Please take a close look at this picture, and identify in your minds, the pieces of the puzzle that comprise the equipment grounding conductor.

I would like to use this thread to identify some of the important parts of the grounding system in an installation, so please add your comments accordingly.

I am not interested in a wire here, my interest is with the recognized equipment grounding conductors permitted by 250.118.

Here we have one and it is Rigid Metal Conduit.

The "T" Condulet is defined as a "conduit body" and the subject of the continuity is where I want to begin.



The EGC originates from the top RMC connected to a panelboard, and the others to the right, and bottom continue the circuit raceway for lighting in stairwells in a parking garage.

The connections at the beginning of the run and at this conduit body are very important and if they are not "wrench" tight may not ensure that 300.10 is complied with.

Question 1: Does the "T" serve to bond the right and bottom raceways?

Question 2: Should the NEC call for at least 5 threads fully engaged here as in hazardous (classified) locations?

Question 3: Could we live with a term to read as follows" "... Equipment grounding and bonding conductor"?

Think about the couplings, and all fittings in a run of RMC, say 200 feet long, do the fittings act to bond the EGC's together?

We may have to wait until 2008 for any changes related to the proposed change from EGC to EBC, so let's start here.

Joe -

Q1: (From 2003 UL 'white book', pg. 21, Conduit Fittings [DWTT]) - 'This category covers metallic....conduit fittings, such as ... conduit bodies...'...towards to bottom of pg. 21, right column is the heading "Grounding" - "All metal fittings for metal cable, conduit and tubing are considered suitable for gounding for use in circuits over and under 250v...". - to answer your question, it looks like UL accepts metal conduit bodies as a satisfactory grounding means. [Soares does a number of tests with conduit fittings but I don't see any testing of conduit bodies].

Q2: ?... if the RMC is threaded properly (346-8) and 'made up tight' as required, it seems to me you'd have the 5 threads engaged regardless of location.

Q3: using the term 'equipment bonding conductor' does seem to help some folks better understand the function of this conductor.

Q4: 200' of pipe and fittings...see Q1.

Joe,
There is a big difference in the threads on the various brands of conduit bodies and conduit. Just because there are a number of threads showing does not mean that the joint has not been made up tight. Some combinations produce a "wrench tight" condition with only 3 or 4 male threads engaged in the fitting.
I do have a problem with the bonding at our conduit couplings. The fact that they are straight thread makes for a poor bonding connection, in my opinion. I think that if our conduit couplings were tapered thread like plumbing couplings, the bonding would be much better.
Don

There is a study that was performed by (I think the University of Georgia) to test the different lengths of various size conduits for grounding purposes. The results list the lengths that would be good for different size conduits for grounding.
The conduit bodies that you have pictured, if made wrench tight would satisfy me for grounding/bonding purposes. IF they are wrench tight, either the threads are tight, or the conduit is tight against the shoulder of the conduit body.
When installing long runs of conduit, there are maximum lengths, when the grounding effectiveness of the conduit will no longer be effective.

Pierre

Joe,

I myself would like to see a hard ground pulled at all times. I don't like using any metal conduit for the grounding conductor. Too many times due to sloppy workmanship or careless helpers, the threads/connectors do not get tighten properly. Most engineers are now requiring hard ground conductors pulled in the metal raceways using 277/480 volt wiring. I think that code change about the rated KO's being able to handle a 277 V fault helped that issue.

how a fault would travel vs. the impedance of the RMC (or any wirerun method) comes to mind here.

so some studies as Pierre alludes may be enlightening...


the two terms can exist soley without the other right? (EGC or BC) so to say EC&BC might end up a terminology stumbling block in the nec

~S~

The Georgia Tech research studies on which the GEMI program is based are included on the disc. This research, plus full-scale testing, provided real-world measurements from which software designers developed the Free copies of the new Windows version of the GEMI software are available to designers, engineers and other electrical professionals from the Steel Tube Institute.
http://www.steelconduit.org/download/gemi-install.exe
http://www.steelconduit.org/pdf/gemivol1.PDF
http://www.steelconduit.org/pdf/groundingpart1.PDF