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#169357 10/02/07 11:26 PM
Joined: Sep 2003
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I have to feed a subpanel for an underground well. It is 250' from the panel and I have calculated with 27.5A 240V load, I'll need to run a #6 feeder. Here is the problem. The contractor installing the well and pump told me that the actual pump will be 200' underground. So, he is going to have a cord all the way down to the bottom and that is where the motor will be. Do I need to calculate this into the line loss? So a total length of 450'??

Please advise.

Thanks,
Byron

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5 HP pump?


Greg Fretwell
Joined: Sep 2003
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I was only told the amps and the voltage but that sounds right.

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you say one run at 250 feet then add 200 more feet down the well if so the total run is 450 feet

my main condseration you will run some issue with voltage drop with this length of wire here

i came up the caluation it will reqired # 3 yeah #3 copper wire for full run with the vd of 6.2 volt drop or 2.6 % vd but with your figures i will post it here

with your figures with #6 wires it will have 5.2% VD or 12.4 volt drop

all the figures are based on 240 volt supply and useing the THHN/THWN wires for other type wires it can change the figures all the figures are based on 75°C termation rating

and i think you are very right on borderline with the VD

Merci, Marc


Pas de problme,il marche n'est-ce pas?"(No problem, it works doesn't it?)

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How about a buck&boost transformer by the subpanel? instead of the large copper wire.


Be kind to your neighbor, he knows where you live

Niko #169366 10/03/07 06:58 AM
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What voltage is the pump rated for? 3% & 5% voltage drop is not an NEC requirement, merely a recommendation for a quality installation so that people get enough voltage at the furthest outlet under all conditions. As this circuit is only running a pump, you only need to worry about the pump and not "what if they plug THIS in?"- can it take 208V? If so, you only really need to calculate cable size to support 208V at 27.5A, and 10% (or even 12%) drop is OK, though you need to have some leeway should the 240V drop a few volts at the panel due to voltage drop in the service feeder, etc.

IF the pump is rated for 208V, #10 (12.4% drop to 210V) would be acceptable for this circuit, but I'd probably run #8 to be safe (7.8% drop to 221V). Otherwise, if it's rated for 220 or 230, etc, just make sure you're meeting the required voltage.

Electric bills are also a concern; the smaller the cable, the higher the resistance, the more energy will be wasted as lost heat, as much as 820W in the case of the #10 feed I outlined. If this pump runs 4 hours a day, that's about $72/year at 0.06/kwh, and more if you're in CA or somewhere with expensive electricity. So, larger cable could easily be cost-justified for that alone.

Last edited by SteveFehr; 10/03/07 07:03 AM.
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The other place to look is typical info from the pump manufacturer. As many pumps use Franklin Electric motors, taking from the "Aplication-Installation-Maintainance" manual, assuming that 5hp, 1-phase, and 240 VAC is correct, then for 450' from the service entrance to the motor they recommend #6 AWG minimum. This is for 5% Vd, for 3% a #4 is close but you would need to go to a #3 AWG.

The 27.5a would be maximum service factor load for "FE's" 5hp. The pump is rated for 3.7 KW and >0.97 power factor running.

I highly recommend getting the pumps data and verifying these numbers versus taking a wild guess on to little data.

Shane

Last edited by sabrown; 10/03/07 10:15 AM. Reason: Added voltage drop info.
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You'll need to look at this as a two headed problem, the drop from the service to the pump house and the drop from the pump house to the pump.

I'll assume the pump guy is providing the wiring and connection from the the ground down to the pump. He has a cable his plans to use. Figure the voltage drop for that part.

To keep within the recommend 5% drop, see what wire size you'll need to have for the feeder you're providing.

Running a pump on lower than designed voltage will cause it to run hotter and thus fail quicker.


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We have a similar installation at a research unit - 450 feet to the well and then 276 feet down. Original installer used #10 copper - the cost to retrench with new wire etc was prohibitive. In the end we stuck a variable speed pump down the well with some sort of a controller panel at the breaker - it was way less expensive.

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I've pressed the owner/well installer for more info particularly if the motor will run at 208V. The well installer insists that #6 will suffice however I'm questioning the effectiveness of the motor with that line loss.

As a contingency plan I do like the Buck & Boost transformer option. Assuming the voltage is around 208 by the time it gets to the panel, and the motor takes 240V which transformer is the best choice? Also, can someone explain the Power Rating and what it means.

Thanks,
Byron


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