Intelligent Technologies QCPort System Install Manual
November 2005
MN05001002E
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Page
22
Sizing Calculation
A power supply that is not end connected creates two sections of cable. Because of this,
each section needs to be evaluated separately. The equation sums the calculated drop
for each device and compares it to 4.65V.
SUM {[(L
n
x Rc) + (N
t
x (0.005))] x I
n
} <= 4.65V
Term Definition
Term
Definition
L
n
L = The distance (ft) between the
device and the power supply,
excluding the drop line distance.
n = The number of the device
being evaluated, starting with 1 for
the device closest to the power
supply and increasing by 1 for the
next device.
(0.005)
The nominal-contact resistance used for
every connection to the trunk line.
Rc
Long Run
0.021 (ohm/ft)
QCPort Interconnect
0.105 (ohm/ft)
4.65V
The maximum voltage drop allowed.
This is the total cable system voltage
drop of 5.00V minus 0.35V reserved for
drop line voltage drop.
N
t
The number of taps between the
device being evaluated and the
power supply. For example:
•
When a device is the first
one closest to the power
supply, this number is 1.
•
When a node has one device
between it and the power
supply, this number is 2.
•
When 10 devices exist
between the evaluated node
and the power supply, this
number is 11.
For a device attached as a drop
from a device, the currents for the
trunk and drop device should be
summed and used with the
equation only once.
I
n
I = The current drawn from the cable
system by the device. For currents
within 90% of the maximum, use the
nominal device current. Otherwise, use
the maximum rated current of the device.
For a device attached as a drop from a
device, the currents for the trunk and
drop device should be summed and
counted as one tap.
n = The number of devices being
evaluated, starting with 1 for the device
closest to the power supply and
increasing by 1 for the next device.
