EMT as ground - For how much amperage?

jim dungar

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Wisconsin
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Engineer
Well .. from what I've understood in the past, for one thing the fault current would run on the surface of the conduit not through it, that's just the nature of electricity, with that said I've heard that some conductors probably major utility power lines have at times been basically hollow tubing, because just like a conductor the circular mils of the diameter is what determines the current flow ease.
What you describe is called skin effect. At 60Hz it is negligible for conductor sizes to about 250kCMIL. At 750kCMIL the multiplier is only around 1.2, this is why we pretty much ignore skin effect for most building circuits.

fixed cable reference.
 
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Another C10

Electrical Contractor 1987 - present
Location
Southern Cal
Occupation
Electrician NEC 2020
we pretty much ignore skin effect for most building circuits.
I was taught 30+ years ago which seemed reasonable and at the same time amazing that current flow, voltage etc .. does not necessarily move from molecule to molecule throughout the entire conductor, it was expressed that, not like water in a hose electricity actually travels on the surface or skin of the object therefor the rest of the conductor is basically useless, the outer diameter of the conductor (circular mils ) aka gauge establishes the safe surface area necessary for transporting specific demands of energy.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
I was taught 30+ years ago which seemed reasonable and at the same time amazing that current flow, voltage etc .. does not necessarily move from molecule to molecule throughout the entire conductor, it was expressed that, not like water in a hose electricity actually travels on the surface or skin of the object therefor the rest of the conductor is basically useless, the outer diameter of the conductor (circular mils ) aka gauge establishes the safe surface area necessary for transporting specific demands of energy.
Yes, but the current 'penetration depth' at 60hz is fairly large, and is taken into account in NEC tables. You need to exceed 1000kcmil before you would even consider hollow conductors. Skin effect is more pronounced in rectangular conductors like bus bar, particularly when they are paralleled for large currents. Skin effect is not an issue for tubular conductors with relatively thin walls, like are used in open air substations or conduit used as a grounding conductor.

Almost every electrician I know has been taught skin effect as something that needs to be considered, but most 'power' engineers I know have never dealt with it. Another, often overlooked concern is proximity effect which also impacts the current density in conductors parallel to each other.
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Skin effect virtually never impacts the work of most inside electricians.

It might barely be an issue for large 400Hz installations.

It certainly is an issue for large scale power distribution.

For RF use (coax and antennas and the like), skin effect is huge.

-Jon
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
For RF use (coax and antennas and the like), skin effect is huge.
Back in high school, I was in the Radio Club, and one of our field trips was to a local TV station. The main transmission line, which we saw, was, we were told, made of 1" copper pipe and soldered fittings inside 3" copper pipe and soldered fittings (I'm not 100% sure of the sizes) with nylon blocks to maintain centering. The visible parts, and the soldering, looked just like standard plumbing pipe and fittings.
 

Another C10

Electrical Contractor 1987 - present
Location
Southern Cal
Occupation
Electrician NEC 2020
Almost every electrician I know has been taught skin effect as something that needs to be considered, but most 'power' engineers I know have never dealt with it.
I may be misunderstanding your valuable input, when I mention skin or surface travel, I'm simply relaying what I was taught about how electricity travels on a conductor, granted I only know what I've learned or how I was taught but it sounds like your mentioning that aside from on the skin of a conductive material, also some depth is included, which is interesting in it self, I remember my ol wise instructor mentioning, " If you could measure the inside of an energized conductor there would be no voltage" obviously the whole class of young minds were very doubtful but he seemed pretty persistent on his past learning and research and pushed it to us, He said " Electricity flows on the outer surface of objects" which is why today I explained what I believe to everyone here, am I right, ..I'd like to think so. If not well fortunately I have a great classroom right here.

Thanks folks for sharing your wisdom and lessons so freely.
 

paulengr

Senior Member
Skin effect virtually never impacts the work of most inside electricians.

It might barely be an issue for large 400Hz installations.

It certainly is an issue for large scale power distribution.

For RF use (coax and antennas and the like), skin effect is huge.

-Jon
It definitely affects 400 Hz installations. You have to oversize the conductors.

It does not affect power distribution except in some cases again bus bar is oversized relative to a theoretical calculation.

But except at the generation plant between the generator and the GSU we don’t care.

Well before that point the issue becomes the economics of building overhead transmission lines. As power goes up, heating is proportional to the square of the current (I squared-R). Conductor size goes up to compensate but thermal transfer is roughly proportional to surface area or diameter while cost and weight is proportional to cross sectional area. So economics suggests more conductors, not larger ones. Then larger poles, then more of them. All the while we are bumping up against all kinds of limitations...fuses and breakers are only so big. Once you get above a few thousand amperes everything becomes difficult at best, and if possible it makes a world of difference to stay in the hundreds of amps. Similarly there is sort of a practical conductor limit on round conductors around 500-1000 kcmil.

Hollow conductors aren’t even in the catalogs except two cases. Tubular bus used to be popular for weight but outdoor overhead bus is not easy to work with or modify so it’s not as popular as it once was. Second some utilities are using OPGW for static lines where the center conductor is a hollow aluminum tube stuffed with fiber. This is a fault current conductor only.

