from the Automation List department...
MV cable size calculation
 Posted by FN on 3 November, 2007 - 10:47 am
Can anyone provide the formula and the details of the factors which is considered in calculating the size of the medium voltage cable.

 Posted by Phil Corso, PE on 3 November, 2007 - 5:01 pm
Responding to FN's 03-Oct (10:47) query, following is a list of the many factors that must be considered:

o Allowable voltage-drop for normal operating load.

o Allowable voltage-drop for future operating load as determined by motor's Service Factor.

o Allowable voltage-drop for starting condition.

o Allowable voltage-drop for motors to be re-accelerated following a voltage interruption.

o Type of starting, i.e., full-voltage, reduced-voltage, part-winding, delta-wye, VFD, etc.

o Short-circuit withstand capability for an in-close fault (eg, motor starter.)

o Ground or earth-fault capability for a far-end fault (eg, motor terminal.)

o Plastic or rubber sheath resistance to chemical attack.

o Metallic screen or shield material withstand capability for excessive heating.

o Derating factor(s) covering installation, i.e., free-air, direct-burial (dry, wet) in conduit (duct bank, steel, plastic.)

o Derating factor(s) covering conductor material (Cu, Al.)

o Derating factor(s) covering construction, round or sector shaped, 3x1/c, 1x3/c, 3-1/2 or 4-core, overall sheath and/or shield.

o Derating factor(s) covering insulation type and voltage level

o Derating factor(s) covering termination type (bolt, lug, clamp, brazed, soldered.)

o If direct-burial, then consider the possibilit of subsidence

If you need additional info, please feel free to contact me. egards, Phil Corso (cepicon@aol.com)

 Posted by FN on 9 November, 2007 - 12:26 am
Dear Phil,

Actually I want to know what values we should consider for these factors. Previously we've taken the factor as 0.5. It derates the cable size by 50%.

Actually we had a 6.3 KV 2 (3X150 mm2) cable for one of our transformer feeder located at a distance of approx. 1500 meters. The transformer rating is 1500 kVA 6.3/0.4 kV 50 Hz. The cable is laid directly into the ground at a depth of approx. 1 meter. There is no other cable in the trench. We've taken the factor 0.5. Do you think it is o.k.? If not, then plz tell me what will be the current rating of this cable.

If you still need any more information, email me on: faysalnaeem @ hotmail. com

 Posted by Phil Corso, PE on 11 November, 2007 - 5:42 pm
Responding to FN's 09-Nov (00:26) reply... the derating factor 0.5 is far too severe for the design criteria provided thus far:

1) Calculated Current.
If the transformer has no forced-cooled rating, then the 8-hour demand (kW & PF) in equivalent kVA is used to calculate feeder current. Assuming 1,500 kVA, then feeder current is 138 A.

2) Assumed Ambient and Installation Conditions.
Plastic insulation; permissible conductor (copper) temperature, 70 degC; metallic shield, none; three single-core, round conductors, arranged in a flat horizontal plane; core separation, 7-10 cm; burial depth, 1-mt is (more than adequate); separate earthing conductor in same trench, 70 mmq (min); soil temperature, 20 degC; soil thermal resistivity, 100 degC cm/W.

3) Cable Ampacity.
Cable capacity for the conditions cited above is about 400-420 A. Thus, 2-conductors per phase are unnecessary.

4) Voltage-Drop.
Because the asymmetrical nature of the flat arrangement impacts inductive reactance, the voltage-drop calculation will be provided later. Please confirm, or if preferred, update, the Design-Basis criteria given above.

Regards, Phil Corso (cepsicon@aol.com)

 Posted by Ghassan Michael Tomeh on 17 August, 2008 - 1:36 pm
Dear Phil,
You cable is more than enough if we asume the following, Power factor 0.85, ambient temperature 25C, YOUR CABLE IS XLPE Type OR EVEN IF IT IS PVC STILL OK. you are ok and you could use 2(3x120mm but well you already have your existing one so no problem at all even the one you have it can carry the expected short circuit which is imporatnt factor for sizing cable. at the end your cable has current carring capacity 450A AND IT IS DE-RATED TO 210 A while your load current is 140 A AT FULL LOAD.

Thanks
Ghassan Tomeh
Electrical; Engineer

 Posted by najeeb on 23 September, 2008 - 8:14 pm
Dear Phil,
I saw your information on web site, I want to calculate feeder cable and its information is:

length # 4124 feet

voltage # 14.4 kv
current on the primary of the transformer is # 182amp

what will be the correct size of cable please provide me in detail calculation?

 Posted by shailesh mahapatra on 16 August, 2012 - 12:37 pm
Hello. I have a similar question. 14.4 kV Cable, Feeding the primary of a 5000/6250 kVA XFMR with primary amps of 200/250 A. What size I can use for this? Wire length is 60 feet.

 Posted by Engineer shafi khan Mohmand on 10 September, 2010 - 6:24 am
if u want calculate the size of the cable, you must have to know the fault current and the duration of fault.

let us consider if I fault current flows for t time, then size of the cable can be given by,

S2= I2 X t/K2

where K is constant and its value depends upon whether u use PVC or XLP insulation.

 Posted by Phil Corso on 10 September, 2010 - 9:25 am
Shafi Khan Mohmand... in most cases it is sufficient to know the supply transformer capacity and its percent impedance!

Regards, Phil Corso

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