cable selection 3

[friend81]

Dear Experts

i have a 5HP, 220V, 60Hz 4ïnch submersible motor whose max.current(SFA) is 19.5A. It´s coupled to a borehole submersible pump and installed at 350m with a supply cable (copper)of 10mm^2. But the actual current consumed by the motor is 23A.

When contacted the supplier they informed that this motor needs a 16mm cable. But earlier also we were using a 5HP pumpset (different manufacturer)with the same cable.The pumpset mentioned above is a replacement.

I need help in cable sizing.Can anyone of you explain is there any formula or how a cable sizing is actually made?

 

[gepman]

For cable sizing there are usually two issues. One is ampacity of the cable. Too small (cross sectional area) of cable will cause overheating of the conductor and is a fire and other hazard. The insulation can actually melt. Minimum conductor sizes are usually given in electrical codes (NEC in the United States).

The other issue is voltage drop. Too much resistance causes the voltage to drop and you have less power available to your motor (or other device).

For your case I would guess that you need to calculate voltage drop. Typical maximum voltage drop is about 3% of the supply voltage but this may change somewhat based upon the actual supply voltage.

There is a voltage drop calculator at

http://www.elec-toolbox.com/calculators/voltdrop.htm

or google "voltage drop calculators".

 

[Valvecrazy]

I usually just use the Franklin Electric wire sizing chart for submersible motors. They list wire size for 200, 230, 380, 460, and 575 volts. You can get more accurate using voltage loss calculations but, this chart works well and saves a lot of time.

The Franklin book also talks about using smaller wire sizes to eliminate the need for a soft starter with submersible motors.

Quote from Franklin AIM manual;
"Reduced-voltage starters may not be required if the maximum recommended cable length is used. With maximum recommended cable length there is a 5% voltage drop in the cable at running amps, resulting in about 20% reduction in starting current, and about 36% reduction in starting torque compared to having rated voltage at the motor. This may be enough reduction in starting current so that reduced-voltage starters are not required."

Reducing the wire size will not only save money on installation cost but, can eliminate the need for any kind of soft starter. This makes a "natural" soft start from Across The Line starters. Has anyone had experience with this before?

See attached page of Franklin Manual;

 

[gepman]

Valvecrazy

Yes it is true. Assuming that the voltage drop in the cable is proportional to the current (not exactly for AC circuits but it is good for an assumption) and assuming that the motor had an inrush of 10X full load current, then the voltage drop in the cable would be 50% (10 x 5%) during startup. This would reduce the voltage at the motor terminals by 50%. This reduced voltage at the motor terminals would of course reduce the inrush. You would need to iteratively calculate to find the actual inrush.

The NEC has a FPN (Fine Print Note) recommending that the voltage drop for feeder and branch circuits total no more than 5%.

I am somewhat surprised that you would go with the 5% voltage drop since it represents 5% wasted energy. The payback of larger conductors should be calculated like every other energy savings device. Since centrifugal pumps are a variable torque load the voltage drop should not get you into trouble but with a constant torque load excessive voltage drop can limit starting torque and prevent the motor from starting.

friend81 should investigate why the motor is pulling more than the service factor amps (SFA). This can damage the motor.

 

[Valvecrazy]

I only work with centrifugal pumps and variable torque so, motor starting should not be an issue. Smaller wire to the motor would increase amperage but, the losses from a 5% voltage drop would still be minimal when compared to the losses from a Drive when it is used as a soft starter. As with friction loss in a pipe, I have always increased wire size when possible. Now I will have to take a closer look at wire size with submersibles. It seems a wire selection chart could be devised to give all submersibles a "natural" soft start.

Friend81 you may still have a 5 HP motor but, the new pump end (different manufacturer and probably different size) is pulling more amps than the old one. Figure your voltage loss with the wire size you have now. If the wire is large enough and the voltage is correct, the new pump end may just be overloading the motor. If you go with larger wire and the amperage does not reduce, the pump end is still the problem and you lost your "natural" soft start.

 

[gepman]

Most of the soft-starts that I use on pumps are to control hydraulic transients both on starting and stopping, which undersizing the motor leads will not necessarily control or at least not allow you to adjust the starting and stopping ramps. If you only need a soft-start then buy a soft-start, not a VFD. A soft-start with bypass contactor will not have any appreciable losses and can be used as a full-voltage starter if the soft-start fails.

