NEC Table 250.122 vs Table 250.66

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We are manufacturing a 4000A/480V distribution switchgear and installing it in a Power Distribution Center (PDC - electrical house). The switchgear has an incoming 4000A breaker from a plant transformer (not utility) and several 1600A feeder breakers going to 480V MCC lineups also in the building. The switchgear has a 1/4"x2" copper ground bus bar. We are connecting the Switchgear ground bus bar to the building ground bus ring (1/4"x2" copper bus run around the inside perimeter of the building). The building ground bus ring is then connected to the system grounding electrode (ground grid).

My questions concern which table to use for
1.) The conductor from the Switchgear ground bus bar to the building ground bus bar ring.
2.) The conductor from the building ground bus bar ring to the system grounding electrode(s).
3.) The conductor from the MCC ground bus bar to the building ground bus bar ring.
 

Dennis Alwon

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My questions concern which table to use for
1.) The conductor from the Switchgear ground bus bar to the building ground bus bar ring.
250.66
2.) The conductor from the building ground bus bar ring to the system grounding electrode(s).
250.66
3.) The conductor from the MCC ground bus bar to the building ground bus bar ring.
250.66

Table 250.122 is the equipment grounding conductor which is run with the circuit conductors to the equipment or outlets that they serve.
 
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don_resqcapt19

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I am not clear on the installation. Where will the system bonding jumper be located?
There needs to be an unspliced GEC from the grounding electrode to the location of the system bonding jumper.
Supply side bonding jumper(s) will be required between the transformer and the switchgear. The supply side bonding jumper is sized per the rules in 250.102(C).
There is no requirement that a conductor be run from the MCC to the building ground bus ring. The code does not specify the size of that conductor.
An EGC, sized per T250.122 must be run from the switchgear grounding bus, with the power conductors, to the MCC.
 
Dennis, I am trying to validate this to my customer. Can I ask what credintials you have? My customer is sighting the word 'Equipment' in table 122 and saying that refers to the Switchgear.

Dennis, we are manufacturer of the switchgear and the PDC. The customer will take possession of the PDC,move it to site and connect the incoming. They are responsible for the connection to the ground electrode. We have connected the building ground ring to a ground pad on the skid of the building. This same ground pad is where the customer will connect the ground electrode.
 

Dennis Alwon

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I am a licensed electrical contractor with 40 years experience but I don't do much commercial work.

Terminology is always an issue. The MCC is the first disconnect for the building so and grounding electrode conductor must terminate in that piece of equipment or at the meter. That is the service equipment. Once you leave the MCC to the switch gear then you would use Table 250.122 for the equipment grounding conductor but no grounding electrode conductor is needed at that point.

The building steel, ground ring, water pipes etc must be run to either the MCC or the meter
 

augie47

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Normally ant time you have overcurrent protection ahead of a conductor, Table 250.122 is the applicabel table. For services and Seperately Derived systems where there is no oveercurrent device directly ahead of the conductors, Table 250.66 applies.
Your situation is a horse of a different color, however, as your gear is the located in a separate building. Even though you have a 4000 amp OCP device ahead of the gear, 250.32(E) requires a grounding electrode conductor base on 250.66 be used to connect your gear EGC to the building grounding electrode.
Anything in the building protected by an overcurrent device would have a EGC based on 250.122 for the respective OCP.
 

iceworm

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We are manufacturing a 4000A/480V distribution switchgear and installing it in a Power Distribution Center ....
JR -
You must have had an Engineer of Record for this job. What does she say?

I'm not understanding the issue. What exactly are you trying to validate to the customer? Its not clear to me.

ice
 

charlie b

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The part that is not clear to me is what is located in this building, and what is not. Let's start with the transformer. I infer that, (1) the transformer is the property of the owner of the building and not the property of the utility company, and (2) the transformer is located outside this building. Many things change if those two statements are not true. But for the moment I will continue with the assumption that both are true.

Since the 4000 amp switchgear is located in a building that is apart from its power source, you need to connect its ground bus to planet earth. Your first two questions are related to this requirement. Table 250.66 is the source of wire sizes for this requirement, as Dennis has already stated. And as Don has already stated, there is no requirement to do what you are asking in your third question.
 
Grounding Diagram

Grounding Diagram

Attached is a sketch of the grounding for the system. The transformer is external to the PDC (building). This building is an unmanned power distribution center only and contains Medium Voltage Switchgear, Low Voltage Switchgear, and MCC's. There are also other ancillary subsystems in the building like lighting panels, battery systems, air conditioning, etc. What we normally do here is run 'bonding jumpers' or 'grounding electrode condutors' from the Switchgear ground bus to the building ground bus ring. We do the same principal with the MV Switchgear, MCC's, and other distribution equipment.
The real question is what table should I reference when sizing these conductors for grounding. My customer is looking at Table 250.122, sizing 'Equipment Grounding Conductors', calling the Switchgear the 'Equipment'. I have historically used Table 250.66, sizing 'Grounding Electrode Conductors'. I just need technical verification for using either Table and I want to be sure I am applying the correct Table to the correct conductor.
 

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Dennis Alwon

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The only time you use T 250.122 is after the MCC. Everything up to and including the MCC gets sized with 250.66. The 2014 has actually added a differrent table for bonding (T 250.102(C)(1) but it is pretty much the same as 250.66
 

don_resqcapt19

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I don't see the drawing as being in compliance with the NEC.

