conduit as GEC replacement..!!!???

[wyedelta]

I have read the post of ryan about conduit calculation for serving as GEC, his concern is about NEC and the replies are quite convincing,my only concern is this, WHY WE HAVE TO USE CONDUIT AS REPLACEMENT AND NOT TO RUN GROUND CONDUCTOR ITSELF.

As for safety reason we can not rely that conduit pipes are 100% connected to each other, there might be loose connection in between which could result to high resistance.sometimes if it really need to use conduit pipe as grounding it is necssary to weld the joint(the coupling etc)to make effective and have no loose connection.

Secondly, i would stress that this is part of equiptment grounding and therefore consider secondary means of grounding and not to be serve as primary means.

any comment on this safety matter is highly appreciated.just for the forum only and nothing else.

 

[iwire]

Re: conduit as GEC replacement..!!!???

Wye I assume you mean a EGC (Equipment Grounding Conductor) not a GEC (Grounding Electrode Conductor)

I do not think that Ryan was suggesting we have to use the raceway, Ryan is an inspector a careful one judging from his posts.

 

[websparky]

Re: conduit as GEC replacement..!!!???

Bob,

Here is something to consider....

250.122(B) Increased in Size. Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to circular mil area of the ungrounded conductors.

Maybe one of the reasons this has been added is just for the concerns that Ryan has mentioned?? Plus there is

Grounded, Effectively. Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons.

which ties into

250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.

which makes me wonder if we will see more specific codes in the future?

 

[wyedelta]

Re: conduit as GEC replacement..!!!???

my sincer apology.i mean EGC not GEC.sorry guys (iwire and ryan).thanks for contructive comment.

 

[charlie]

Re: conduit as GEC replacement..!!!???

Something else to consider is how the conduit system is put together and where it is located. If the installer has done a god job, the run is not over 300 feet, and it is in a location that is not subject to movement, the run of conduit is automatically OK. What is the difference between a run of this nature with all the couplings, boxes, conduit bodies, and fittings that the same run with an EGC with all the connections with different types of connectors?

The reference to 300 feet is from a study by Georgia Tech that looked at the impedance of EMT, IMC, and GRC used for grounding conductors. Various sized circuits were used in different sizes and lengths of conduit runs. The minimum length that was OK was a little over 300 feet. That left a general rule that any circuit over 300 feet should have an EGC installed or the size and type of conduit and circuit should be looked at for its impedance.

 

[ryan_618]

Re: conduit as GEC replacement..!!!???

I think a nice addition to the NEC might be a FPN after 250.4 (A)(5), something to the effect of "compliane with (part VI??? no book here) of article 250 may not always ensure compliane with this section".

 

[websparky]

Re: conduit as GEC replacement..!!!???

I agree!

 

[iwire]

Re: conduit as GEC replacement..!!!???

I do not agree unless some evidence shows the way we are doing it now is causing unsafe conditions.

I would not want to see the code change only because of conjecture and hunches.

 

[ryan_618]

Re: conduit as GEC replacement..!!!???

Bob: If I'm not mistaken, there is a note to table 250.122 that already talks about it.

 

[iwire]

Re: conduit as GEC replacement..!!!???

Note to table 250.122

Note: Where necessary to comply with 250.4(A)(5) or 250.4(B)(4), the equipment grounding conductor shall be sized larger than given in this table.
*See installation restrictions in 250.120.

This note seems to be a waste of ink because 250.122(B) tells us the same thing.

This note has nothing to do with raceways as grounding conductors.

It is very rare at least in the work I do that you would have over 300' of single isolated conduit.

Most times at the end of this conduit there are many more paths back to the source than the conduit, building steel, plumbing pipes etc.

No we can not intentionally use plumbing pipes as EGC but in reality they end up being one.

Wire a boiler, or water heater and you have tied the EGC to the piping and any fault current will go all directions back to the source.


All I am asking is there any evidence from the field to support more restrictive grounding procedures?

If there is no evidence that breakers are not opening due to conduit impedance IMO leave the code as it is.

[ April 11, 2004, 05:36 PM: Message edited by: iwire ]

 

[websparky]

Re: conduit as GEC replacement..!!!???

Over the years I have seen several long conduit runs become lose and/or damaged and as a result the grounding impedance was high enough to not trip the feeder OCD. The result was a small fire due to heat generating at the weak joints. The only reason the fires did not grow larger was that personell saw the smoke and threw the breaker!

Now these were older installs that were subject to moisture, vibrarion and poor air circulation.
Chance are, on the original install, set screws were not tightened adequately.

I'm not suggesting that a copper conductor be installed in every conduit run but maybe there should be some kind of minimum standard to follow?

 

[tom baker]

Re: conduit as GEC replacement..!!!???

The Steel Tube Institute has a free software program called GEMI that is based on the study that Charlie mentions. It analyizes a circuit, with and with out a EGC, and metallic and non metallic conduit, and determines the max length. The size of the ciruit OCPD is not used, but rather the voltage at the fault. About 300 feet is the longest you can go with out increasing the size of the EGC.
There is an overlooked requirement in Art 250:

250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.

What this says if we follow the rules (prescriptive-like baking a cake) in Art 240 we comply with the performance part of this section.

In some cases we need to increase the size of the EGC, or install an EGC to satisfy 250.4. Mike Holts Grounding and Bonding Text has information on this as it relates to 250.122, as does Soares Book on Grounding.

 

[charlie]

Re: conduit as GEC replacement..!!!???

This note seems to be a waste of ink because 250.122(B) tells us the same thing.

Bob, the note is directing the user of the table back to 250.122(B) . . . period. We are trying to make the Code "user friendly", when you are required to write in legalese, it is difficult to do. The CMP realizes that most people tend to just use the Code for the tables and never pay any attention to the text.

 

[iwire]

Re: conduit as GEC replacement..!!!???

Originally posted by charlie:
Bob, the note is directing the user of the table back to 250.122(B) . . . period. We are trying to make the Code "user friendly"

That would be great, except the note does not direct us to 250.122(B) it directs us to 250.120 Equipment Grounding Conductor Installation.

[ April 12, 2004, 09:04 AM: Message edited by: iwire ]

 

[charlie]

Re: conduit as GEC replacement..!!!???

OOPS!

The intent is still the same even if I am stupid enough to read through the note and miss the reference.