Grounding in a Parallel Installation with a twists

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1972Grady

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[FONT=&quot] So this is a very specific question that requires additional thought beyond what you might first think. 250.122(F) Conductors in Parallel. We all know you size your grounding conductors based off of table 250.122. My question is this. If I have a 4000A OCP and I'm using (12) sets of 600kcmil AL then what size AL ground do I need to pull in each raceway?. If you look at 250.122(A) it states that "equipment grounding conductors of the wire type shall not be smaller than shown in Table 260.122, but in no case shall they be required to be larger than the circuit conductors supplying the equipment." The second line creates the confusion. What size are my circuit conductors. I have 12 sets of 600 does that mean my circuit conductors are 12 X 600kcmil which equals 7200kcmil meaning if I select the 750kcmil AL ground in table 250.122 then in reality my ground conductor is not larger than my circuit conductors. Or would my circuit conductors be 600kcmil meaning I don't have to use the 750kcmil AL wire listed in table 250.122 for the 4000A OCP?[/FONT]
[FONT=&quot] We know when we parallel ungrounded conductors we are adding separate conductors together to make a larger conductor that can handle the load. But with grounds each conductor should be sized according to its individual ability to open the OCP.[/FONT]
[FONT=&quot] So which size would I be required to use in this scenario? A 750kcmil or a 600kcmil?[/FONT]
 
Your ungrounded conductors are 12 X 600

Roger
 
You would use the 4000 amps and T250.122 to find the size of each parallel EGC. The EGC can be larger than the conductors in each raceway and in this case they would be required to be larger.

Welcome to the Forum. :)
 
infinity said:
You would use the 4000 amps and T250.122 to find the size of each parallel EGC. The EGC can be larger than the conductors in each raceway and in this case they would be required to be larger.

Welcome to the Forum. :)
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Then what does the code mean when it says the grounding conductor shall not be required to be larger than the ungrounded conductors?
 
Grady_G said:
Then what does the code mean when it says the grounding conductor shall not be required to be larger than the ungrounded conductors?
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It says that the EGC in each parallel raceway is not required to be larger than the ungrounded conductors which Roger pointed out is equivalent to 12*600 Kcmil or 7200 Kcmil. The 750 Kcmil you mention is still smaller than the ungrounded conductors. All of this is assuming that you have a feeder.
 
Grady_G said:
Then what does the code mean when it says the grounding conductor shall not be required to be larger than the ungrounded conductors?
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A group of wires connected in parallel is "one" conductor and the size of the conductor is the sum of all connected together, think about stranded conductors, they are groups of smaller conductors.

Roger
 
So let me ask a few follow up questions if I may.
Question #1
If I'm using (12) sets 600 KCMIL AL MC cable as a feeder off a 4000A breaker would I still need a 750 KCMIL AL ground?

Question #2
Say I have a 4000A busway rising up the building fed from a 4000A OCP. Im tapping off the busway to a 200A MCB panel less than 10' away. I'm using (1) set of 250 KCMIL AL to feed the 200A panel. Would a 250 KCMIL AL ground be large enough or would i need a 750 KCMIL ground?

Question #3
Say I have a 4000A busway rising up the building fed from a 4000A OCP. Im tapping off the busway to a 400A MCB panel less than 10' away. I'm using (2) sets of 250 KCMIL AL to feed the 400A panel. Would a 250 KCMIL AL ground in each conduit be the correct size or should I add the (2) sets of 250 KCMIL together to equal 500 KCMIL and use a 500 KCMIL AL ground in each tap raceway or would each tap raceway need a 750 KCMIL ground.

Thank you all very much for your knowledgeable and quick responses to all my questions. The way 250.122A in the 2011 code was written has led some people to believe that you only size the ECG off of table 250.122 until you reach the size of the ungrounded conductors then you no longer have to increase its size unless you're adjusting for voltage drop. This proves sound in a single run application as the fault would be limited by the size of the ungrounded conductor. But in a parallel application the fault is limited by the combined size of the ungrounded conductors in parallel with each other. Therefore a full size ground would make sense to me to pull in each raceway.
If anyone could support any answers with theory or good examples please feel free to include that in your explanation. Or if you have any good reading material by leading experts to point me in the right direction please let me know. It has been easy finding answers based off of copper where you wouldn't typically have these questions because you would be using a 500 KCMIL ground to begin with. But I live in the AL world so I have to ask these odd questions.


 
infinity said:
It says that the EGC in each parallel raceway is not required to be larger than the ungrounded conductors which Roger pointed out is equivalent to 12*600 Kcmil or 7200 Kcmil. The 750 Kcmil you mention is still smaller than the ungrounded conductors. All of this is assuming that you have a feeder.
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I asked further questions but so far no one responded. Perhaps its because I didn't reply back in the proper way and you didn't get an email. I hope responding like this will notify you in order to get my last few questions clarified.
 
