Page 1 of 2 12 LastLast
Results 1 to 10 of 15

Thread: Utility Voltage - What is acceptable?

  1. #1
    Join Date
    May 2005
    Posts
    20

    Utility Voltage - What is acceptable?

    I am inspecting a 480VAC pump station that appears to be at the end of the utility feed. The system voltage measured (unloaded) is 500VAC. When I run a single 250HP pump, the voltage sags to 480VAC. When multiple pumps are running it sags to as low as 455VAC. This is causing a problem with the solid state drives. The pumps are having a problem reaching full speed with this voltage sag. I need to find some standards to compare these findings with.

    Does the NEC address power quality?

    The 120VAC power derived from this source varies from 123VAC to 116VAC. Are there any standards for how much this voltage may vary?

  2. #2
    Join Date
    May 2004
    Location
    Indianapolis, IN
    Posts
    701
    Do these pumps sag the voltage on start, or also during run ? Around here, we have to inform the POCO if we connect anything larger than a 100 HP motor and it can not be started across the line. You said you are using drives, so I assume you are ramping up the start ?

    Most POCO's that I have dealt with say 5 to 10 % is an acceptable voltage tolerance.
    Brad Darnell
    Master Electrician -Electrical Contractor
    IN-NJ-WI-CT-LA-OH-KY-TX-FL-NV-GA-TN-MS-OK-IL-NC-AL-KS

    "The Code doesn't say what you think it says" - Charles E. Beck, P.E.

  3. #3
    Join Date
    May 2005
    Posts
    20
    Yes, we are attempting to ramp up the start using A/B soft starter drives. The problem when using utility power, the 2nd pump never reaches full speed and indicated by the drive automatically switching over to it's internal bypass contacts. Under generator power, it switches over in a matter of seconds, and there is negligible (3 VAC) voltage sag.

  4. #4
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    18,977
    The Illinois Commerce sets the voltage requirements and specifies +/- 10% of nominal. For a 480 volt system that would mean any voltage between 432 and 528 is acceptable. The system should work at 455. You need to check your equipment.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  5. #5
    Join Date
    Feb 2003
    Location
    Alabama
    Posts
    2,570
    Are you measuring the voltage at the motor or at the utility service?

  6. #6
    Join Date
    May 2005
    Posts
    20
    I'm feeling very confident in the equipment, as it runs great on genertor power.

    The voltage was measured at the switchboard and the MCC section.

  7. #7
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    18,977
    There must be something else going on...the equipment should run fine at 455 volts.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  8. #8
    Join Date
    May 2005
    Posts
    20
    All indications point to a utility problem. I'm pretty sure with better equipment, the voltage sagging will turn out to be much more than 10% during motor starting.

  9. #9
    How close is your equipment to the utility transformer? Normally the generator is in very close proximity to the load and thus very little voltage drop is experienced. The length of the distribution of the 480 V as well as every connection will influence the efficiency of your operation. IMHO, these factors should be checked out .

    Fred

  10. #10
    Join Date
    Nov 2004
    Location
    Las Cruces N.M.
    Posts
    1,090
    I believe that ANSI C84 specifies a service voltage variation of +/- 5%, and utilization voltages of +/-10%. If a utility service voltage varies by 10% from nominal for a long period of time, I believe it could cause problems. I am surprised to find that public utilities vary more than 5%.

    Jim T

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •