Ampacity Discussion & Correction Factors

The ampacity tables in this section cover the installation conditions most commonly encountered. The actual current carrying capacities tables are derived from AIEE-IPCEA “Power-Cable Ampacities”, joint publication S-135-1 and P-46-426 which includes more complete tables covering additional earth resistivities and load factors.

The following tables relate to insulated cables in underground ducts, in free air, in conduit in air, and directly buried in earth. The values are based on 90°C and 105°C conductor temperatures and an ambient temperature of 20°C for all cables in underground duct or directly buried in the ground and 40°C for all cables in air.

Ampacity values are based on a 100% load factor. By definition the load factor is the ratio of the average load over a designated period of time to the peak load occurring in that period. For variable continuous loading the base period is 24 hours. These apply for cables in conventional underground duct installations since there is a time lag between the temperature rise of the cable and the temperature rise of the duct structure and surrounding earth. This heat-time-lag characteristic permits assigning higher current ratings for cables in ducts which do not carry full load continuously. For in-air installations 100% load factor is used. These ratings are used to any load factor due to the relatively low thermal capacity of the surrounding air.

Emergency Overloads

Operation at the emergency overload temperature of 130°C or 140°C shall not exceed 100 hours in any twelve consecutive months nor more than 500 hours during the lifetime of the cable.

Lower temperatures for emergency overload conditions may be required because of the type of material used in the cable, joints and terminations or because of cable environmental conditions.

Temperature Correction Factors

To determine ampacities for ambient temperatures and conductor temperatures other than those indicated on the individual tables, multiply table values by the correction factors shown in the Ambient Air Temperature or Ambient Earth Temperature tables.

 

Correction Factors For Various
Ambient Air Temperatures

AMBIENT AIR TEMPERATURE

 

30°C

35°C

40°C

45°C

50°C

Conductor
Temperature
in C

75
85

0.97
1.06

0.92
1.01

0.86
0.96

0.79
0.90

0.72
0.84

90

1.10

1.05

1.00

0.95

0.89

100
105
110
125
130
140

1.17
1.20
1.23
1.31
1.33
1.38

1.12
1.16
1.19
1.27
1.30
1.35

1.08
1.11
1.15
1.24
1.27
1.32

1.03
1.07
1.11
1.20
1.23
1.28

0.98
1.03
1.06
1.16
1.19
1.25

....  

Correction Factors For Various
Ambient Earth Temperatures

AMBIENT EARTH TEMPERATURE

 

10°C

15°C

20°C

25°C

30°C

Conductor
Temperature
in C

75
85

0.99
1.04

0.95
1.02

0.91
0.97

0.87
0.93

0.82
0.89

90

1.07

1.04

1.00

0.96

0.93

100
105
110
125
130
140

1.12
1.14
1.16
1.22
1.24
1.27

1.09
1.11
1.13
1.19
1.21
1.24

1.05
1.08
1.10
1.16
1.18
1.22

1.02
1.05
1.07
1.14
1.16
1.19

0.98
1.01
1.04
1.11
1.13
1.17

 

Effect of Grouping

Ampacities for cable in air or conduit in air are based on a single isolated cable or conduit. Where the spacing between cable or conduit surfaces is not greater than the cable or conduit diameter, the current rating should be reduced in accordance with values given in the table. Spacings less than one quarter of cable or conduit diameter are not covered.

Group Correction Factors

NUMBER OF CABLES IN AIR

Horizontally

1

2

3

4

5

6

Vertically

 

1
2
3

1.00
0.89
0.80

0.93
0.83
0.76

0.87
0.79
0.72

0.84
0.76
0.70

0.83
0.75
0.69

0.82
0.74
0.68

....

Group Correction Factors

NUMBER OF CONDUITS IN AIR

Horizontally

1

2

3

4

5

6

Vertically

 

1
2
3

1.00
0.92
0.85

0.94
0.87
0.81

0.91
0.84
0.78

0.88
0.81
0.76

0.87
0.80
0.75

0.86
0.79
0.74