The economics drives us towards higher voltages. Then the GSU cost goes up and the insulators and pole heights increase modestly but it’s not even a linear increase compared to fighting I-squared. So transmission lines in the US run 115 kv, 230 kv, and even higher voltages. Currents are reasonable.

So skin effect is normally an issue but a minor one taken into account in ampacity tables because we rarely get to the currents and dimensions where it matters.
 

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
Engineer
He said " Electricity flows on the outer surface of objects" which is why today I explained what I believe to everyone here, am I right, ..I'd like to think so. If not well fortunately I have a great classroom right here.
His was likely just a simplified explanation for a complicated concept. Like current follows the path of least resistance or is always trying to get to ground.

You ahave the right concept, but think of it as current flows on the outer 1/2" of a conductor's skin, not just the surface. So it doesn't have much impact until the conductors are about 1" in dia.
 

GoldDigger

Moderator
Staff member
Well .. from what I've understood in the past, for one thing the fault current would run on the surface of the conduit not through it, that's just the nature of electricity, with that said I've heard that some conductors probably major utility power lines have at times been basically hollow tubing, because just like a conductor the circular mils of the diameter is what determines the current flow ease.
This phenomenon is called the skin effect, and it is caused by the changing magnetic field within the conductor causing a repulsion of the moving electrons from the center of the conductor. The effect is frequency dependent, and at 60Hz can be ignored except for very large diameter conductors. And even then it is only a few percent difference in resistance between AC and DC.
At the high frequencies involved with lightning related currents, on the other hand, it can be orders of magnitude in impedance.
A secondary benefit to using hollow tubes is that it provides a proportionally greater surface area for dissipation of heat. This is a practical effect which has to be taken into account when calculating the ampacity of conductors.
 

GoldDigger

Moderator
Staff member
I may be misunderstanding your valuable input, when I mention skin or surface travel, I'm simply relaying what I was taught about how electricity travels on a conductor, granted I only know what I've learned or how I was taught but it sounds like your mentioning that aside from on the skin of a conductive material, also some depth is included, which is interesting in it self, I remember my ol wise instructor mentioning, " If you could measure the inside of an energized conductor there would be no voltage" obviously the whole class of young minds were very doubtful but he seemed pretty persistent on his past learning and research and pushed it to us, He said " Electricity flows on the outer surface of objects" which is why today I explained what I believe to everyone here, am I right, ..I'd like to think so. If not well fortunately I have a great classroom right here.

Thanks folks for sharing your wisdom and lessons so freely.
Your teacher was wrong. Sounding plausible, but wrong.
Now in the case of static electricity the charge on a conductive object does reside entirely at the surface of the object. But that is not a consideration here.

P.S: The question of how current can flow in a conductor with no electric field to drive it is actually a very sophisticated question that I do not want to try to answer here. :)
 

tom baker

First Chief Moderator
Staff member
At one time the Steel Tube Industry had nice software program "GEMI" that calculated the length of RMC, EMT with and with out a supplemental EGC. It was based on studies done at the Georgia Tech on EMT grounding, some of this was in Soares Book on Grounding. I don't know if the GEMI program is still available, I have copy but it probably won't run on Win 10.
 
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tom baker

First Chief Moderator
Staff member
Well, here is the GEMI program
And read this report on the development of GEMI by Richard Loyd (who recently passed away)
 

Another C10

Electrical Contractor 1987 - present
Location
Southern Cal
Occupation
Electrician NEC 2020
Your teacher was wrong. Sounding plausible, but wrong.
Now out of respect for a person I though and still do think was a pretty intelligent educator of electricity, granted products get upgraded and theories turn more into facts based on further research studies.

This is a product that to me contradicts the depth theory involved upon a conductor regarding current flow and its nature. I'm just posting this for your input regarding the concept were in this case a 20 A current flow can travel on a paper thin conductor. the diameter is not the concern in this design but notice the circular mills are basically transformed or translated into a wide flat mass. seems pretty Interesting ..

flat cable.jpg
 

kwired

Electron manager
Location
NE Nebraska
Now out of respect for a person I though and still do think was a pretty intelligent educator of electricity, granted products get upgraded and theories turn more into facts based on further research studies.

This is a product that to me contradicts the depth theory involved upon a conductor regarding current flow and its nature. I'm just posting this for your input regarding the concept were in this case a 20 A current flow can travel on a paper thin conductor. the diameter is not the concern in this design but notice the circular mills are basically transformed or translated into a wide flat mass. seems pretty Interesting ..

View attachment 2552844
If still same CSA you still have same amount of copper. If flattened out you have more surface area, but less "depth".

Then as mentioned this effect is fairly negligible especially at 60Hz until you get into larger conductor sizes. We typically start paralleling conductors below the sizes mentioned anyway as it usually cost less than a single large conductor.
 
I looked it up a couple of days ago- at 60Hz, the skin effect is around 8mm (0.31")- the conductor would have to be over around 5/8" diameter, which looks like over 1500 kcir, for it to matter. (I may be off there, not enough coffee.)

Backing up the thread, the table tells us that over around 400', the conduit alone can't be relied upon for fault clearing. OTOH, if conduits are attached to building steel or are run/used in groups, it might be OK.
 
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