Does ground wire conduit need to be bonded at both ends?

[Dennis Alwon]

do you mean the risers are PVC also. You wrote underground twice.

 

[roger]

Hello Ed.

Roger,

I have a question regarding this situation.

If the conductor is in PVC underground with GRC 90's on each end and PVC underground.

1.) Is this an acceptable installation?

No it's not, there is no way to bond the GRC 90's correctly for a GEC installation

2.) if it is acceptable do you need to bond the ends of the GRC that stub up through the poured slab? or should this be either all PVC or All GRC (or some other metal conduit)?

It should be either PVC or Metal, not a combination.

Just curious because I see this quite a bit on large projects that require (per the specifications) that GRC 90's be used to turn up through a slab.

Thanks,
Ed

I see this spec on a regular basis too, but this is where the electrician and inspector (those in the know) have to take over and then explain why the spec can not be followed.

Roger

 

[W6SJK]

OK I'll bite. Why does it make any difference to electrons whether it's a EGC or GEC conduit when bonding? One carries high fault current and one carries higher lightning strikes. Is that enough of a reason?

 

[FirmlyGrounded]

Help me out here with the achronyms..GRC stands for what?

 

[Smart $]

GRC?

GRC = RMC, but specifically stands for Galvanized Rigid Conduit

 

[don_resqcapt19]

sparkie001,

Why does it make any difference to electrons whether it's a EGC or GEC conduit when bonding?

The choke effect only occurs when a single conductor of an AC circuit it run through a magnetic raceway. The GEC is a single conductor of an AC circuit and to eliminate the choke effect problems, you must bond the GEC to both ends of every section of magnetic raceway. This same problem does not occur with EGC because the other conductors of the circuit are withing the same raceway. Of course, all metal parts of the electrical system are required to be bonded by the code rules, however in the case of conduits with an EGC and the circuit conductors this bonding is only required at one point, normally the supply end of the raceway.
Don

 

[W6SJK]

I understood the choke effect but forgot that the conductors supplying the fault current would be in the same conduit and cancel the field. Thanks.

 

[tom baker]

sparkie001,

The choke effect only occurs when a single conductor of an AC circuit it run through a magnetic raceway. Don

Don: I hadn't considered this issue with a EGC, however it would occur, thats why 300.5 requires all conductors in the same raceway, except for bonding jumpers, limited to 6 ft
But, functionally speaking, wouldn't the problem be more acute with a GEC due to the high (20,000 amps) current and higher lightning frequecy?

Whats interesting is the code recoginzes the problem for all circuits and picks the magic number of 6 ft. Probably this number is from the same proposal that set up 25 ohms for ground rod resistance. That documentation and reasoning is long gone.

 

[vince440]

Thanks Everybody

Thanks Everybody

Thanks for the input from everybody. I don't know if any of you have ever dealt with OSHPD in CA, but they are the state agency that inspects hospitals. They will not allow PVC above slab anywhere as a health issue not a NEC issue. We should have run GRC contiuous underground but it was an oversight on our part. No big deal, we'll just have to run it overhead in EMT. Too bad because it's in! Oh well, now I have a spare loop between electric rooms.

 

[benaround]

Vince,

Sounds like a good spot to install Alum. sweeps.

 

[Pierre C Belarge]

Don: I hadn't considered this issue with a EGC, however it would occur, thats why 300.5 requires all conductors in the same raceway, except for bonding jumpers, limited to 6 ft
But, functionally speaking, wouldn't the problem be more acute with a GEC due to the high (20,000 amps) current and higher lightning frequecy?

Whats interesting is the code recoginzes the problem for all circuits and picks the magic number of 6 ft. Probably this number is from the same proposal that set up 25 ohms for ground rod resistance. That documentation and reasoning is long gone.

Tom
Answering your question, yes. The high frequency lends toward "skin effect". More current will travel on the outer surface of the conductor and the current will have a tendency to "jump" from the conductor to the raceway. As a matter of fact, a study performed showed that approximately 93% of the current will "jump" from the conductor to the ferrous raceway.

 

[Luketrician]

sparkie001,

you must bond the GEC to both ends of every section of magnetic raceway.
Don

Both ends of every section, yeah that would be impossible to do when the other ends of the stub up 90's are burried in the slab. Makes sense to me now. Which is why it is a violation right?

 

[roger]

Which is why it is a violation right?

You will find the answer on the first page of this thread.

Roger

 

[Luketrician]

Ok, 250.64(E)

Thanks!