pvc conduit

[edl]

i was told that you need to "bring the ground with you" when using pvc conduit

i am a rank ameteur regarding electricity - what does this mean?

thanks!

 

[nehog]

If you are pulling stranded wire, you need to pull at least a black, a white, and a green. The green is your ground. If using metallic conduit, you can sometimes get away without the green wire. (Personally I'd not, it is just too easy to pull the additional green and be safe.)

 

[oleguy]

metal conduit cannot be used for the ground anymore.you have to run a seperate ground,and bond all metal boxes and conduit.

 

[walrus]

metal conduit cannot be used for the ground anymore.you have to run a seperate ground,and bond all metal boxes and conduit.

Huh, what code section is that? and when did it change?

 

[Friartuck]

Sections 250-70 through 250-80 in the NEC show options for grounding with enclosures, piping, and raceways. Remember the intent of the ground is to provide a path if the piping gets disconnected somehow, like when an earthquake flattens the building.

 

[oleguy]

code does say it can be used for eq.grounding conductor.but most places won't allow it.beacause lets say it is clamped to a brick wall,and a coupling is loose or comes apart,you have no ground protection.hence you could get hurt.thats why i won't do it.just bond it and don't be a cheap scate with your life and run a ground.

 

[fusionspecialists]

oleguy,

I understand that a drilled and tapped ground screw must be used to bond a ground conductor to a box -- a self tapping screw cannot be used nor can a screw that mounts the box to the wall. It appears that the NEC does not require the cover to be bonded, but some people prefer to bond it when receptacles are mounted to the cover. Do these people drill and tap a hole in the cover? Or do they bond to the ground terminal on the receptacle and rely on the self-grounding receptacle to bond the cover?

 

[oleguy]

the recp mounting screw will bond the cover.that is the screw that holds the cover to the recp.when the box is bonded the cover is grounded also.unless the cover is plastic.
if you need to drill and tap,use a #10-32 tap and #21 bit.

 

[walrus]

code does say it can be used for eq.grounding conductor.but most places won't allow it.beacause lets say it is clamped to a brick wall,and a coupling is loose or comes apart,you have no ground protection.hence you could get hurt.thats why i won't do it.just bond it and don't be a cheap scate with your life and run a ground.

a #12 copper wire or properly installed 1/2 emt used as ground, which one clears a fault better?

And if its allowed by code how can it be denied?

 

[oleguy]

if you read what i said,the code says it is ok.but most industrial places i have worked specifie you need to run a seperate ground.and depoending on the number of ckts involved,more than one ground may be required.such as when you pull several ckts to a common pull point that go to other conduit runs.do what you want but i belive in running a ground wire.

 

[walrus]

metal conduit cannot be used for the ground anymore.you have to run a seperate ground,and bond all metal boxes and conduit.

I can read what you said, it's pretty clear

 

[oleguy]

i agreed with that it is ok in the book.ialso said most places i have worked want a ground wire run.you did read that also did you not?just because it says you can use emt for an eq ground,doesn't mean i can't run a ground wire.

 

[walrus]

i agreed with that it is ok in the book.ialso said most places i have worked want a ground wire run.you did read that also did you not?just because it says you can use emt for an eq ground,doesn't mean i can't run a ground wire.

I quoted what you said, run all the ground wires you want

 

[Charles (in GA)]

I've always argued that its too easy for the conduit to become disconnected in some way. The nut that holds the conduit end in the box, the set screw in the end works loose and the conduit slips out, whatever, and you lose the ground. It may be way up high where you cannot see it or find it. I run a ground wire directly to the ground screw on the receptacle, which is attached to the metal covers with three screws (not just the center one like most lazy folks will do).

Charles

 

[walrus]

I've always argued that its too easy for the conduit to become disconnected in some way. The nut that holds the conduit end in the box, the set screw in the end works loose and the conduit slips out, whatever, and you lose the ground. It may be way up high where you cannot see it or find it. I run a ground wire directly to the ground screw on the receptacle, which is attached to the metal covers with three screws (not just the center one like most lazy folks will do).

