Tap Conductors - Tech Spec by dfsiopmhy6


									    September 2003
    Tap Conductors
                                              Tech Spec                                                 Tech Spec 25: Volume 7 - Issue 2

                                                 A Newsletter for the Design Professional
    Part I

When considering protection of conductors, the general rule per              The length of the tap often determines the requirements for
NEC 240.21 is to provide overcurrent protection in each                     feeder taps. The rules for feeder taps with lengths of 10’ per
ungrounded circuit conductor at the point where the conductors               240.21(B)(1) and 25’ per 240.21(B)(2) seem to be the most
receive their supply. For branch circuits, the overcurrent protective        common. Other feeder tap rules cover; taps supplying a
device must comply with 210.20. For feeder circuits, the                     transformer, taps over 25’, and outside taps of unlimited
overcurrent protective device must comply with 215.3. For service            length.
conductors, overcurrent protection is not required at the point
where the conductor receives it supply, but overcurrent protection           Feeder Taps Not Over 10’
requirements are detailed in Part VII of Article 230. In addition,           Per 240.21(B)(1), feeder taps 10’ or less are permitted
240.4 provides additional provisions and requirements for branch,            provided:
feeder, and service conductors. If a conductor does not adhere to              1. The ampacity is not less than the combined computed
the requirements above, it is then considered a “tap conductor”.                  loads supplied and not less than the rating of the device or
                                                                                  overcurrent protective device supplied.
Tap Conductors                                                                 2. Does not extend beyond the switchboard, panelboard or
A Tap conductor is defined in 240.2 as a conductor, other than a                  control device which it supplies, and
service conductor, that has overcurrent protection ahead of its                3. Is enclosed in raceway, except at the point of connection
point of supply that exceeds the value permitted for similar                      to the feeder, and
conductors that are protected as described in 240.4. Tap                       4. For field installed taps, where the tap conductors leave the
conductors are permitted provided they adhere to the requirements                 enclosure, the ampacity of the tap conductor must be at
of 240.21(A)-(G). In addition, 240.21 prohibits tap conductors                    least 10% of the overcurrent device rating.
from supplying other conductors, except through an overcurrent
protective device meeting the requirements of 240.4. This means              An example of acceptable application of the 10’ tap rule is
“you can’t tap a tap”.                                                       shown below:
One common misconception about tap conductors is that
overcurrent protection is not required. However, it should be
noted that in most instances, overcurrent protection is required at
the termination point of the tap conductor by either 240.21(A)-
(G), transformer protection requirements per 450.3, or panelboard
protection requirements per 408.16. In addition, the use of tap
conductors should be limited where possible since tap conductors
have limited short-circuit protection. In fact, besides meeting the
requirements of 240.21, a short-circuit protection analysis of the
tap conductor should also be done per NEC 110.10. The primary
applications of interest to most include feeder taps per 240.21(B)
and transformer taps per 240.21(C). This issue will focus on
feeder taps, the next issue will focus on transformer taps.

Feeder Taps                                                                  The example above complies with 240.21(B)(1), but is the tap
As stated above, 240.21(B) covers the requirements for feeder taps.          conductor protected against short circuits? In this example the
Five possibilities exist as listed in 240.21(B)(1) - 240.21(B)(5).           tap conductor is a 10 AWG conductor. Let’s assume that the
                                                                             available fault current is 10,000A at the location of the tap
                                                                             conductor. The question now is, can the 200A overcurrent
                                                                             protective device protect the 10AWG tap conductor (assuming
                                                                             the fault is ahead of the 30A fuse)?

                                                                             According to the ICEA (Insulated Cable Engineers
                                                                             Association) protection chart, a 10AWG conductor with 75°C
                                                                             thermoplastic insulation can withstand 6,020 A for ½ cycle or
                                                                             4,257A for 1 cycle. This represents the amount of current for
                                                                             a given period of time that can begin to damage the conductor
                                                                             insulation (raises the temperature in the conductor from 75°C
                                                                             to 150°C).

