Multiconductor Cable Ampacity

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charlie b said:
Here is another approach: Table 430.22(E). The application is a sump pump, and the on/off cycles are driven by water level. I watched the existing system operate a couple weeks ago. It seemed to me that it took less than 2 minutes for the pump to bring the water level from the "on" setpoint level to the "off" setpoint level. Under worst case conditions, I am made to understand that the pump would cycle on every 11 minutes or so. We are looking at ways to extend that, so that the pump doesn't experience that frequent a start/stop cycle.

With that said, I think I can use the "intermittent duty" 15 minute rated value of 85%. So my minimum ampacity would be 85% of 124, or 106 amps. A #2 would do that job. I would still have to deal with VD issues, but this could resolve the "conduit fill" problem.

Any comments?
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using a larger or higher horsepower sump pump is only going to make the line I bolded worse not better. unless they are flooding the sump with the existing pump not keeping up, I am completely failing to understand why they want to increase the size so much
 
charlie b said:
But you don't use the 125% if you are applying table 430.22(E). It says that in the opening paragraph of 430.22.
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note to table 430.22(E)

Note: Any motor application shall be considered as continuous duty
unless the nature of the apparatus it drives is such that the motor will
not operate continuously with load under any condition of use.
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Is there any condition of use where the motor might run for 3 hours? Short of some control interlock preventing it, I don't see how you can claim it is not continuous. Will is a pretty strong word.

would 310.15(C) help any?

ETA: In the end I think you are going to be constrained by VD if you have to deal with that issue. maybe run a smaller set of conductors inside the conduit and splice on larger ones once you escape the conduit.
 
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petersonra said:
Is there any condition of use where the motor might run for 3 hours? Short of some control interlock preventing it, I don't see how you can claim it is not continuous. Will is a pretty strong word.
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I had noticed that as well. I will need to discuss this with the mechanical engineer. I think we can claim that the motor will never run more than a couple minutes at a time. That is based on the maximum rate at which the sump has ever taken in water, the rate at which the new 100 HP pump will remove water, and the fact that there are three other pumps that would automatically start if one pump alone cannot keep up with the inflow. But that is a calculation I would still have to formalize.

 
One more tidbit to consider: The power supply is at 480V and the motor is rated at 460V. So we have 20 volts, or about 4%, to throw away, before we even start talking about the impact of a 3 or 4% VD.

Has anyone ever used this to your advantage?
 
charlie b said:
Here is another approach: Table 430.22(E). The application is a sump pump, and the on/off cycles are driven by water level. I watched the existing system operate a couple weeks ago. It seemed to me that it took less than 2 minutes for the pump to bring the water level from the "on" setpoint level to the "off" setpoint level. Under worst case conditions, I am made to understand that the pump would cycle on every 11 minutes or so. We are looking at ways to extend that, so that the pump doesn't experience that frequent a start/stop cycle.

With that said, I think I can use the "intermittent duty" 15 minute rated value of 85%. So my minimum ampacity would be 85% of 124, or 106 amps. A #2 would do that job. I would still have to deal with VD issues, but this could resolve the "conduit fill" problem.

Any comments?
Click to expand...
Look carefully at what that table says.

If motor isn't identified as an intermittent duty motor, you actually need to use 140% or even 200% of the motor full load current for conductor ampacity if you are not calling it continuous duty, where a 5 or 15 minute duty rated motor can have 85%.

This actually is something that maybe is overlooked a lot - I'm sure I have done it on motors that frequently start-stop, even reverse. Most time you get away with little or no trouble - but effects of that frequent starting is more noticeable on contactors, soft starters or even to VFD's.
 
charlie b said:
One more tidbit to consider: The power supply is at 480V and the motor is rated at 460V. So we have 20 volts, or about 4%, to throw away, before we even start talking about the impact of a 3 or 4% VD.

Has anyone ever used this to your advantage?
Click to expand...

