Issue in Cable Sizing and control cable selection
Hi All,
I have a query regarding the cable sizing and motor starting.
I have a 200HP motor to feed Low voltage power @480V. The distance between the substation and the motor is 2000m. Voltage drop limitations are 5% normal and 15% Starting. Control voltage is 120V.
Query 1.
What could be the size of power cable. What if I use intermediate Junction boxes.
Query 2
How could we do the control cabling as the length is quite long. due to which voltage drop in control cable will not pick up the control coil.
Does someone faces such issues?
1 answer
200 HP is 147.1 kW. At 480 V, it takes 306 A to transfer that much power.
You didn't say, but your 480 V power feed is probably three-phase, and the motor is three-phase too. I don't remember how exactly that reduces the current requirements in each of the three conductors, so let's just say you need to support 200 A per line. You will have to scale the result according to the real current requirement.
A 5% voltage drop starting at 480 V is 24 V. That's the total out and back drop, so 12 V per line. That means the resistance of each line can't be more than (24 V)/(200 A) = 120 mΩ. That's going to require absurdly thick cables.
I'm not going to go further because this makes no sense. For that much power over that much distance, you really need to use higher voltage. Find the 480 V transformer and maybe you can tap into whatever is driving its primary.
Another possibility is to use a transformer to increase the 480 V to a few kV or something, then convert back down at the other end. At 150 kW, you will need a fork lift to move them around.
This is the kind of thing you need to get the power company involved in. Anyone that has to ask here shouldn't be anywhere near this project. There are serious safety concerns, and most likely regulatory ones too. Get someone qualified and certified to work on this.
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