Many times on pump services the transformer sizing by the utility will limit the inrush current on the motor due to voltage drop across the transformer at high currents.

I ran friend81's data through an online voltage drop calculator at

http://www.monachos.gr/eng/calculators/voltage_drop.htm

(which I have not used before but it was the only one that I could find with mm2 wire sizing) and the voltage drop came out to be 9% which is way to high. This brought his voltage down to 199V at the motor. It took 25 mm2 conductors to bring his voltage drop down to less than 5%. I used 3 phase, 220V, 19.5A, copper, and 350m as the inputs.

His overcurrent problems are probably due to undervoltage at the motor.

 

[Valvecrazy]

If this is true, then his old pump must have been worn out from day one, or it was only a 3 HP pump on a 5 HP motor to start with. Otherwise, why would it have worked with the old motor and not the new one?

I like the soft starter with a bypass contact idea. You really need to have a way to keep water flowing when the electronics crap out. I have never had much luck using soft starters for hydraulic transients. Since head is lost by the square of the speed, the pump is usually not moving any fluid until the RPM gets up to about 80% speed. Submersibles must get to 50% speed in one second for bearing lubrication, then 80% speed before it starts moving water. This gives very little room or time (from 80% to 100%) speed for a soft start or VFD to try to control hydraulic transients both starting and stopping. I only use a soft start when I have to for inrush currents, and I use a control valve, which works much better, to more easily control hydraulic transients.

 

[friend81]

We have already tested the pumpset in test pit & the max.current consumed by it is 19.5A. But when installed in well only, its consuming more Amps. In the begining itself i have doubted about the cable sizing only. Both the pump & motor we are talking about are new units & from the same manufacturer.

But what i have wondered is that if a product of same rating manufactured by a different manufacturer can work within its limits using the same cable then why not the other?

 

[friend81]

On the whole thanks for both of your suggestions. I have tried almost all the online calculators as per gepman instructions & also tried Franklin manual but the results are only available in AWG. Using that i can only arrive a nearest approximate sizing & not the actual.

I have tried the new metric calculator

http://www.monachos.gr/eng/calculators/voltage_drop.htm

but i m not able to view it completly on my screen.

Even if there is any formula available with you guys it will be appreciable.

The fact is that if i can arrive the actual requirement in Sq.mm then i can use an additioanl cable along with the existing cable in parallel to compensate the voltage drop.

 

[waross]

Some pumps will overload the motor if there is not sufficient head against the discharge. Your motor may be taking more current than the old motor. An undersized cable will undersized cable will have more voltage drop and that may cause a further increase in current.
Have you considered that using a 10mm cable where another manufacturer recommends a 16mm cable for the same HP may be part (or all) of the reason you are installing a replacement pump?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[gepman]

Valvecrazy remember it is the pump not the motor that determines the performance characteristics and power requirements. You would need to look at the intersection of the system curve and pump curve for both the old pump and the new pump to compare. His old five horsepower pump may not have pumped as much water or it may have been more efficient (at least at the operating point).

Your statement regarding 50% speed at least answers why the Gould pump sizer wouldn't let me put in less than 50% speed. I don't have much experience in submersible pumps but I have a fair amount in other types of pumps especially ANSI chemical process pumps.

The 13% voltage drop will take about 13% more amperage (not including other effects like changes in power factor) in order to obtain the required power. This very closely corresponds to the 23A that friend81 has measured.

friend81 in the US you could not parallel a 10mm2 wire (about a #7) per NEC. I don't know about the codes where you are at. You can also calculate the voltage drop using US units and then convert to mm2. There is an AWG to mm2 conversion table at

http://www.engineeringtoolbox.com/awg-wire-gauge-d_731.html

I would like to give you a formula for voltage drop but all of my references are in US units plus you still need a table of impedances for the different wire sizes. Try this voltage drop calculator at

http://www.southwire.com/voltagedropcalculator.jsp

and convert to mm2.

 

[Valvecrazy]

Most submersibles of this size are non-overloading. Even with no head on them they will usually not draw too many amps, they will upthrust with no head but, not pull to many amps. You could have left the wire attached when you put it in the test pit. Then you could cut the wire off and try again. This would tell you if the wire is the problem or not.

You could also check the voltage while the pump is off and while it is on. A big difference in these two numbers may mean that your supply or transformers are the problem.