cable selection
cable selection
(OP)
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?
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?





RE: cable selection
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 ht
RE: cable selection
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;
RE: cable selection
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.
RE: cable selection
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.
RE: cable selection
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:
His overcurrent problems are probably due to undervoltage at the motor.
RE: cable selection
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.
RE: cable selection
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?
RE: cable selection
I have tried the new metric calculator http:
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.
RE: cable selection
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
RE: cable selection
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://ww
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.
RE: cable selection
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.