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.
