I have a well head that is 560 feet from my panels. The well is at a depth of 130 feet for a total of 690 feet. The pump motor is 3/4 horsepower 240 volt. What wire is allowable by code to allow for voltage drop? Thank you. Every time I try a voltage drop calc my number always seems way overkill.
Help with voltage drop to well head
- Thread starter crtemp
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What are you coming up with that seems like overkill? I don't see any reason to go bigger than #8AWG and #10 would work. I have seen wells at similar distances running on 12AWG.
Table 430.248 puts it at 6.9A 1?.
cadpoint
Senior Member
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- Durham, NC
You need to have the proper power at the point of usage, what is wrong with your calculations?
The wire size difference is compensation to assure that the total correct power can flow that distance and operate the equipment correctly.
I want to say that the requirements of the type of motor that's being used and the cycling of the motor might be well considered, one still has to have to meet all the requirements, it's just tough getting over what's involved!
The wire size difference is compensation to assure that the total correct power can flow that distance and operate the equipment correctly.
I want to say that the requirements of the type of motor that's being used and the cycling of the motor might be well considered, one still has to have to meet all the requirements, it's just tough getting over what's involved!
gar
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- Ann Arbor, Michigan
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- EE
130530-2202 EDT
crtemp:
Here is my wild guess of a solution.
Your total loop length is 2*690 = about 1400 ft.
A ballpark figure for full load current is 746 /240 = 3.1 A. 746 = 1 HP and using that as a ballpark for a 3/4 HP to account for inefficiency and PF may be low, but this I will compensate for by over estimating peak inrush at 10 times running load.
So that is 30 A at startup.
An A. O. Smith B642 3/4 HP 230 V motor is rated about 4.6 A full load. About 30% higher than my wild guess.
If we assume you really have 240 normally and don't want more than 20 V drop at startup, then maximum wire resistance would be 20/30 = 0.67 ohms. #6 is about 0.4 ohms per 1000 ft or about 0.56 ohms for your loop length. I think this is larger than what you really need for several reasons.
You need to know what your nominal voltage really is.
You need to know what a realistic maximum series resistance is for motor startup with your normally expected minimum nominal voltage. This will determine your wire size. In a long run like this running current is not a factor. So motor performance at startup is the criteria. Your motor running load will not overheat the wire under run conditions, because voltage drop under startup is the dominate criteria.
Interesting side subject --- http://www.datasheetcatalog.org/datasheet/vishay/305cseri.pdf
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crtemp:
Here is my wild guess of a solution.
Your total loop length is 2*690 = about 1400 ft.
A ballpark figure for full load current is 746 /240 = 3.1 A. 746 = 1 HP and using that as a ballpark for a 3/4 HP to account for inefficiency and PF may be low, but this I will compensate for by over estimating peak inrush at 10 times running load.
So that is 30 A at startup.
An A. O. Smith B642 3/4 HP 230 V motor is rated about 4.6 A full load. About 30% higher than my wild guess.
If we assume you really have 240 normally and don't want more than 20 V drop at startup, then maximum wire resistance would be 20/30 = 0.67 ohms. #6 is about 0.4 ohms per 1000 ft or about 0.56 ohms for your loop length. I think this is larger than what you really need for several reasons.
You need to know what your nominal voltage really is.
You need to know what a realistic maximum series resistance is for motor startup with your normally expected minimum nominal voltage. This will determine your wire size. In a long run like this running current is not a factor. So motor performance at startup is the criteria. Your motor running load will not overheat the wire under run conditions, because voltage drop under startup is the dominate criteria.
Interesting side subject --- http://www.datasheetcatalog.org/datasheet/vishay/305cseri.pdf
.
eHunter
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- North Texas & Georgia
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Last edited:
broadgage
Senior Member
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- London, England
3% is a widely accepted figure for voltage drop but is not actually a code requirement.
If a motor is run on less than the optimum voltage then it will draw more amps, run hotter and probably have a reduced life.
A lot depends IMHO on the expected use of the well pump. If it is to run only briefly, on demand for household water then as much as 10% voltage drop might be acceptable, the reduced life might still be decades of normal use, and the wasted energy very little $
If however the pump is to run continually, perhaps for filling a reservoir or for crop irrigation, then I would be far more cautios as regards voltage drop and limit this to 3% at the very most.
Remember that a loss of 10% could be as much as 100 watts being paid for but achieving nothing useful. For a long hour load, this loss of 100 watts could be perhaps 800KWH a year, or from $80 a year. The reduction in energy waste would help pay for the larger cable.
The longer motor life might mean 5 yearly replacement instead of yearly.
If a motor is run on less than the optimum voltage then it will draw more amps, run hotter and probably have a reduced life.
A lot depends IMHO on the expected use of the well pump. If it is to run only briefly, on demand for household water then as much as 10% voltage drop might be acceptable, the reduced life might still be decades of normal use, and the wasted energy very little $
If however the pump is to run continually, perhaps for filling a reservoir or for crop irrigation, then I would be far more cautios as regards voltage drop and limit this to 3% at the very most.
Remember that a loss of 10% could be as much as 100 watts being paid for but achieving nothing useful. For a long hour load, this loss of 100 watts could be perhaps 800KWH a year, or from $80 a year. The reduction in energy waste would help pay for the larger cable.
The longer motor life might mean 5 yearly replacement instead of yearly.
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