Component Protection
Wire & Cable
The circuit shown originates at a distribution panel where 40,000 amps RMS
symmetrical is available. To determine the proper fuse, first establish the shortcircuit withstand data for the 10 AWG THW copper cable shown in the diagram.
Short-Circuit Current Withstand Chart for
Copper Cables with Thermoplastic Insulation
Short-Circuit Protection of Wire and Cable
The following table shows the short-circuit withstand of copper cable with
75°C thermoplastic insulation based on Insulated Cable Engineers Association
(ICEA) formulae.
The short-circuit withstand of the 10 AWG THW copper conductor is 4300A for
one cycle (0.0167 seconds). Short-circuit protection of this conductor requires
the selection of an overcurrent device which will limit the 40,000A RMS
symmetrical available to a value less than 4300A, and clear the fault in one
cycle or less.
The Low-Peak dual-element fuse let-through chart shows that the
LPS-RK30SP Low-Peak dual-element fuse will let-through an apparent
prospective RMS current of less than 1800A, when 40,000A is
available (and would clear the fault in less than 1⁄2 cycle).
Short-Circuit Currents for Insulated Cables
The increase in kVA capacity of power distribution systems has resulted in
possible short-circuit currents of extremely high magnitude. Conductor
insulation may be seriously damaged by fault induced, high conductor
temperatures. As a guide in preventing such serious damage, maximum
allowable short circuit temperatures, which damage the insulation to a slight
extent only, have been established for various insulation as follows:
• Paper, rubber and varnished cloth
200°C
• Thermoplastic
150°C
The chart at the top of next column shows the currents which, after flowing for
the times indicated, will produce these maximum temperatures for each
conductor size. The system short circuit capacity, the conductor crosssectional area and the overcurrent protective device opening time should be
such that these maximum allowable short-circuit currents are not exceeded.
Using the formula shown on the ICEA protection table will allow calculating
withstand ratings of conductors. It may be advantageous to calculate
withstand ratings below one cycle, when the opening time of the currentlimiting device is known; see table below.
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CONDUCTOR SIZE
*Copyright 1969 (reaffirmed March, 1992) by the Insulated Cable Engineers
Association (ICEA). Permission has been given by ICEA to reprint this chart
Protecting Equipment Grounding Conductors
Safety issues arise when the analysis of equipment grounding conductors
(EGC) is discussed. Table 250.122 of the NEC® offers minimum sizing for
equipment grounding conductors.
Equipment grounding conductors are much more difficult to protect than phase
conductors because the overcurrent protective device is most often several
sizes larger than the ampacity of equipment grounding conductor.
The problem of protecting equipment grounding conductors was recognized
more than 30 years ago when Eustace Soares, wrote his famous grounding
book “Grounding Electrical Distribution Systems for Safety.” In his book he
states that the “validity” rating corresponds to the amount of energy required to
cause the copper to become loose under a lug after the conductor has had a
chance to cool back down. This validity rating is based upon raising the
copper temperature from 75°C to 250°C.
In addition to this and the ICEA charts, a third method promoted by
Onderdonk allows the calculation of the energy necessary to cause the
conductor to melt (75°C to 1083°C).
The table on the next page offers a summary of these values associated with
various size copper conductors.
©2005 Cooper Bussmann
Component Protection
Wire & Cable
It becomes obvious that the word “Minimum” in the heading of NEC® Table
250.122 means just that - the values in the table are a minimum - they may
have to be increased due to the available short-circuit current and the currentlimiting, or non-current-limiting ability of the overcurrent protective device.
250.4(A)(5) and 250.4(B)(4) require grounding conductors sized adequately for
the short-circuit current that could be let-through. This means that based on
the available short-circuit current, the overcurrent protective device
characteristics (it’s let-through current), the grounding conductor may have to
be sized larger than the minimum size in Table 250.122.
Good engineering practice requires the calculation of the available short-circuit
currents (3-phase and phase-to-ground values) wherever equipment
grounding conductors are used. Overcurrent protective device (fuse or circuit
breaker) manufacturers’ literature must be consulted. Let-through energies for
these devices should be compared with the short circuit ratings of the
equipment grounding conductors. Wherever let-through energies exceed the
“minimum” equipment grounding conductor withstand ratings, the equipment
grounding conductor size must be increased until the withstand ratings are not
exceeded.
Take the example below. The EGC must be protected from damage. It can
withstand 4300A of current for 1 cycle. The 1 cycle opening time of the circuit
breaker will cause damage to the 10 AWG EGC. However, a current-limiting
fuse will limit the current to within the withstand rating of the EGC. An LPSRK60SP will limit the line to ground current to approximately 3300A, providing
protection.
©2005 Cooper Bussmann
71
Component Protection
Small Wire, NFPA 79
16 and 18 AWG Conductors
For Industrial Machinery Power Circuits
The 2002 edition of NFPA 79, Electrical Standard for Industrial Machinery,
allows the use of 16 and 18 AWG as branch circuit conductors if specific
protection requirements are met. Note, in general 14 AWG is the smallest
branch circuit conductor allowed in the NEC®;16 and 18 AWG branch circuit
conductors are not allowed per the NEC® for building system conductors.