The conductor from the switchgear, must go directly to the "building ground pad". That conductor and the conductor from the building ground pad are the only two conductors on that drawing that are required by the NEC. Those two conductors are sized per the rules in 250.66.

The rest of the bonding jumpers are not required by the NEC and can be any size the designer wants them to be.

Note that the feeders from the switchgear to the MCCs must include an EGC that is sized per the rules in 250.122.
 

nhee2

Senior Member
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NH
Attached is a sketch of the grounding for the system. The transformer is external to the PDC (building). This building is an unmanned power distribution center only and contains Medium Voltage Switchgear, Low Voltage Switchgear, and MCC's. There are also other ancillary subsystems in the building like lighting panels, battery systems, air conditioning, etc. What we normally do here is run 'bonding jumpers' or 'grounding electrode condutors' from the Switchgear ground bus to the building ground bus ring. We do the same principal with the MV Switchgear, MCC's, and other distribution equipment.
The real question is what table should I reference when sizing these conductors for grounding. My customer is looking at Table 250.122, sizing 'Equipment Grounding Conductors', calling the Switchgear the 'Equipment'. I have historically used Table 250.66, sizing 'Grounding Electrode Conductors'. I just need technical verification for using either Table and I want to be sure I am applying the correct Table to the correct conductor.

Where/what is the MV switchgear? The original post said 480V/4000A switchgear. I would agree that 480 VAC switchgear GEC sized to 250.66 and a EGC sized per 250.122 run with feeders from switchgear to MCCs located in same building.
 

don_resqcapt19

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The underlined is the only statement of yours I do not agree with. 250.102(D) requires them to be sized per 250.122.
I don't agree that they are covered by 250.102(D). They are not EGCs nor GECs and they are not equipment bonding jumpers that are required by the NEC. I don't see these jumpers as being equipment bonding jumpers as defined in Article 100.
 

augie47

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FWIW I'm on the fence with this one. I agree that the jumpers from MCC to the Buss are not needed and I also don't see them fitting the definition of equipment bonding jumpers thus not having to comply with 250.102D, however, if they were installed I would like to see them sized per 250.122 based on the 1200 amp feed so we would not have an undersized ground path.
(in the same manner that we don't accept undersize conductor type EGCs when they parallel a 250.118 approved conduit)
 

don_resqcapt19

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FWIW I'm on the fence with this one. I agree that the jumpers from MCC to the Buss are not needed and I also don't see them fitting the definition of equipment bonding jumpers thus not having to comply with 250.102D, however, if they were installed I would like to see them sized per 250.122 based on the 1200 amp feed so we would not have an undersized ground path.
(in the same manner that we don't accept undersize conductor type EGCs when they parallel a 250.118 approved conduit)
Why do we care about the size of this remote bonding path? Even if sized per T250.122 or larger, the vast majority of any ground fault current will flow on the EGC that is required to be installed with the feeder conductors. The impedance of a remote fault clearing path for an AC system is much much greater than the impedance of the EGC that is installed with the feeder conductors.

I don't see this as being even remotely related to the size of an EGC of the wire type that is installed in a raceway that is permitted to be an EGC by 250.118.
 

augie47

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Why do we care about the size of this remote bonding path? Even if sized per T250.122 or larger, the vast majority of any ground fault current will flow on the EGC that is required to be installed with the feeder conductors. The impedance of a remote fault clearing path for an AC system is much much greater than the impedance of the EGC that is installed with the feeder conductors.

I don't see this as being even remotely related to the size of an EGC of the wire type that is installed in a raceway that is permitted to be an EGC by 250.118.

I guess I have to plead ignorance due to my lack of knowledge concerning fault current paths and I'm certainly not equipped to argue with your position that size is irrelevant. The coloration is when we have a conduit system that qualifies as a EGC and someone installs an undersize wire type conductor those folks here with knowledge on the subject take the stand that we have a hazardous situation.. I can't recall why, perhaps the presence of an undersized path if you lose the conduit. What ever the reason, I see the same pattern. If the OP elects to install the bonding jumper described and the primary EGC is lost, in the event of a fault do we not have a piece or primacord ??
 

Smart $

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Ohio
I don't agree that they are covered by 250.102(D). They are not EGCs nor GECs and they are not equipment bonding jumpers that are required by the NEC. I don't see these jumpers as being equipment bonding jumpers as defined in Article 100.
I agree they are not required per 250.90... but no Part V section including 250.90 says bonding shall not be provided where NOT necessary.

Additionally, the Article 100 definition does not say an equipment bonding jumper is the only grounding path between the two or more portions of the equipment grounding conductor. In a few perhaps rare cases it may, but in most it does not. If all the grounding requirements are followed, there are only a few specific cases where an equipment bonding jumper completes the path of a required equipment grounding conductor. The only one I can think of offhand is around an expansion joint, and even then it shouldn't be the sole path as the design of the expansion joint should provide a grounding path, just not a solidly connected path, so the jumper is only to provide assurance.
 

Smart $

Esteemed Member
Location
Ohio
... The impedance of a remote fault clearing path for an AC system is much much greater than the impedance of the EGC that is installed with the feeder conductors.

...
True, but both together provide a lower impedance than either alone. Additionally, a remote path having greater impedance than an EGC is justification for it to be sized not less than the required EGC.
 
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