The use of fine stranded, flexible cables
appears to be increasing each year. This is particularly true
with relatively "young" industries like the
photovoltaic (PV) industry, the fuel cell industry, and the
uninterruptible power supply (UPS) industries. In many cases,
technicians and installers in these fields prefer to use
fine-stranded flexible cables in the larger sizes (1/0 AWG and
up) due to the perceived easier installation of these cables
compared to the more rigid conventional cables.
Photo
1 shows the differences between a typical standard Class B
cable and a typical fine stranded cable (sometimes incorrectly
known as diesel locomotive cable or welding cable).
Both cables are 2/0 AWG (67.4 mm2). The THHN Class
B cable on the left has 19 separate conductors, each with a
diameter of 0.084 in. (2.13 mm). The THW fine stranded cable
on the right has 1330 separate conductors, each with a
diameter of 0.01 inch (0.25 mm).
Note that both types of cables are listed
in NEC Table 310.13 as suitable for code-compliant
installations. Cables marked with only the DLO (Diesel
Locomotive) marking are not suitable for code-compliant
installations, and listed welding cables are only to be used
when attached to the secondary of welding machines under the
requirements of NEC Article 630.
Also of note, based on the values in NEC Table 5, chapter 9, is the overall diameter of the 2/0
THHN cable at 0.532 in. (13.51 mm) compared with an overall
diameter of 0.610 in. (15.49 mm) for the THW. The greater
diameter of the fine stranded THW cable is mainly due to the
thicker insulting jacket required for THW cables. This
generally indicates that fewer THW cables will fit in a given
size of conduit.
Reports (unfortunately, mostly anecdotal)
have been received over the last several years about
field-made connections in PV and UPS systems that have failed
when flexible, fine-stranded cables have been used with
mechanical terminals or lugs that use a set screw to hold the
wire in the terminal.
These terminals are found on nearly all
circuit breakers (except those with stud-type terminals), fuse
holders, disconnects, PV inverters, charge controllers, power
distribution blocks, some PV modules, and many other types of
electrical equipment. Photo
2 shows examples of a few of these set screw types of
terminals.
Fine-stranded conductors and cables are
considered as those cables having stranding more numerous than
Class B or C stranding. Class B stranding (the most common)
will normally have 7 strands of wire per conductor in sizes
18-2 AWG, 19 strands in sizes 1-4/0 AWG, and 37 strands in
sizes 250-500 kcmil. Conductors having more strands than these
are widely available and are in different classes such as K
and M used for portable power cords and welding cables.
Commonly used building-wire conductors such as USE, THW, RHW,
THHN and the like are most commonly available with Class B
stranding but are also readily available (in some
locations) with higher quantities of stranding. Fine-stranded cables are frequently used by PV installers to
ease installation and are used in PV systems for battery
cables, power conductors to large utility-interactive
inverters and elsewhere.
Some PV modules are supplied with
fine-stranded interconnecting cables (14 AWG–10 AWG) with
attached irreversible compression connectors. While these
crimped-on connectors listed with the module are suitable for
use with the fine-stranded conductors, an end-of-string
conductor with mating connector may also be supplied with the
fine-stranded conductor, and the unterminated end of that
conductor will not be compatible with mechanical terminals.
According to Underwriters Laboratories (UL)
Standard 486 A-B, a terminal/lug/connector must be listed and
marked for use with conductors stranded in other than Class B
and C. With no marking or factory literature/instructions to
the contrary, the terminal may only be used with
conductors with the most common Class B and C stranded
conductors. These terminals and lugs are not suitable and
should not be used with fine-stranded cables. UL engineers
have said that few (if any) of the normal screw-type
mechanical terminals that the PV industry commonly uses have
been listed for use with fine stranded wires. The terminal
must be marked or labeled specifically for use with
fine-stranded conductors [see NEC 110.3(A) and (B)].
UL suggests two problems, both of which
have been experienced in PV systems. First, the tightening
screw tends to break the fine wire strands, reducing the
amount of copper available to meet the listed ampacity.
Second, the initial torque setting does not hold and the fine
strands continue to compress (creep) after the initial
tightening. Even after subsequent retorquing, the connection
may still loosen. The loosening connection creates a
higher-than-normal resistance connection that heats, loosens
even further, and may eventually fail. A recent example of a
failed mechanical terminal from a large PV system is shown in
photos 3 and 4.
The terminal had been torqued properly less than three months
before the failure.
Over-tighten or
Retighten?
Some installers over-tighten or retighten a connection to get
fine stranded cable to hold in screw-type terminals. UL
standards for connectors require that the terminal be
tightened once to the specified torque and there is no
retightening specified. Tightening the terminal beyond the
specified torque value may cause binding of the threads
thereby giving a false torque reading. Both over-tightening
and retightening of listed connectors and terminals on
overcurrent devices and other equipment would appear to
violate the provisions of the listing and therefore be a
violation of NEC Section 110.3(B).
A quick review of NFPA Standard 70B-2002, Recommended
Practice for Electrical Equipment Maintenance, does not
find any suggestions that electrical equipment terminals be
periodically retorqued. The terminals are to be inspected and
examined for signs of looseness or overheating and that
situation should be corrected where found. There is a
retorquing recommendation for mechanical fasteners on box
covers and the like.
