Current Carrying Capacity of 630 Sq mm Cable


Thread Starter


I wish to know what should be the maximum safe limit of current that can be allowed through a cable with following specs:

630 sq. mm; XLPE Aluminium 11kV laid over the ground and is about 25 years old

The cable is single core;

Average Ambient Temp is around 45 DegC in Summers and about 20 Deg C in Winters and Monsoon;

The cable is not Armoured (if that is meant by mechanical protection)

It is not covered. It is laid overground.

Brief about the application where the cable is used:
The alternator has three output points RYB.
With R, three such 630 Sq. mm cables are connected. With Y also three such 630 Sq. mm cables are connected and with B also.

Thanks and regards
Ashish... it is very important that I correctly interpret what you have presented about arrangement and grouping. Is the following correct?

a) Arrangement: The 9 single-core are laid out horizontally.

b) Grouping: R-R-R-Y-Y-Y-B-B-B.

c) Spacing: Separation distance between cores in cm or mm, if any?

d) Transposition: st specific distances? Or there is none?

e) Service Factor: need to know the total current carried! Continuously (24/7)? Cyclical(day/night variation)?

Phil Corso
Thanks Phil for your prompt reply:

I'll try my best to answer the queries as I understand them:

Arrangement is as follows:
The R1Y1B1 R2Y2B2 R3Y3B3 cables are arranged in a Tripod arrangement and taken from the Alternator to the Breaker through cable trays.

Grouping: Immediately after leaving the alternator, at the first available spacing, the cables are bunched as RiYiBi (i = 1-3) in a tripod arrangement as detailed above.

Spacing: Spacing between the cables R1Y1B1 is very little. But I have no specific mm/cm distance to quote here but they are very closely knit in the tripod arrangement. However between, R1Y1B1 tripod bunch and R2Y2B2 Tripod bunch the distance is about 10 Inches and same between R2Y2B2 and R3Y3B3 also as it travels through the cable trays to the Breaker about 50 Meters away.

Transposition: No

Service Factor: 1200 Amps per phase, Continuously and the variations are minimal. Actually, the variations in current depend on the load demand of the plant but mostly our turbines are running at base load.

thanks and regards
Ashish - the following comments and calculations are presented to answer to your question:

1. Load Amperes. [Ref 1] Load = 22.9MVA => IL = 1,200A/phase.

2. Cable Specification.[Ref 1] Single-core, Aluminum Conductor, 11kV; XLPE-insulated, Unarmored.

3. Cable Installation. [ref 1] Three trefoil groups, mounted on a cable-trough having non-restricted air circulation. Group separation is 10-inches, and air circulation is not inhibited by any obstruction.

4. Nominal Rating. [Ref 2] Single-conductor, 260mmq, 900A in Air @30DegC, Maximum Insulation Rating 90DegC.

5. Rating Factor Due to Trefoil Arrangement. Ref [3] Ka = 0.90.

6. Rating Factor Due to Grouping, [Ref 3] Kb = 0.88

7. Rating Factor Due to 45DegC Ambient. [Ref 4] Kc = 0.87

8. Overall Rating Factor = Ka x Kb x Kc = 0.69 or Permissible Amperes, Ip = 623 A.

Although the present Load current is 400 A/conductor, which suggests that the cable current has not reached its maximum current limit, I cannot claim to have calculated the cabl's true maximum permissible rating. Also, I want to emphasis the study is only qualitative because several factors have not been included. They are the effects of screens, cross-bonding, transposition of conductors, skin-effect, and charging currents. Furthermore, I have no history of insulation failures)!
Finally, I cannot provide you with an estimation of its life expectancy because of its age, some 25 years in operation! The reason is shown in the following table, produced by General Cable [5]. It illustrates that life expectancy is very much impacted by the date of manufacture:

Manufactured Expectancy
1960’s & 1970’s 20 yrs
1980’s 30 yrs
1990’s & 2000’s 40 yrs

I strongly suggest that if you are contemplating a Load increase or expect many more years of operation then you should have the cable tested using the newer Non-Destructive Techniques developed over the last few years!

[1] Ashish 25-Sep-10 Post
[2] Relemac Technology Ltd
[3] IEEE-IPCA Power Cable Ampacities (Aluminum)
[4] Siemens, Power Cable Book
[5] General Cable, NZ