DE-RATING FACTORS

 

CURRENT RATINGS

KEI Recommendations for Current Ratings

The Current rating of power cables is defined by the maximum intensity of current (amperes) which can flow continuously through the cable, under permanent loading conditions, without any risk of damaging the cable or deterioration or its electrical properties.

The value given in the tables are valid for one circuit in a three phase system under conditions specified. For grouping cables rating factors must be used.

The current carrying capacities mentioned in KEI technical data are intended as a guide, to assist operating engineers in selecting cables for safety and reliability.

  • Max. Conductor Temperature:90°C
  • Ambient Ground Temperature:30° C
  • Ambient Air Temperature:40° C
  • Thermal resistivity of soil:150°C cm/W
  • Depth of laying
    • 3.3, 6.6 & 11 kV Cables:90 cm
    • 22 and 33 kV Cables:105 cm
  • Max. Conductor Temperature
    • for Short Circuit:250° C

To obtain the maximum current carrying capacity of a cable operating at different conditions from the standard, various rating factors are to be multiplied, as follows :-

  • I a = K x I s
  • in amperes
  • I a : Current rating at actual operating conditions (amperes)
  • I s : Current rating at standard operating conditions (amperes) K : Rating factor, as applicable

DE- RATING FACTORS

For Air And Ground Temperature

Rating Factors For Variation In Ambient Air Temperature

Air Temperature, 00C 25 30 35 40 45 50 55 60
Rating Factor (Maximum Conductor Temperature 900C) 1.16 1.11 1.06 1.00 0.94 0.88 0.81 0.74

Rating Factors For Variation In Ground Temperature For Cables Laid Direct In The Ground

Ground Temperature, 00C 15 20 25 30 35 40 45 50
Rating Factor (Maximum Conductor Temperature 900C) 1.12 1.08 1.04 1.00 0.96 0.91 0.87 0.82

Rating Factors For Variation In Ground Temperature For Cables In Ducts

Ground Temperature, 00C 15 20 25 30 35 40 45 50
Rating Factor (Maximum Conductor Temperature 900C) 1.12 1.08 1.04 1.00 0.96 0.91 0.87 0.82

Rating Factors For Depth Of Laying For Cables Laid Direct In The Ground

Depth of Laying 3.3, 6.6 & 11 kV Cables 22 & 33 kV Cables
mm
900 1.00 –
1050 0.99 1.00
1200 0.97 0.99
1500 0.95 0.97
1800 0.94 0.95
2000 0.93 0.94
2500 0.91 0.92
3000 or above 0.90 0.91

GROUP RATING FACTORS

FOR SINGLE CORE CABLES

Group Rating Factors For Three Core Cables, In Horizontal Formation Laid Direct In The Ground

Number of Cables in Group Spacing between trefoil group centres
Touching 200 400 600 800
2 0.79 0.86 0.90 0.92 0.94
3 0.67 0.77 0.82 0.86 0.89
4 0.61 0.72 0.79 0.83 0.87
5 0.56 0.68 0.76 0.81 0.85
6 0.53 0.65 0.74 0.80 0.84
7 0.50 0.63 0.72 0.78 0.83
8 0.48 0.61 0.71 0.78 –
9 0.46 0.60 0.70 0.77 –
10 0.44 0.59 0.69 – –
11 0.43 0.58 0.69 – –
12 0.42 0.57 0.68 – –
2 0.79 0.86 0.90 0.92 0.94

Cables laid on Racks / Trays in covered trench with removable covers where air circulation is restricted, Trefoils are separated by two cable diameter horizontally and the trays are in tiers having 300 mm distance.

No. Racks / Trays in Tiers No. of Trefoils in Horizontal Formation
1 2 3
1 0.95 0.90 0.88
1 0.95 0.90 0.88
2 0.90 0.85 0.83
3 0.88 0.83 0.81
6 0.86 0.81 0.79

As above B. but cables laid in open air.

1 1.00 0.98 0.96
2 1.00 0.95 0.93
3 1.00 0.94 0.92
6 1.00 0.93 0.90

FOR MULTI CORE CABLES

Cables laid Inside concrete trench with removable covers, on cable trays where air circulation is restricted. The Cables spaced by one cable diameter and trays are in tiers spaced by 300 mm. The clearance Between the wall and the cable is 25 mm.

No. of cables trays in tier Distance between Trefoils
1 2 3 6 9
1 0.95 0.90 0.88 0.85 0.84
2 0.90 0.85 0.83 0.81 0.80
3 0.88 0.83 0.81 0.79 0.78
6 0.86 0.81 0.79 0.77 0.76

Cable laid on cable trays exposed to air, the cables spaced by one cable diameter and trays are in tiers spaced by 300 mm. The clearance of the cable from the wall is 25 mm.

No. of cables trays in tier No. of cables
1 2 3 6 9
1 1 0.98 0.80 0.75 0.73
2 1 0.95 0.76 0.71 0.69
3 1 0.94 0.74 0.70 0.68
6 1 0.93 0.72 0.68 0.66

Cables laid on cable trays exposed to air, the cables touching and trays are in tiers spaced by 300 mm. The clearance between the wall and the cable is 25 mm.

No. of cables trays No. of cables per tray
1 2 3 6 9
1 1 0.84 0.80 0.75 0.73
2 1 0.80 0.76 0.71 0.69
3 1 0.78 0.74 0.70 0.68
6 1 0.76 0.72 0.68 0.66

Cables laid direct in ground in horizontal formation.

No. of cables in Group Distance of cables
Touching 15cm 30cm 45 cm
2 0.79 0.82 0.87 0.90
3 0.69 0.75 0.79 0.83
4 0.62 0.69 0.74 0.79
5 0.58 0.65 0.72 0.76
6 0.54 0.61 0.69 0.75