Charles

Argue that all you want, you may be right? But to say its not legal to use metal conduit as a ground is wrong.

 

[Aceman]

If you are pulling stranded wire, you need to pull at least a black, a white, and a green. The green is your ground. If using metallic conduit, you can sometimes get away without the green wire. (Personally I'd not, it is just too easy to pull the additional green and be safe.)

So if I pull solid wire, I don't need a ground in pvc conduit?

 

[BackAgain]

I've always argued that its too easy for the conduit to become disconnected in some way. The nut that holds the conduit end in the box, the set screw in the end works loose and the conduit slips out, whatever, and you lose the ground. It may be way up high where you cannot see it or find it. I run a ground wire directly to the ground screw on the receptacle, which is attached to the metal covers with three screws (not just the center one like most lazy folks will do).

Charles

I agree, in my shop they used the 1/2" EMT as the ground, and I don't like it. Way too easy to come apart. Sure it meets code, but what's another 10 cents per foot? I like the idea of using EMT as the ground AND having a ground wire, beat that!

I'm really surprised the NEC allows the EMT to be used as ground, as picky as they are on everything else. For example, the neutral wire for an outlet has to be pigtailed to the next one, you can't just screw it into the outlet and continue. Not "good enough".

 

[Charles (in GA)]

Argue that all you want, you may be right? But to say its not legal to use metal conduit as a ground is wrong.

Never said it wasn't.

Charles

 

[Steve from Socal]

In PVC it is a moot point; you would need a ground in each run. As another data point all my machine wiring is home runs in flex or EMT and they all have ground wires to the ground rod. If you plan on using a Tig welder with high frequency a hardened ground system is needed. My lathe's DC drive is 10 grand and ground wires are cheap!

Steve

 

[dwilliams35]

Copper wire presents a lot better ground anyway: corrosion will degrade the continuity in the pipe a lot quicker than it would a copper ground.. I haven't run EMT without a separate ground in years...

 

[Norcal]

In PVC it is a moot point; you would need a ground in each run. As another data point all my machine wiring is home runs in flex or EMT and they all have ground wires to the ground rod. If you plan on using a Tig welder with high frequency a hardened ground system is needed. My lathe's DC drive is 10 grand and ground wires are cheap!

Steve

If they are just running grounding conductors to a rod they are cruising to kill someone,NEC requires the grounding conductors all to be bonded together, as there is no way in h*** it will ever open a overcurrent device going to just a ground rod.

 

[Steve from Socal]

If they are just running grounding conductors to a rod they are cruising to kill someone,NEC requires the grounding conductors all to be bonded together, as there is no way in h*** it will ever open a overcurrent device going to just a ground rod.

The ground cables all run to the service ground rod, they are bonded at that point. I don't know of any residential service that allows multiple ground rods?

Steve

 

[Norcal]

The ground cables all run to the service ground rod, they are bonded at that point. I don't know of any residential service that allows multiple ground rods?

Steve

A stunt is for some CNC manufacturers to require grounding to a driven ground rod at the machine, a bad practice & that is what I thought you were refering to.

 

[Steve from Socal]

A stunt is for some CNC manufacturers to require grounding to a driven ground rod at the machine, a bad practice & that is what I thought you were refering to.

I had a feeling you were talking about that; I had three machines that did require that, all were rather big, 80-120amp service on 480v.

I was talking about my own home garage in the original comment.

Steve

 

[fusionspecialists]

Norcal and Steve in SoCal; you both sound like you are electricians. I am not. I ask this only to get more education on this subject.

Why are you saying that a device that is grounded to a rod and not bonded back to the panel will not have a sufficient ground? That's what a ground is, isn't it? The earth seems to me like a pretty good return path for any ground faults. That panel itself is connected to the earth, and so is the grounding rod of a piece of equipment like the ones you described.

