Electrical Question.... Transformer Cabling

Easy. Open British Standard 7671, and start reading. Once digested, apply the contents.

What is the primary and secondary voltage?

For this high voltage cable criteria is short-circuit current. Usually- at 11 kV- 350 MVA could be the short-circuit power. I"k=350/11/sqrt(3)=18.37 kA.

For CU conductor and xlpe ins.[250 dgr.C short-circuit temp.] 0.45 sec. [I prefer 0.45 sec. for reserve protection fault clearing] 3*75 sqr.mm CU 6/10 (12) kV- according to IEC 60502-2- will be good.

Low voltage side: first of all -according to cable raceway-underground, free air, duct bank and so on and according to maximum permissible temperature of the equipment connected with-you can chose the appropriate cable[it depends also on the your national standard-NEC ,BS or else]. A short-circuit criteria applying it is good in order to check if this cable can withstand the short-circuit current.

The voltage drop also has to be checked[By the way ,NEC recommends 3% for the feeder]

Sorry! No conductor cross section of 75 sqr.mm. is mentioned in IEC 60228 so the cross section has to be 70 sqr.mm and then the time permissible will be only 0.4 sec. or could be 95 sqr.mm and the duration up to fault clearance could be 0.7 sec.

The low voltage feeder-according to IEC 60364-5-52 [the same as BS]for instance-running in embedded duct 3*185 sqr.mm will withstand 243A[90 dgr.C].For 70 dgr.C it has to be 3*240 sqr.mm.

Answering questions such as this are like giving a small child a machine gun and telling him to go play.

The likelihood that this censored kills someone is pretty high. Only problem is it probably won't be the OP.

I agree with you lyn. I was very naive, as usually, and I forgot to ask myself what could be the o.p. knowledge level. Thanks again!

It's probably just homework. Maybe your answer will help educate.

If it's not, your answer will probably not change the amount that OP is dangerous anyway. Just cross fingers and hope for the best.

For selecting primary cable, check rating of cable for two conditions:

1. System fault level (......kA for .......Secs).

2. Continuous Current and Laying conditions (3 phase, ambient, in air or in trench or buried in soil, or in duct etc and cables laid side by side etc.)

For Secondary size just size according to criteria 2 above as fault level is low (limited by 160kVA rating of Transformer. For example for 4% Short Ckt Impedance of transformer this shall be 25 times full load current, which is approximately 5.5kA).

The current carrying capacity of cable is provided by all cable manufacturers depending up on their voltage rating and type of cable laying. The size of the cable required for your specific job is to be chosen from the chart depending up on your specific conditions like UG directly , cable trench, on OH cable tray.
Other conditions are Ambient temperature ,Depth of cable trench ,Thermal resistivity of soil ,ground temperature and other parameters. After considering the de rating factor and specified rating of the cable manufacturer you can decide the size of the cable yourself. Only important factor is you should use only cable confirming to standard Specification and as far as possible of reputed make so that you get the correct rating as they specify.
Get the current carrying capacity of cables by down loading the chart from the site of any reputed cable manufacture

Didnt you find anybody else or any books other than this forum to get the basic knowledege of cable sizing!! It shows that you are a lazy man just spending time in internet.

Believe me,..it is good for you if you can read the books again, find a experienced Engineer or atleast search in google.

Start by doing some calculations as follows:

160kVA is the transformer rating.

Apply Ip = KVA/vP and Is = KVA/Vs

I take that Full load primary current (Ip) = 8.40A

And that Full Load secondary current (Is) = 222A

Primary voltage = 160kVA/8.40A = 19.047kV or 19.05kV

Secondary voltage = 160kVA/222A = 0.7207kV or 0.721 X 10^3 = 721V

Now the above is known you can refer to IEE regulations or other relevant specification documents, manufacturers guide, etc to determine how to size the cables. Remember to take into consideration the ambient temperature of the cable, correction factor for thermal insulation, etc. I hope that this will give you some help.

You have to divide by sqrt(3) then medium voltage is 11 kV and L.V. 0.415.

Another word of caution; Voltage rating of Primary side cable shall be selected carefully.

If the Neutral of Primary is solidly connected to earth, then go for E grade cable. For example it will be 11kV/6.6kV for 11kV 3 phase System.

If the Neutral of Primary is connected to earth via a Transformer or Resistance or any impedance (Floating Neutral) then use UE Grade Cable. For 11kV system it is 11kV (UE), which is 22kV/11kV for 11kV System. That is phase to phase 22kV and phase to earth 11kV.

According to IEC 60502-2 ch.4.1 Rated voltages there are 3 categories A,B,C.

A-if a phase-to-earth contact will be cleared in 1 min or less.

B- if a phase-to-earth contact will be cleared in more than 1 min but less than 1 hour.

C- if a phase-to-earth contact will be cleared in more than 1 hour.

Table 1 states:

for categories A and B the recommended rated voltage [phase to earth]=6kV.For C it is 8.7 kV.

I think all the other European [harmonized] including BS will recommend the same.

7anoter4, pls expalin, because NEC Tables 310 and Tables 360 give ampacities based on for nominal voltages and currents, and clearance time isnt a factor. However, clearance time is a factor in selecting fuses and circuit breakers.

You are right. NEC is not so clear about short-circuit current withstand issue for cables.

What NEC cares it is only cable ampacity.

NEC Art.100 Definitions

Ampacity. The current, in amperes, that a conductor can

carry continuously under the conditions of use without exceeding

its temperature rating.

Short-Circuit Current Rating. The prospective symmetrical

fault current at a nominal voltage to which an apparatus

or system is able to be connected without sustaining damage

exceeding defined acceptance criteria.

Art.310 refers only to ampacity.

Art. 310.15 Ampacities for Conductors Rated 0-2000 Volts.

Art 310.60 Conductors Rated 2001 to 35,000 Volts.

There are 3 [usually] criteria in order to chose the right size of a conductor :

1)Ampacity

2)Short-circuit withstand

3) Voltage drop

See[for instance]:

http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/short_circuit_current-ratings/BUS_Ele_2008_NEC_SCCR_Requirements.pdf

7anoter4 - understood, thank you Sir!