Station Service A.C.

[corbin801]

I am working on a situation that we haven't had to deal with before. I'm installing a Cap bank across the road of one of our switching station. To get station service power to the cap bank, I want to run Quadruplex cable from the switching station 120 V station service AC box to the Cap Bank AC box. This run is 470' and will be ran in PVC conduit the entire way, with exception to 100' span off of two 45-2 poles. The cap bank in question will have four 20 Amp breakers for the Switcher A.C. (Motor, Light, Heaters), Control Cab (Heaters), Battery Cab (charger, Heaters) and Scada. My question is pertaining to sizing this cable for adequate voltage drop. I received some help from system protection that suggested 3/0 cables w/ 3/0 neutral, which works out to about 8.2% voltage drop, using an on-line voltage calculator. I heard you should never go above 8% for AC motors. So, is this acceptable? If I’m missing anything, how big should I size the Quadruplex if this isn't adequate?

Thank you,

 

[oldfieldguy]

corbin

What's the voltage on the cap bank?

In most of the situations I've run into, we hung a little pole mounted transformer for the AC supply rather than deal with long runs of large cable.

old field guy

 

[waross]

I could suggest using about 120/240 Volts. The pat answer that this will reduce your percentage voltage drop to 1/4 of the voltage drop at 120 Volts, but for such a small application, you need to consider and evaluate the load currents. Do the calculations with a variety of loads connected considering the resulting neutral currents. If you consider using 480 Volts and a pair of transformers the saving in cable may pay for the transformers, but again, the actual loads must be evaluated.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Overvoltage]

Why don't you just hang a small transformer on the same pole or a nearby one to supply the AC? That's what I've typically seen done for pole-mounted cap banks (which is what it sounds like you're talking about).

 

[corbin801]

Oldfieldguy: It's a 69kV Station. I was in communication with the CO-OP to install a station service pole on a distribution line that runs close, but my boss wants to explore this option, to keep from paying the CO-OP and from hanging a 69kV station service transformer.

Waross: I can't get a real direct answer from our system protection guy, so I have been assuming full load current at 80 amps. I never considered 480 volt transformers, I'm going to dig around on that this afternoon.

 

[dpc]

I doubt that the current will be anywhere close to 80 A. The heaters won't care about the low voltage. Generally the motor operators are designed to handle a pretty wide voltage range - you might check with switch manufacturer for motor operating specs.

 

[PHovnanian]

corbin

Run a 120/240V circuit and split the loads between both legs.

Use the FLA of each load when calculating circuit current. Not the breaker rating or circuit ampacity.

Run the voltage drop calculation again (beware of on-line calculators).

 

[corbin801]

DPC: your right, I got to talking with a few guys that where in a meeting with me and it seems like the full load will be much closer to 15 amps(i never really had to deal with small AC load devices), which will make this problem almost go away. I figure I'll design for around a 30 amp load and call it quits. Thanks all. If I find out I'm wrong on the load, a couple of 120/240 transformers will solve the problem in a heart beat.

Thanks all.