Voltage Drop and Transformer

[djr3203]

I am a assisting a senior engineer on a project where we are renovating the emergency system of a commercial building. Due to the distances involved, the plan is to install a 1250KW/480V genset, step up to 12KV and then step back down to 480V in the electrical transformer room at the northwest corner of the building. I have been requested to calculate the voltage drop requirements, and while discussing the topic with another engineer, I came up with the following questions:

1. When calculating the voltage drop, another engineer stated that I only had to account for the voltage drop from the step up transformer to the step down transformer. However, If the primary voltage on the transformer drops, won't the secondary voltage also drop proportionally? I know transformer's have different taps for different voltage ranges, but won't a voltage drop on the primary still result in a voltage drop on the secondary?

2. To keep to the NEC recommendation of 5% from feeders down to the branch circuit level, I am under the assumption I need to do the following: Voltage drop from 480-12KV XFMR to 12KV-480V XFMR, from 12KV-480XFMR to 120/208V XFMR, and then from 120/208V XFMR all the way to the farthest branch circuit device.

Please let me know if I am missing anything in my considerations.

As always, I appreciate all the help!

DJR

 

[DRWeig]

My opinion: the NEC voltage-drop rule applies only to the conductors, not the transformers. Voltage changes from changing taps (or utility supply fluctuations) are voltage changes, not voltage drops.

Wait for others' opinions, though. Or ask the fellow who is going to review your drawings for the authority having jurisdiction.

Good on ya,

Goober Dave

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[iop995]

By changing taps on XFMR can compensate voltage drop.
Regarding solution, to step up to 12kV, use 2 XFMR and 12kV cables, it's not cheaper to split emergengy genset unit in 2 or more? In such way increase redundancy at genset level units and use more sub-segment in distribution energy. How much is longest circuit?

 

[djr3203]

From the Generator to the Electrical room is approximately 500 feet. The 12kv will be stepped down to 480V and feed a switchboard, which will then feed a 480V distribution panel which will then feed a 480V-120/208V transformer (this all happens in the same electrical room). The next farthest run is from the 480V-120/208V transformer to the 120/208V panelboard which is approximately 250 feet. From the panelboard I would say the farthest circuit would be in some rooms approximately 125 feet away from the panelboard.

The total emergency service design load is estimated at approximately 720KVA and we are running 3#2 15KV 133% insulated feeders.

Are the voltage taps on a transformer adjustable or in predetermined/unchangeable percentages of the primary voltage? I read a lot of transformers come with a 5% tap. Is it common to design the system so you have around a 5% drop and then use the 5% voltage tap to compensate for this voltage drop? If so, then this should kind of 'wipe the slate clean' and you can then design the system to have a 5% drop from the transformer to the farthest branch circuit?

Are there any drawbacks for relying on the voltage taps to compensate for the voltage drop? It seems to me that if you aren't drawing full load, you won't have as high of a voltage drop and then you could have a higher voltage on the secondary than expected. I assume though that this wouldn't be that much of a big deal because you would then have the voltage drop through the rest of the system which would bring you back down to your nominal voltage.

DRWeig, it is still super early in the design process and we haven't really started any communications with an AHJ yet.

Iop995, can you expand a bit more I don't exactly grasp your idea.

Thanks for the ideas and comments everyone.

David

 

[DRWeig]

No, it's not common to size feeder voltage drop around taps. Taps are for compensating for primary voltage being a bit too high or a bit too low. I usually specify taps at ±2.5% and ±5%.

Remember, the standard utilization voltages are a little less than 5% below supply voltages. Motors for a building whose supply is 480V say 460 on the nameplate. If service is 208V, you'll see 200 on the nameplate. Same with 120/115, 240/230, etc... We usually don't compensate for 5% voltage drop on the feeders and branch circuits. For a very long run, a buck/boost transformer might make better economic sense than bigger conductors.

Good on ya,

Goober Dave

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[pwrtran]

Or use fixed shunt capacitor bank, the cost is even lower than using a voltage regulator

 

[djr3203]

So best design practice would be to keep the voltage drop from the generator all the way to the branch circuit motor or receptacle to within 5% and not account for taps on the transformer to correct for voltage drop in the feeders?

BTW DRWeig, super good input. Thanks.

 

[DRWeig]

Thanks but one more point: NEC limits the voltage drop in an advisory note (FPN), which some do not consider mandatory. However,local codes and AJHs are free to make it mandatory and they often do. Also, ASHRAE 90.1 (energy code) has mandatory requirements of 2% voltage drop on feeders and 3% on branch circuits at design load.

5% drop from the generator to point of use is good, but that is only so if your generator voltage is as-specified (in this case 480). Generator voltage should be easy to tweak, though. If needed, you can do transformer tap changes.

Good on ya,

Goober Dave

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[waross]

You can use a CT(s) on the generator with a suitable load resister to subtract a bias voltage from your sense voltage and raise the voltage at the generator when under load. I wouldn't recommend this level of complexity for most installations, but it may solve a problem if voltage drops become a greater than normal problem (due to distances or other complications).

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