Ungrounded 11KV system

[LarryFine]

Six sets of 300 kcmil conductors can carry 350 A over a distance of 1400 feet. The voltage drop would be approximately 26.25 V, which represents a 5.47% voltage drop.

What voltage are you using in this calculation, the original 480v?

 

[julianov]

Yes, it's 480V. That's why we're using a transformer—to avoid having that current running over 1400 feet.

 

[electrofelon]

Six sets of 300 kcmil conductors can carry 350 A over a distance of 1400 feet. The voltage drop would be approximately 26.25 V, which represents a 5.47% voltage drop. So we are not good. It should be six of 400 kcmil. I thinks its a lot.

Did you catch post #25? 5.47% will have about the same performance as having two transformers. Do you have the transformers or do you need to purchase them? If the latter, What is the cost?

 

[zbang]

Yes, it's 480V. That's why we're using a transformer—to avoid having that current running over 1400 feet.

Then why 11kv and not 4160v (and use a lower class of MV cable)? All of these things are part of the engineering & cost analysis, including running fat wires and using 480v the whole way. And none of the voltage discussions get into the un-grounded part, which with a 480 wye source becomes a non-problem; see also my post #10.

The OP might also find something useful in this thread- https://forums.mikeholt.com/threads/4160-vesus-480-volt.99996/

 

[julianov]

Since no one has come out and said it, No you do not need to ground MV systems, and you do not even need ground detectors.

Thanks. That's the conclusion: there's no need to ground the MV system or install ground detectors. For 480V motors powering water pumps over 100 HP, especially when they're far from the panel and large wire sizes aren't feasible, transformers are the only viable option to prevent significant voltage drops.

Thanks to all for your responses.

 

[tortuga]

Just dont forget for sizing the branch circuit and feeder ampacity has to be per 430.6 and table 430.250. So you'd use 361A for a 300HP.
So I'd think you need 361 X 125% 452A conductor size to the motor on the 1-line.

 

[kwired]

Interesting to see what people do in different areas, Franklin Electric makes '575 volt' pumps.
They have a 8" 75HP 2396136241 submersible. It would run off a typical 600V 3ph pump service.

Most these pumps are not submersibles they are an assembly with a shaft that runs from top of casing all the way down to the pump. The motors are "hollow shaft" motors and the pump shaft extends through the motor. Most common is 60, 75 and 100 HP. Sometimes you will run into smaller motors, all depends on volume of water that is needed to be pumped as well as how far it needs lifted from the well. Have seen submersible type pumps a time or two but those were only like 20 hp. I'm sure there likely is 575 volt versions of these motors but I don't know you will find a 600 volt service anywhere around here either, therefore there isn't any local stock of any 600 volt motors. Kind of no point in stepping 480 up to 600 either will likely cost more to do so than to increase conductor size if voltage drop is the main reason for going with 600 volts instead of 480.

And like I said I never seen a center pivot machine that wasn't 480 volts either. They do make export machines that very well may be other voltages, but I'd guess the dealers around here would rather not deal with other voltages, keeps it simpler for their guys who IMO most are not true electricians, but do know these systems fairly well.

 

[kwired]

Just dont forget for sizing the branch circuit and feeder ampacity has to be per 430.6 and table 430.250. So you'd use 361A for a 300HP.
So I'd think you need 361 X 125% 452A conductor size to the motor on the 1-line.

That would give you minimum conductor ampacity. There isn't any NEC requirements on voltage drop, just a suggestion. So would you base VD calculations on the minimum ampacity of 452 or use actual current of normal running conditions? I don't think NEC cares. Some energy code possibly could care. This motor may not normally draw the 361, and if it has power factor correction (near the motor) that could lower what the conductor actually sees as well. NEC still wants the 452 for min ampacity but VD is dependent on actual current along with actual conductor impedance and not minimum required ampacity.

 

[tortuga]

Good points to clarify I was just saying in his hypothetical 480- 11kv - 480 setup the 480V conductor segments should be at least 452 Amps