Sizing a circuit for a 600 HP 460v motor

Status
Not open for further replies.
W

WPiper

Member
The NEC tables stop at 500 HP. We encounter 600 and higher horsepower 460 v motors all the time on high pressure air compressors. There seems to be no clear rule for how to size the branch circuit and feeders for these motors. Maybe I just haven't looked hard enough but I can't find any place in the code that addresses this.

The motors DO have HP marked on the nameplate. (Sometimes, they are marked in KW because many of these compressors are European. There aren't that many companies that make them.) But the HP is beyond the top value listed in the tables.

Extrapolate?
 
WPiper said:
The NEC tables stop at 500 HP. We encounter 600 and higher horsepower 460 v motors all the time on high pressure air compressors. There seems to be no clear rule for how to size the branch circuit and feeders for these motors. Maybe I just haven't looked hard enough but I can't find any place in the code that addresses this.

The motors DO have HP marked on the nameplate. (Sometimes, they are marked in KW because many of these compressors are European. There aren't that many companies that make them.) But the HP is beyond the top value listed in the tables.

Extrapolate?
Click to expand...

Size it as per the nameplate current.
 
Any motor that does not have a corresponding HP rating listed in the tables goes by name plate rating, whether it is 4, 6, 16, 600, etc.
 
hillbilly1 said:
With such a large motor, it probably has a soft start starter ...
Click to expand...
but not universally ... my first job out of college was with a company with 12 each 9000HP at 7200V, started (NOT SIMULTANEOUSLY) across the line at one plant ...
 
GeorgeB said:
but not universally ... my first job out of college was with a company with 12 each 9000HP at 7200V, started (NOT SIMULTANEOUSLY) across the line at one plant ...
Click to expand...

Yeah, but the OP was saying 460, the POCO would be very upset with you line voltage starting a motor at that voltage.:)
 
Last edited:
Soft starts are becoming very common on them but Y-delta's are also still used. But neither of those things changes the branch circuit requirements. The Y-delta just changes the conductors between the contactors and the motor. The soft start doesn't change anything in the circuit.

Sometimes you can get the info from the equipment manufacturer but frequently it is very hard to get in a timely fashion. The circuits to supply them generally have to be designed and the materials ordered long before the equipment is built or shipped and the compressor manufacturers are reluctant to say what info WILL be on a motor nameplate until they actually see it. And of course they don't buy the motor until they have to and it only shows up a bit before they ship the compressor. They don't keep these beasts or even all the parts to make one sitting around in a warehouse. They start when you place an order.

It's really sort of a rhetorical question. Practically speaking, it is easy enough to set down safe circuit requirements for it although at 600 HP, you could easily end up running wire a size bigger than you need. If you have to guess and you look at the NEC tables for motors up to 500 HP, you have to guess about 700 amps for a 600 HP motor. The nameplate, though, could actually be only 670. That would take you down a wire size. It's only money.
 
WPiper said:
The amps for 9000 at 7200V and 600 at 460V are about the same.
Click to expand...

I would say that a plant with 7200 has it's own substation, where a plant with 480 most likely comes of a general distribution line. No comparison. The wire size for the running amps does not change between the line started or the soft start, but there is a big difference in the feeder breaker size. The only portion of starting amps that may call for a larger wire size would be voltage drop. Code does not require a larger size wire because starting amps is greater than running amps.
 
I don't see what difference the voltage of the motor will make to the POCO when it comes to across the line starting. It is still going to take the same amount of KVA from their distribution system to start the same load regardless of what the actual motor voltage is.
 
I was involved with a 750 HP 460/3 motor in western NY. It had Y-delta but the POCO still wanted to be notified ahead of time the first time we started it. I believe their issue was fuses or circuit breakers in their system that might not handle it. They did though. They had actually committed to upgrading their service to the plant or maybe a whole industrial park but had not done it yet so they were getting close.

It's probably true that a 7200 V system in a plant means a substation for the plant. I guess the POCO's wire would be at 14.4? Higher? If you have your own substation, does that somehow protect the grid from your inrush?

Every plant I work with has their own transformers to provide the 480/3, usually several.

"there is a big difference in the feeder breaker size."
Explain please. A TM breaker is sized for the FLA, not the inrush current.
 
WPiper said:
I was involved with a 750 HP 460/3 motor in western NY. It had Y-delta but the POCO still wanted to be notified ahead of time the first time we started it. I believe their issue was fuses or circuit breakers in their system that might not handle it. They did though. They had actually committed to upgrading their service to the plant or maybe a whole industrial park but had not done it yet so they were getting close.

It's probably true that a 7200 V system in a plant means a substation for the plant. I guess the POCO's wire would be at 14.4? Higher? If you have your own substation, does that somehow protect the grid from your inrush?

Every plant I work with has their own transformers to provide the 480/3, usually several.

"there is a big difference in the feeder breaker size."
Explain please. A TM breaker is sized for the FLA, not the inrush current.
Click to expand...

If you have an inrush of current at the utilization voltage shouldn't the current at all upstream sources increase by the same proportion? KVA is KVA no matter what the voltage is. Now if you have an undersized transformer, or conductors somewhere along the line it may cause some voltage problems, but motor is still going to demand power to accelerate the load.
 
kwired said:
I don't see what difference the voltage of the motor will make to the POCO when it comes to across the line starting. It is still going to take the same amount of KVA from their distribution system to start the same load regardless of what the actual motor voltage is.
Click to expand...
Except that maybe the supply capacity in kVA will be greater with a higher supply voltage.
 
Besoeker said:
Except that maybe the supply capacity in kVA will be greater with a higher supply voltage.
Click to expand...

Doesn't that really depend on the transformer supplying the lower voltage? Yes the transformer will be a current limiter to some extent, change the transformer to a different size and the limiting capabilities changes also. If you were to assume infinite supply being available it will take the same kVA to start the motor regardless of the voltage.

The reason the POCO says no across the line starting of motors over X horsepower is not because of voltage problems on the load side of the transformer supplying the customer, it is because of voltage drop or overcurrent protection issues on their distribution system.

It is about keeping their system operating withing certain performance standards while trying to start these large motors.
 
Status
Not open for further replies.
Top