The way I see it, ground is ground; the lowest potential that an electrical system bound to the Earth's surface can have.

Please tell me why this hypothetical situation would be unlikely and would therefore be a safety hazard: a single phase machine (for simplicity's sake) is wired with a neutral and a phase conductor coming from the panel. The neutral conductor is firmly connected to the neutral buss bar and the phase conductor is connected to an overcurrent device. There is a ground conductor, but it doesn't go back to the panel. Instead, it goes into a grounding rod. Let's assume that the grounding rod meets the minimum requirements that the main panel's grounding rod meets, i.e. equal or lesser impedance as measured by a three- or four-pole ground impedance tester.

Now let's pretend that the insulation in the single phase machine fails and the phase conductor contacts the machine's housing. In my mind, the current would flow through the housing, through the ground conductor, and into the earth via grounding rod. This would be a very low impedance path (same or lower impedance than a ground path that first goes all the way back to the main panel), causing a large amount of current to flow and quickly tripping the overcurrent device.

That's my theoretical situation/question, I am open for someone who actually installs electrical systems every day to explain what practical reasons there are for this not to be true.

NEC requires the grounding conductors all to be bonded together

I am guessing that there is a practical reason for this; so that periodic ground rod testing involves testing just one ground rod, not dozens of them?

Thanks!

 

[VHF]

I'm not an electrician but I'll chime in anyway (and then let the real electricians come along and straighten things out!)

In most places, the ground is not that good of conductor. The impendence of a ground rod might be 25 ohms... or several hundred ohms--even with two ground rods. When the insulation fails in the hot phase conductor and energizes the outside of the metal machine, it is probable that not enough current will flow through the earth to trip the breaker.

To complete a circuit, the fault current needs to get back to its source, which is ultimately the utility transformer. Therefore, what is needed is a good low impedance (as in close to zero ohms) connection back to the transformer. Earth just isn’t a good enough conductor. That is why grounding conductors eventually tie to the neutral at the service entrance—so that enough current will flow back to the transformer to trip the breaker and clear the fault.

In fact, ground rods don’t really play a part in clearing faults—their purpose is to ensure that the grounding conductors (and the metal outsides of grounded equipment) remain at the same potential as the earth—otherwise the whole system could “float” relative to earth and you might get a tingle when you touched your table saw. Ground rods also play a role in lightening protection.

Hope this helps.

 

[fusionspecialists]

Thanks VHF,

I'm just making some observations here:

1. The transformer is grounded to the earth just like the building is. I don't think there is a metallic ground conductor that "bonds" the building to the transformer. I think the earth provides that link (maybe if the transformer is very close to the building, they do run a wire to it for good measure).

2. Each transformer is grounded to the earth with a rod.

3. The utility only runs three wires from the power plant, for three phases. They rely on the earth as the return path. Some towers do have a ground wire running along them, but it is on the top of the tower and primarily for the purposes of attracting lightning. I don't think they rely on this for a return path.

4. I think the earth is a good conductor, but as you mentioned, sometimes the ground rod isn't in good contact with the earth. One of the conditions of my previous argument was that the ground rods were all confirmed to be good by accepted testing methods, i.e. low impedance. NEC requires 25 ohms and some other bodies require it to be no more than a few ohms.

 

[rockwithjason]

I am a wireman by trade so i will throw my 2 cents in

1. the transformer is bonded to the building via a grounding electrode conductor. this is required

2. it's true that the transformer is grounded but not always by means of a ground rod. in large buildings we often carry a ground from the main service to transformer in another area and then bond that ground with a building steel ground or a ufer ground. the important part is that they are all bonded together with the proper size wire.

3. three phase does not require a separate return path. the ground on the towers is for static aura control and for lightning protection only.

VHF is correct in his statements

4. the earth is an unreliable ground path at best. the resistance of the earth ground path varies with moisture, soil chemistry and the quality of the ground method. here in vegas even with ufer grounds and chemical ground pots we have a hard time getting below 25 ohms

 

[fusionspecialists]

Cool. Thanks Jason. Just the info I was looking for.

