Circuit Size for 380/400/415/460/480V

[fifty60]

I am trying to size the circuit for equipment that consists of compressors, heaters, and a transformer. I am trying to prove that what I see at 480V will be my worst case current draw due to the heater load. Or at least show that the amps are close enough to not be a concern.

I know the compressors will increase their amp draw when the voltage drops by anything other than the V/F ratio. So I I go from 460 to 380, I should not see a change. But if I go from 460 to 415V, there would definitely be an increase.

The heaters will increase/decrease by the square of their voltages. So there is definitely a change between 460V and 415V. The current will decrease.

I have a 3KVA transformer that has primary taps for 380/400/460/480V and a secondary tap of 110V. With the primary taps, I will be getting the full 3KVA (nominal) regardless whether I use 380/400/460/480V, assuming I use the correct taps.

I know 3000/380V =7.9A
and 3000/460V= 6.52A

So initially, on startup, the inrush and magnetization current will be more at 380V. After the initial inrush, the difference in current draw by the primary would only be the difference in current drawn from the loads?

Actually, it would still be the reverse of the primary to secondary ratio right? So if I am stepping down from 380 to 110V from primary to secondary, when I go to secondary to primary looking at current I would then step the current down by 3.5 times. So if my current increases by 10A on the secondary, it would only increase by 2.8A on the primary side.

So looking at 480V stepping down to 110V, I have a ratio of 4.36. So an increase of 10A from my secondary would result in an increase of 2.2A on my secondary.

If anyone could help me try to analyze this, I would appreciate. I want to convince myself that if I size the circuit for 460V + or - 5%, that I can use the same sized circuit for 400V + or - 5%.
So it is still more net amps at 380V.

 

[eesonny]

Starting 8 480 volt 1/2 hp motors, nameplate indicate 6 amps inrush.

Starting 8 480 volt 1/2 hp motors, nameplate indicate 6 amps inrush.

I am having difficulty sizing the fused feeder for starting 8- 480 volt 1/2 hp motors (FLA .9 amps) with nameplate indicating 6 amps inrush. I have sized a 15 amp fused feeder. Is that correct?

 

[GoldDigger]

I am trying to size the circuit for equipment that consists of compressors, heaters, and a transformer. I am trying to prove that what I see at 480V will be my worst case current draw due to the heater load. Or at least show that the amps are close enough to not be a concern.

I know the compressors will increase their amp draw when the voltage drops by anything other than the V/F ratio. So I I go from 460 to 380, I should not see a change. But if I go from 460 to 415V, there would definitely be an increase..

I am not sure just what you meant to say here, but I sincerely hope that it was not what you said.

You have not mentioned 50/60Hz or presence of a VFD, so I really do not understand your reference to maintaining the V/F ratio.
And the current consumption of the motor will be dependent on the load on the motor, not just the V/F ratio.
When you take a 60Hz motor and run it off 50Hz at a lower voltage, you are NOT keeping the current the same if you are expecting to get the same power output from the motor. You are blending too many separate concepts in a haphazard way.

The first question you need to ask is whether the motor driving the compressor will be asked to deliver the same output horsepower at each of the voltage options. If it will be, then the lowest voltage will be the one which will correspond to the highest current at the same loading.
So now you will have to determine whether the increase in motor current is greater or lesser than the decrease in the draw of the resistive loads. This would need to be determined on a case by case basis for different machine models and designs.

 

[fifty60]

The compressors have an RLA. The RLA is for 380/460. There is a + or - 10% voltage tolerance on the RLA rating. As long as my operational voltage is within the + or - 10% of the rated compressor voltage, then I can still be justified in using the RLA of the compressor. What else would I use? So even at 415V, the compressor will draw more current, but it will still be operating within the + or - 10% of the 380V, so I would still use the same RLA to size my branch and main equipment breaker.

I think that is kind of obvious, but it really did not make sense until now.

The heater will draw less amps, and produce less watts as the voltage drops. The current will draw more amps on the low voltage primary taps. Depending on the size of either, the amps for the equipment could go up, down, or cancel.

If my amp rating for all of my voltages are within 20% of the mean amp rating, then I do not have to list each voltage and corresponding FLA for the equipment. The marked value shall not be less than 90% of the maximum value. That is what the UL standard i am working with dictates. Using the 460V FLA achieves this.

 

[fifty60]

• GoldDigger, the rated HP of the compressors will decrease with the decrease in Voltage and frequency.