Selection of Nominal Voltage to Be Used

As far as I know switchgear nominal voltage should be selected same to transformer rated voltage. While load voltage of switchgear shall be selected lower than switchgear nominal voltage to compensate voltage drop.

ex. transformer 13.8kV/0.48kv the switchgear must be selected Un=480volts while motor voltage connected to switchgear must be Urated=400V to compensate voltage drop.

How much would the whole set of circuitry cost, compare to the switch?

I bet this Experienced User could explain

The secondary voltage you are referring to is the open circuit voltage of the trafo which will fall when load is applied on the secondary due to voltage regulation. This voltage (after loading) shall be the same as the rated bus voltage of the downstream switchgear.

What if 90% is unloaded? Voltage will increase consequently switchgear will become overvoltage.

Yes. But that should not be a problem as the basic insulation level for the 480 V switchgear is 600V.

I understand now why they call it 480V nominal voltage cause actual voltage may increase or decrease to nominal voltage that's why insulation level should always much higher than nominal voltage. thanks for sharing.

Do you get many motor burn outs?

What are the next up Un ratings?

Those ratings are pretty close to the service voltages.

Maybe the next ones up are (or were at time of build) just too much......?

Have they given you any grief?

If switchgear is 480v and motor is 460volts the motor will not burn because we have 5% voltage drop on motor feeder ex. 480-(480x.05)=456v besides motor has +-10% voltage range. And in addition, 480volts bus switchgear in reality will have lower value than nominal because of voltage drop on upper stream unless if we adjust tap changer of transformer.

In IEC standard they use to make motor voltage equal to switchgear nominal voltage but transformer secondary voltage is higher than switchgear voltage.

ex.transformer UrTHV/UrTLV = 21kV/10.5kV selected nominal bus voltage of switchgear is Un=10kV

Trasformer and switchgear are almost on the same room, if actual voltage of secondary xformer is really 10.5kV then switchgear is undersized.

Any explanation why IEC do it like this?