mbrooke
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Where did these impedance values come from? Are they resistance or Impedance (R+Jx)?
Where did these values come from?
- Thread starter mbrooke
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ptonsparky
Tom
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I typed the given information into MH spreadsheet for Fault current and came up with the 583 amps. I used the 8 as what would normally be the Neutral conductor in the SS calculation. Used 10,000 amp as Utility fault current and Cu in Nonmetallic raceway.
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mbrooke
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Do you have a link to the spread sheet? Is it the one in the free stuff?
ptonsparky
Tom
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Yes, it is.
mbrooke
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Nice, I will give it a look. Can I ask what methods the spread sheet uses to get these ohm values?
mwm1752
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I thought resistance can be formulated using Chapter 9 table 8 - working on it
mbrooke
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Nuber
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I am betting resistance. See table 9 in chapter 9 under alternating current resistance for copper wires. .25 ohms per thousand feet. 200 feet is 1/5 of the length and also 1/5 of the resistance = .05 ohms.
Side note - is Jx common for reactance in your experience? Not busting on you but I was taught 25 years ago that Z is impedance, R is resistance, X is reactance. Z=R+X
I am aware that there are a multitude of symbols that are used are not universal. Just curious - I very well might just be out of touch.
mbrooke
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I want to make a table for each wire method and gauge in simple ohm values. The issue I'm having is:
1) 3 phase systems with single phase line to ground faults.
2) Parallel ground current paths which lower resistance but raise reactance.
3) deriving an ohm value for 3 phase and single phase transformers.
mbrooke
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I might be thinking of this: https://en.wikipedia.org/wiki/Cartesian_coordinate_system
Its common for transmission lines. Not sure if calling it Z= R+jX would be appropraite here.
The thing about AC or DC resistance only is that it produces high fault current values (vs actual) especially with larger conductors.
ptonsparky
Tom
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Yes, Table 8, Chapter 9.
mwm1752
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Where is the info that this is a 3-phase circuit? -
mbrooke
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Ahhhh. {}
Light bulb given!
Ideally I'd like to develop a set tables where I can simply add two or more ohm values together for single phase line to ground faults. Both for breaker clearing time and short circuit calcs... in addition to labeling requirements.
mbrooke
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Single phase in the graphic, but my understanding is that line to ground faults in a 3 phase system vary in fault current vs phase-phase faults.
ptonsparky
Tom
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or Turn page and use Table 9
mbrooke
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or Turn page and use Table 9
I'm not sure how though... I mean how do I apply these numbers for various scenarios... for example, in a commercial building parallel current paths would change the reactance of the fault current path.
ptonsparky
Tom
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That is what the SS indicates.
mbrooke
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If I'm right, (forgive me folks) reactance changes for L-G, L-L-G, L-L, and L-L-L.
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