Hello !
At what length do you start increasing you wire length due to distance?? This question might differ my country-country but im interested in the USA code, none the less i would still like to know how it is done on your country.
So assuming i have a 100meter run on a 3phase 3 wire 380V 200kW system. How thick the wire would be for it to be up to code (lets say for a THHN wire)?
Question 1: The 200kW system would be outputting 302.36 Amps, Do you gauge each of the 3 wires for that current? or you split it up?
Question 2: If it were a 4 wire system (with neutral), should all 4 wires be the same gauge, and should each wire be rated for the full 300A of current?
Question 3: Based on the answer on questions 1 and 2. I have read that you derating depending on the voltage drop , what is the allowable voltage drop for the system? (5% , 3%, or 1%, or something else).
You wouldn't discuss a 200kW system, for electrical wiring you mean kVA. Also, circuits are typically in even, round current values, not kW or kVA because the wiring doesn't care if it is at 380V or 400V--it's the current that matters when sizing it. A 3-phase circuit with 300A on each leg and 380V phase-to-phase would be 300 x 380 x 1.732 (square root of 3) or about 197kVA.
The size of the neutral depends on the situation, but the most common situations require it to be the same gauge as the phases. In rare instances it needs to be a lot larger.
The allowable voltage drop could vary quite a bit and it depends on whether you are talking about regulatory or practical considerations. For regulatory concerns, consult your local AHJ. For a very long run, you could have a huge voltage drop and as long as the ampacity of the wire wasn't exceeded and the voltage is still sufficient for the load, there's no issue other than wasting a bit of power. For intermittent uses where you don't care too much about power loss, you can add a boost transformer on one end if your load would be undervoltage. In any case, if you need the voltage drop to be under a certain amount for long runs, you have to upsize.
One additional issue is that ampacities are given with a temperature rating. All components connected to the wires must be rated for that temperature to use the full rating. THHN is rated at 90C, but many circuit breakers may be only rated for 60 or 75C. In that case, you have to assign a lower ampacity to the THHN if you use it. Also, if multiple wires are placed in conduit, they may need to be derated. From this chart you can see that you would need 300MCM (kcmil) wire if you can tolerate 90C, otherwise 350MCM. That would apply no matter how short the branch is.
https://www.cerrowire.com/products/resources/tables-calculators/ampacity-charts/Using this handy online calculator, you can enter your values and see that even with a 100-meter run (330 feet) with the minimum 300MCM cable in steel conduit, 380V @ 300A, your voltage drop is only 3.1%. Voltage drops are absolute, only resistance and current matter. The higher the voltage, the more (absolute) voltage drop is permissible. A 208V 3-PH 300A circuit with the same wires would be 5.7%, which might be unacceptable.
http://www.mcgowanelectric.com/etools/voltage-drop-calculator