present to
the industry and the customers it serves.
Given the expense involved, the emphasis for
network growth is frequently placed on the
architecture of the lines, yet industry dynamics
are leaving some telcos vulnerable. Prior to
industry deregulation, much of the engineer,
furnish, and install (EFI) was performed by
specialized manpower within the telco com-
pany. Today, competitive forces have led many
to out-source EFI. Equipment vendors are
increasingly called upon to configure equip-
ment, including power distribution. While
Bellcore standards exist for these procedures,
in actuality DC installation practices have been
known to vary.
This paper will present guidelines that
Telect considers “best-practice” for power
distribution and wiring. Adhering to these
recommendations is an important measure for
protecting the system investment over its
service life. As the system changes, these
practices can also help telcos control lost
revenue costs due to power-related downtime.
Protecting Investments With
Proper Power Distribution
“Best-practice”
guidelines for power
management and wiring
contribute to effective
power protection. Telect Doc #WP23; Copyright © 2001 Telect, Inc.
2
Best-Practice Power Distribution
By design, the cumulative output rating of
virtually every power distribution panel can
exceed the maximum input rating of the panel.
While this provides users with load design
flexibility, it is imperative that system engineers
design for the cumulative load current require-
ments. If a load drawn by connecting equip-
ment surpasses either the maximum rated
amperage of any individual fuse, or the maxi-
mum rated input value for a group of fuses,
system down-time costs will be incurred.
Consequently, when selecting a secondary
power distribution system, it is important to
give consideration to the overall load which
will be drawn by connecting equipment, as
well as to the maximum amperage rating of the
individual loads, and to the DC input level.
Fuses & Magnetic Breakers
Fuses or magnetic breakers are two of the
most commonly used devices in power distri-
bution panels. It can be useful to understand
how best to leverage the advantages offered by
the various circuit interrupt protection devices.
This can help improve system integrity, and
maximize the cost-effectiveness of operating
the panel.
Fuses
Two widely-used fuses for low-amp,
equipment-feed applications are the Type 70
and the GMT. A cylindrical-shaped fuse, Type
70’s are typically rated for a maximum output
of 10-amps. Color-coded GMT fuses are
available in variety of sizes up to a maximum
of 15-amps. (Refer to Figure 1.) A popular
advantage to the GMT fuses is that they
require less space than Type 70 fuses, which
allows greater fuse density on a given power
distribution panel.
Several other fuse types are available for
larger loads. Intermediate fuses are commonly
used to supply power to equipment, or to the
small fuses noted above. Included in this
category is the KLM fuse. It is suitable for
applications up to 30-amps. Where even larger
capacities are required, the 50-amp TPA fuses
may be used.
Regardless of the type of fuse selected for
an application, adhering to a few best-practice
guidelines can help ensure trouble-free opera-
tion. Within their respective class types, the
physical fuse dimensions are the same irrespec-
tive of the specific fuse amperage. Although
this makes it physically possible to use a higher
amperage fuse in a fuse position rated for a
lower amperage fuse, this must be avoided.
Additionally, while wiring to the fuse is se-
lected based on the fuse’s maximum value, a
best-practice policy is to ensure that the load
supported by the fuse is not more than 75% of
the fuse’s rated capacity.
Magnetic Breakers
Another option for power protection
involves the use of magnetic breakers. They too
are available in a variety of sizes, however unlike
Maximum
Amperage
Rating
GMT Color Code
1/4 A
violet
1/2 A
red
3/4 A
brown
1 A
gray
1-1/3 A
white
1-1/2 A
white/yellow
2 A
orange
3 A
blue
4 A
white/brown
5A
green
7-1/2 A
black/white
10 A
red/white
12 A
yellow/green
15 A
red/blue
Figure 1: GMT color-coding of fuse ratings Telect Doc #WP23; Copyright © 2001 Telect, Inc.
