Sizing the Neutral for a 21 Kv Four Wire Multi Grounded System?

One thought is how long is the circuit? What kind of load? If it is all 3phase your neutral current is zero. If you have a multiple of different kinds of load and the system is balanced, the neutral current is small. After all that I would suggest 2/0 AL.

The circuit is about 115 miles. The load is about 450A and it has single phase load, two phase and three phase load. The circuit feeds a faily big area, you can say big section of a small town. The circuit is also unbalanced. So it is not a balanced circuit.

The circuit is about 115 miles. The load is about 450A and it has single phase load, two phase and three phase load. The circuit feeds a faily big area, you can say big section of a small town. The circuit is also unbalanced. So it is not a balanced circuit.

I give up! DO NOT contact a licensed electrician to meet your wiring code. Put in any kind of scrap wire you can find. When a fault happens with this circuit you'll be long gone and nobody will be able to trace back to the license you do not have. You will be long gone. You might even kill somebody from the fire you make with this installation but you won't care because you'll never know.

Let me take a wild guess here. You were the one that got the lowest bid to rework the power distribution for the Carnival Cruise ship the Triumph. Which web site told you how to wire that ship?

Eddie I have some bad news for you. You say the load is 450 amps. The site below gives the ampacity of 4/0 AL at 360 amps. Your load won't fit.

http://www.google.com/url?sa=t&rct=j&q=ampacity%20of%20acsr%20conductor&source=web&cd=1&ved=0CDIQFjAA&url=http%3A%2F%2Fwww.phoenixwc.com%2Fdocuments%2FBare%2FACSR-AluminumConductorSteelReinforced.pdf&ei=qrUeUeXJEpHc8wSy-YDYDw&usg=AFQjCNFHFMcO6dlNWebGkr8BFxFdjip1PA&bvm=bv.42553238,d.eWU

This site also shows the resistance for 4/0AL to be 0.1157 ohms per 1000 ft. The resistance for 115 miles is 5.280 x 115 x 0.1157 = 70.2 ohms. The voltage drop will be 450 amps x 70.2 x 1.73 = 54692 volts. Do you see the problem. Of course you started with only 21000 volts. I suggest you go with 230 kv or maybe 115 kv and see if that works.

I saw the ship you wired. It docked at my home port last night. Everyone was safe.

Dear Engineer,

The line is a 21 kv, it start from station with 719 Al, then down the line we have 397Al, then goes to 4/0Al and then to 4ACSR and then 6C. So, as you know, you don't size your line from start to finish with big conductors unless you are fresh out of school or you are looking for a new career.

You also use line regulators and cap banks to conpensate for the losses. Anyhow thanks for your help and go war eagle and Auburn university.

By the way it is a good school and my relative is a student there. I am far away from Alabama but it is a good place and again go war eagle.

Thanks,

Eddie