Voltage Drop

[dray]

Man I hate that if I don't work problems for a while I have a hard time with the most trivial things. Use it or lose it. I have been working for a utility for a while and I came from the consulting world. We calculate voltage drop for services and here are my questions:

We try to keep our voltage drop to 5% which is the NEC recommendation. We include the voltage drop across the transformer along with the drop in the conductors. I have no problem with the voltage drop across the conductors using NEC's way. I am not sure if anyone else includes the drop across the xfmr and how accurate does it need to be.

We don't get X and R values just %Z. Say I have a 50kVA, 3phase, 4%Z, 208/120V xfmr. The load is 3 phase, with 20kVA demand.

One way I have seen is (.04)(20/50)(208)=3.3. Another way would be using FLA and Z. 20kVA = 55.5A. (55.5)(.04)=2.22

What is correct or incorrect about these calcs. I am definitely missing something and this should be super easy.

Another thing we calculate is Flicker. It seems to me that it is voltage drop due to inrush of a motor starting. It seems like it should be similar to one of the above calcs except the load is the FLA of the motor multiplied by 5 or 6 to get inrush amps.

Thanks in advance.

 

[Flyer_PE]

The first answer is correct. It's a per unit calculation.

The second answer doesn't account for the 4% transformer impedance being on a 50 kVA base.

 

[dray]

Thanks for the reply. What about the start up or cold load voltage drop? Say I had a 10hp motor which is approx 10kVA. At start up assume 50kVA. So it would just be .04x208?

Does anyone know a way to get x and r data from core and load losses? We get %z and loss data but that's it. Thanks.

 

[knight1fox3]

Does anyone know a way to get x and r data from core and load losses? We get %z and loss data but that's it. Thanks.

I believe you can get that information from the corresponding transformer manufacturer.

 

[dray]

You are probably correct. I was trying to use what I had though. I guess I could try giving them a call. Our xfmrs are bought through purchasing and it seems like it would be easy to have them request that info every time we receive a test report but there may be a lot of hoops to jump through to ask them to do that.

 

[knight1fox3]

You are probably correct. I was trying to use what I had though. I guess I could try giving them a call. Our xfmrs are bought through purchasing and it seems like it would be easy to have them request that info every time we receive a test report but there may be a lot of hoops to jump through to ask them to do that.

I typically had 2 or 3 main manufacturers that I used when indicating which transformers to buy. And at each company I had a personal technical contact I could email direct with specific information I needed. In addition, purchasing could stipulate that a transformer test data sheet be provided once the unit has been manufactured. You will find more detailed information in that document versus the nameplate data. Shouldn't be too much hoop-jumping since the mfr. should have that data on hand and filed somewhere. Usually the hassle was going through our purchasing dept. Hence why I established my "go-to" tech. contact.