Grady_G said:
So let me ask a few follow up questions if I may.
Question #1
If I'm using (12) sets 600 KCMIL AL MC cable as a feeder off a 4000A breaker would I still need a 750 KCMIL AL ground?
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Yes, each cable would require an internal 750 kcmil equipment grounding conductor. If this is being installed in cable tray and under the 2017 code, you would be permitted to connect all of the internal EGCs in parallel with a single external 750 kcmil EGC.
Question #2
Say I have a 4000A busway rising up the building fed from a 4000A OCP. Im tapping off the busway to a 200A MCB panel less than 10' away. I'm using (1) set of 250 KCMIL AL to feed the 200A panel. Would a 250 KCMIL AL ground be large enough or would i need a 750 KCMIL ground?
Click to expand...
That is an example of where "not required to be larger than the ungrounded conductors" comes into play. The tap would be permitted to use a 250 kcmil EGC.
Question #3
Say I have a 4000A busway rising up the building fed from a 4000A OCP. Im tapping off the busway to a 400A MCB panel less than 10' away. I'm using (2) sets of 250 KCMIL AL to feed the 400A panel. Would a 250 KCMIL AL ground in each conduit be the correct size or should I add the (2) sets of 250 KCMIL together to equal 500 KCMIL and use a 500 KCMIL AL ground in each tap raceway or would each tap raceway need a 750 KCMIL ground.
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The conductor size is 500 kcmil and the required EGC is 750 kcmil. Each raceway will require a 500 kcmil EGC for that installation.
 
Grady_G said:
So let me ask a few follow up questions if I may.
Question #1
If I'm using (12) sets 600 KCMIL AL MC cable as a feeder off a 4000A breaker would I still need a 750 KCMIL AL ground?

Question #2
Say I have a 4000A busway rising up the building fed from a 4000A OCP. Im tapping off the busway to a 200A MCB panel less than 10' away. I'm using (1) set of 250 KCMIL AL to feed the 200A panel. Would a 250 KCMIL AL ground be large enough or would i need a 750 KCMIL ground?

Question #3
Say I have a 4000A busway rising up the building fed from a 4000A OCP. Im tapping off the busway to a 400A MCB panel less than 10' away. I'm using (2) sets of 250 KCMIL AL to feed the 400A panel. Would a 250 KCMIL AL ground in each conduit be the correct size or should I add the (2) sets of 250 KCMIL together to equal 500 KCMIL and use a 500 KCMIL AL ground in each tap raceway or would each tap raceway need a 750 KCMIL ground.




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Don gave you the correct responses, I would add that for question #2 your tap conductors would need to be a minimum of 400 amps (10%) meaning either 600 kcmil copper or 900 kcmil Al.
 
infinity said:
Don gave you the correct responses, I would add that for question #2 your tap conductors would need to be a minimum of 400 amps (10%) meaning either 600 kcmil copper or 900 kcmil Al.
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What code are you quoting for the 10%. I was unaware there were any restrictions for a tap that is less than 10 feet in length.
 
Grady_G said:
So let me ask a few follow up questions if I may.
Question #1
If I'm using (12) sets 600 KCMIL AL MC cable as a feeder off a 4000A breaker would I still need a 750 KCMIL AL ground?



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So as you have certainly noticed, paralleling cables can be problematic. Usually ypu can parallel several before the standard egc size bites you. After that you would need to specially order the cable with your needed egc size.
 
Grady_G said:
What code are you quoting for the 10%. I was unaware there were any restrictions for a tap that is less than 10 feet in length.
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Rob already answered, but off top of my head tap conductors must be at least 10% for 10 foot rule and 1/3rd for the 25 foot rule of feeder current rating.
 
kwired said:
Rob already answered, but off top of my head tap conductors must be at least 10% for 10 foot rule and 1/3rd for the 25 foot rule of feeder current rating.
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Yup. :thumbsup:

Now can someone figure out why there are two Grady's posting in this thread?
 
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