Lucent Technologies Lineage
®
2000 ECS/GPS Battery Plant J85500G-2
3 - 12 Engineering, Planning and Ordering
Issue 6 May 1999
ac power, low-voltage disconnect/reconnect may not be
necessary.
Calculating
Voltage Drop
A useful formula to relate voltage drop, cable length and cable
size is:
VD = (K x I x L) / CM , or CM = (K x I x L) / VD
where:
VD = allowable voltage drop, in volts
CM = conductor size in circular mils
K = 11.1 for copper at 78°F (25.5°C)
I = appropriate current drain, in amperes
L = conductor length, in feet
The formula may be applied to one-way conductors or to loop
circuits (i.e. paired power and return conductors). The value of
K in the above expression increases with increasing conductor
temperature.
Conductor
Ampacity
Two criteria are used to select the actual wire gauge of a given
conductor. These two criteria are ampacity and voltage drop.
Ampacity is the current that may be carried safely without
overheating. In relatively low voltage/high current systems, such
as dc distribution, voltage drop limitations are often the
determining factors in sizing conductors. In systems, such as ac
distribution, with relatively high voltage and low current,
ampacity usually determines minimum conductor size. All
conductors, however, must be large enough to safely carry the
intended current.
Allowable ampacity is provided in Article 310 of the NEC
(National Electrical Code), and it is a function of the following:
•
wire size,
•
ambient temperature,
•
type of insulation, and
•
proximity to other conductors.
