The National Electrical Safety Code (NESC)
only addresses conductor ampacity directly relative to
grounding conductors. Rule 93C, page 19, requires grounding
conductors to have a short-time ampacity adequate to handle
the available fault current magnitude for the time it takes
the source protection device to operate without melting or
otherwise affecting the design characteristics of the
conductor. One source for calculation of short-time ampacity
is Aluminum Electrical Conductor Handbook published by
The Aluminum Association, Inc., chapter six under
short-circuit performance. Steady state conductor ampacity is
indirectly addressed through the code’s clearance
requirements in that some of the NESC’s clearance
requirements apply with the conductor at its maximum operating
temperature. To keep from exceeding the conductor’s maximum
temperature the engineering personnel who design the conductor
overcurrent protection and the operations personnel who
monitor conductor currents must know the associated ampacity
of the conductor.
Bare
Conductors
One
source for calculating the steady-state ampacity of bare
aerial conductors is IEEE Standard 738-1993. The calculation
method presented in the standard and the associated computer
program (included with the standard) can be used to calculate
the conductor temperature based upon the current or calculate
the ampacity based upon the maximum temperature. In both
computations, thirteen pieces of information including the
weather conditions under which the temperature or ampacity is
to be calculated and the conductor physical characteristics
must be entered into the calculation/program.
Insulated
Conductors (Cables)
One
source for the calculation of the steady-state ampacity of
insulated conductors is IEEE Standard 835-1994, "IEEE
Standard Power Cable Ampacity Tables." This 3086- page
hardbound book is not just about tables. The standard goes
into great detail explaining where the numbers came from.
First there is a listing of the types of cables for which
ampacities have been calculated. They range from simple
cross-linked polyethylene insulated 600-volt cables up to
complex 500 kV high-pressure, paper-insulated, liquid-filled,
pipe-type cables. The Technical Introduction gets into cable
construction, installation conditions, calculation methods,
calculation examples and details of the assumptions made in
calculating the ampacities that appear in the tables.
Computer
Programs
There
are various computer programs available for calculating cable
ampacities. Don’t be surprised to pay as much as $5000 for a
good program. Make sure the software is compatible with your
computer and operating system. Get a hands-on demonstration of
the program’s use before you put out the money. Some
programs are harder than others to use. Where bare conductor
ampacity calculations require thirteen pieces of information
to make the calculation, cable ampacity calculation require
twice as much for simple calculations like 600-volt cables
direct buried in earth. Complex calculations involving
multiple high-voltage cables may require the input of a dozen
pieces of information for each cable and take hours even with
a computer program.
Different
Results
Don’t
be surprised if the ampacity you come up with is very
different from the ampacity quoted by a manufacturer or the
ampacity calculated by another program. Thermodynamics is not
rocket science. We can calculate space science parameters so
accurately that we can land a craft on a planet millions of
miles away. But when it comes to cable ampacity, we are only
making a guesstimate. First, there are two generally accepted
methods for calculating steady-state cable ampacity: the Neher
and McGrath method originally published in 1957 and the method
detailed in IEC Standard 287 last revised in 1995. Given
identical input data, the two methods will often come up with
two very different answers. One source of information on the
similarities and differences between the two methods is Rating
of Electric Power Cable by George J. Anders, an IEEE Press
and McGraw-Hill publication. Second, the input data that must
be entered into the calculations varies from one cable
manufacturer to another, with installation location and with
time. For example: Earth thermal resistivity varies with the
moisture content of the soil. Ambient earth temperature varies
with the season of the year. Another alternative is to let the
cable manufacturer that you buy your cable from do the
calculations for you.
If you have any general questions about the
NESC, please e-mail me at dave.young@conectiv.com .
National Electrical Safety Code, NESC and
IEEE are registered trademarks of the Institute of Electrical
and Electronics Engineers. IEC is a registered trademark of
the International Electrrotechnical Commission.
Dave is a consulting engineer with Conectiv Power Delivery of
Wilmington, Delaware where he has been working with and teaching all
aspects of the NESC for over 32 years. He is a member of the NESC
Interpretations Subcommittee and represents the Edison Electric
Institute on the NESC Overhead Line Clearances Subcommittee 4. Dave is
also vice-chairman of the Delmarva Division of the Chesapeake Chapter of
the IAEI
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