Maximum Voltage Drop in Cable Size for Transformer Feeder

8%? What do you think it is?

Where is your location?

Why is it important to you? Is there a possibilty of people getting hurt?

What is the code telling you?

Internet search came up with 8% on a quick glance!

I need to calculate the cable size based on voltage drop for Primary side of transformer.

The location of the Transformer is transformer yard of indoor substation.

I think the voltage drop shall be maximum 5 % of source voltage .

can u tell me the reason for 8% voltage drop & any reference ?

Then get out a copy of the local standard, such as British Standard 7671 for UK, Eire and Singapore installations, and stop wasting time on CR4.

On the primary side of the transformer Mr Slack. For once I don't think you can quote the IEE regs. Is that kettle still on??

You'll not find it in BS7671 Mr. Slack.

http://www.eon-uk.com/downloads/network_design_manual.pdf

The Voltage Drop shall be limited to 3% of the rated voltage as per IS 7098 (Part II), IEC-502.

Like many questions on here we don't really have enough information to give you an answer.

The answers so far seem to assume your primary is LV i.e. less than 1000V.

I'm thinking your primary is more likely HV in which case volt drop is the least of your worries. You need to be looking to the fault current a cable will withstand and how it is protected. Totally different approach.

Sir, I believe that the maximum voltage drop acceptable shall be 3% during running condition and shall be limited to 15% during starting. We calculated using IS 7098 or IEC 502, Although cable shall be sized to withstand Fault level of 50kA for LT system (in our case), considering the Cable Derating factors and the above criteria.

I agree, but I'm not qualified to answer. OP said "any transformer". I think cable sizing for transmission transformer, where tap changer is available, would be different from sizing for LV usage.

That would involve an On Load Tap Changer, not a cheap piece of kit and not really economical for a smallish transformer. The OP also mentioned starting loads, an OLTC doesn't react that quickly. (Not the ones I've had dealings with anyway.)

It would help if the OP could give an indication of the size of the transformer.

Another consideration is available cable sizes. Recently I was costing the installation of a 11/.433KV transformer. I could have got away with a 35mm XLPE cable, 95mm is more readily available and worked out cheaper.

The way the original question is worded I get the feeling we are doing someone's homework.

I was agreeing with silverfox that, as is too often the case, too little information was given. OP said 'all transformers' which, even this dumb old ME knows, encompasses a w-i-d-e variety. (BTW, I don't think OP mentioned starting loads. As usually happens when too little info is given, folks started making assumptions and others kept following that rabbit. )

For medium voltage or high-voltage cable no voltage drop is to be considered. You have to check the short-circuit withstanding and the ampacity for the maximum circuit conditions. The limits are the maximum permissible temperature-for long time loading 70 [pvc] 90 [EPR, XLPE] for short-time 130 [pvc] 250[EPR, XLPE].Of course, the connected with apparatus maximum allowable temperature has to be considered. At a low voltage system you may consider the total voltage drop -income cable-transformer-outgoing cables-up to motor terminals in steady state load and in starting conditions.

Is this a homework question? If your wondering about allowable voltage drop on any transformer and your working in the field, please let us know your location! I will make sure to stay miles away from you and your craftsmanship or work.

If a large induction motor is to be started-namely D.O.L.-the voltage drop has to be carefully checked. However, in my opinion, most of the voltage drop will be in transformer and in low-voltage cable. See [for instance]:

http://www.idc-online.com/conferences/PAPERMichaelHamilton.pdf