Role of Equipment Grounding Conductors in 250.36 and 250.187

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srtelando

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Location
Cincinnati
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Electrical Engineer
Hello,

I'm working on a medium voltage project right now where we are installing a neutral ground resister to limit fault current.
The NGR sits right outside the transformer, but in a separate enclosure.

I noticed that the Impedance Bonding Jumper is defined as the conductor which connects the equipment grounding conductors at the first point of disconnect to the NGR grounded location and must not be spliced.

We are running two parallel sets of 500KCMIL current carrying conductors from the transformer secondary to the switchgear breaker (first point of disconnect.) This means that each set of 500KCMIL conductors will have their own Equipment Grounding Conductor, which normally I would connect to Transformer Frame and Grounding Electrode Conductor. The grounding electrode conductor in this case is connected at the NGR.

My question: Can I bond both equipment grounding conductors from the parallel feeds at the transformer; then run one impedance bonding jumper to the NGR grounded connection? Do I need to run both Equipment Grounding Conductors to the transformer with the current carrying conductors and the run straight to the NGR grounded connection? (In this scenario I would run an additional equipment grounding conductor to the NGR from the transformer frame.)

Any insight on this? Attached is what I think makes the most sense, however, I feel like the violates the rule that the impedance bonding jumper must originate at the first point of disconnect and land at the grounded side of the grounding impedance device without spliced.

(trying to figure out how to attach a picture of my drawing, sorry if you can't see it)
 
The connection on the low side of the transformer is not typically referred to as a equipment grounding conductor since there is no OCPD on the low side. The first OCPD is down stream of that point. So it is typically either referred to as a supply side bonding jumper or a system bonding jumper (depending on configuration).

To attach an image, you can post it on a 3 party site (that they allow) like imgur. Or you can try and reduce the file size / image resolution.

The ground resistor is used to limit the current back on the neutral offering a method of observing the fault without it needing to directly operate a OCPD.

250.187 tells you how this should look.

Primary EGC + GEC + Low side bonding jumpers to the neutral resistor to the neutral point on the transformer. So I would assume the wire would need to be sized based on the low side wire sizing like a System bonding jumper or Supply side bonding jumper.

Mainly 250.187 (D) says to connect the equipment grounding provisions to the grounding electrode conductor
 
Welcome to the forum.

Normally, there is no equipment grounding conductor ahead of the first OCPD; all bonding is done directly to/by the secondary neutral.

The neutral and transformer have a grounding electrode system, and the OCPD/service disconnect have their own electrode system.
 
srtelando said:
Hello,

I'm working on a medium voltage project right now where we are installing a neutral ground resister to limit fault current.
The NGR sits right outside the transformer, but in a separate enclosure.

I noticed that the Impedance Bonding Jumper is defined as the conductor which connects the equipment grounding conductors at the first point of disconnect to the NGR grounded location and must not be spliced.

We are running two parallel sets of 500KCMIL current carrying conductors from the transformer secondary to the switchgear breaker (first point of disconnect.) This means that each set of 500KCMIL conductors will have their own Equipment Grounding Conductor, which normally I would connect to Transformer Frame and Grounding Electrode Conductor. The grounding electrode conductor in this case is connected at the NGR.

My question: Can I bond both equipment grounding conductors from the parallel feeds at the transformer; then run one impedance bonding jumper to the NGR grounded connection? Do I need to run both Equipment Grounding Conductors to the transformer with the current carrying conductors and the run straight to the NGR grounded connection? (In this scenario I would run an additional equipment grounding conductor to the NGR from the transformer frame.)

Any insight on this? Attached is what I think makes the most sense, however, I feel like the violates the rule that the impedance bonding jumper must originate at the first point of disconnect and land at the grounded side of the grounding impedance device without spliced.

(trying to figure out how to attach a picture of my drawing, sorry if you can't see it)
Click to expand...
https://imgur.com/a/JVWgCJk
 
Elect117 said:
The connection on the low side of the transformer is not typically referred to as a equipment grounding conductor since there is no OCPD on the low side. The first OCPD is down stream of that point. So it is typically either referred to as a supply side bonding jumper or a system bonding jumper (depending on configuration).

To attach an image, you can post it on a 3 party site (that they allow) like imgur. Or you can try and reduce the file size / image resolution.

The ground resistor is used to limit the current back on the neutral offering a method of observing the fault without it needing to directly operate a OCPD.

250.187 tells you how this should look.

Primary EGC + GEC + Low side bonding jumpers to the neutral resistor to the neutral point on the transformer. So I would assume the wire would need to be sized based on the low side wire sizing like a System bonding jumper or Supply side bonding jumper.

Mainly 250.187 (D) says to connect the equipment grounding provisions to the grounding electrode conductor
Click to expand...
Ok thanks for clearing that up on the EGC and the Supply Side Bonding Jumper. My drawing uses the word "Equipment Grounding Conductor" but I really have a Supply Side Bonding Jumper, and in this case, the Supply Side Bonding Jumper seems to be reclassified as the Impedance Bonding Jumper according to 250.36.

The Impedance Bonding Jumper is described as: "The impedance bonding jumper (the connection between the equipment grounding conductors and the grounding impedance device) shall be an unspliced conductor run from the first system disconnecting means or over current device to the grounded side of the grounding impedance device."

My grounding impedance device is located a few feet from the transformer. So I ran unspliced impedance bonding jumpers with the current carrying conductors from the first system disconnecting means into the transformer, but then routed the impedance bonding jumpers back out of the transformer to the grounded side of the grounding impedance device. (So I did not end up attaching the impedance bonding jumper to the transformer frame as shown in the drawing. It is continuous out to the Neutral Grounding Resistor.)

I still attached the transformer frame to ground through a separate grounded conductor.

My question is this how you would interpret this routing of the impedance bonding jumper? It just seems kinda clunky/inconvenient to not attach the transformer frame as I had originally intended as shown in my drawing.
https://imgur.com/a/JVWgCJk
 
The transformer frame is not rated to carry current but should be bonded incase it becomes energized. Most panelboards and outlet boxes are tested and rated as a bonding method but for some reason transformer frames / enclosures are not.

So you should bond the metal enclosure of the transformer to this GEC / bonding jumper but the fault current path should not be through the enclosure. If that makes sense.
 
Yes that makes sense. Thank you for your time and your responses. I did run a separate grounded conductor from the transformer frame to the GEC so its protected if the frame got energized for any reason. But any other ground fault path does not include the transformer frame and must travel along impedance bonding jumper through the NGR back to the transformer neutral. I suppose that's why this code is written the way that it is with the requirement for the impedance bonding jumper being continuous/unspliced.

Thanks again!
 
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