53
3
Selection and application
3-4 Wiring protection
Fig. 3-12 Current-time characteristics in which 600V PVC insulated conductors reach a temperature of 100
°
C (rise of 70
°
C)
10
2
3
4
5
6
0.1
0.2
0.3 0.4
0.6 0.8 1
2
3
4 5 6
8 10
Current (
u
10
2
A)
50
20
200
1
100
20
30
40
50
60
1000
200
300
400
500
600
800
30 40
60 80 100
300
500
1000
Time (sec)
1.0
1.5
2.5
4.0
6.0
10
16
25
35
50
70
95
120
150
185
240
300
400
500
630
Wire sizes (mm
2
)
The relationship of current to the rise in conductor temperature
in the continuous and short-time regions makes heat
dissipation too important to ignore, yet it varies with conditions
like the type of installation. Although this is not impossible to
calculate, it is not commonly done. IEC standards stipulate
allowable current for insulated wiring in the continuous region
using an ambient temperature of 30°C and a rise in conductor
temperature of 40°C.
The IEEE uses a minimum of 20s for the short-time region
where a conductor temperature as high as 100°C is allowable.
This temperature is sustainable in the conductor because of
the inverse time-delay trip time of the breaker. Fig. 3-12 shows
current-time characteristics for 600V PVC-insulated wiring
where conductor temperatures reach 100°C starting from
no-load conditions at an ambient temperature for the wiring of
30°C.
Table 3-9 (b) Conductor specifications
Resistor temperature coefficient
D
1/
D
0.00427 (1/°C)
234 (°C)
Initial conductor temperature *
T
0
IEC wiring: 70°C
JIS wiring: 60°C
Ultimate conductor temperature *
T
IEC wiring: 160°C
JIS wiring: 150°C
Specific conductor resistance
U
0.0000016 (
:
cm)
Mechanical equivalent of heat
J
4.19 (J/cal)
Specific heat of the conductor
C
0.092 (J/cm
3
°C)
Specific gravity of the conductor
V
8.93 (g/cm
3
)
K
2
=
D
p/JC
V
1.985E–09
Note: * Ambient temperature: 30°C
Table 3-9 (a) Current squared time i
2
t=5.05 log
e
((234+
T
)/(234+
T
0
)
u
S
2
u
10
4
IEC wiring values
JIS wiring values (Reference)
Wire cross section (S)
(mm
2
)
Current squared time (
u
10
6
A
2
s)
Wire cross section (S)
(mm
2
)
Current squared time (
u
10
6
A
2
s)
Starting at 70°C (i
2
t)
Starting at 30°C (i
2
t)
Starting at 60°C (i
2
t)
Starting at 30°C (i
2
t)
1
1.5
2.5
4
6
10
16
25
35
50
70
95
120
150
185
240
300
400
500
630
0.013
0.029
0.082
0.210
0.471
1.31
3.35
8.18
16.0
32.7
64.2
118
189
295
448
754
1179
2095
3274
5198
0.020
0.045
0.126
0.324
0.728
2.02
5.18
12.6
24.8
50.6
99.1
182
291
455
692
1165
1820
3235
5055
8025
2
3.1
3.5
5.5
8
14
22
30
38
50
60
80
100
125
150
200
250
325
400
500
0.054
0.133
0.165
0.408
0.863
2.64
6.53
12.1
19.5
33.7
48.6
86.3
135
211
303
539
843
1425
2158
3372
0.076
0.187
0.232
0.572
1.21
3.71
9.16
17.0
27.3
47.3
68.1
121
189
296
426
757
1183
1999
3028
4731
