short circut calculation

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eric dutenhoeffer

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Location
Gettysburg,sd usa
Hello, I am new to this forum so I hope I am following proper protocol.
I have a new facility that I need to do a short circut calculation for at the main switchboard location.
The following are the specs:

This is a 3-wire, 480 volt, 3 phase installation.
utility transformer size is 750kva with an impedance of 5.4%
utility conductor size from the xfmr to our ct cabinet is 4 paralleled sets of 350kcmil aluminum wire in pvc conduit. Not adequate for this instalation but is utilitys call.
from the ct cabinet to our main switchboard is a buss duct system rated at 2000amp. it is of a double alluminum construction. it is 10' in length with two 90 degree elbows.
our switchboard is rated at 2000 amp and does not have a main breaker. Its buss is rated at 65k. I need a short circuit value for this location. Your help is greatly appericiated. If possiable could you provide something I could print out for posting at the switchboard purposes.
Thank You,
Eric Dutenhoeffer
 
charlie b

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
It sounds to me that the utility owns the conductors at least as far as the CT cabinet. They should tell you the fault current that their equipment would impose at that location. There will be very little reduction in fault current along the busway, so you can post their value on your switchboard.

By the way, how can your switchboard not have a main breaker? What device serves as the required main disconnect for the building? :?
 
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
LMAO said:
16.71 kA @ 480V
Click to expand...

Just for fun, in accordance with mathematical rules of significant figures; the answer cannot be more precise than the least precision measured numbers used in the calculations. Therefore, 5.4% is two significant figures which means 16.71 has to be rounded up to; wait for it........................................................................17kA. :thumbsup:
 
Smart $

Smart $

Esteemed Member
Location
Ohio
kingpb said:
Just for fun, in accordance with mathematical rules of significant figures; the answer cannot be more precise than the least precision measured numbers used in the calculations. Therefore, 5.4% is two significant figures which means 16.71 has to be rounded up to; wait for it........................................................................17kA. :thumbsup:
Click to expand...
I agree with your assessment.

Just wondering though about sig fig rounding for electrical calculations. Should we do conventional rounding (up or down) to last sig fig... or should we always round up to achieve the "safer" result?

As an example, let's say the calcualtion result before sig fig rounding was 16.49?
 
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Smart $ said:
I agree with your assessment.

Just wondering though about sig fig rounding for electrical calculations. Should we do conventional rounding (up or down) to last sig fig... or should we always round up to achieve the "safer" result?

As an example, let's say the calcualtion result before sig fig rounding was 16.49?
Click to expand...

From a pure math perspective I would say that the 9 becomes a 10, and thus the 4 becomes a 5. The 5 is not followed by a zero because the trailing zero has no significance, thus in math rules a number ending in 5 is rounded up. Therefore, on technicality the 16.49 becomes 17 anyway.

Practically speaking, anything over 16, I would probably go with 17, simply because of the conservative nature of sizing duty cycle ratings for equipment.

Now say you calculate 16.44; based on the rules of math, which were developed by much smarter individuals than myself, your number would be 16. Practicality says I still go with 17, but the reality is that 16.44 is not any better than 16 from an accuracy standpoint. The difficulty for us is in separating our brains from what we think is correct versus what is mathematically correct.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
I don't think your method of rounding the 9 up is a valid implementation of rounding to the least significant figure. I was taught one rounds based solely on the digit to its right (4). 0 through 4 rounds down; 5 through 9 rounds up.

But ultimately it appears you are agreeing that, for practicality of electrical calculations, the last significant figure should be rounded up* if non-significant figures are anything greater than trailing zeroes.

* down if the practicality provides for a safer condition.
 
kingpb

kingpb

Senior Member
Location
SE USA as far as you can go
Occupation
Engineer, Registered
Smart $ said:
I don't think your method of rounding the 9 up is a valid implementation of rounding to the least significant figure. I was taught one rounds based solely on the digit to its right (4). 0 through 4 rounds down; 5 through 9 rounds up.

But ultimately it appears you are agreeing that, for practicality of electrical calculations, the last significant figure should be rounded up* if non-significant figures are anything greater than trailing zeroes.

* down if the practicality provides for a safer condition.
Click to expand...


I may have to agree on both points.
 
E

eric dutenhoeffer

Member
Location
Gettysburg,sd usa
charlie b said:
It sounds to me that the utility owns the conductors at least as far as the CT cabinet. They should tell you the fault current that their equipment would impose at that location. There will be very little reduction in fault current along the busway, so you can post their value on your switchboard.

By the way, how can your switchboard not have a main breaker? What device serves as the required main disconnect for the building? :?
Click to expand...

I can check with the utility to see what value they come up with.

our switchboard has 5 breakers so the 6 disconnect rule applies, no main required.
 
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