Civil Engineering Reference
In-Depth Information
Fig. 7.2
Current density of different copper cables: (
a
) Single-core paper insulated in air; (
a
0
)
single-core paper insulated underground; (
b
) three-core paper insulated in air; (
b
0
) three-core paper
insulated underground; (
c
) non-insulated conductor in air; (
d
) single-core rubber insulated cables
The main parameters to consider in designing electrical systems are
current density and voltage drop through lines and transformers, to
which losses are related. Widely accepted criteria for the design of
electric power distribution systems in industrial plants are based on a
predetermined voltage drop along the lines, generally fixed at less than
5 % in normal operating conditions, to which corresponds less than
2-3 % of losses along the same line. These values are related to the
ratio (resistance
) of the cable and to the power factor of
the load. The maximum current density in this situation depends on
operating temperature and conductor cross section. As a general rule,
the larger the cross section of the conductor the lower the acceptable
current density is (see Fig.
7.2
).
R
/reactance
X
For low-voltage cables (0.6-1 kV), and for various modes of installation, the
following formula may be used to calculate electric current capacity:
S
m
S
n
(A)
I
¼
A
B
where
A
,
B
,
m
, and
n
are parameters defined by IEC Standard and
S
is the
conductor's cross section.
A simplified formula for non-buried cables is
S
b
(A)
I
¼
a
where
a
current capacity of a conductor with a cross section of 1 mm
2
. Typical values
for copper conductors are 13 A/mm
2
for PVC insulation and 17 A/mm
2
for EPR,
XLPE insulation.
¼
