Both our Canadian Electrical Code
(CEC) and the U.S. National Electrical Code (NEC),
when followed, are designed to eliminate undue
electrical fire and shock hazards in electrical
installations. Our intentions are identical to those of
our neighbours. Our electrical codes vary only in the
details and sometimes in how the rules are applied. We
discussed some of the differences between the CEC and
the NEC in earlier articles. In this one, let’s
look at a few more.
CEC Rule 14-100(d) permits the
secondary conductors supplied by a high voltage power
transformer to be protected by the transformer’s
primary overcurrent protection (with no restriction in
conductor length through the building). The rule
requires that the wiring be mechanically protected and
terminates at a single circuit-breaker or set of fuses
set to protect the tap conductors against overloading.
The NEC wisely restricts this
application by permitting high-voltage primary
overcurrent protection for secondary conductors only for
single-phase or delta-delta wound transformers. (We can
assume the delta-delta restriction applies so as to
reduce the possibility of arcing ground faults.)
CEC Rules 10-700 to 10-710 provide a
list of permissible electrical system grounding
electrodes. The list includes metal water piping
systems, metal well casings, ground rods,
concrete-encased conductors, concrete-encased metal
plates and buried metal plates.
The NEC expands the CEC list
considerably to include an effectively grounded metal
building frame, a ground ring around a building,
concrete-encased reinforcing bars, underground metallic
objects such as piping and storage tanks.
CEC Rules 4-004(3) and (4) allow us
to consider the neutrals of normally balanced
three-phase, four-wire circuits as non-current-carrying
conductors when determining the ampacities of cables or
conductors in raceways.
The NEC takes this to a
different level, considering the neutral conductor to be
current-carrying when a major portion of the load is
non-linear (contains harmonics).
CEC Rule 14-014 permits series rated
electrical equipment in locations where the fault level
exceeds the equipment rating, when installed as a
component of a tested and approved series combination.
Appendix B - Notes on the Rules tells us that motor
loads connected between the upstream and downstream
protective devices of series rated overcurrent
protection must be restricted to no greater than 1% of
the interrupting rating of the downstream devices. Here
the intention is to ensure that the motor contribution
during a fault does not affect the ability of the
downstream protection to safely isolate the fault. Since
this note is located in Appendix B, it becomes a
"good installation practice" rather than a
code requirement.
The NEC makes this an
electrical code requirement by writing it as a rule.
CEC Rule 18-000 requires our using
the Zone method of classifying all new locations
containing flammable gases and vapours in quantities
sufficient to produce an explosive gas atmosphere. The
CEC permits the Division system of classification only
for additions, modifications, renovations, operation and
maintenance of existing facilities.
The NEC takes a more
laissez-faire view, permitting either classification
method to be used in any situation.
CEC Rules 28-100 to 28-210 provide
requirements for determining minimum motor conductor
ampacities and maximum overcurrent protection. Although
the CEC provides Tables 44 and 45, listing motor
currents for single-phase and three-phase motors, it is
silent on whether the tables or actual motor nameplate
ratings are to be used for these purposes.
The NEC also provides motor
current tables, but specifically requires they be used
for determining motor conductor sizes and overcurrent
protection. However, this does not apply to motor
overload protection, where the NEC requires that
actual nameplate data be used.
CEC Rule 14-100(c) permits reduced
size feeder taps up to 7.5 m as long as each tap
conductor:
· Has
an ampacity not less than 1/3 of the ampacity of the
feeder
· Is
protected against damage
· Terminates
in a single set of fuses or circuit breaker, set at a
value not exceeding the ampacity of the tap conductor
The NEC has an identical
rule but goes on to expand this practice even further,
to permit 1/3 size taps up to 30 m in length, but with
an abundance of conditions:
· The feeder
is located in a high bay manufacturing building
minimum 11 m high
· Qualified
persons service the electrical system
· Tap
conductors are not over 7.5 m long horizontally, not
over 30 m in total length and terminate in a single set
of fuses or circuit-breaker set at a value not exceeding
the ampacity of the tap
· The tap is
protected against mechanical damage and without splices
· Minimum tap
conductor size is 6 AWG copper or 4 AWG aluminum
· Taps are
made not less than 9 m above the floor and do not
penetrate through any floors, walls or ceilings
As with earlier articles, you should
always consult the electrical inspection authority in
each province or territory as applicable to obtain a
more specific interpretation of any of the above.
Leslie Stoch, P.E. is principal of L.
Stoch & Associates, providing electrical engineering
and ISO 9000 quality systems consulting. Prior to that,
he spent over 20 years with Ontario Hydro as an
electrical inspection manager and engineer. Les holds a
B.S. in electrical engineering from Concordia University
in Montreal.
|
