Bob Deroeck
02-04-2011, 6:19 PM
Hi,
My workshop is located away from my house and the power supply line runs underground in conduit. The total distance from the main panel in the house to the subpanel in the workshop is about 170 ft.
I'm curious as to how many horsepower worth of machines I could operate through this power line and the possible negative effects on motor life due to high voltage drop during motor startup.
I've done voltage drop calculations and there is no problem running considerable amps at 240 v through this line during continuous motor operation. My question pertains to starting up a large motor. My understanding is that instantaneous amp draw when starting a motor ranges from 3-8 times the continuous load amps. Can any of you guys confirm if this is true? If so, what would be a "typical" number, or is the variation so great that there really is no "typical" number? It's quite likely that the largest motors I would run would be 5 HP, and likely to be Baldor or Lesson single-phase motors, but that a number of these motors could be operating simultaneously.
I've used the advanced voltage drop calculator at electrician.com, which includes the effect of conductor temperature on resistivity. Cases where starting up the motor at 3 or 5 or 8 times the continuous motor amps result in high amps and considerable voltage drop. While I recognize that this high amp draw is for a relatively short amount of time, I wonder what the effect would be on the lifetime of the motor. What would be the maximum voltage drop you guys would recommend for motor startup currents before you would be concerned about possibly damaging the motor?
Thanks for your input.
Bob
My workshop is located away from my house and the power supply line runs underground in conduit. The total distance from the main panel in the house to the subpanel in the workshop is about 170 ft.
I'm curious as to how many horsepower worth of machines I could operate through this power line and the possible negative effects on motor life due to high voltage drop during motor startup.
I've done voltage drop calculations and there is no problem running considerable amps at 240 v through this line during continuous motor operation. My question pertains to starting up a large motor. My understanding is that instantaneous amp draw when starting a motor ranges from 3-8 times the continuous load amps. Can any of you guys confirm if this is true? If so, what would be a "typical" number, or is the variation so great that there really is no "typical" number? It's quite likely that the largest motors I would run would be 5 HP, and likely to be Baldor or Lesson single-phase motors, but that a number of these motors could be operating simultaneously.
I've used the advanced voltage drop calculator at electrician.com, which includes the effect of conductor temperature on resistivity. Cases where starting up the motor at 3 or 5 or 8 times the continuous motor amps result in high amps and considerable voltage drop. While I recognize that this high amp draw is for a relatively short amount of time, I wonder what the effect would be on the lifetime of the motor. What would be the maximum voltage drop you guys would recommend for motor startup currents before you would be concerned about possibly damaging the motor?
Thanks for your input.
Bob