grounding transformer

[jkpyke]

If a transformer supplies a 120/240v, single phase, 200amp panelboard from a 480v, 3 phase supply and if the secondary ungrounded conductors from the transformer to the panleboard are sized 3/0 awg copper. This transformer is near the service disconnect to the building and it is grounded to the metal underground water pipe. The min. size copper grounding electrode conductor permitted for this transformer installation is:

For this problem would I refer to Table 250.122 and would 6 awg be correct?

 

[augie47]

Nope. Refer to 250.30

 

[Volta]

Which should point you back to 250.66.

As a rule of thumb, you can think of the two tables as 250.122 for circuits with overcurrent protection, and 250.66 for those without.

To use 250.122, you need to know the size of the OCP device on the line side. If there is none, you probably should be in the other table.

 

[jkpyke]

Thank you 4 awg copper is what should be used then. Thanks

 

[raider1]

Thank you 4 awg copper is what should be used then. Thanks

Correct.

Chris

 

[myrshub]

I have the same question for another XFMR. I'll ask here.

The secondaries call for parallel 300 mcm's and a #1 ground. How do you convert MCM to kcmil's to figure the the size of the bonding jumper?

Additionally table 250.66 doesn't offer a #1 as an option for the bonding jumper. I'm a little confused.

 

[Volta]

I have the same question for another XFMR. I'll ask here.

The secondaries call for parallel 300 mcm's and a #1 ground. How do you convert MCM to kcmil's to figure the the size of the bonding jumper?

Additionally table 250.66 doesn't offer a #1 as an option for the bonding jumper. I'm a little confused.

Same thing. MCM is what we used to call it- before we (the industry) decided that 'M' shouldn't be used for thousand. So 'K' is the modern usage, but habits are hard to break.

So are you reading job spec's that ask for a #1 bonding jumper? That would be compliant, if the secondaries are 300 mcm, since the jumper shall be not less that given in Table 250.66, which is a #2, so a #1 is ok.

 

[myrshub]

Same thing. MCM is what we used to call it- before we (the industry) decided that 'M' shouldn't be used for thousand. So 'K' is the modern usage, but habits are hard to break.

So are you reading job spec's that ask for a #1 bonding jumper? That would be compliant, if the secondaries are 300 mcm, since the jumper shall be not less that given in Table 250.66, which is a #2, so a #1 is ok.

But they are parallel 300 mcm which = 600 kcmil. Wouldn't I need a 1/0? Or would that only apply to the electrode conductor to building steel?

 

[Volta]

But they are parallel 300 mcm which = 600 kcmil. Wouldn't I need a 1/0? Or would that only apply to the electrode conductor to building steel?

Sorry, I missed parallel :roll:. You are right, 300+300=600, so 1/0 min.

That would be the EBJ to the disconnect. 250.30(A)(2) > 250.102(C) > T250.66.

It would be the SBJ between grounded and grounding 250.30(A)(1) > 250.28(D) > T250.66.

It would be the GEC to the steel. 250.30(A)(3) > 250.66.

 

[myrshub]

Thanks for the help Volta.

So the Engineer is wrong in this case?

It looks like he/she sized it (EBJ) to each individual set? Even though it would only require a #2 in that case and not a #1 but that would be the incorrect method because the secondaries are in parallel. Am I right?

 

[hurk27]

Thanks for the help Volta.

So the Engineer is wrong in this case?

It looks like he/she sized it (EBJ) to each individual set? Even though it would only require a #2 in that case and not a #1 but that would be the incorrect method because the secondaries are in parallel. Am I right?

I'm not so sure as I just learned in another thread, which I kind of don't agree with (the code part) but it is there.

if these bonding conductors are run with each set of parallel conductors then 250.102(C) last sentance, will allow the bonding jumper to be sized upon the ungrounded conductors in each raceway,

Grounding versus Bonding
Check out this thread: Grounding versus Bonding

It will depend upon how they are ran and between what two points are they connected, IE: are they ran in each of the raceways containing the parallel conductors and connect in the transformer and first disconnect?

 

[myrshub]

Hurk, I believe you are correct. My book refers me to that section in order to size the Eq. bonding jumper. The way I interpret that (250.102 c), I would size per conductor separately.