3
fuses, the breaker description represents the
maximum output the circuit will hold. Conse-
quently, a 50-amp breaker will typically trip if the
output required reaches or exceeds approxi-
mately 125% (62.5 amps) of the breaker’s rated
value. This trip point is determined by the breaker
manufacturer. An advantage to the use of break-
ers is that in the event the circuit becomes over-
loaded and trips, resuming operation may be as
simple as resetting the switch after the fault has
been corrected; unlike fuses, which when over-
loaded need to be replaced. This feature makes
breakers an attractive power distribution method
where it may be inconvenient to replace fuses, or
where fuse availability may be limited. However,
time and overloads will unpredictably degrade the
trip point of a breaker. This makes the identifica-
tion of degraded breakers difficult during proac-
tive maintenance. Additionally, it is more difficult
and time consuming to replace a defective
breaker versus a faulty fuse.
Output, Input & Load Considerations
Electing to use breakers or fuses is an
initial step in determining what type of power
distribution panel is best suited for the applica-
tion. Evaluating the specific input power needs
of the equipment that will connect to the
distribution panel will help determine suitable
fuse/breaker sizes.
Load requirements determine the output
fuse/breaker sizes required on the power
distribution panel. Load is determined by the
maximum continuous operating load drawn by
either the equipment or the equipment shelf, as
determined by the equipment manufacturer.
Distribution fuses must be equal to, but no
greater than, 1.5 times the equipment’s, or the
equipment shelf’s load fuse. While it may be
necessary to use a protective device with a
time delay to accommodate some surge condi-
tions, generally the following formula can help
approximate the output fuse/breaker size
required to allow for the upper voltage limit:
Output Breaker/Fuse Size =
(maximum continuous operating current) x 1.5
When powering switch-mode devices (also
known as power cards), the maximum input
current rating should be determined by the
lowest operating voltage of the equipment.
This is usually 42 volts. Switch-mode power
devices increase their current demands as the
input voltage decreases in order to maintain
their output voltage and wattage demands.
Using the following equations can help define
the highest maximum input current.
Operating Watts =
Float Voltage x (maximum continuous
operating current)
Maximum Low Voltage Operating Current =
Operating Watts
-42 Low Voltage
Given the heat sensitive nature of fuses and
thermal breakers, the conditions in which the
fuse/breaker will be used can cause the rated
trip points of the devices to deteriorate. Ambi-
ent temperature, heat dissipated by surround-
ing equipment, or placing high amperage loads
next to each other in the power distribution
panel may produce thermal conditions which
can hinder part performance. To minimize the
effect of heat generated by these sources, best-
practice guidelines recommend increasing the
spacing between components to promote air
circulation, or using a magnetic breaker that is
not as effected by thermal conditions.
The function of output fuse/breaker is
primarily to provide required wire protection,
since in most instances with active devices, the
equipment connected to the output load is
required to be fused for circuit protection.
However, best-practice policy recommends Telect Doc #WP23; Copyright © 2001 Telect, Inc.
4
that individual output fuses/breakers should not be
operated continuously at more than 75% of their
rated amperage, nor should the combined amper-
age of individual fuses/breakers exceed 75% of
the input from the interrupting device feeding the
power distribution panel.
Using these guidelines, it is then possible to
determine the type of fuse best suited for the
application. Local practice or application may
dictate the standardization on a specific distribu-
tion panel featuring only one fuse type; for ex-
ample GMTs or KLMs.
Applications requiring a mix of high- and
low-amp outputs may find the use of two-in-
one type fuse panel advantageous. These
panels, for example, provide both GMT fuses
for low-amp needs, and KLM fuses for higher
amp requirements.
There are also both single-feed-type- and
dual-feed-type-panels designed to offer power
redundancy. If a single feed power source is
used for a dual feed panel, the power bay
breaker/fuse size cannot be greater than one of
the duel inputs. If using a single feed source
greater than one of the duel inputs, input fuses/
breakers are required in the panel.
It is important to remember that the load is
likely to be effected if the connecting equip-
ment configuration is changed. Should any
component of the configuration change, it is
imperative that the power requirements and
installation be reviewed and revised as needed.
Best-Practice Wiring
Having defined a sound power distribution
system in accordance with the best-practice
guidelines mentioned above, the next step in
assuring network integrity is to implement the
power distribution system using best-practice
wiring methods. These guidelines recommend that
wire installations be rated for worst-case applica-
tions, making it important to understand the factors
which influence wire size selection.
Voltage Drop & Wire Length
Voltage drop occurs in every configuration
and is attributed to the physical length of the
wire and the application’s requirements. In
every installation, the inherent voltage drop
from the power bay to the equipment