ryan_618
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- Salt Lake City, Utah
How do you do a ground-fault calculation if you are using a raceway as an equipment ground? Is there somewhere that you can find the resistance properties of a particular type of conduit?
Raceway as EGC
- Thread starter ryan_618
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Re: Raceway as EGC
Ryan:
When you say ground-fault calculation, I assume you mean the calculation to determine the needed AIC rating of a circuit breaker. The way I do the calculations, the size of the equipment ground doesn't matter (at least it doesn't enter the calculation anywhere).
Why doesn't it come into play? I'm not 100% sure, but I think in the case of a line to ground fault, it is assumed that there is a perfect ground at the location of the fault. In other words, the resistance and inductance of the ground path is simply ignored. Also, if a line-line short occurs, the current doesn't travel on the equipment ground.
On the other hand, enclosing the conductors in a metal raceway does affect the ability of current to flow during a fault, and different numbers are used for conductors enclosed in metal conduit than in PVC conduit.
Steve
Ryan:
When you say ground-fault calculation, I assume you mean the calculation to determine the needed AIC rating of a circuit breaker. The way I do the calculations, the size of the equipment ground doesn't matter (at least it doesn't enter the calculation anywhere).
Why doesn't it come into play? I'm not 100% sure, but I think in the case of a line to ground fault, it is assumed that there is a perfect ground at the location of the fault. In other words, the resistance and inductance of the ground path is simply ignored. Also, if a line-line short occurs, the current doesn't travel on the equipment ground.
On the other hand, enclosing the conductors in a metal raceway does affect the ability of current to flow during a fault, and different numbers are used for conductors enclosed in metal conduit than in PVC conduit.
Steve
ron
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- New York, 40.7514,-73.9925
Re: Raceway as EGC
This is a neat article on the topic.
http://www.electrician.com/electa1/ground2.html
Steve is correct, that a normal short circuit calc, does not include ground impedance.
Once you calculate the fault current available, you can then work on the actual ground impedance, which has many many variables that are difficult to include in a calculation to get an accurate answer.
This is a neat article on the topic.
http://www.electrician.com/electa1/ground2.html
Steve is correct, that a normal short circuit calc, does not include ground impedance.
Once you calculate the fault current available, you can then work on the actual ground impedance, which has many many variables that are difficult to include in a calculation to get an accurate answer.
- Location
- Illinois
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- retired electrician
Re: Raceway as EGC
Ryan,
You can download the GEMI software from the Steel Tube Institute. The software does conduit grounding length calculations.
Don
Ryan,
You can download the GEMI software from the Steel Tube Institute. The software does conduit grounding length calculations.
Don
ryan_618
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- Salt Lake City, Utah
Re: Raceway as EGC
Hi guys. I agree that the AIC rating should be determined by a line-to-line fault (short), but I was just curious as to how to determine it (the ground fault). I'm no engineer by any stretch...in fact I'm just a code geek and not a theory guy, but it seems to me that the fault current generated would have to be dependant upon the length and resistance(impedance) of the grounding means. Am I off on this?
BTW: Thanks for the links...I'll be sure to check those out.
Edited for clarity
[ March 19, 2004, 06:41 PM: Message edited by: ryan_618 ]
Hi guys. I agree that the AIC rating should be determined by a line-to-line fault (short), but I was just curious as to how to determine it (the ground fault). I'm no engineer by any stretch...in fact I'm just a code geek and not a theory guy, but it seems to me that the fault current generated would have to be dependant upon the length and resistance(impedance) of the grounding means. Am I off on this?
BTW: Thanks for the links...I'll be sure to check those out.
Edited for clarity
[ March 19, 2004, 06:41 PM: Message edited by: ryan_618 ]
- Location
- Massachusetts
Re: Raceway as EGC
If you are a code geek what the heck does that make me.
ryan_618
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- Salt Lake City, Utah
Re: Raceway as EGC
A code geekOriginally posted by iwire:
If you are a code geek what the heck does that make me.
charlie
Senior Member
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- Indianapolis
Re: Raceway as EGC
HMMMM
HMMMM
ryan_618
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- Salt Lake City, Utah
Re: Raceway as EGC
All right code geeks ( ), lets try this question.
If I have a 500' length of EMT that is being used as the EGC, with no supplemental equipment grounding conductors, and a 120-V circuit in it, have I satisfied the requirement250.4(A)(5)???
All right code geeks (
), lets try this question.If I have a 500' length of EMT that is being used as the EGC, with no supplemental equipment grounding conductors, and a 120-V circuit in it, have I satisfied the requirement250.4(A)(5)???
