. If the movement on either side equals or exceeds 0.25
inch, an expansion fitting would be required.
Let’s take a step by step look at how to determine
whether or not an expansion fitting is required. The first
step is to determine the minimum and maximum temperatures to
which the RNC will be exposed. As an aid, you may contact the
local official weather station to obtain the maximum and
minimum temperatures. For the sake of developing an
illustration, we’ll utilize the same temperature extremes I
use in my locality. The temperature range I use for RNC
installations outdoors is from 15°F below zero in the winter
to 115°F above zero in the summer. This means that RNC
installed outdoors will be exposed to a total temperature
range of 130°F. I am aware of some manufacturers who
recommend that an additional 30°F be added for installations
where the RNC will be exposed to radiant heating from sunlight
exposure. This is illustrated by a rooftop installation where
the RNC is subjected to the changes in the ambient temperature
as well as direct sunlight exposure resulting in additional
heating of the RNC.
The next step is to determine the amount of change in
length the RNC will have based on this temperature range.
Remember that Section 347-9 requires that we use the tables in
Chapter 9 to determine the length change of RNC. There is only
one table currently in Chapter 9 which details the information
needed to determine the length change. Table 10 is divided
into alternating columns of temperature change and length
change. We begin by finding our temperature range in the
appropriate temperature change column. Counting from left to
right, the fifth column contains the 130°F temperature range
we are looking for. Moving to the immediate right of this
entry we find that the length change for PVC RNC is 5.3-inches
per 100 feet of PVC RNC. This means that the PVC RNC will
expand or contract 5.3-inches for each 100 feet of length when
it is exposed to a temperature change of 130°F. This is a
length change of .053-inch per foot [5.3-in/100 ft = .053-in
per foot]. For a run of 20-feet "between securely mounted
items such as boxes, cabinets, elbows, or other conduit
terminations," there would be a total length change of
1.06-inches [.053-in x 20-ft = 1.06-inches] for a temperature
change of 130°F.
Having determined that the total length change exceeds the
threshold of 0.25-in., our next step is to install the
expansion fitting. The expansion fitting consists of two
pieces, a barrel and a piston. Referring to Figure
D we see an illustration of a typical expansion fitting
installation. Because the piston moves inside the barrel, it
is important that the piston and barrel are aligned to prevent
the piston from binding. The barrel portion of the fitting is
to be securely fastened in place and the piston is then
installed so it will move, as the RNC expands and contracts,
inside the barrel. This means the length of conduit solvent
welded to the piston must be allowed to move. I am aware of at
least one manufacture of RNC who makes clamps that are
designed to allow movement in conduit trade sizes up through
2-inches. Metal conduit and tubing straps may be utilized if
they are sized slightly larger than the outside diameter of
the RNC so movement will be allowed and they are installed so
the conduit will not bind on the strap as it expands and
contracts. It is important here to remember the language in
Section 347-8 requires "Rigid nonmetallic conduit shall
be fastened so that movement from thermal expansion or
contraction will be permitted." However, it is equally
important to remember the language also requires "conduit
shall be securely fastened within 3 ft (914 mm) of each outlet
box, junction box, device box, conduit body, or other conduit
termination." This means the RNC must be securely
fastened within 3-feet of a termination while doing so in a
manner which still allows for any necessary linear expansion
and contraction to take place.
The piston is equipped with at least one o-ring to seal
against the entrance of dirt, water, insects, etc. into the
system. There are at least some expansion fittings I am aware
of which are listed as being water-tight as long as the piston
is installed so it moves horizontally or upward, rather than
downward, into the barrel. This installation configuration
will prevent water from possibly entering the conduit system
by way of the small gap between the piston and the barrel.
This is an unlikely occurrence due to the o-ring seal and the
close tolerances between the barrel and the piston. However,
it is a remote possibility and the installation of the fitting
so the piston is not located above the barrel will preclude
the entrance of water under normal conditions.
The expansion fitting must be installed based on the
temperature at the time of installation relative to the total
temperature change to which the conduit will be exposed. For
instance, if the PVC RNC is being installed at a time when the
temperature is 60°F, the straps, boxes, elbows, etc. will be
installed with the PVC RNC length at the 60°F length. As the
PVC RNC is exposed to temperatures above or below 60°F then
the length of the PVC RNC will change based on its coefficient
of linear expansion. So, we have to determine at what point
the expansion fitting piston has to be set relative to its
total movement.
Expansion fittings are available with different maximum
expansions such as 2-inch or 6-inch. Let’s use the
information provided above to determine what the setting would
be for an expansion fitting with a total movement of 6-inches.
Remember, we have a temperature range of 130°F and a
temperature at the time of installation of 60°F. To determine
the piston opening we subtract the temperature at the time of
installation from the maximum temperature to which the PVC RNC
will be exposed, divide it by the total temperature range to
which the PVC RNC will be exposed, and multiply the result by
the total movement of the expansion fitting. Plugging in the
numbers for our installation we do the following:
Piston Opening = [(Max Temp - Installation Temp) / Total
Temp Range] x Fitting Length
Piston Opening = [(115°F - 60°F) / 130°F] x 6-inches
Piston Opening = [(55°F) / 130°F] x 6-inches
Piston Opening = .42 x 6-inches
Piston Opening = 2.52 inches
The next step is to insert the piston into the barrel to
its maximum depth and make a mark on the piston at the end of
the barrel. (Refer to Figure
E) This mark represents the maximum depth to which the
piston can go into the barrel. This is unnecessary in some
cases as there are manufacturers who have already placed a
mark on the piston to indicate the maximum depth. To set the
piston based on the information given above, we pull the
piston out a distance of 2.52-inches from the mark we made
earlier. This setting will allow the PVC RNC to expand and
contract within its range as he piston slides in and out of
the barrel.
I hope this article has helped in giving you an
understanding of how to apply the provisions of Section 347-9.
It is important to remember that PVC RNC is an excellent
product and needs to be installed in conformance with the Code requirements to address the possible damage to PVC RNC
installations.
Wayne Lilly, senior building codes inspector for the city
of Harrisburg, Virginia, has been the principal representative
for IAEI on Code-Making Panel No. 8 since 1991. He is a
certified master electrician, a member of the IAEI Education
Committee, chairman of the Southern Section Education
Committee, a past chairman of the National Certification
Program for Construction Code Inspectors Electrical Test
Development Committee, and a member of the American National
Standards Institute C-80 Committee. |