   For questions, contact Bussmann® Application Engineering at 636-527-1270, fax: 636-527-1607, or email: fusetech@buss.com

              ã 2003 Cooper Bussmann, Inc.                                                                                Page 1
    September 2003
    Tap Conductors
                                              Tech Spec                                                 Tech Spec 25: Volume 7 - Issue 2

                                                 A Newsletter for the Design Professional
    Part I

Thus, in order to provide short-circuit protection and avoid               Feeder Taps Not Over 25’
insulation damage to the tap conductor, the 200A overcurrent               Per 240.21(B)(2), feeder taps 25’ or less are permitted provided:
protective device must:                                                      1. Has an ampacity not less than 1/3 that of the feeder
     • Limit the current to below 6,020A if the device clears                   overcurrent protective devices from which the tap
         the fault in ½ cycle or                                                conductors receive their supply, and
     • Limit the current to below 4,257A if the device clears in             2. Terminate in a single set of fuses or circuit breakers sized
         1 cycle.                                                               not more than the tap conductor ampacity, and
                                                                             3. Is suitably protected from physical damage.
If the 200A device is a Bussmann FRN-R (Class RK5), LPN-
RK (Class RK1), LPJ (Class J), or JJN (Class T), all will limit the        In the previous example, the tap could be extended up to 25’ if
current below 6,000A in ½ cycle (per the current-limiting effects          the tap conductor size was increased to 4AWG, Cu, 75° C.
charts as shown in the Bussmann SPD) and thus protection is
assured.                                                                   As shown before, the question now remains; is the tap
                                                                           conductor protected from an available fault current of
If the 200A device is a standard circuit breaker (non current-             10,000A?
limiting), the opening time can be determined. Most likely the
opening time will be approximately 1 cycle. However, unless the            Since the conductor has increased, it is expected that it can
circuit breaker manufacturer provides let-through data, it must be         handle more current for the same duration of time, but it must
assumed that the full available fault current could be let through.        be verified. Below are the protection considerations for 4
Thus, the analysis then shows that the standard circuit breaker            AWG conductors per ICEA:
could potentially let through 10,000A which is greater than the                      • 24,207A for ½ cycle
conductor withstand of 4,257A.                                                       • 17,117A for 1 cycle

Taking it a step further, there is a second and third damage level         Since the available fault current is below the protection
for conductors that could be explored. The second is the Soares            threshold for either the current limiting fuses mentioned above
Validity Rating. This represents the amount of current for a               or a circuit breaker that clears within 1 cycle, either could
given time that can cause the conductor to anneal (raises the              protect the tap conductor from a short circuit of 10,000A.
temperature in the conductor from 75°C to 250°C). When a                   However, if the available short-circuit current is greater than
conductor becomes annealed, it can loosen in the terminals                 10,000A, an analysis would be required.
causing heating issues. The third damage level is given by
Onderdonk. This damage level represents the amount of current              Conclusions
for a given time that can cause the conductor to melt or vaporize          When determining the protection required for conductors,
(raises the temperature in the conductor from 75°C to 1,083°C).            protection must be applied in accordance with branch, feeder,
                                                                           service or tap conductor requirements. When applying tap
For a 10 AWG conductor, the Soares Validity Rating is:                     conductors, care must be taken to not only meet the
         • 8,651A for ½ cycle or                                           requirements of the 240.21, but also the requirements of 450.3
         • 6,117A for 1 cycle                                              and 408.16 if applicable. In addition, additional steps should be
                                                                           taken to assure short-circuit protection of conductors per NEC
For a 10 AWG conductor, the Onderdonk rating is:                           110.10.
         • 15,549A for ½ cycle or
         • 10,995A for 1 cycle                                             For more information on tap conductor rules, see the
                                                                           Bussmann NE02 (p. 25-29) at:
Again, looking at the example above, the fuses mentioned before            http://www.bussmann.com/library/docs/NE02.pdf
will all protect within these levels as well. However, a standard
circuit breaker (non current-limiting) potentially could cause the         For information on short-circuit protection of conductors, see
tap conductor to anneal (result in a loose termination and cause           Tech Spec 6 and a white paper on conductor protection at:
heating issues) and be almost enough to vaporize the conductor.            http://www.bussmann.com/library/techspec/TechSpec6.pdf

Register for the Bussmann® Industrial & Commercial Power System Seminar on Overcurrent Protection October 23-24. Seminar is
in St Louis and includes demonstrations in our high power test lab. 1.6 CEUs will be offered for attending the seminar.
See informational flyer at: http://www.bussmann.com/apen/seminars

   For questions, contact Bussmann® Application Engineering at 636-527-1270, fax: 636-527-1607, or email: fusetech@buss.com

              ã 2003 Cooper Bussmann, Inc.                                                                                Page 1

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