Do you understand this table?

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Dennis Alwon

Dennis Alwon

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The title of the table mentions the length of steel conduit used as an equipment grounding conductor however the table mentions a wire equipment grounding
conductor. Are they trying to say the steel conduit is equivalent to the egc?

Steel Conduit.png
 
Odd that nowhere on the table is steel conduit mentioned. Steel is supposed to have much less resistance than a copper wire used for the circuits in a particular conduit.
 
The header (part in black) says steel conduit/EMT.

What I find odd is the size of the conduit isn't mentioned. I could run 3 #12 wires 300 feet in 1/2" EMT but what if I wanted to use 3/4". Shouldn't that allow for a longer run?
 
Take a look at figure 3 in this article. It gives the lengths of conduit that are acceptable for use as an EGC and the acceptable lengths of wire type equipment grounding conductors.
This information was developed by the Georgia Institute of Technology for the Steel Tube Institute. The Steel Tube Institute also has a free software program named GEMI to do these types of comparison calculations for equipment grounding paths.
 
Dennis Alwon said:
Odd that nowhere on the table is steel conduit mentioned. Steel is supposed to have much less resistance than a copper wire used for the circuits in a particular conduit.
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That may also depend on the type of construction.

Isolated steel conduit is one thing (ran across a brick or masonry wall, for example), but steel conduit fastened every 10' to steel framing can be completely different.
 
don_resqcapt19 said:
Take a look at figure 3 in this article. It gives the lengths of conduit that are acceptable for use as an EGC and the acceptable lengths of wire type equipment grounding conductors.
This information was developed by the Georgia Institute of Technology for the Steel Tube Institute. The Steel Tube Institute also has a free software program named GEMI to do these types of comparison calculations for equipment grounding paths.
Click to expand...
so, is this another 'study' by a manufacturer , of it's own product Don ?

~RJ~
 
I saw something similar a long time ago except it was in an IEEE Color Book and the table values were from performance testing. They built it in an industrial plant and ran the actual current throught it. Conduit, EMT RGS IMC, was much better than equivalent copper wire because the impedance is lower. My guess would be that the magnetic field cancellation is much better in conduit than the cable or open conductor (they tested cable tray which was worst).

Current flow over the length of the wire drops Voltage and has a Voltage raise (of the EGC or neutral above 0 or Earth), but the calculated values can only be a guide. It's an abritrary exercise to say the fault current will be 400% and the Voltage raise cutoff is 40 Volts.

More likely in the field, the fault will not be bolted and have its own high impedance, while the remote outlet or load could be either attached to plywood or bolted to the I beam. The fault may not be a nice 60 cycle sinewave but have its own high frequency components.

What you want is either faster clearing time or less Voltage raise at the remote point. Conduit is superior in all cases. The IEEE testing table I saw was for remote Voltage raise under bolted fault.

I load tested a very iffy EGC to a remote outdoor point that had multiple materials, very corroded EMT inside the plant, an original very old RGS run under the road, then probably 800 ft of triplex on poles. I was very surprised how well it tested. The EMT attached to the steel plant and then the RGS was probably 0, and any drop was in the triplex. That was after I attached the triplex EGC to the RGS pole riser.
 
drcampbell said:
I also find it odd that there's no indication of how the conduit segments are joined. I would expect the couplers to have a significant influence on the overall impedance.
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I agree. I think it makes a huge difference. Also, I would never use conduit as EGC simply because of the number of occasions I've seen EMT disconnected at a coupling.

Now threaded GRC I wouldn't have a problem with.

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winnie said:
Whereas I use EMT for the neutral whenever I can! (Tong firmly in cheek)
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Lol. Seriously, though, that could theoretically happen. Say a ground wire touches a neutral wire somewhere, that could potentially put a 120 volts on the load side of a disconnect conduit.

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