User Tag List

Page 3 of 4 FirstFirst 1 2 3 4 LastLast
Results 21 to 30 of 37

Thread: Short Circuit Calculations Help

  1. #21
    Join Date
    Jan 2019
    Location
    NJ
    Posts
    11
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Awesome. Thanks for the help

    Now how do I add the 600 AMP MDP Panel to Busmann to the system since I will need to add another Panel which is about 150 ft away (225 AMP)

    Do I select Bus Run and select add as feeder with the distance of 100 ft

    FYI...I just need the hand calculation and I got 18074.

  2. #22
    Join Date
    Feb 2003
    Location
    Connecticut
    Posts
    3,240
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by BatmanisWatching1987 View Post
    Awesome. Thanks for the help

    Now how do I add the 600 AMP MDP Panel to Busmann to the system since I will need to add another Panel which is about 150 ft away (225 AMP)

    Do I select Bus Run and select add as feeder with the distance of 100 ft

    FYI...I just need the hand calculation and I got 18074.
    Just add another Conductor Run for the next panel.

  3. #23
    Join Date
    Jan 2019
    Location
    NJ
    Posts
    11
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by BatmanisWatching1987 View Post
    Awesome. Thanks for the help

    Now how do I add the 600 AMP MDP Panel to Busmann to the system since I will need to add another Panel which is about 150 ft away (225 AMP)

    Do I select Bus Run and select add as feeder with the distance of 100 ft

    FYI...I just need the hand calculation and I got 18074.
    I re did my hand calculation and I got the same answer as the Busmann Program now of 17191 Amps

  4. #24
    Join Date
    Feb 2003
    Location
    Connecticut
    Posts
    3,240
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by BatmanisWatching1987 View Post
    I re did my hand calculation and I got the same answer as the Busmann Program now of 17191 Amps
    18068 is correct for PVC conduit. (The 17191 is for metallic conduit.)

  5. #25
    Join Date
    Jul 2005
    Location
    Tennessee
    Posts
    16,184
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Its an entirely different conversation and you may already be familiar with the method but, for downstream panels, there is a process called "series rating". If you are not familiar, it will be worth your time to explore.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  6. #26
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
    Posts
    5,038
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by augie47 View Post
    IMO, once the utility gives you the available current at their transformer terminals, the transformer, impedance, etc. become irrelevant. All you need is their number.
    Although, it is my opinion that nothing requires us to use that number, I am free to calculate from the transformer info if I wish. I know some disagree with me on that.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  7. #27
    Join Date
    Jul 2005
    Location
    Tennessee
    Posts
    16,184
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by electrofelon View Post
    Although, it is my opinion that nothing requires us to use that number, I am free to calculate from the transformer info if I wish. I know some disagree with me on that.
    I don't disagree but, locally, our POCO advises they arrive at their number by taking into account the transformers they keep on hand in case of a failure so they will be in the clear if they have to replace the one actually in place so we use their number in inspecting.
    At my age, I'm accustomed to restaurants asking me to pay in advance, but now my bank has started sending me their calendar one month at a time.

  8. #28
    Join Date
    Jun 2004
    Location
    Cherry Valley NY, Seattle, WA
    Posts
    5,038
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by augie47 View Post
    I don't disagree but, locally, our POCO advises they arrive at their number by taking into account the transformers they keep on hand in case of a failure so they will be in the clear if they have to replace the one actually in place so we use their number in inspecting.
    I usually go with their number too, because it is rare I get to see the transformer or get accurate specs. I have used my own calc a few times when there number was unreasonably high.
    Ethan Brush - East West Electric. NY, WA. MA

    "You can't generalize"

  9. #29
    Join Date
    Dec 2008
    Posts
    549
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Calculating Zsys for 208 V and 27 kA Zsys=208/sqrt(3)/27000=0.004448 ohm
    According to NEC Table 9 3*350 MCM copper in p.v.c. conduit:
    r=0.038 and x=0.040 ohm/1000ft then for 100 ft two parallel cables:
    Rcb=0.038*100/1000/2=0.0019 ohm; Xcb=0.040*100/1000/2=0.002 ohm
    If Xsys=Zsys [Rsys=0] the short circuit current will be maximum:
    I=208/SQRT(3)/SQRT(0.0019^2+0.006448^2)=17865 A
    If Rsys>0 then for Rsys=Xsys I=16666 A for instance.

  10. #30
    Join Date
    Jul 2006
    Location
    SE USA as far as you can go
    Posts
    2,731
    Mentioned
    0 Post(s)
    Tagged
    0 Thread(s)
    Quote Originally Posted by electrofelon View Post
    I usually go with their number too, because it is rare I get to see the transformer or get accurate specs. I have used my own calc a few times when there number was unreasonably high.
    That is a risky proposition. Utilities use system planning numbers that have future load growth and many other parameters that would not be known to someone not having access to their information. So, what may seem "unreasonably high" to you today, it could very well be down the road a few years that changes to the distribution system means those numbers are now accurate.

    For my own knowledge, what would be the advantage of not using the utility numbers; simply to save a few bucks on equipment ratings? That makes no sense to me. The risk is too high that someone could be injured.
    "Just because you're paranoid, doesn't mean they're not out to get you"

Posting Permissions

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