- Location
- Massachusetts
Re: Raceway as EGC
Darn DIYs never give enough info.
What is the amperage of the circuit?
What is the size of the EMT?
Darn DIYs never give enough info.
What is the amperage of the circuit?
What is the size of the EMT?
ryan_618
Senior Member
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- Salt Lake City, Utah
Re: Raceway as EGC
Lets say.... 1/2" EMT with 10 AWG on a 20 Amp OCPD.
Lets say.... 1/2" EMT with 10 AWG on a 20 Amp OCPD.
- Location
- Massachusetts
Re: Raceway as EGC
Ryan I think given that description the wire size and length will be as much of a problem as the conduit.
A line to line fault on this circuit would only provide 99.1 amps of fault clearing current.
At 100 amps of fault clearing current the 1/2" EMT is good for about 425'
[ March 20, 2004, 12:54 PM: Message edited by: iwire ]
Ryan I think given that description the wire size and length will be as much of a problem as the conduit.
A line to line fault on this circuit would only provide 99.1 amps of fault clearing current.
At 100 amps of fault clearing current the 1/2" EMT is good for about 425'
[ March 20, 2004, 12:54 PM: Message edited by: iwire ]
Re: Raceway as EGC
"If I have a 500' length of EMT that is being used as the EGC, with no supplemental equipment grounding conductors, and a 120-V circuit in it, have I satisfied the requirement250.4(A)(5)?"
Probably Not. This one area that may be often overlooked when designing systems. In the 96 code there is a chart that provides information regarding this question. The chart was a product of a study done by Professor A.P. Sakis Meliopoulos at Georgia Tech. I had the good fortune to talk with him on a number of occasions about the project. As mentioned, the Soares Book on Grounding has a good discussion on this subject.
There are several assumptions that were made in the Soares study. 1. The fault current was assumed to be 5 times the OC device. 2. The fault arc was given a value of 50 volts.
In making fault caculations, a problem that occurs is that the impedance of the conduit is nonlinear and affected by the magnitude of the fault. The fault magnetic field affects the iron in the conduit causing the impedance to vary.
In the Georgia Tech study, it was found that when using todays OC devices that arc voltage of 40 volts and a fault current of 4 times the OC device
provide adequate safety margins.
[ March 20, 2004, 02:20 PM: Message edited by: bob ]
"If I have a 500' length of EMT that is being used as the EGC, with no supplemental equipment grounding conductors, and a 120-V circuit in it, have I satisfied the requirement250.4(A)(5)?"
Probably Not. This one area that may be often overlooked when designing systems. In the 96 code there is a chart that provides information regarding this question. The chart was a product of a study done by Professor A.P. Sakis Meliopoulos at Georgia Tech. I had the good fortune to talk with him on a number of occasions about the project. As mentioned, the Soares Book on Grounding has a good discussion on this subject.
There are several assumptions that were made in the Soares study. 1. The fault current was assumed to be 5 times the OC device. 2. The fault arc was given a value of 50 volts.
In making fault caculations, a problem that occurs is that the impedance of the conduit is nonlinear and affected by the magnitude of the fault. The fault magnetic field affects the iron in the conduit causing the impedance to vary.
In the Georgia Tech study, it was found that when using todays OC devices that arc voltage of 40 volts and a fault current of 4 times the OC device
provide adequate safety margins.
[ March 20, 2004, 02:20 PM: Message edited by: bob ]
ryan_618
Senior Member
- Location
- Salt Lake City, Utah
Re: Raceway as EGC
Looking at the language of the above reference, does it imply that by following the rest of article 250, compliance is automatically achieved?
If not, is the only way to ensure compliance through resistance testing?
So, the rest of article 250 would allow the installation described in my previous posts.
Looking at the language of the above reference, does it imply that by following the rest of article 250, compliance is automatically achieved?
If not, is the only way to ensure compliance through resistance testing?
pierre
Senior Member
- Location
- Westchester County, New York
Re: Raceway as EGC
250.4(A)(5) is important in this example. As phase conductors are increased in size for voltage drop and the conduit is employed as the EGC, it will become an issue. The link in an earlier post and the Soares book have charts to show the effectiveness of different conduits used as equipment grounding conductors. I would think that the code sections here would have to be interpreted as saying some method that would be considered as 'effective' would have to be employed.
Pierre
250.4(A)(5) is important in this example. As phase conductors are increased in size for voltage drop and the conduit is employed as the EGC, it will become an issue. The link in an earlier post and the Soares book have charts to show the effectiveness of different conduits used as equipment grounding conductors. I would think that the code sections here would have to be interpreted as saying some method that would be considered as 'effective' would have to be employed.
