Horsepower versus Amperage Rating

[Leo1]

When are we required to use the horsepower rating of a safety switch versus its amperage rating? For example if a 600hp motor with an integral solid state starter has a FLA current of the motor is 652 AMP, then at 125% that is 815 AMP for service conductors and main disconnect switch. For 480VAC, 3 phase safety switches I would select a 1,200 AMP safety switch and fuse it at 900 AMP. This all seems correct but I have a question, "The rating of the most safety switch at 1200 amps is 500 hp"? Is this a concern? I think not since the 600hp motor has a starter. Correct?

 

[jim dungar]

Only when the device is being used as the motor controller (e.g. it performs start-stop functions) does it need to be HP rated.

 

[Leo1]

Only when the device is being used as the motor controller (e.g. it performs start-stop functions) does it need to be HP rated.

Thank you very much.

 

[GoldDigger]

Only when the device is being used as the motor controller (e.g. it performs start-stop functions) does it need to be HP rated.

And the logic behind that is that for just carrying current the nature of the load does not really matter (starting amps of the motor not normally being a problem for contact heating.)
But to start a motor the contacts must close onto the surge/starting current without welding or pitting and to stop a motor the contacts must quench the arc fed by an inductive load, again without damaging the contacts.
That is why the HP load break rating of the switch is so much lower than the carrying amps.

Tapatalk!

 

[Besoeker]

And the logic behind that is that for just carrying current the nature of the load does not really matter (starting amps of the motor not normally being a problem for contact heating.)
But to start a motor the contacts must close onto the surge/starting current without welding or pitting and to stop a motor the contacts must quench the arc fed by an inductive load, again without damaging the contacts.
That is why the HP load break rating of the switch is so much lower than the carrying amps.

Tapatalk!

I accept that NEC code may require you to do that.
But, in my opinion (and I'll probably get lambasted for this) it isn't entirely logical.
Yes, for motor duty the contactor rating is lower than the thermal current rating for the reasons you give.
But not all 600HP motors take the same current. You might expect a significant difference in the full load current between a 2-pole motor and an 8-pole motor.

Here that is dealt with by assigning different current ratings for the contactor. AC1 is is the thermal rating good for resistive loads or where the logic (eg from a VSD) dictates that the contactor will not switch load current. AC3 is the rating for motors based on the rated motor current rather that its rated power.

 

[GoldDigger]

I'm sorry, that just makes too much sense.
For smaller motors the NEC takes the position that the motor may someday be replaced with a more or less efficient motor with the same HP rating, and that the motor overload will be changed as necessary but nobody will even glance at the rest of the circuit.

For large motors it is much more likely that any change will be engineered, but the Code does not make that distinction.

Tapatalk!

 

[kwired]

I'm sorry, that just makes too much sense.
For smaller motors the NEC takes the position that the motor may someday be replaced with a more or less efficient motor with the same HP rating, and that the motor overload will be changed as necessary but nobody will even glance at the rest of the circuit.

For large motors it is much more likely that any change will be engineered, but the Code does not make that distinction.

Tapatalk!

Then you sometimes have owners that want to replace with a larger motor and can't understand why you need to re do the circuit

Once had a dairy farmer that had a waste water pump go bad. 50 hp. He thought since it went bad maybe a 60 would be better - of course conductors were too small for a 60, but he already had the motor replaced and wanted me to connect it.

After running new conductors and connecting it I figured out why the 50 hp failed. This pump takes a little time to fill the discharge line as it is pumping to a center pivot irrigation 1/4 mile away. During this time there is little restriction on the line and the pump is overloaded until the line is filled, then the current drops to a level that would be acceptable for the 50 HP motor. When it tripped long ago when first installed they didn't like that so like any farmer they cranked the overloads as high as they will go - no more tripping until there is a problem involving needing motor rewind/replacement

 

[GoldDigger]

Sounds like the original problem was that the motor was mismatched to the pump body. Proper design would have allowed the pump to run indefinitely with zero back pressure.

Tapatalk!

 

[kwired]

Sounds like the original problem was that the motor was mismatched to the pump body. Proper design would have allowed the pump to run indefinitely with zero back pressure.

Tapatalk!

That would be equipment salesperson problem more so then my problem. I would be willing to sell him a VFD though to run at a lower speed until the pipe is fully primed:happyyes:

 

[Jraef]

Sounds like the original problem was that the motor was mismatched to the pump body. Proper design would have allowed the pump to run indefinitely with zero back pressure.

Tapatalk!

Sizing every pump for open channel flow conditions would be a very expensive proposition. In this case he may have needed a >1000 pump, even though when at full pressure and flow the system only needed <600HP. Then you have to size everything for that larger motor, so everything gets exponentially more expensive.

More likely there was originally a hydraulically controlled valve that opened slowly to prevent overloading the pump, but someone got tired of waiting for it, or the valve broke and they didn't replace it, they just took it out. I used to see that a LOT with circle-jerk irrigators. The people hired to maintain irrigation equipment are often completely unqualified to understand all the ramifications of their mods and fixes. Remember, these are the same guys who will be flattening the ends of some EMT to fit in the fuse holders...

 

[kwired]

Sizing every pump for open channel flow conditions would be a very expensive proposition. In this case he may have needed a >1000 pump, even though when at full pressure and flow the system only needed <600HP. Then you have to size everything for that larger motor, so everything gets exponentially more expensive.

More likely there was originally a hydraulically controlled valve that opened slowly to prevent overloading the pump, but someone got tired of waiting for it, or the valve broke and they didn't replace it, they just took it out. I used to see that a LOT with circle-jerk irrigators. The people hired to maintain irrigation equipment are often completely unqualified to understand all the ramifications of their mods and fixes. Remember, these are the same guys who will be flattening the ends of some EMT to fit in the fuse holders...

All that system had was manually actuated valves to select which irrigation machine to send the water to. I talked them into putting a panel mount ammeter on the pump panel so they could throttle the valve and watch current until the system is filled. When up to pressure the pump was fine, it was just pumping too much volume before then and took too long to fill before motor overload would trip, if we did not slow down the flow somehow until it was filled.

Have to remember the simpler the better for these kind of people, because as you mentioned they don't understand the ramifications of their fixes. Even with what I did they will eventually forget how to properly use it. First time motor overload trips they may reset it. Second time they will adjust it higher. Third time they are replacing motor again.:roll:

 

[Jraef]

All that system had was manually actuated valves to select which irrigation machine to send the water to. I talked them into putting a panel mount ammeter on the pump panel so they could throttle the valve and watch current until the system is filled. When up to pressure the pump was fine, it was just pumping too much volume before then and took too long to fill before motor overload would trip, if we did not slow down the flow somehow until it was filled.

Have to remember the simpler the better for these kind of people, because as you mentioned they don't understand the ramifications of their fixes. Even with what I did they will eventually forget how to properly use it. First time motor overload trips they may reset it. Second time they will adjust it higher. Third time they are replacing motor again.:roll:

Meh... job security... :angel: