September 2003
Tap Conductors
Part I
Tech Spec
Tech Spec 25: Volume 7 - Issue 2
A Newsletter for the Design Professional
Introduction
When considering protection of conductors, the general rule per
NEC 240.21 is to provide overcurrent protection in each
ungrounded circuit conductor at the point where the conductors
receive their supply. For branch circuits, the overcurrent protective
device must comply with 210.20. For feeder circuits, the
overcurrent protective device must comply with 215.3. For service
conductors, overcurrent protection is not required at the point
where the conductor receives it supply, but overcurrent protection
requirements are detailed in Part VII of Article 230. In addition,
240.4 provides additional provisions and requirements for branch,
feeder, and service conductors. If a conductor does not adhere to
the requirements above, it is then considered a “tap conductor”.
Tap Conductors
A Tap conductor is defined in 240.2 as a conductor, other than a
service conductor, that has overcurrent protection ahead of its
point of supply that exceeds the value permitted for similar
conductors that are protected as described in 240.4. Tap
conductors are permitted provided they adhere to the requirements
of 240.21(A)-(G). In addition, 240.21 prohibits tap conductors
from supplying other conductors, except through an overcurrent
protective device meeting the requirements of 240.4. This means
“you can’t tap a tap”.
The length of the tap often determines the requirements for
feeder taps. The rules for feeder taps with lengths of 10’ per
240.21(B)(1) and 25’ per 240.21(B)(2) seem to be the most
common. Other feeder tap rules cover; taps supplying a
transformer, taps over 25’, and outside taps of unlimited
length.
Feeder Taps Not Over 10’
Per 240.21(B)(1), feeder taps 10’ or less are permitted
provided:
1. The ampacity is not less than the combined computed
loads supplied and not less than the rating of the device or
overcurrent protective device supplied.
2. Does not extend beyond the switchboard, panelboard or
control device which it supplies, and
3. Is enclosed in raceway, except at the point of connection
to the feeder, and
4. For field installed taps, where the tap conductors leave the
enclosure, the ampacity of the tap conductor must be at
least 10% of the overcurrent device rating.
An example of acceptable application of the 10’ tap rule is
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
As stated above, 240.21(B) covers the requirements for feeder taps.
Five possibilities exist as listed in 240.21(B)(1) - 240.21(B)(5).
The example above complies with 240.21(B)(1), but is the tap
conductor protected against short circuits? In this example the
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
Part I
Tech Spec
Tech Spec 25: Volume 7 - Issue 2
A Newsletter for the Design Professional
Thus, in order to provide short-circuit protection and avoid
insulation damage to the tap conductor, the 200A overcurrent
protective device must:
• Limit the current to below 6,020A if the device clears
the fault in ½ cycle or
• Limit the current to below 4,257A if the device clears in
1 cycle.
If the 200A device is a Bussmann FRN-R (Class RK5), LPNRK (Class RK1), LPJ (Class J), or JJN (Class T), all will limit the
current below 6,000A in ½ cycle (per the current-limiting effects
charts as shown in the Bussmann SPD) and thus protection is
assured.
If the 200A device is a standard circuit breaker (non currentlimiting), the opening time can be determined. Most likely the
opening time will be approximately 1 cycle. However, unless the
circuit breaker manufacturer provides let-through data, it must be
assumed that the full available fault current could be let through.
Thus, the analysis then shows that the standard circuit breaker
could potentially let through 10,000A which is greater than the
conductor withstand of 4,257A.
Taking it a step further, there is a second and third damage level
for conductors that could be explored. The second is the Soares
Validity Rating. This represents the amount of current for a
given time that can cause the conductor to anneal (raises the
temperature in the conductor from 75°C to 250°C). When a
conductor becomes annealed, it can loosen in the terminals
causing heating issues. The third damage level is given by
Onderdonk. This damage level represents the amount of current
for a given time that can cause the conductor to melt or vaporize
(raises the temperature in the conductor from 75°C to 1,083°C).
For a 10 AWG conductor, the Soares Validity Rating is:
• 8,651A for ½ cycle or
• 6,117A for 1 cycle
For a 10 AWG conductor, the Onderdonk rating is:
• 15,549A for ½ cycle or
• 10,995A for 1 cycle
Again, looking at the example above, the fuses mentioned before
will all protect within these levels as well. However, a standard
circuit breaker (non current-limiting) potentially could cause the
tap conductor to anneal (result in a loose termination and cause
heating issues) and be almost enough to vaporize the conductor.
Feeder Taps Not Over 25’
Per 240.21(B)(2), feeder taps 25’ or less are permitted provided:
1. Has an ampacity not less than 1/3 that of the feeder
overcurrent protective devices from which the tap
conductors receive their supply, and
2. Terminate in a single set of fuses or circuit breakers sized
not more than the tap conductor ampacity, and
3. Is suitably protected from physical damage.
In the previous example, the tap could be extended up to 25’ if
the tap conductor size was increased to 4AWG, Cu, 75° C.
As shown before, the question now remains; is the tap
conductor protected from an available fault current of
10,000A?
Since the conductor has increased, it is expected that it can
handle more current for the same duration of time, but it must
be verified. Below are the protection considerations for 4
AWG conductors per ICEA:
• 24,207A for ½ cycle
• 17,117A for 1 cycle
Since the available fault current is below the protection
threshold for either the current limiting fuses mentioned above
or a circuit breaker that clears within 1 cycle, either could
protect the tap conductor from a short circuit of 10,000A.
However, if the available short-circuit current is greater than
10,000A, an analysis would be required.
Conclusions
When determining the protection required for conductors,
protection must be applied in accordance with branch, feeder,
service or tap conductor requirements. When applying tap
conductors, care must be taken to not only meet the
requirements of the 240.21, but also the requirements of 450.3
and 408.16 if applicable. In addition, additional steps should be
taken to assure short-circuit protection of conductors per NEC
110.10.
For more information on tap conductor rules, see the
Bussmann NE02 (p. 25-29) at:
http://www.bussmann.com/library/docs/NE02.pdf
For information on short-circuit protection of conductors, see
Tech Spec 6 and a white paper on conductor protection at:
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