Automatic Transfer Switch/Service Panel & Sizing Electrode Conductors

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Bigbri0104

Member
Location
New Jersey
Occupation
Student
Hello Everyone,

I'm working on a job that entails installing a 24 KW Generac automatic natural gas generator. The electrical service to the home is a 300-amp underground service with two 150-amp main breaker panels inside the basement, fed off a double lugged electrical meter. The service was installed prior to NEC 2020 code cycle. We are only installing one Automatic transfer switch, converting one main panel to a feeder panel, and updating the grounding and bonding system.

I already contacted the electric inspector and received the green light to mount our 150-amp automatic transfer switch inside the basement without installing it outside as an "Emergency Disconnect. The inspector also wanted to ensure the electrode system was updated. During our conversation about grounding and bonding, he wanted to make sure that the installation included inversible crimps, if I were to utilize the existing 1/0 common grounding electrode.

1st question - Does table 310.12 play into the initial installation, or must the SEU cable be sized to a min of 150 amp for each service panel that was installed? Table 310.12 was not used in the 2017 code cycle I think, which was around the time this house was built. However, for my own education would the electrician install 2/0 AL SEU cable or 3/0 AL SEU cable to each service panel.

2nd question - 250.64 (C) states the grounding electrode shall be installed in one continuous length without a splice or joint. If necessary, splices or connections shall be made as permitted in (1) though (4).

- Does this allow the electrician to install one large continuous wire by daisy chaining it through both service panels as shown in the picture below, Or must you follow the language in 250.64 (D) because of the charging language stating multiple disconnecting means in separate enclosures?

- My other question pertains to 250.64 (D) (2). This sub section states the electrician is allowed to install one continuous electrode from one service panel/service
disconnect based on the size of the largest ungrounded conductor in the enclosure, and terminate to let's say a water pipe, then install another continuous electrode from the second service panel/service disconnect enclosure to the same water pipe. The reason for the electrode example is because there is one present at the property. Am I reading this and interrupting it correctly?

Thanks,

Brian
 
Locally the inspectors won;t technically allow you to use 310.12 to each disconnect with multiple service disconnects however it is generally a moot point as the "next size up rule" (240.4) ends up with the same conductor (load calculation permitting)
As to your GEC. If I understand your question, there is no problem installing a separate GEC from each service panel to the GES with each based on the serviced conductors for the particular panel.
 
augie47 said:
Locally the inspectors won;t technically allow you to use 310.12 to each disconnect with multiple service disconnects however it is generally a moot point as the "next size up rule" (240.4) ends up with the same conductor (load calculation permitting)
As to your GEC. If I understand your question, there is no problem installing a separate GEC from each service panel to the GES with each based on the serviced conductors for the particular panel.
Click to expand...
Thank you for answering my question concerning table 310.12.

There is no problem installing two separate GEC from each panel; however, I will contact the inspector again and ask him if that's permitted in his jurisdiction. I myself have never used this rule in article 250. The reason I ask is due to the fact I don't own a crimping tool for that size conductor to make a tap from each of the 150-amp panels. Additionally, it would be more economically to install two separate GEC from each panel in the basement to the water main instead of purchasing a crimp tool.

I want to ensure that I'm interrupting this section of the code correctly 250.64(D)(2) for this install.
 
You need to identify what is the GEC and what is a bonding jumper. Per the NEC, the GEC must use irreversible splices, but other connections to it can be split bolts or a similar tapping mechanism that doesnt cut the GEC.

So lets say the worst case (biggest wire required) ground electrode is a water pipe in the ground. If I was doing this from scratch, I would run a #2 copper from one of the panels to the water pipe. This is the GEC. Then I would run a #4 copper from the other panel to the GEC using a split bolt. I would then run a #6 copper also split bolt off that GEC to the ground rods.

Note that the water pipe must be metal in the earth for 10 feet to be a GEC. If the buried pipe is plastic, then you are just bonding the pipe and that wire is not a GEC even though they make you size it like one.

If all you had was ground rods, then a #6 from one panel to the rods would be the GEC. Then a #6 bonding jumper from the second panel to the GEC via a split bolt would be fine.

The hard thing is finding the electrode that needs the largest conductor and running an unspliced GEC to it. All the other connections can be taps off of it.

And the GEC probably only has to be #2 copper unless the service entrance conductors were oversized.
 
Bigbri0104 said:
Thank you for answering my question concerning table 310.12.

There is no problem installing two separate GEC from each panel; however, I will contact the inspector again and ask him if that's permitted in his jurisdiction. I myself have never used this rule in article 250. The reason I ask is due to the fact I don't own a crimping tool for that size conductor to make a tap from each of the 150-amp panels. Additionally, it would be more economically to install two separate GEC from each panel in the basement to the water main instead of purchasing a crimp tool.

I want to ensure that I'm interrupting this section of the code correctly 250.64(D)(2) for this install.
Click to expand...
A lot of electrical supply houses will rent you the tool, sometimes for free if you buy enough crimps from them.
 
suemarkp said:
You need to identify what is the GEC and what is a bonding jumper. Per the NEC, the GEC must use irreversible splices, but other connections to it can be split bolts or a similar tapping mechanism that doesnt cut the GEC.

So lets say the worst case (biggest wire required) ground electrode is a water pipe in the ground. If I was doing this from scratch, I would run a #2 copper from one of the panels to the water pipe. This is the GEC. Then I would run a #4 copper from the other panel to the GEC using a split bolt. I would then run a #6 copper also split bolt off that GEC to the ground rods.

Note that the water pipe must be metal in the earth for 10 feet to be a GEC. If the buried pipe is plastic, then you are just bonding the pipe and that wire is not a GEC even though they make you size it like one.

If all you had was ground rods, then a #6 from one panel to the rods would be the GEC. Then a #6 bonding jumper from the second panel to the GEC via a split bolt would be fine.

The hard thing is finding the electrode that needs the largest conductor and running an unspliced GEC to it. All the other connections can be taps off of it.

And the GEC probably only has to be #2 copper unless the service entrance conductors were oversized.
Click to expand...
Thank you for the response. I think the largest size underground conductor being feed from the transformer appears to be somewhere in the range of 3/0 copper to 350 KCMIL which terminates in the meter enclosure. The original installer put a 1/0 aluminum GEC daisy changing it through both panels and having it terminate at the water pipe.
 
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