How to Calculate Conductor Size

British Standard 7671.

It depends particularly on what the voltage is. (Half voltage requires double current.)

the voltage for the system is 3-phase 415v

It always depends on site location and the variety of environmental concerns. These will be spelled out in the wiring standards for your location. But knowing what cabling reference is required and being authorized to select from the reference those cables are two different things. Hire a licensed electrician or electrical engineer to do this work. To many facilities have burned to the ground because untrained people thought that they understood these cryptic codes.

It would be unprofessional and irresponsible to do the math for you, since you obviously are not qualified to perform the work.

I agree with redfred but would add that people's lives are at stake here. Do you want to be responsible for that if your wiring burns the building down.

Ask your insurance carrier, if you have one, what they think of unqualified people performing electrical installations.

hello boss if u know the answer tell me or else no need to give such advices for me anyway thanks for ur valuable suggestion

tel4fun,

This web site has for a good reason a rule against doing student's homework. Now the members do occasionally answer a student's question when the student clearly shows that the student is doing most of the work already and just needs a little guidance. You've not demonstrated this.

Now your question might not be homework. In this case there are a variety of wiring standards that are location specific. There are also a collection of logistic questions that need to asked and answered for a proper installation. You've not mentioned any of these concerns. The only information that you've offered is the length of your cable run and that this is a lethal amount of power. You don't even identify what voltage you have at your source or how much of a voltage drop you can accept at your load. This does demonstrate that you know nothing about electricity. This is why you need a licensed electrician or electrical engineer to help. Anybody around this work can be killed by a poor installation.

As I typed this long reply, it dawned on me that there is a third possibility. You may have a bar bet that there's a simple formula for calculating which cables are needed for this run. Well there isn't one, you lose the bet. Pay the bar tab.

First Calculate the Current of the Load (i.e) 800kW from the Power Equation P = 1.732 V I Cos(theta) in Watts, convert to kilo watts accordingly. Then take the distance (i.e) 50 mtrs. Calculate the Voltage drop from MCC to PDB for the distance.

If there is a voltage drop, then work accordingly to the new voltage reducing the voltage drop. Now your current will go up.

Now Refer any reputed Cable manufacturer's data sheet for the final current you arrived after the voltage drop calculations. Confirm your cable laying medium (Air, Ground or Concealed in pipes) which decides your derating factor.

From the Mfr's Cable Selection Tables, now can get the Cable Size.

Also you have to consider the Short Circuit kVA or MVA and Short Circuit Current that is going to be involved in that Cable route.

I am sure, but you can take 3 to 6 times the current value you got after voltage drop calculation as Short Circuit Current. For the Short Circuit kVA / MVA you have to consider the kVA / MVA and Voltage rating of the Power Transformer.

DISREGARD THIS POST.

While the basic calculations are correct, LITTLE things like the available short circuit current and the required appropriately sized breakers/switches cannot be guessed at or use rules of thumb.

Go back to the two previous posts and DO THE RIGHT THING. There is no way ANY engineer should try to do what he/she is not qualified to do, and reading a manual on how to do it DOES NOT QUALIFY YOU TO DO DESIGN WORK. It will help you assist the qualified designer to do their work for you.

This is just like an attorney representing himself- he has a fool for a client. However, in this case, foolishness can really hurt someone else.

Thankyou very much

I think you'd better calculate the size of your load, then according to where you use it, find a suitable size in the cable, or conductor wire catalogs of suppliers. You will need stiff or more flexible, isolation X,Y,Z. The conductors have been calculated for you. Take one that handles your load.