Voltage Drop

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megawatt

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Central Kentucky
I am finishing up an irrigation project for a horse farm, and the Irrigation Contractor can't get his controls to operate the system.
It is not part of My work, I bypassed His control circuit, and water flows from the sprinkler heads. Payday for Me.
...but I would like to help Him get this fixed if possible.
Here is what He has engineered.
Rain Bird control box with a 24vac transformer.
He has installed 525ft of 14-2 UF, to close a 24 vac coil, at the well head. This should close the contacts, & start the well pump.
Trouble is, it doesn't work as planned. Now I haven't seen any of His programing charts, specs, or operation manual for this irrigation control, but I wanted to know if anyone knows if a 525' run of 14-2 @ 24vac would close this coil ? again i don't have any specs on any of the components, but was curious if anyone thought it would work. I know He used this wire, because He had it left over from another job 8 yrs ago.
He keeps hovering over this control box, scratching His head, & I'm wondering if there may be too much of a voltage drop at that distance to close a coil ?
Any takers ?
 
So the 24V xfmr and controller is 525ft away from the 24V coil? Is that correct? Also is 525ft a oneway distance or is that the total length of the loop? I'm curious what is the line voltage rated at? What is the amp load @ 24V for the well pump? Also if the contacts are not operating correctly could the wires have been switched accidently?
 
What is the voltage at the coil when energized? What about with the coil disconnected? I'll bet there is a large difference.

You can decrease the voltage drop by sending 120v on the UF and stepping down at the coil. The 5-fold increase in voltage will result in 1/5th the current, with the same wattage coil.

If the control box only puts out 24v, you can use that to control a relay that in turn switches the 120v to the coil. For that matter, if you have 240v handy, you could even use a 240v coil.
 
Yes, The easy thing to do is check the voltage at the coil since we do not know the transformer or coil current. It will probably just hum when trying to pull in the coil. He could try a bigger transformer. For example if it's 24V 1200ma go up to a 1500ma. Altenatively,the contactor's coil may be rated more current than expected. If he's desparate, tell him you'll put in a 240v coil and ss relay for a nominal fee. ;)

I don't know about sending 120V on that uf, probably not kosher.
 
I have another job to quote for Him, and I may offer to help then.
I know that the transformer is part of the irrigation control box, & wouldn't be easy to replace... but I still have My doubts about 24 vac closing a coil 525 ft away.
Off the top of My head, I'd guess even a half amp of load would be too much voltage drop at that distance. Anybody have an idea what range a 24 vac coil would have ? ie 20-28 volts ??
 
Voltage drop

Voltage drop

Megawatt

Looks like it could be close in one of the controller guides it calls for 24-27 VAC at solenoid to call the solenoid bad if not operating. In the Irrigation wire sizing it expects a transformer putting out 26.5 VAC. Most of the valves or pilot valve solenoids are pulling less than .5 amps even on inrush. Comes down to how many valves how much current what?s the voltage exact equipment specs probably a call to RainBird if voltage at valve is not between 24-27VAC. Also don?t forget summer voltage running a little lower. Hope
Some Rain Bird support info general
http://www.rainbird.com/contractor/literature/valves.htm
http://www.rainbird.com/ag/literature/valves.htm
Rain Bird wire sizing
http://www.rainbird.com/pdf/ag/ts_Valves.pdf
Voltage drop versus amps just a table
http://www.paigewire.com/volt_loss_awg.htm

Hope it helps.
 
Like I said you need to know what the load is to calculate it. If the load is 1/2amp it would be 22.4v at the pump. Because of the inrush or the coil I would guess 3/4A might be needed to pull in the contactor at 24V. That would be 21.6V or a 10% voltage drop. Wire looks a bit undersized.
 
I come up with a 7.5%(1.8 v) drop using the #12, 4.6%(1.1 v) with #10, 2.9%(0.7 v) with #8 and 1.7%(0.4 v) with #6. These are based on a 1 amp load, 525' and a 24 volt supply. If the output of the transformer is 26.5 volts, even the #12 would supply over 24 volts to the load.
Don
 
I was using 24V, thats why we differ. Anyway, It apears that the wire is undersized. I guess the question is; do you want to go thru the trouble of replacing the wire for 525Ft. Or, put just put a SS relay and 240coil in the pump box?
 
You are all leaning in the same direction as I was. Too much voltage drop @ 525 ft to close the coil.
I still haven't been asked to look at it, but I feel it's going to end up with Me working out a solution.
For starters using a contactor with a 120 volt coil should take care of the remote starting situation, but the Rainbird control will have to be used to alternate the sprinkler heads...If I'm asked to fix this I will use the link to rainbird's help site. Thanks to all
 
For starters using a contactor with a 120 volt coil should take care of the remote starting situation,
Click to expand...
Be careful here, the 120 should have no problem pulling the coil in, but you may have a problem getting it to drop out. Take a look at this Square D document. It is possible that the shunt capacitance of the control circuit could prevent the remote coil from droping out if the power source is at the motor starter.
Don
 
I'd be very careful about changing to 120V. First off the cover requirements are likely 6" more than the trench that was dug, and it would require GFCI protection.
 
I would switch to 24 volt DC controls.........
Low voltage AC controls are not good for long distance.....
Any capacitance in the cables results in current loss before it reaches the coil, this has to be factored in the control scheme and the voltage drop. It is actually possible to have a PLC input read as "true" when a switch is open, due to the capacitance of the cable allowing enough current flow to enable the input to sense "on". For all control circuits over 1000 ft, I switch to 24-32 volt DC controls, from the traditional 120 v AC. I am doing a 9,000 ft conveyor right now on 24 v DC control (I/O not full 9,000, as I have remote racks).
 
Well I got My call today, went to look at the control, did some trouble shooting & here's what I found....The Rainbird control would not close the 24vac coil, on the pump start relay, even after I moved it from the well, to directly beside the Rainbird control box, ( Eliminating the 525' run ) it would try to close it several times but could never hold it in.
I got some info on the 24 v coil.... 39 watts to close, and 3 watts to hold.
I felt like the Rainbird control was a little undersized for this application, ( info on the panel said output 1 amp max ) by My calculations it is taking 1.56 amps to close this relay.....so I took a couple of test leads, and went straight from the sec side of the transformer to the coil. Worked perfectly ! Water everywhere ! Now I am going to replace this relay with a smaller, less than 1 amp, variety and all should be well.
I too found the info about the relay not opening when called to, due to capacitance. Franklin Electric said installing an extra capacitor would take care of this, and 120 volts would then be OK to use for the control circuit !
Thanks for everyones help.
 
megawatt said:
Now I am going to replace this relay with a smaller, less than 1 amp, variety and all should be well.
Click to expand...
Make sure you don't overload the contacts. If you have one with a marginally-adequate contact rating, you can place two poles in series (yes, series, not parallel) to increase interrupting capacity.
 
megawatt said:
Franklin Electric said installing an extra capacitor would take care of this, and 120 volts would then be OK to use for the control circuit !
Click to expand...

Did Franklin Electric say it was NEC compliant to do this? What about wiring method differences discussed earlier?
 
Completed today. Replaced the original contactor with one that required less wattage to hold closed, and it worked perfectly as originally designed.
The irrigation contractor had chosen a contactor that was designed for a larger load & the Rainbird control module was listed with a 1 amp max output...and that's why it wouldn't hold the contacts closed.
It was simple to trouble shoot after looking at the specs on the different components.
 
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