Pierre
scott thompson
Senior Member
- Location
- Anaheim, California
Re: Raceway as EGC
I guess the best way to cover this would be to figure the Conducivity (MHOs) of the Metallic Conduit, along with its Cross Sectional (CM) size - and apply this VS the MHOs of the Circuit Conductors.
Sounds kind of funny, doesn't it?
Even if we include an Insulated EGC for Bonding the Raceways and Equipment, (sized equal or larger than listed in 250-122 [99 NEC]), if a Ground Fault occurs the majority of Fault Current will still be flowing on the Metallic Raceway.
Reason: Skin Effect.
In a way, having a little lower MHO value could be a nice thing - limiting the intensity of the Ground Fault Current - as long as it does not limit it to something so low the OCPD will not open for like 5 Minutes - or never open!
To determine the SCA for Ground Faults to Metallic Conduits, this could be figured by:
* If Steel (Magnetic or Ferrous) Conduits:
</font>
* If Aluminum (Non-Magnetic or Non-Ferrous) Conduits:
</font>
Chances are on a 20 Amp circuit with 1/2" Steel Conduit (and tightly set couplings + connectors), the Conduit will have an equal - or even greater Conductive ability as opposed to a #12 cu Conductor.
I will check into this later, and maybe post a "Comparitive Sheet" at ECN to reference this stuff.
Scott
I guess the best way to cover this would be to figure the Conducivity (MHOs) of the Metallic Conduit, along with its Cross Sectional (CM) size - and apply this VS the MHOs of the Circuit Conductors.
Sounds kind of funny, doesn't it?
Even if we include an Insulated EGC for Bonding the Raceways and Equipment, (sized equal or larger than listed in 250-122 [99 NEC]), if a Ground Fault occurs the majority of Fault Current will still be flowing on the Metallic Raceway.
Reason: Skin Effect.
In a way, having a little lower MHO value could be a nice thing - limiting the intensity of the Ground Fault Current - as long as it does not limit it to something so low the OCPD will not open for like 5 Minutes - or never open!
To determine the SCA for Ground Faults to Metallic Conduits, this could be figured by:
* If Steel (Magnetic or Ferrous) Conduits:
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Conductive Equivalent VS Copper of Steel per Conductive area, with no EGC,</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Conductive Equivalent VS Copper of Steel per Conductive area, with an included Copper EGC in Parallel.</font>
* If Aluminum (Non-Magnetic or Non-Ferrous) Conduits:
</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Conductive area and percentage of Conductivity of Aluminum VS Copper - with no Copper EGC,</font>
- <font size="2" face="Verdana, Helvetica, sans-serif">Conductive area and percentage of Conductivity of Aluminum VS Copper - with an included Copper EGC in Parallel.</font>
Chances are on a 20 Amp circuit with 1/2" Steel Conduit (and tightly set couplings + connectors), the Conduit will have an equal - or even greater Conductive ability as opposed to a #12 cu Conductor.
I will check into this later, and maybe post a "Comparitive Sheet" at ECN to reference this stuff.
Scott
pierre
Senior Member
- Location
- Westchester County, New York
Re: Raceway as EGC
Scott
For a 500 foot run, one will most likely increase the size of the phase conductors. With that increase, a proportionate increase in the EGC as well. The raceway itself does not change in resistance, but the increased size of the EGC will have less resistance. Ohms Law tells us most current will travel through the least resistance, with some current flowing on the smaller resistance as well - no??
Pierre
Scott
For a 500 foot run, one will most likely increase the size of the phase conductors. With that increase, a proportionate increase in the EGC as well. The raceway itself does not change in resistance, but the increased size of the EGC will have less resistance. Ohms Law tells us most current will travel through the least resistance, with some current flowing on the smaller resistance as well - no??
Pierre
- Location
- Massachusetts
Re: Raceway as EGC
Pierre that was the same thing I was trying to point out earlier.
A 20 amp 120 volt circuit 500' long on 10 AWG is almost a worthless circuit in the first place.
This circuit has 3% voltage drop if loaded to only 3 amps.
I find the wording of 2504(A)(5) a little strange.
I assume that would mean until the OCPD opens but I believe the CMP carefully chooses the words they use or don't use.
There must be a reason it does not say:
[ March 21, 2004, 09:07 AM: Message edited by: iwire ]
Pierre that was the same thing I was trying to point out earlier.
A 20 amp 120 volt circuit 500' long on 10 AWG is almost a worthless circuit in the first place.
This circuit has 3% voltage drop if loaded to only 3 amps.
