Voltage Drop Before and After a transformer

[wyattlewis]

I was doing some calculations and I ran into a problem I cannot find the solution to which is basically...

It is allowable to have a 5% voltage drop from your service entrance to the device that is being powered...

If that device is a 480-to-208 step down transformer, does the calculation become 5% from the service to the transformer then the 208V side resets?

Is there an inherent drop in the 208V side of the transformer if I have dropped 5% on my 480V feeder circuit?


Thanks for any help that can be offered.

 

[Npstewart]

Seems like this may be a complicated answer, and im not really sure what the answer would be. The transformer doesn't really know what voltage it is receiving. I would first figure out the voltage drop just before the transformer. Say you lose maybe 5 volts so your voltage is 475V. Then I would use that voltage and see what the secondary voltage would be given the ratio of windings in the transformer and see if that voltage is acceptable to the device on the other side.

 

[wyattlewis]

The way I think about it is similar to what you said... The transformer does not know what voltage it is receiving. It also exists only to create a new voltage of 208V.


How do you find the new voltage on the other side of the transformer? 480:208 == 1:0.433?

so the 475V on the primary creates 205.6V on the secondary?

 

[Npstewart]

It is directly proportional to the number of windings. So yes I beleive you are correct, if the input voltage was 475 the output voltage would be 205.6. The only reason why we can figure this out is because we know the transformer was originally designed as a 480 to 208. Otherwise you would need the transformer data.

Like I said though, I wouldn't worry about being under the 5% number or any other percent, that is a FPN requirement only. Do a voltage drop calc with your NEW voltage, and the FLA of the motor or device, then see if that is within the acceptable range for that motor or device. I believe the only time you really have to be under the 5% is for a fire pump. And I seriously doubt anyone would use a step down xfrmr prior to a fire pump.

 

[wyattlewis]

I have gotten to the bottom of this. Npstewart is 100% correct. The output voltage is a function of the winding ratio x the input voltage.

however, in situations where the input voltage in less or more than desirable, there are taps on the input and output terminals that can help you attain the voltages you need. For example, on square-d EE Three phase transformers, the standard tap configuration for 480-to-208 is 6-2.5%2+4-. This means there are six taps in increments of 2.5%. Two above 480V and four below 480V.

In the case of a full 5% voltage drop, you place the inputs at the correct tap and you can create 208V with no voltage dip.

NOW... This creates a new question from me.

If i have a 5% voltage drop (456V) does the feeder breaker need to change because of the increased current that will flow through the wire?

480V*rad(3) ~= 830
456*rad(3) ~= 790

Normally my FLA of a 150KVA transformer would be 180 amps. I would then use a 225A/3P breaker.

Does this voltage drop situation force me down the path of... 150KVA/790=189A * 125% = 237A. The next breaker size being 250A/3p?

(I understand that 150 kva did not quite cause mind blowing results, but as the transformers increase in size, the disparity of the breaker sizes will be more pronounced)

I'm interested in hearing some input on the matter!

 

[david luchini]

NOW... This creates a new question from me.

If i have a 5% voltage drop (456V) does the feeder breaker need to change because of the increased current that will flow through the wire?

480V*rad(3) ~= 830
456*rad(3) ~= 790

I'm interested in hearing some input on the matter!

The voltage drop will not cause increased current to flow through the wire. Voltage drop is caused by the load current flowing through the wire. So if you have a 150kVA transformer and the load connected to the secondary side is 150kVA, then the current flowing on the primary side would be 180 Amps. (If the secondary load is less than 150kVA, then the primary current would be lower.)

Now lets assume you feed the primary from a 225A c/b, using #4/0 conductors, and the conductor length is such that the voltage drop at the transformer is 5% (or 24V, leaving 456V at the transformer primary.) The current at the transformer primary is still 180A.

 

[Smart $]

The voltage drop will not cause increased current to flow through the wire. Voltage drop is caused by the load current flowing through the wire. So if you have a 150kVA transformer and the load connected to the secondary side is 150kVA, then the current flowing on the primary side would be 180 Amps. (If the secondary load is less than 150kVA, then the primary current would be lower.)

...

Changing the primary voltage tap will change the primary current for a given secondary load...

 

[david luchini]

Changing the primary voltage tap will change the primary current for a given secondary load...

Yes...I read his question as does he need to change the breaker due to increase in current due to voltage drop, not increase in current due to changing taps.

I'd probably size the primary c/b to the 250% allowed by 450.3 in the first place, rather than 125% to accommodate inrush. That would probably mitigate the concern for increased current due to changing taps for because of voltage drop.

 

[kingpb]

I'd probably size the primary c/b to the 250% allowed by 450.3 in the first place, rather than 125% to accommodate inrush. That would probably mitigate the concern for increased current due to changing taps for because of voltage drop.

But that could also mean that the transformer is not protected, because 250% could be above the thermal damage curve of the XFMR.

Also, the fact that the current will go up or down utilizing taps assumes that the taps above and below nominal have full KVA rating. A safe assumption for coordinating protection, but from a utilization point of view, may not always be the case.

Finally, for the OP, 5% VD is not a requirement, I believe it is a recommendation.

 

[gar]

110715-1343 EDT

Several general comments:

1. The secondary voltage of a typical transformer when fully loaded will not be directly defined by the turns ratio. The transformer's internal impedance is also important. However, the unloaded secondary voltage of a tightly coupled ferromagnetic transformer may be closely related to the turns ratio.

I measured an unlabeled, I do not know where the cover is, transformer, about 0.5 KVA, and measured the input and out voltage with no load. Input 122.9, and output 129.1 . This ratio is about 1.05 . So the turns ratio is close to 1.05 to 1 rather than 1. This type of desgn is to be expected to compensate for transformer drop under full load. About 5% impedance for a typical power transformer could be expected.

2. The voltage change vs load current, source impedance is a result of all of the various impedances in series with the load going back to the original source of the electrical power. But usually the most important impedance, major contributor, is that which is close to the load.

3. What is probably more important than "voltage drop" is the absolute voltage to the load under full load conditions, and start up. Inter-related with this is the no-load voltage.

Suppose you have a no-load to full-load voltage change of 5%. Then no-load to start-up for a motor might be 25%. Change the 5% to 10% and the 25% may become 50%. Would the motor start? Probably not. This start-up problem possibly could be modified by running a much higher running voltage, but that would be undesirable.

The no-load to full-load voltage change is a result of the load current and all of the above mentioned components that contribute to the source impedance.


So the voltage drop on the last piece of wire is only part of what is importnat to you.

.