Running a separate wire for grounding

Although I do not understand the reasoning for this code rule, I always understood that if you have the old 2 prong outlets, fed from old cable, which does not have a ground wire, that you can NOT run a single green wire from the breaker box ground buss, to the ourlet(s), to ground them. Yet, I saw someone suggest doing this on a blog. Of course blogs are like this newsgroup, and are not always good information.
As I said, I think this rule is sort of senseless, because as long as the added wire was of the same gauge, connected to the ground buss in the panel, it would make safer wiring. Of course replacing the old cable is the preferred method, that is not always possible without ripping a house apart.
Just wondering if anyone has any further information about doing this? Is this ever allowed for any reasons?
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If the old wiring is in conduit(metal), I have seen the conduit itself used as the ground for the new three-holers.
I'm not sure what code has to say about that.
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On Tue, 15 Nov 2016 05:23:07 -0800 (PST), snipped-for-privacy@gmail.com wrote:

Conduit has always been legal for grounding, but it has to be connected properly.
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On 11/15/2016 08:14 AM, snipped-for-privacy@unlisted.moo wrote:

Correct. It is legal and code to due so, but I have seen so many instances where the fittings come apart...usually due to jarring....that when I wire anything with conduit I also use a bare copper wire inside as an additional ground.
I had one connection where the conduit was physically intact but through the years oxidized and gave a less than satisfactory ground of several ohms. It might still have been safe but obviously I re-did it and made sure all the grounds in my house were approx .1 ohms or so
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On Tuesday, November 15, 2016 at 7:42:34 AM UTC-5, snipped-for-privacy@unlisted.moo wrote:

Why do you think it is not allowed by code?
As far as I know, and this is as of 2013, when I asked a similar question, a separate ground was allowed to be run to a junction box to supply a ground that didn't exist, as opposed to running a new length of grounded Romex.
Please cite the section of code that doesn't allow it.
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On Tuesday, November 15, 2016 at 9:24:40 AM UTC-5, DerbyDad03 wrote:

+1
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Although I do not understand the reasoning for this code rule, I always

Actually I think the code does allow a separate ground wire to be installed in order to ground non-grounded boxes and devices. However for the work i nvolved to install the separate ground wire you might as well install a new cable with the ground wire in it.
John Grabowski http://www.MrElectrician.TV
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On Tue, 15 Nov 2016 06:41:33 -0600, snipped-for-privacy@unlisted.moo wrote:

You can run a separate grounding wire in old work but if it is smaller than #6, it needs physical protection if it is not fished into a wall cavity. I have seen a #6 solid run through attics and crawl spaces with #12 drops going to the boxes in the walls but, as John says it is almost as much work as totally rewiring. This was an historic building. There was a time 40 years or so ago that you could just run a wire to any cold water pipe and be done. It is in the 75 code. I am not sure exactly when that changed but it was gone in 87.
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On Tuesday, November 15, 2016 at 7:42:34 AM UTC-5, snipped-for-privacy@unlisted.moo wrote:

That would depend on how you run that additional ground wire. As long as it's compliant with code, eg not a 20 gauge wire run around the whole house, it's OK.

There is no such rule.
because as long as

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