voltage drop

[smur]

I have transformer 45kva. My load is 19kva, 0.8pf 208V and is a rectifier.
The transformer regulation at unity pf is 5.5%.
What size of conductor must I provide to feed the load ?
Other laods are about 10kva, combintaion of single phase and 3 phase. Pl assume some unbalance of 15% due to single phase loads. The voltage at load end may not be below 200v.
thanks

 

[DanDel]

Per the NEC, the conductors and overcurrent device should be sized to 125% of the load. Your 45kVA transformer is capable of supplying more than the load, so you can install larger conductors and a larger overcurrent device.

If you are looking for some advice, there it is. If you are looking for someone to design your system, you may have to hire an electrician or engineer (and maybe supply the distance between the transformer and the load to calulate the voltage drop?)

 

[smur]

We are having a trip problem at site. I suspect it is voltage as I raise the tap, the problem goes away. I cannot raise the tap to solve the problem. So for a load length of 60 feet for the 19kva load,0.75 pf, 208v ( transformer is 480/208v) what is the correct cable size? The transformer is having a regualtion of 5.5% at upf. the instrument voltage cannot go below 200v thanks Other loads on the system is 10kva, both single and three phase loads. The intrument is 3 phase.

 

[DanDel]

What is actually tripping?

 

[waross]

Are you sure that this is not homework? Your constrictions sound like the type of conditions typically stated by instructors.
5.5% sounds like poor regulation. At full load, the terminal voltage will be 196.6 volts. At 19 kva + 10 kav = 29 kva, the transformer terminal voltage will be 200.67 volts. When you adjust for unbalance you will be below 200 volts. An adjustment for power factor may bring the voltage slightly above 200 volts.
208 volt rated equipment will typically function on a voltage tolerance of +/- 10%. That would be 187.2 volts.
120/208 volt systems are often operated at 2% or 3% over voltage.
Your solution may end up with conductors sized for 0.5% voltage drop, rather than the normal 3% or 5%.
You may:
Replace the 45 KVA transformer with a unit with better regulation.
Add an auto transformer to boost the voltage at the rectifier.
Re-evaluate the possibility of running at a slightly higher voltage.
yours
This sounds like a no win homework problem.

 

[jghrist]

I suspect it is voltage as I raise the tap, the problem goes away. I cannot raise the tap to solve the problem.

Why can't you raise the tap?

 

[smur]

Hello
The reason we cannot change the tap is because these tools go all over the world. And we want to maintain some commonality and not change taps all the time to suit our needs. They are fixed at a certain tap. Small transformers have poor regulation. The problem I am having is that this load has to drive a plasma generator which has a rectifier in it. The rectifier dumps in harmonics in the line and folks say that is the cause for the instrument to fail at full power. Raising taps lets the problem go away. The theory is that the harmonics is causing resonance and this trips the instrument. I believe that the voltage drop along the line which is further enhanced by the harmonic content to further drops may be cause of the trippings. The cable size is 6AWG and the length is 40 feet. Should I increase cable size and shorten the line ? Do you think that line drop is significant?
Any ideas welcomed.
thanksin advance

 

[DanDel]

smur, you said "We are having a trip problem at site." What is actually tripping?