Bus Rating 6

[mikeengurs]

Hi there. I have a question regarding bus rating. Does higher actual bus voltage mean current rating should be lowered? For example, if a 4160v bus is rated at 2000A, would that rating decrease if the XFMR feeding the bus is at a higher tap? In other words, should I worry about Amps or MVAs?

 

[dpc]

Continuous bus ratings are a current rating and are based on heating and bus temperature.

The bus also has a maximum voltage rating.

Changing transformer taps doesn't change the switchgear bus rating. But it does change the transformer rated amps.

David Castor

http://www.cvoes.com

 

[mikeengurs]

Thanks dpc. This means if I stay under bus max voltage rating, I can get away with more amps if the voltage is slightly higher (which is the case).

 

[jraef]

No.

Read what he said, don't read more into it than exactly what he said.

Bus has a current rating. So a 2000A bus is rated 2000A at 1V, or 480V, 0r 4160V

Voltage on the bus has only to do with the spacing and / or insulation rating, hence a maximum voltage rating as he said. That just means that if you have a bus system designed to be used in 4160V gear it will likely be 5kV rated bus, you cannot use it for 15kV gear. But you can use it at 3300V or 2400V or 1500V, anything lower.

That still has nothing to do with the current rating. It's still 2000A no matter what.


"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[mikeengurs]

Jraef, thanks for the reply. when I said slightly higher voltage, I didn't mean a higher voltage class. I meant something like 4300v for a 4160v bus. This will allow the load to draw little less current. We are at a situation where we're very close to the 2000A limit and any little saving counts.

 

[davidbeach]

You can't get away with more amps. The rating is the rating. At higher voltages, but below the voltage rating of the bus, you can get more VA or more Watts but never more amps.

 

[mikeengurs]

I guess my confusion is, would induction motors draw less amps at higher voltage? I appreciate everybody's response...

 

[davidbeach]

At slightly higher voltages, an induction motor will draw slightly lower current. Larger increases in voltage may result in increased current.

 

[jraef]

This is pretty much the accepted standard for motor performance with regards to voltage changes.

Notice that at +2-3% the current drops ever so slightly, but at +10% it goes well above what it would have been at rated voltage. So for your 4160V motor, the motor nameplate voltage is likely 4000V (to allow for voltage drop), so at 4300V you would be in that range where it will be drawing more current.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[mikeengurs]

Thanks everybody! I appreciate everyone's input...this was very helpful.