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'??
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.
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.
Last edited by sabrown; 10/03/0709:15 AM. Reason: Added voltage drop info.
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.
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.