 

[Lovely World]

Metal conduit cannot be used for the ground anymore.you have to run a seperate ground,and bond all metal boxes and conduit.

 

[walrus]

Metal conduit cannot be used for the ground anymore.you have to run a seperate ground,and bond all metal boxes and conduit.

Lets see the code section that says that?

 

[Falcon67]

Lets see the code section that says that?

Maybe this 2005 revision sparks the comment?? -

"A substantial addition to Article 250.118(5)(D) from the 2002 edition pertaining to flexible metal conduit is also included. The code requirement for using flexible metal conduit as a grounding conductor was enhanced to include the following: a. The conduit is terminated in fittings listed for grounding.
b. The circuit conductors contained in the conduit are protected by over-current devices rated at 20 amperes or less.
c. The combined length of flexible mental conduit and flexible metallic tubing and liquid-tight path does not exceed 6 feet (1.8m)."
d. Where used to connect equipment where flexibility is necessary, an equipment-grounding conductor shall be installed.
These enhancements increase the specificity of the code requirement.



Might be others - hard to Google this stuff.

 

[JBurgess]

My local amendments on grounding

Revision to NEC Article 250.118
250.118. Types of Equipment Grounding Conductors
The equipment grounding conductor run with or enclosing the circuit conductors shall be one or
more or a combination of the following:
1. A copper, aluminum, or copper-clad aluminum conductor. This conductor shall be solid or
stranded; insulated, covered, or bare; and in the form of a wire or a busbar of any shape.
2. Threaded Rigid metal conduit and fittings.
3. Threaded Intermediate metal conduit and fittings.
metal conduit in the same ground return path does not exceed 6 ft
(1.83 m).

DELETE 4 through 8
4. Electrical metallic tubing.
5. Flexible metal conduit with an individual equipment grounding conductor and where both
the conduit and fittings are listed for grounding.
6. Listed flexible metal conduit that is not listed for grounding and meeting all the following
conditions.
a. The conduit is terminated in fittings listed for grounding.
b. The circuit conductors contained in the conduit are protected by overcurrent devices
rated at 20 amperes or less.
c. The combined length of flexible metal conduit and flexible metallic tubing and
liquidtight flexible metal conduit in the same ground return path does not exceed 6 ft
(1.83 m).
d. The conduit is not installed for flexibility.
7. Listed liquidight flexible metal conduit meeting all the following conditions.
a. The conduit is terminated in fittings listed for grounding.
b. For trade sizes 3/8 in. through ½ in., the circuit conductors contained in the conduit
are protected by overcurrent devices rated at 20 amperes or less.
c. For trade sizes ¾ in. through 1¼ in., the circuit conductors contained in the conduit
are protected by overcurrent devices rated not more than 60 amperes and there is no
flexible metal conduit, flexible metallic tubing, or liquidtight flexible metal conduit in
trade sizes 3/8 in. or ½ in. in the grounding path.
d. The combined length of flexible metal conduit and flexible metallic tubing and
liquidtight flexible metal conduit in the same ground return path does not exceed 6 ft
(1.83 m).
e. The conduit is not installed for flexibility.
8. Flexible metallic tubing where the tubing is terminated in fittings listed for grounding and
meeting all the following conditions.
a. The circuit conductors contained in the tubing are protected by overcurrent devices
rated at 20 amperes or less.
b. The combined length of flexible metal conduit and flexible metallic tubing and
liquidtight flexible

9. Armor of Type AC cable as provided in Section 333-21.
10. The copper sheath of mineral-insulated, metal-sheathed cable.
11. The metallic sheath or the combined metallic sheath and grounding conductors of Type MC
cable with an individual equipment grounding conductor.
12. Cable trays as permitted in Sections 318-3(c) and 318-7.
13. Cablebus framework as permitted in Section 365-2(a).
14. Other electrically continuous metal raceways listed for grounding