old romex - smaller ground conductor

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Mr. Spinosa

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Tacoma, Washington, USA
I've worked on several projects where the older wiring has a very small equipment ground conductor, probably a size 16 AWG. - I have always just connected this existing ground conductor, when present, to my replacement receptacles. However, I am now re-thinking this. It is definitely undersized per current code standards as 250.122 lists a size 14 AWG for 15 amp rated circuits. Anyone have any thoughts on this? Have I done something dangerous? I am thinking I could take a resistance measurement from the furthest receptacle back to the service panel and see what I get. Then what would the maximum safe resistance be? Hmmm.... I would appreciate any thoughts on this. Thanks in advance.
 
Thank you! I've got a Fluke 1507 insulation tester. (I think that's the correct model number.) However, I don't recall using it for that type of situation. Maybe I can look into that. (If so, I'll have to figure out how to do a proper test.)
 
Mr. Spinosa said:
I've worked on several projects where the older wiring has a very small equipment ground conductor, probably a size 16 AWG. - I have always just connected this existing ground conductor, when present, to my replacement receptacles. However, I am now re-thinking this. It is definitely undersized per current code standards as 250.122 lists a size 14 AWG for 15 amp rated circuits. Anyone have any thoughts on this? Have I done something dangerous? I am thinking I could take a resistance measurement from the furthest receptacle back to the service panel and see what I get. Then what would the maximum safe resistance be? Hmmm.... I would appreciate any thoughts on this. Thanks in advance.
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well, it needs to safely flow enough to trip the breaker. that's its job.

a bare 16ga. will handle 20 amps pretty well. it takes 117 amps to fuse it.
#14 is 166 amps, and a #12 copper is 235 amps.

and NOT hooking it up, is a significant safety hazard.

you'll not be about to measure the delta between #14 and #16 copper with any
equipment most of us are likely to have on hand. you are looking at maybe an
ohm and a half divided by 20, so .08 ohms or so. the leads on your fluke are a
quarter ohm usually.

AWG wire size (solid)Diameter (inches)Resistance per 1000 ft (ohms)
160.05084.016
140.06402.525
120.08081.588
100.10190.999
 
That was legal until sometime in the late 60's. There is no code requirement to replace the existing wiring. It is my understanding the the NM used in Canada still has the smaller EGC.
 
Fulthrotl said:
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well, it needs to safely flow enough to trip the breaker. that's its job.

a bare 16ga. will handle 20 amps pretty well. it takes 117 amps to fuse it.
#14 is 166 amps, and a #12 copper is 235 amps.
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That is a bit lower than what the table I have shows for the 5 second fusing current.
I have 158 for the #16, 253 for the #14 and 401 for the #12.

I also have the 3 cycle damage current for insulated conductors. This is the current that will not raise the temperature of the copper above 150°C where the current flows for 3 cycles.

Those currents are 600, 1000 and 1550. It would be expected that the OCPD would clear the fault in less than 3 cycles for currents that high.
 
Mr. Spinosa said:
Fulthrotl, I greatly appreciate your reply and the information you sent me! - Thank you!
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I would look at it this way, though goodness that you even have an EGC. Just so it is intact and functional all the way back to the service entrance. If it is open or compromised then that would present an issue.
 
Mr. Spinosa said:
I've worked on several projects where the older wiring has a very small equipment ground conductor, probably a size 16 AWG. - I have always just connected this existing ground conductor, when present, to my replacement receptacles. However, I am now re-thinking this. It is definitely undersized per current code standards as 250.122 lists a size 14 AWG for 15 amp rated circuits. Anyone have any thoughts on this? Have I done something dangerous? I am thinking I could take a resistance measurement from the furthest receptacle back to the service panel and see what I get. Then what would the maximum safe resistance be? Hmmm.... I would appreciate any thoughts on this. Thanks in advance.
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What size EGC is required for a 60 amp circuit? How about a 100 amp circuit?

This is the exact same concept, the EGC only carries current for a short time during a groundfault.


As far as the change from reduced sized EGCs for smaller circuits to full sized one of our past moderators was on a CMP and his recollection was it had to due with physical damage and not an electrcal reason.
 
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