2#10 with 1#12 EGC?

Status
Not open for further replies.
J

JDBrown

Senior Member
Location
California
Occupation
Electrical Engineer
Is there some Code section that allows the use of a #12 AWG (copper) EGC with #10 AWG line and neutral conductors? According to one of the older guys I work with it used to be allowed (he still marks plans that way out of habit, and then has to go back and correct himself), although I'm not sure how long ago that was. The oldest NEC book I have available is 1993, and it seems to have pretty much the same requirements as the 2008, which is the code cycle I'm working under right now (although in 1993 it was Section 250-95). The only real difference I see is that the 1993 NEC only required increasing the size of the EGC when the ungrounded conductors were increased due to voltage drop, whereas beginning in 2002 NEC doesn't care why the ungrounded conductors were increased in size.

As I understand it, Table 250.122 requires a #10 copper EGC for a 25 or 30 amp circuit (i.e. a circuit that would require #10 copper line and neutral conductors without any derating). For a 20 amp circuit with #10 copper line and neutral conductors, the EGC would also have to be increased to #10 per NEC 250.122(B). These requirements haven't changed since 2002.

However, lately I've seen several sets of approved plans calling for circuits with 2#10 and 1#12 EGC. If memory serves, all of these circuits have been 120 volts, 20 amps, feeding general use receptacles. These plans are all pretty recent (between 2005 and 2012), and I'm starting to wonder if there's something I've missed in my reading of the Code regarding this issue.

So... do you know of anything in the NEC that would permit running a #12 EGC with #10 line and neutral?
 
JDBrown said:
.............................................

So... do you know of anything in the NEC that would permit running a #12 EGC with #10 line and neutral?
Click to expand...

Know of no provision that allows.
 
jumper said:
Are these engineered drawings according to Annex B perhaps?
Click to expand...

I don't believe so -- at least, there weren't any additional calculations included with the drawings to indicate that the EE used Annex B. Would it matter if the ampacities were engineered according to Annex B? In my quick skimming I didn't see anything in Annex B (or in 310.15(C)) that would exempt him from 250.122.
 
Is your install using anther EGC per 250.118? If the wire ECG is not isolated from the metal raceway then the combination of both is the EGC. The metal raceway per 250.118 complying with 250.4(A)(5) or (B)(4) would not need a parallel EGC to be compliant. Possible option to the OP ?.
 
mwm1752 said:
Is your install using anther EGC per 250.118? If the wire ECG is not isolated from the metal raceway then the combination of both is the EGC. The metal raceway per 250.118 complying with 250.4(A)(5) or (B)(4) would not need a parallel EGC to be compliant. Possible option to the OP ?.
Click to expand...

That's a good possibility that I hadn't thought of. I was under the impression that when 250.122(A) says "... equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122 ..." it means that, if you install a ground wire, it can't be smaller than the sizes listed in Table 250.122.

In other words, I understood 250.122(A) to mean that, on a single-phase 20 amp circuit you could install 2#12 and 1#12 ground in EMT or 2#12 in EMT with the EMT serving as the EGC, but you could NOT install 2#12 and 1#14 ground in EMT. Have I misunderstood?
 
mwm1752 said:
Is your install using anther EGC per 250.118? If the wire ECG is not isolated from the metal raceway then the combination of both is the EGC. The metal raceway per 250.118 complying with 250.4(A)(5) or (B)(4) would not need a parallel EGC to be compliant. Possible option to the OP ?.
Click to expand...

If the wire EGC in parallel with the raceway EGC is too small, just don't connect it, and avoid the problem?
 
If the combination of the two result in the EGC the there may be no issue. IMO the wire EGC should be sized properly but not sure code would require in this particular situation. Good point on overcurrent protection size -- does most restrictive come into play?.
 
Last edited:
JDBrown said:
That's a good possibility that I hadn't thought of. I was under the impression that when 250.122(A) says "... equipment grounding conductors of the wire type shall not be smaller than shown in Table 250.122 ..." it means that, if you install a ground wire, it can't be smaller than the sizes listed in Table 250.122.

In other words, I understood 250.122(A) to mean that, on a single-phase 20 amp circuit you could install 2#12 and 1#12 ground in EMT or 2#12 in EMT with the EMT serving as the EGC, but you could NOT install 2#12 and 1#14 ground in EMT. Have I misunderstood?
Click to expand...

The inspectors I work with will not accept an undersized equipment ground even if it is supplemental an approved conduit ground path.
 
Status
Not open for further replies.
Top