Cable Size Calculation Problem

Since the length is substantial, i would suggest using 6 runs of 300 sq mm single core copper (SWA/XLPE) for each phase.

Cheers.

I think as far for the selection of cable for 1500 kva transformer we have considered even the voltage drop because the distance is quite far from the main distribution panel so i would suggest you better go for 1c-630sq mm (11 cables) 3 cables for each phase and 2 neutrals. Correct me if iam wrong.

hi,

more than answering ur question I am curious to know how u managed to insert the diagram?

Thanks for the answers, but could you guys show me how do you do all the calculations to decide the size of cable to use?

-> Just click on the camera icon to add pictures

Voltage drop in the UK is limited to 4 % (I believe -- correct me if I'm wrong, PW or Del), so you want no more than that in 520 m.

Copper has a resistivity of 1.7 X 10^-8 Ohm - m. Resistivity = Resistance X Cross Sectional Area / length. Rearranging, Resistance = Resistivity X Length / Cross Sectional Area, or Cross Sectional area = Resistivity X Length / Resistance.

Voltage drop = Current X Resistance. Current in a 500 kVA (1 of 3 phases) line at 220 Volts is 2273 Amps. Maximum resistance of conductor for 4 % drop at 220 volts is 8.8 volts / 2273 amps = 0.00387 ohms.

Minimum Cross Sectional Area (sq. meters) = 1.7 X 10^-8 ohm-m X 520 meters / 0.00387 ohms, or 0.00228 sq. meters, or 2283 sq. mm. for no more than 4 % voltage drop.

Of course, it's better to have less voltage drop.

thank you for the detailed calculation Bill, I really appreciate it, but, according to the AWG table, there is no such wire with the cross section area of 2283 sq. mm. it seems too big.

True, there's not a conductor that I know of which has a cross-section of 2283 sq. mm., but 2_2000 MCM has a cross-section of 2700 sq. mm., or 3_1500 MCM has a cross-section of 2526 sq. mm.

This similar to what ahmed952 suggested: use 3_630 sq. mm. cables for each phase, however doing so will yield a voltage drop of a little over 4 % at full load. Four 630 sq. mm. cables for each phase will keep you under 4 % at full load.

Would it be easier to move the location of the transformer, using either overhead or underground HV cables?

I guess the supply is fed from the street to a transformer just inside the boundry of your property by your electricty supplier?

At the levels your talking about, don't just calculate on the cable guage (I generally run on about 10A per mm² )

You also have to look at the effect of voltage drops on each and every termination point.

There is a company close to here, they measured the voltage drop from 1 end to the other, about 1/2Km, similar distance to what your using, and calculating the voltage drop in the wiring, they were still falling short, until they measured the drop across each termination, Transformer -> wire, wire -> wire, wire -> load.

6 set of (4) 600kcmils copper with 75 degree c. insulation. Will give you 2,520 amps of conductor.

The 1500kVa transformer will have 2,275 amp output on the secondary side of the transformer

Am I to assume this is an industrial setup and not domestic as suggested by an earlier poster? If it's industrial, it maybe cheaper to install a new transformer closer to the MDB! I may be wrong ( so please correct me if I am ) but coming directly off the secondaries of the transformer would require some kind of switch gear/fuses directly after the transformer so that the LV line could be isolated without touching the HV side and also to protect the transformer if a fault should happen on the LV line!

Would be good to know what you intend doing with all of those Amps and if you intend to use them all at the same time but as yet, you haven't told us so I won't be able to offer you more than 2cents worth!

Moving the transformer closer to where the power is required and extending the high voltage primary feeder would substantially reduce the amount of copper required on the secondary of the transformer to the incoming main circuit protective device and reduce copper required and the installation cost.

Less distance, less copper, less cost.

Hi,

In your case you have to consider only the voltage drop. I assume that you will install the cables on ladder or tray, therefore the temperature and laying factor are not really important (but not extrem situation) because of the length.

To solve this Problem, please give following data

1. Max. allowable voltage drop

2. According which regulation; IEC, VDE, BS?

3. What net you have? TN-S; TN-C?

4. Just for information; what cable type you use for? XLPE, PVC, EPR

Best wishes from black forest

Wormnhaque

1500 KVA 1000 KVA 3000KVA

CABLE COMPLETE DETAILS SINGLE CORE AND MULTI CORE

1500 KVA 1000 KVA 3000KVA

CABLE COMPLETE DETAILS SINGLE CORE AND MULTI CORE

1500 KVA 1000 KVA 3000KVA TRANSFORMER COMPLTE CABLE DEATILS WITH DIAGRAM

Very interesting.would like to know any CT/PT are to be installed at the outlet of the secondary transformer immediately or near the MDB which is away at around 35 meters away from the transformer.The transformer rating is 120 kva and how much voltage drop can be anticipated and maximum current can be considered.Pl. excuse if anything wrong in furnishing the details as i have very little knowledge in ele. eng.

I=S/1.732*Vsec, WHERE S=VA,

therefore, I=1500kVA*1000/1.732*380Vsec

=2279A.

Consult NEC Cable sizes, for example: 250mm2 has 354amps.

I=2279A/354A=6.4

Say 7 sets of (4C 250mm2).

That's it!

Also consider Voltage Drop of the Cable, cable manufacturer has a complete definition regarding this matter..

Vd should not exceed 5% from transformer down to the last equipment.

Let say, 1% from transformer, 2% from MDB to DB, then 2% from DB to last utilizing equipment..That's the standard i usually used. When exceed get higher size.

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thank you!

Dear Mr.kreyszig,

The cable selection is decided on the following.

1. Voltage Drop

2. Type of Cable that is to be used.

3. De-Rating Factor applicable

4. Thermal Rating or Temp. Rise allowable.

5. Power Factor in the system.

6. Loading pattern - whether steady load or intermittent over load/no.of starts if motor is connected.

DHAYANANDHAN.S