Even a 10% VD is not a big issue with a pump application like this. The motor will work a little harder and maybe draw a little more current but it is not like it is going to matter all that much.
 
In addition to other problems mentioned, the idea of having to re-pull a larger conductor in an existing conduit for that distance is a formidable task.
Have you looked at the possibility of using the existing #4 and boosting your power on the supply end with a 480-600v transformer and using a 600v drive at the load end of reducing the power back to 480 ? (Yiou would have roughly 150 amps of 480 available based on the #4 at 125 amps.)
With this method you might also use your transformer taps to address your voltage drop concern.
 
Interesting concept, Gus. I'll look into it. This specific application cannot work with a VFD. But raising the voltage at one end and reducing it at the other end is worth a review.
 
charlie b said:
Interesting concept, Gus. I'll look into it. This specific application cannot work with a VFD. But raising the voltage at one end and reducing it at the other end is worth a review.
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You would end up with roughly 150 amps of 480 available based on 125 amp rating of #4 (not considering transformer losses)
 
charlie b said:
Interesting concept, Gus. I'll look into it. This specific application cannot work with a VFD. But raising the voltage at one end and reducing it at the other end is worth a review.
Click to expand...

Can you step up your voltage to 600V, and use a 600V (575V) motor at the pump, to save the cost of a second transformer?


SceneryDriver
 
Nothing has been purchased yet, and all ideas are on the table. Reliability is a very important aspect. Adding two transformers brings two possible failure points into the equation. Adding one transformer only brings one failure point. So it is worth considering.
 
100 hp is in the range of needing around 112.5 kVA transformer. Actually a pair of them or one transformer and one 575 volt motor - which unless you are in Canada is likely higher cost and maybe even longer lead time to get it. If you smoke that motor for whatever reason and don't have a spare - possibly longer down time to get a 575 volt replacement then a 460 volt replacement. Special frame size or other spec feature - maybe don't matter, maybe even stuck with an OEM being the most easy to obtain and don't get to choose other voltage or other characteristics. NEMA standard motor - just about any 460 volt motor with same speed and enclosure type suitable for the application would work and is likey not too difficult to come up with on short notice.

I'd probably still look harder at how to install larger raceway/conductors even if it has to take a different route then what is existing. Go even higher then 600 volts and necessary conductor gets even smaller, but insulation gets thicker.
 
kwired said:
100 hp is in the range of needing around 112.5 kVA transformer. Actually a pair of them or one transformer and one 575 volt motor - which unless you are in Canada is likely higher cost and maybe even longer lead time to get it. If you smoke that motor for whatever reason and don't have a spare - possibly longer down time to get a 575 volt replacement then a 460 volt replacement. Special frame size or other spec feature - maybe don't matter, maybe even stuck with an OEM being the most easy to obtain and don't get to choose other voltage or other characteristics. NEMA standard motor - just about any 460 volt motor with same speed and enclosure type suitable for the application would work and is likey not too difficult to come up with on short notice.

I'd probably still look harder at how to install larger raceway/conductors even if it has to take a different route then what is existing. Go even higher then 600 volts and necessary conductor gets even smaller, but insulation gets thicker.
Click to expand...

I think it is a pump down inside a well so would not be a standard motor anyway.
 
Something else you may consider is using a smaller sump pump that runs continuously or nearly so. No problem then with raceway fill, voltage drop, etc.

I would double check to make sure there is not room for 90*C terminations on each end before writing it off.

as for the pump not running continuously, I would think a mechanical interlock or timer to prevent that is the only acceptable solution. Just because a worst case scenario would not require the pump to run for 3 hours, you have to consider failure of float switches, maintenance on other pumps that takes them offline, somebody leaving the H - 0 - A switch in hand or manual operation, etc.

If variable speed is not possible, throttling discharge valves would run the pumps longer, however it would reduce the number of start cycles per hour
 
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