Equipment Ground Through Ferrous Conduit

[winnie]

I believe that Ron's answer in post #2 is the best for the OP.

A single conductor in a ferromagnetic raceway is subject to a 'choke effect' where the the raceway increases the impedance of the conductor. We overcome this choke effect by bonding the GEC to the raceway at both ends, permitting current to use the raceway itself as the conductor. (Which also means that ferromagnetic non-conductive raceway systems would be a problem, but I don't think we will see such anytime soon...)

An isolated EGC would be subject to the same choke effect, but at lower frequency and current relative to a GEC handling a lightning surge.

The other reasonable explanation is that the code writers working on the section about adding an EGCs to existing wiring simply never considered the choking effect.

The choking effect is certainly considered with new installations where all conductors are required to be run together, so the situation the OP describes only comes up in retrofit situations.

A third explanation is that since this only comes up in retrofit situations perhaps the code panel felt that any EGC is better than none and didn't want to introduce barriers to adding one.

Jon

 

[Carultch]

(Which also means that ferromagnetic non-conductive raceway systems would be a problem, but I don't think we will see such anytime soon...)

Ferromagnetic insulators are still in scientific research, so it isn't practical for them to become a readily available building material. I don't see any advantage for such a material to ever be used for a raceway. Maybe a transformer core, but not a raceway.

Also, for those who don't know what the trade slang term "choke" means, the formal name is inductance. More specifically, self-inductance.

 

[suemarkp]

Is the OP thinking about 250.130(C) where you run a separate EGC to old ungrounded receptacles to upgrade them to grounding ones? That would be one case where conductors are not run following the same path as the original circuit.

That then drives the question can the individual EGC be run by itself or must it be in one of the approved wiring methods? 300.3(B)(2) and 250.120(C) says you can run the individual EGC without a raceway if you protect it from damage. If you need to protect it from damage, a raceway is permitted (ferrous or non ferrous or even plastic). It doesn't say anything about bonding that raceway at one or both ends. I would expect that even with a fault, the current will be high enough to trip on magnetic trip even if in a ferrous raceway. May not trip in one cycle, but that implies the damage is more limited if the ground path impedance is limited.

 

[Carultch]

That then drives the question can the individual EGC be run by itself or must it be in one of the approved wiring methods?

EGC's that are #6 Cu or #4AL and larger, do not require a raceway for physical protection. #8Cu or #6AL and smaller, require being in a raceway to protect from physical damage. It may require being in a raceway for other reasons, like where it is required to run with the current-carrying conductors.

 

[suemarkp]

You've got a few more options in 250.120(C), but the wording is kind of convoluted...
250.120 (C) Equipment Grounding Conductors Smaller Than 6 AWG. Where not routed with circuit conductors as permitted in 250.130(C) and 250.134(B) Exception No. 2, equipment grounding conductors smaller than 6 AWG shall be protected from physical damage by an identified raceway or cable armor unless installed within hollow spaces of the framing members of buildings or structures and where not subject to physical damage.

If you can run it between open joists or fish it in the wall, you can run a single EGC smaller than #6. You still may need to protect open joist runs from damage with guard boards or some other mechanism. If run on the surface, I agree you need a raceway or cable armor. I wonder what constitutes cable armor in this case? Could it be FMC? Can you buy single conductor AC or MC cable that is green and in a smaller size like #12 or #10?

Using runs of #6 to ground ungrounded outlet boxes seems kind of difficult to terminate in the old small metal boxes. You'd probably want splice points somewhere to run legs of #14 or #12 which just makes more ugly. I'd like to see a bit more leniency in the code for this section such as using whatever methods or protection that are allowed for NM cable should be allowed for a single small EGC if #12 or larger.

 

[steve66]

An engineer once told me many years ago that, like lightning, a dead short (or maybe just an arc) has "a component of all frequencies." I don't understand it and maybe he didn't either.

Any "step" or "square" waveform will include multiple harmonics of the fundamental frequency. That applies to both a repeating square wave, or a single pulse.

If we look at the square corner of a waveform on a scope, and zoom in enough, you are going to see the corner is actually rounded, and there will probably be some ringing of some sort at the corner.

