Grounding electrode conductor size

[ktm400sx]

As a grounded conductor, still have to question the setup, as the OP says the #4's land on the MCC ground bus. Of course the OPer could be mistaken by thinking it is a grounding conductor because that is also where the GEC terminates... but instead lands on a neutral bus (should be mounted with stand-off insulators and have a main bonding jumper). Could also be an MCC designed for 3? 3-wire (non-compliant).

You are correct, it is the neutral, not the ground that has been brought to the MCC. The customers GEC lands on the same bus within the MCC.. and the MCC does not have a neutral bus.

For right now my main concern is the size of the GEC. I assumed that table 250.66 is used but from Denis' post (he says use #6) I must be mistaken...

 

[Smart $]

You are correct, it is the neutral, not the ground that has been brought to the MCC. The customers GEC lands on the same bus within the MCC.. and the MCC does not have a neutral bus.

For right now my main concern is the size of the GEC. I assumed that table 250.66 is used but from Denis' post (he says use #6) I must be mistaken...

You need to tell us what electrode(s) is(are) connected to this GEC before we can definitively tell you the correct size...

 

[ktm400sx]

You need to tell us what electrode(s) is(are) connected to this GEC before we can definitively tell you the correct size...

It is a driven ground rod in soil, 8' long galvanised 5/8" thick ... this is the only electrode connected to the GEC

 

[Smart $]

It is a driven ground rod in soil, 8' long galvanised 5/8" thick ... this is the only electrode connected to the GEC

All that is required for a ground rod electrode is a #6 copper GEC (at #2 it is already sufficient). If you want to bring it up to current Code, drive a second rod 6' or more away from first rod and also connect to bus or existing GEC with #6.

But the question arises whether any other electrodes are present...??? Is the MCC inside or outside -- standing by itself or up against a building or other structure. We need all the pertinent details that we cannot see from here

 

[ktm400sx]

All that is required for a ground rod electrode is a #6 copper GEC (at #2 it is already sufficient). If you want to bring it up to current Code, drive a second rod 6' or more away from first rod and also connect to bus or existing GEC with #6.

But the question arises whether any other electrodes are present...??? Is the MCC inside or outside -- standing by itself or up against a building or other structure. We need all the pertinent details that we cannot see from here

Yes, there is another electrode present in the building that is used by a secondary from a transformer lighting panel. This is an emergency generator building and the MCC has a transfer switch. I do not know if this electrode in the building is rebar attached to the foundation or something else. Thanks for your help
Its a small building about 10' by 15' with only the MCC and generator inside

 

[Dennis Alwon]

Dennis, where are you getting this from?
Thank you

Look at 250.66(A) for ground rod and 250.66(B) for the concrete encased electrode.

 

[Dennis Alwon]

As a grounded conductor, still have to question the setup, as the OP says the #4's land on the MCC ground bus.

As I stated in an earlier post #13-- it's a NO NO.

 

[MarineTech]

You do not parallel the grounding electrode conductor. Note 1 states to add the cir. mil of the 3/0 conductors. Table 8 Chapter 9 shows the cir mil for 3/0 at 167,800. Multiple by 2 for the parallel run and you get 335,600-- closest size to that is 350 kcm. Thus you need a 1/0 copper grounding electrode conductor

Dennis,

Just a quick question on the use of NEC 2011 Table 250.66 as per the notes. For the ungrounded cross sectional sum of 335,600, do we go to the larger available size conductor, 350 kcmil and then reference the table, giving 1/0.

Or do I read the table as a range of cross sectional areas. That is, from the table a range of Over 3/0 (167,800 kcmil) through 350 kcmil. Between which we would find 335,600 kcmil, resulting in #2 AWG.

Thanks,

MarineTech

 

[Smart $]

As I stated in an earlier post #13-- it's a NO NO.

And also noted at the end of the post which you quoted only in part. However, is it really a major concern? A main bonding jumper is permitted to be busbar. What if we considered the entire bus as the main bonding jumper? Is it really much different than using the same bus for neutral and grounding in a residential service panelboard?

 

[Smart $]

Dennis,

Just a quick question on the use of NEC 2011 Table 250.66 as per the notes. For the ungrounded cross sectional sum of 335,600, do we go to the larger available size conductor, 350 kcmil and then reference the table, giving 1/0.

Or do I read the table as a range of cross sectional areas. That is, from the table a range of Over 3/0 (167,800 kcmil) through 350 kcmil. Between which we would find 335,600 kcmil, resulting in #2 AWG.

Thanks,

MarineTech

The sum is the sum. Code does not require us to round up to a standard conductor size cmil area.

You read the table as ranges.

 

[ktm400sx]

Thanks for all the help so far. There is still a question in my mind as to why table 250.66 does not apply. It seems quite clear that the GEC must be sized according to the table...is the #6 for a bonding conductor?

The NEC handbook shows an example (exhibit 250.31 and 250.30) that shows what appears to be #4 GEC for one 3/0 service conducter.

 

[Smart $]

Thanks for all the help so far. There is still a question in my mind as to why table 250.66 does not apply. It seems quite clear that the GEC must be sized according to the table...is the #6 for a bonding conductor? ...

Look at 250.66 general statement. It says the Table applies except as permitted in 250.66(A) through (C).

 

[ktm400sx]

Look at 250.66 general statement. It says the Table applies except as permitted in 250.66(A) through (C).

Of course...I'm feeling a bit dense! Thanks again.

 

[Dennis Alwon]

Dennis,

Just a quick question on the use of NEC 2011 Table 250.66 as per the notes. For the ungrounded cross sectional sum of 335,600, do we go to the larger available size conductor, 350 kcmil and then reference the table, giving 1/0.

Or do I read the table as a range of cross sectional areas. That is, from the table a range of Over 3/0 (167,800 kcmil) through 350 kcmil. Between which we would find 335,600 kcmil, resulting in #2 AWG.

Thanks,

MarineTech

Yes you are correct--- I made a visual error reading the result in my earlier post #3. A #2 not 1/0 is needed. Thank you