Why the change to permit 16 and 18 AWG conductors? One of the major
objectives of NFPA 79-2002 was to harmonize, where possible, with
IEC60204. This IEC standard allows for smaller conductors.
What is the challenge with using smaller conductors? Small conductors are a
challenge to protect under short circuit conditions; some overcurrent protective
devices are unable to provide protection. The smaller the conductor size, the
lower the short-circuit current withstand. 16 and 18 AWG conductors have
extremely low short-circuit current withstandability. Based on the ICEA
(Insulated Cable Engineers Association) conductor withstand equation
presented in a previous section, copper conductors, 75°C thermoplastic
insulation have these short-circuit withstands:
The NFPA 79 proposal for allowing 16 and 18 AWG for branch circuits was
accepted based on a study conducted by the Small Wire Working Group
formed by the NFPA 79 Committee. The Small Wire Working Group studied
the critical application considerations for small conductors in industrial
machinery, proposed the requirements and conducted UL witnessed tests to
prove the proposed requirements are acceptable. After considering several
damage criteria, the group decided to use the ICEA damage levels because
they were the most conservative. All other methods allowed certain amounts
of damage.
The Small Wire Working Group had to determine the requirements for a
proposal to NFPA 79 on how the conductors would be protected. Their analysis and testing resulted in proposing different approaches or requirements for
different type overcurrent protective devices. This allowance is conditional and
limited to certain requirements such as: limited ampacity circuits, specific
overcurrent protective device criteria, and limits for physical damage. Specific
overcurrent protective devices must be selected in order to use 16 and 18
AWG for branch circuit conductors. These devices are:
Fuse Protection
1. Class CC, J, or T fuses, or
2. Other fuses listed and marked for protection of 16 or 18 AWG wire
They chose to segregate the Class J, CC and T fuses because the published
UL/CSA/ANCE 248 Fuse Standards for these fuses (for applicable amp
ratings) have established performance short circuit let-through limits that are
lower than the ICEA conductor short-circuit withstand limits. This means that
these fuses will let-through less energy than the ICEA damage threshold, so
conductor protection is assured. Therefore, the task in this case was to
prove this hypothesis. Testing was performed using special “umbrella” fuses
with let-through values above the UL/CSA/ANCE fuse standard limits. The
results: these fuses protected the 16 and 18 AWG conductors, passing both
visual inspections and dielectric testing. UL issued a Special Service
Investigation, An Investigation of the use of 16 and 18 AWG Conductors for
Power Branch Circuits in Industrial Machinery Applications, file number E4273
to verify the test results. The analysis, test program and results can also be
viewed in an IEEE paper presented at the 2002 IEEE Industrial and
Commercial Power Systems Technical Conference titled, An Investigation of
the Use of 16 and 18 AWG Conductors for Branch Circuits in Industrial
Machinery Built to NFPA 79 2002. The report and paper can be found on
www.cooperbussmann.com.
What about overcurrent protective devices other than Class J, CC, and T
fuses? Other Classes of fuses, or circuit breakers must be evaluated, tested
and listed to protect 16 and 18 AWG conductors in branch circuits. The
electrical industry will revise the appropriate standards with procedures that
evaluate these other overcurrent protective devices to the application
considerations that the NFPA 79 Small Wire Working Group utilized.
The Small Wire Working Group noted some of the specific criteria that is
necessary to evaluate overcurrent protection for protection of 16 and 18 AWG
conductors for industrial machinery applications:
1. Conductor length can vary greatly: 1 foot, 4 feet and 100 feet lengths of conductor
were tested to cover a wide variety of installations.
2. Short-circuit current tests with full L-L voltage on one pole of a circuit to insure the
overcurrent protective device can protect conductors for various types of
grounding schemes. L-G faults are very prevalent fault conditions. And full L-L
system voltage can be present for single or multiple L-G faults on some grounding
systems. For example, the let-through I2t for a 480 volt fault opened by only one
pole is much greater than that same fault opened by two poles of that same
device.
3. The damage level for conductors was selected as the ICEA Allowable ShortCircuit Currents For Insulated Conductors. After the overcurrent tests, to assess
the conductor insulation integrity, a reliable test method (not just visual inspection)
via dielectric insulation testing was used.
4. It is necessary to protect these small conductors for all levels of short-circuit
current. Tests were conducted with 5000A, 10,000A and 50,000A of available
short-circuit current to ensure protection at low, intermediate and high fault levels.
At the time of this updated publication, a new SPD for Industrial Control
Panels and Industrial Machinery is being written. The specifics on the small
wire requirements and sizing will be in this new publication. When completed
this new publication will be available at www.cooperbussmann.com.
Circuit Breaker Protection
3. Circuit breakers listed and marked for protection of 16 or 18 AWG conductors
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©2005 Cooper Bussmann