The only way to get a perfect square corner is to have harmonics out to infinity, which is impossible. But many waveforms contain harmonics high enough that they look square when we zoom out enough.

Without higher harmonics, we can only make nice, smooth, curvy waveforms.

 

[LarryFine]

Any "step" or "square" waveform will include multiple harmonics of the fundamental frequency. That applies to both a repeating square wave, or a single pulse.

If we look at the square corner of a waveform on a scope, and zoom in enough, you are going to see the corner is actually rounded, and there will probably be some ringing of some sort at the corner.

The only way to get a perfect square corner is to have harmonics out to infinity, which is impossible. But many waveforms contain harmonics high enough that they look square when we zoom out enough.

Without higher harmonics, we can only make nice, smooth, curvy waveforms.

That's why amplifier "clipping" damages tweeters in speakers.

 

[wyreman]

You've got a few more options in 250.120(C), but the wording is kind of convoluted...
250.120 (C) Equipment Grounding Conductors Smaller Than 6 AWG. Where not routed with circuit conductors as permitted in 250.130(C) and 250.134(B) Exception No. 2, equipment grounding conductors smaller than 6 AWG shall be protected from physical damage by an identified raceway or cable armor unless installed within hollow spaces of the framing members of buildings or structures and where not subject to physical damage.

If you can run it between open joists or fish it in the wall, you can run a single EGC smaller than #6. You still may need to protect open joist runs from damage with guard boards or some other mechanism. If run on the surface, I agree you need a raceway or cable armor. I wonder what constitutes cable armor in this case? Could it be FMC? Can you buy single conductor AC or MC cable that is green and in a smaller size like #12 or #10?

Using runs of #6 to ground ungrounded outlet boxes seems kind of difficult to terminate in the old small metal boxes. You'd probably want splice points somewhere to run legs of #14 or #12 which just makes more ugly. I'd like to see a bit more leniency in the code for this section such as using whatever methods or protection that are allowed for NM cable should be allowed for a single small EGC if #12 or larger.

I thought you can't run to any old cold water pipe for the 2 prong like they used to do, for many reasons, but not the least that you have to run all the way to the water entrance. And then everybody is using PEX tubing now as well

 

[suemarkp]

Did anyone mention water pipe?? 250.130(C) lists the following places where you can terminate this separate equipment ground used to change old ungrounded circuits to grounded:
(1) Any accessible point on the grounding electrode system as described in 250.50 (this could include the main water pipe if within 5' of where it enters and it has a GEC run there).
(2) Any accessible point on the grounding electrode conductor
(3) The equipment grounding terminal bar within the enclosure where the branch circuit for the receptacle or branch circuit originates
(4) An equipment grounding conductor that is part of another branch circuit that originates from the enclosure where the branch circuit for the receptacle or branch circuit originates
(5) For grounded systems, the grounded service conductor within the service equipment enclosure
(6) For ungrounded systems, the grounding terminal bar within the service equipment enclosure

 

[wyreman]

I like it when they wrap a big clamshell around the service riser w a #14 green for the cable guy.

but you know I've seen those #14's carry the whole unbalanced load when the white wire goes away, until the green wire goes away =0

 

[nickelec]

So if you were to install the GEC in aluminum conduit it would not need to be bonded

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[Carultch]

So if you were to install the GEC in aluminum conduit it would not need to be bonded

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You'd still have to make it electrically continuous with some part of the either the EGC or GEC system, since it is a non-current carrier with a chance of becoming energized. Just as you'd have to do with any metal raceway regardless of application.

The difference it, with an aluminum raceway, it doesn't need a bonding bushing on both ends, like a steel raceway would, when carrying the GEC. Aluminum is not ferromagnetic, and doesn't multiply the inductance of the GEC in the same way that a steel raceway would. That is the reason why the GEC ferrous raceway gets special bonding requirements, that aren't ordinarily required. Standard locknuts alone could bond an aluminum raceway of the GEC to an enclosure wall. Whereas with a GEC in a ferrous raceway, the standard locknuts are only used mechanically, and the bonding bushing is required for the GEC application.