I find the wording of 2504(A)(5) a little strange.
It does not say it must be capable of operating the OCPD, only that it must safely carry all the current likely to be imposed on it.
I assume that would mean until the OCPD opens but I believe the CMP carefully chooses the words they use or don't use.
There must be a reason it does not say:
Why is that?
[ March 21, 2004, 09:07 AM: Message edited by: iwire ]
scott thompson
Senior Member
- Location
- Anaheim, California
Re: Raceway as EGC
Pierre;
If the run's length is 500', then the size of the Circuit Conductors + EGC will be most likely increased by at least two full sizes - like from #12 cu. to #8 cu. - if not even larger (depending on the System Voltage and the Load Current).
With this increase comes an increase in the size of the Raceway - and a corresponding larger cross section (more like diameter).
Now the Conduit has a much larger Conductive area, and outermost Conduction path (outer and inner walls of the Conduit) which makes for a change in the Skin Effect scenario.
Even with the included EGC, the Raceway (Conduit) will carry some - if not most - of the Fault Current during a Ground fault.
If the Raceway is IMC or GRC, the Conductive area is larger - since the Conduit's wall is much thicker than with EMT or Reduced Wall FMC / LFMC.
Using Non-Ferrous Materials - like Aluminum Conduits - increases the Conductive aspects of the Raceway.
Only time this is different is when there's a section of PVC - or other types of Non-Metallic Raceways - separating the Fault point from the "Grounded Metallic Conduit" point; such as having a ground fault in a pull box, and the last 10 foot stick of Conduit is PVC, with EMT running from the Panel upto the last 10 feet where it terminates to the PVC.
Of course an EGC is included to bond the pull box, due to the PVC section.
An included EGC is not a waste of time and money - it reduces the overall Z in the Grounding Equipment's path, and allows for an alternate path, in the event that someone breaks the Raceway.
If the Ground Fault Current would flow only flow on the EGC, I would not be throwing all this baloney out - and simple Ohm's Law (I= E?R) would likely show the largest current on the EGC.
Only with the Skin Effect scenarios, the Currents will tend to accumulate on the extents of the Conductive bodies - which in this case the Raceway has a larger Diameter than the Conductors, so Current flows tend to be higher on it during the Ground Fault.
Sorry if this message is difficult to follow (Read: Whaddaheck???), so let me know where things become "Clear As Mud"!
Scott
Just for reference, here's Pierre's message I am replying to:
[ March 21, 2004, 09:56 AM: Message edited by: scott thompson ]
Pierre;
If the run's length is 500', then the size of the Circuit Conductors + EGC will be most likely increased by at least two full sizes - like from #12 cu. to #8 cu. - if not even larger (depending on the System Voltage and the Load Current).
With this increase comes an increase in the size of the Raceway - and a corresponding larger cross section (more like diameter).
Now the Conduit has a much larger Conductive area, and outermost Conduction path (outer and inner walls of the Conduit) which makes for a change in the Skin Effect scenario.
Even with the included EGC, the Raceway (Conduit) will carry some - if not most - of the Fault Current during a Ground fault.
If the Raceway is IMC or GRC, the Conductive area is larger - since the Conduit's wall is much thicker than with EMT or Reduced Wall FMC / LFMC.
Using Non-Ferrous Materials - like Aluminum Conduits - increases the Conductive aspects of the Raceway.
Only time this is different is when there's a section of PVC - or other types of Non-Metallic Raceways - separating the Fault point from the "Grounded Metallic Conduit" point; such as having a ground fault in a pull box, and the last 10 foot stick of Conduit is PVC, with EMT running from the Panel upto the last 10 feet where it terminates to the PVC.
Of course an EGC is included to bond the pull box, due to the PVC section.
An included EGC is not a waste of time and money - it reduces the overall Z in the Grounding Equipment's path, and allows for an alternate path, in the event that someone breaks the Raceway.
If the Ground Fault Current would flow only flow on the EGC, I would not be throwing all this baloney out - and simple Ohm's Law (I= E?R) would likely show the largest current on the EGC.
Only with the Skin Effect scenarios, the Currents will tend to accumulate on the extents of the Conductive bodies - which in this case the Raceway has a larger Diameter than the Conductors, so Current flows tend to be higher on it during the Ground Fault.
Sorry if this message is difficult to follow (Read: Whaddaheck???), so let me know where things become "Clear As Mud"!
Scott
Just for reference, here's Pierre's message I am replying to:
edited to include Pierre's quoted text!
[ March 21, 2004, 09:56 AM: Message edited by: scott thompson ]
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