Table 250.122 NEC 2017 derivation

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hhsting

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I have a question in regards to if anyone know how are equipment grounding cable sizes derived from breaker sizes in Table 250.122? From which books or other standards?
 
I don't know the history but it has been the same for decades as far as I remember. If you are wondering why they are so small, it is because the EGC only carries current for a very brief period and as such heat and overloading are greatly reduced.
 
texie said:
I don't know the history but it has been the same for decades as far as I remember. If you are wondering why they are so small, it is because the EGC only carries current for a very brief period and as such heat and overloading are greatly reduced.
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I have coworker who states that equipment grounding conductor need not be larger than main bonding jumper Table 250.102(c)(1).

He states that in circuit main bonding jumper carries same current as equipment grounding conductor under fault.

I fail to see his logic since there are two different tables and so wondered how table 250.122 is derived?
 
They are based on a couple of things....large enough so that the voltage drop under fault conditions does not provide an unsafe touch condition, and large enough that the temperature of the conductor will not rise to the point it will cause damage to the insulation of other conductors.
I expect that they are based on actual testing done decades ago.

While there are other considerations involved, your coworker is 100% correct that the main bonding jumper carries the same current as the EGC under fault conditions. That is its purpose...to connect the EGCs to the grounded conductor so the fault current can get back to the utility transformer.
Until you get into the larger sizes with parallel conductors, in many cases the main bonding jumper is larger than the EGC.
 
hhsting said:
I have coworker who states that equipment grounding conductor need not be larger than main bonding jumper Table 250.102(c)(1).

He states that in circuit main bonding jumper carries same current as equipment grounding conductor under fault.

I fail to see his logic since there are two different tables and so wondered how table 250.122 is derived?
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I may be overlooking something, but is there a condition in the Tables where the EGC is larger than the MBJ ??
 
Dons post agreed with what I have learned. table is based on insulated cable mfgs info and IEEE green book.
 
This issue came up in the following scenario i have service 4#750kcmil copper into service rated fire pump ats/controller/disconnect all in one.

The conductors label x in attached sketch on the load side of service rated fire pump ats/controller/disconnect all in one feeding the fire pump ground cable size the co worker is saying cannot be larger than Table 250.102(C)(1) main bonding jumper.

Please note the conductors X is on the load side of service rated fire pump ats/controller/disconnect all in one is equipment grounding conductor.

Should it not be based off of Table 250.122? Also the breaker trip can be different then breaker frame size since its inside the fire pump controller/ats/disco/.

3d06bacc5bdb2a3599f05832b0b1c08b.jpg
 
hhsting said:
Don do you have example of where EGC would be larger than MBJ?
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When a feeder ungrounded conductors have been increased in size for voltage drop the proportional increase in the EGC could possibly make it larger than the MBJ.
 
hhsting said:
This issue came up in the following scenario i have service 4#750kcmil copper into service rated fire pump ats/controller/disconnect all in one.

The conductors label x in attached sketch on the load side of service rated fire pump ats/controller/disconnect all in one feeding the fire pump ground cable size the co worker is saying cannot be larger than Table 250.102(C)(1) main bonding jumper.

Please note the conductors X is on the load side of service rated fire pump ats/controller/disconnect all in one is equipment grounding conductor.

Should it not be based off of Table 250.122? Also the breaker trip can be different then breaker frame size since its inside the fire pump controller/ats/disco/.

3d06bacc5bdb2a3599f05832b0b1c08b.jpg
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What makes you think that the conductor sized per 250.122 would be larger than one sized from 250.102?
Note that 250.122(D)(2) would likely apply to the selection of the EGC between the controller and the motor.

Also you can always install a conductor larger than what the code requires.
 
hhsting said:
I gotta get IEEE green book. Where is insulated cable mfgs?
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Ices.org
But on second thought I would start with Soares Book on grounding. Its a great resource and considered a classic. Start with Mikes Bonding and Grounding then Soares, then the green book. You’ll be out about $300, and then won’t need to use the forum
 
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don_resqcapt19 said:
What makes you think that the conductor sized per 250.122 would be larger than one sized from 250.102?
Note that 250.122(D)(2) would likely apply to the selection of the EGC between the controller and the motor.

Also you can always install a conductor larger than what the code requires.
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Assume this scenario fire pump ats/controller/disco service rated has internal breaker provided by manufacturer of 250A frame with trip set at 800A trip. It feeds 100HP 480V three phase fire pump with phase conductors of 2/0 awg cu.

According to Table 250.122 EGC would be 1/0awg cu based on 800A trip however what u say is true regarding MBJ is larger then EGC then this not the case.

According to Table 250.102(C)(1) based on 2/0 awg cu then MBJ is #1 awg cu.

In this case MBJ is smaller then EGC. How?
 
tom baker said:
Ices.org
But on second thought I would start with Soares Book on grounding. Its a great resource and considered a classic. Start with Mikes Bonding and Grounding then Soares, then the green book. You’ll be out about $300, and then won’t need to use the forum
Click to expand...

Ices.org sorry i looked but didnt find?
 
hhsting said:
Assume this scenario fire pump ats/controller/disco service rated has internal breaker provided by manufacturer of 250A frame with trip set at 800A trip. It feeds 100HP 480V three phase fire pump with phase conductors of 2/0 awg cu.

According to Table 250.122 EGC would be 1/0awg cu based on 800A trip however what u say is true regarding MBJ is larger then EGC then this not the case.

According to Table 250.102(C)(1) based on 2/0 awg cu then MBJ is #1 awg cu.

In this case MBJ is smaller then EGC. How?
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I cited a code section that I say applies to this installation...read that code section and select a new EGC size based on that section.
 
Using the word always or never in regards to the code doesn't work often. If you're going to use the requirement that conductors be upsized due to voltage drop, then the EGC could be larger than the MBJ. Take a 100A service with a #8 MBJ. From that service, run a long 30A branch circuit with the conductors upsized to #6. The EGC is required to be #6 in that case and that is larger than the MBJ.
 
suemarkp said:
Using the word always or never in regards to the code doesn't work often. If you're going to use the requirement that conductors be upsized due to voltage drop, then the EGC could be larger than the MBJ. Take a 100A service with a #8 MBJ. From that service, run a long 30A branch circuit with the conductors upsized to #6. The EGC is required to be #6 in that case and that is larger than the MBJ.
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See post #15 scenario where No voltage drop is involved EGC is larger then MBJ
 
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