Solutions
Electrical equipment listed to UL Standards has:
• Terminals rated for the required
current and sized to accept the proper conductors
• Sufficient wire bending space to
accommodate the Class B stranded conductors in a manner that
meets the wire bending requirements of the NEC
• Provisions to accept the appropriate
conduit size for these conductors where conduit is required.
It is therefore unnecessary to use the
fine-stranded cables except possibly when dealing with
conductors 4/0 AWG and larger. Experienced electricians and
electrical contractors routinely install the normal,
relatively stiff Class B conductors without difficulty and use
parallel-connected smaller conductors where very large
conductors are required.
In those cases where a fine-stranded cable
must be used, a few manufacturers make a limited number of
crimp-on compression lugs in various sizes that are suitable
for use with fine-stranded cables. These lugs are attached to
a stud on the device using a washer and nut. Most of the
commonly used overcurrent devices (both circuit breakers and
fuse holders/terminals) come with screw-type terminals so
there is no stud available. Most of these special crimp-on
lugs are solid copper or tinned solid copper. Photo
5 shows a typical copper light-duty crimp-on lug that is
not marked as being suitable for use with fine stranded
cables.
Factory-supplied markings and literature
indicate which lugs are suitable. An example is the ILSCO FE
series of lugs in sizes 2/0 AWG and larger (see photo
6). Burndy makes the YA-FX series of lugs in sizes 8 AWG
and larger that have been listed for use with fine stranded
cables. In both cases the lugs are solid copper. It should be
emphasized: Most crimp-on lugs are not listed for
use with fine-stranded wire. Where the crimp-on
compression lugs can be used, they must be installed
using the tools recommended by the manufacturer and, of
course, they must be attached to a stud with a nut and washer.
Other terminal manufacturers also make pin
adapters (a.k.a. pigtail adapters) that can be
crimped on fine-stranded cables. These pin adapters provide a
protruding pin (solid or stranded) that can be inserted into a
standard screw-type mechanical connector. Again, not all pin
adapters/pigtail adapters are listed for use with
fine-stranded conductors; some are intended for use with
aluminum wire and others provide only a conversion to a
smaller AWG size for a B Class conductor.
It is suggested that the use of
fine-stranded conductors be avoided wherever possible. Where
such cables must be used, they should only be terminated with
the appropriate connectors/lugs. Previously installed systems
should be revisited and the cables replaced where possible or
terminated properly.
For Further
Thought
Some of the requirements established by UL Standards are of
the form: "Don’t do something unless it is specifically
allowed by markings on the product or in the
instructions." An example is the use of fine-stranded
cables discussed above. They are not to be used with a
connector or terminal unless that connector or terminal is
specifically marked allowing their use. Another example may be
the Line and Load markings on circuit breakers
where the absence of such markings indicate they are
deemed suitable for backfeeding. Most of the dc circuit
breakers used in the PV industry are marked with Line and Load, but are routinely used in a backfeed
configuration on listed equipment.
Many of these "invisible"
requirements were developed decades ago in the early days of
the electrical power industry. Old-line firms like Square D,
GE, T&B, Westinghouse, and the other manufacturers and
users of electrical equipment have developed in-house
procedures and standards to preserve the "old"
corporate knowledge of these hidden requirements over the
years as people come and go. It appears that
"youngster" industries like PV, fuel cells,
uninterruptible power systems and the like, may not have
developed a means of first discovering and then preserving
these "hidden" requirements. Additionally, the
testing agencies may be overlooking some of these "’invisible"
requirements in the testing and listing of equipment as their
corporate memory retires.
The PV Industry (and possibly others) may
have to implement a "search and discover" activity
to ferret out these hidden requirements, develop methods to
preserve the knowledge, and then ensure that they are met by
our equipment and systems that must remain safe, reliable, and
durable for 30+ years.
Installed or inspected any fine conductor
cables recently?
For Additional
Information
If this article has raised questions, do not hesitate to
contact the author by phone or e-mail. E-mail: jwiles@nmsu.edu,
Phone: 505-646-6105
A PV Systems Inspector/Installer Checklist
will be sent via e-mail to those requesting it. A copy of the
100-page Photovoltaic Power Systems and the National
Electrical Code: Suggested Practices, published by Sandia
National Laboratories and written by the author, will be sent
at no charge to those requesting a copy with their address by
e-mail. The Southwest Technology Development web site (http://nmsu.edu/
tdi) maintains all copies of the "Code Corner
Columns" written by the author and published in Home
Power Magazine over the last 10 years.
The author makes 6–8 hour presentations
on "PV Systems and the NEC" to groups of 40
or more inspectors, electricians, electrical contractors, and
PV professionals for a very nominal cost on an as-requested
basis.
John Wiles works at the Southwest
Technology Development Institute (SWTDI) at New Mexico
State University. SWTDI has a contract with the US
Department of Energy to provide engineering support to
the PV industry and to provide that industry, electrical
contractors, electricians, and electrical inspectors
with a focal point for code issues related to PV
systems. He serves as the secretary of the PV Industry
Forum that will be submitting 30+ proposals for Article
690 in the 2008 NEC. He provides draft comments to NFPA
for Article 690 in the NEC Handbook. As an old solar
pioneer, he lives in a stand-alone PV-power home in
suburbia with his wife, two dogs, and a cat—permitted
and inspected, of course.
This work was supported by the United
States Department of Energy under Contract
DE-FC04-00AL66794 |
