NM with undersized EGC


Old 06-11-06, 09:51 AM
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Location: Oregon
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NM with undersized EGC

Does anyone know the _history_ of required EGC size in NM cables?

I am in the process of replacing a 10-30R dryer receptacle circuit with a 14-30R dryer receptacle circuit. The 10-30R is the old style where the grounded conductor (neutral) serves double duty as the equipment grounding conductor. I had assumed that this circuit was wired with some form of ungrounded cable or possibly 10/2 cable with the bare ground improperly used as a neutral.

However when I checked the cable, I found that it was real 10/3 NM, with three insulated conductors and a bare ground. However the bare ground appears to be 14ga, not a proper full sized 10ga conductor.

Does anyone know if this was at some point in the past a legal equipment grounding conductor? If this was a legal EGC in the past, would it be considered 'grandfathered' as a suitable EGC so that I could simply replace the receptacle with a proper grounding receptacle?

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Old 06-11-06, 11:33 AM
Join Date: Sep 2003
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I do not know the exact history or the dates involved, but I do know that at one point the ground in NM type cable was allowed to be smaller than the current carrying conductors. The kitchen in my house (wired in 1959) had 12-2 with a smaller ground.

While wiring that was originally code compliant can continued to be used, you cannot replace part of a circuit and not the other. Since you are replacing the receptacle, you must replace the cable as well.
Old 06-12-06, 09:01 AM
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I don't know when the change to full size EGC was, but undersized conductors used to be legal. EGC #16 was allowed for a 20A circuit, and #14 for a 30A circuit.
Old 06-12-06, 09:16 AM
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Thanks for the replies. This is a receptacle replacement, not a circuit extension. I've been doing some more reading on this topic.

406.3(D)(1) states that if an equipment grounding conductor is present, then when a non-grounding receptacle is replaced, a grounding receptacle _must_ be used.

Thus I believe that _if_ this NM cable was legal when installed, it would be appropriate to change to the desired 14-30R grounding receptacle.

However if this NM cable was not legal when installed, then not only could I not use this (undersized by modern standards) EGC for a grounding receptacle, but because of the presence of the undersized EGC I could also not replace the receptacle with a 10-30R. An illegal install is never 'grandfathered'.

That said, if time and money are not constraints on this project, I'll just run new, modern 10/3 MC.

Old 06-12-06, 10:52 AM
Join Date: Apr 2005
Location: Houston, TX
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Undersized EGC

Are we saying that the bare or green wire must match the current carrying conductors?
What if the ground conductor meets the breaker size requirement e.g. 12 AWG conductor with 20A breaker but with 10 AWG black & white?
Seems I've recently seen (especially higher capacity) cables with one size smaller grounds.
Old 06-12-06, 11:12 AM
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The _current_ NEC standard is given in table 250.122. For 15,20, and 30A circuits, the EGC (the bare or green insulated wire) must be the same size as the conductors. But for larger circuits the required EGC is proportionately smaller. For example, a 60A circuit (requiring #6 conductors) is fine with a #10 EGC, and a 200A circuit only requires a #6 EGC, far smaller than the circuit conductors.

The rules are quite clear for a _new_ installation. The EGC must be sized per the breaker rating, and than increased in size depending upon certain situations. For example, if you install a 20A circuit with #10 conductors, then you must install a #10 EGC.


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