NEC Table 250.122

Status
Not open for further replies.
F

farmaped

Member
Location
PA - USA
Occupation
Engineer
Can the minimum ground size be met by paralell grounding conductors (1/0AWG and larger)?
In other words, if 500MCM is the specified minimum (per 250.122), is the calculated equivilant total area acceptable, for example - (2) runs of 250MCM?
Thanks.
 
farmaped said:
Can the minimum ground size be met by paralell grounding conductors (1/0AWG and larger)?
In other words, if 500MCM is the specified minimum (per 250.122), is the calculated equivilant total area acceptable, for example - (2) runs of 250MCM?
Thanks.
Click to expand...

No.

See 250.122(F) in the 2005.
 
As Bob said no. And if 500 kcmil is the minimum required you'll need one of those in each raceway.
 
First 310.4 does not apply to EGCs, so there is no size restriction on the installation of parallel EGCs. The previous posts seem to be addressing an installation of parallel runs of conduits, but I am not sure that is what the question is asking. If the 4000 amp circuit was run in a cable tray, it is my opinion that you could install a single 500kcmil EGC or multiple smaller EGCs as long as the total cross sectional area of the multiple EGCs is at least 500 kcmil.
 
Don if the egc is specified at 500 mcm that would mean we are looking at a 4000 amp service. How would this not be a parallel setup?
 
Dennis Alwon said:
Don if the egc is specified at 500 mcm that would mean we are looking at a 4000 amp service. How would this not be a parallel setup?
Click to expand...
That is why I picked a cable tray installation....you can use a single EGC when the circuit conductors are installed in tray.
 
don_resqcapt19 said:
That is why I picked a cable tray installation....you can use a single EGC when the circuit conductors are installed in tray.
Click to expand...

Got ya, sorry I missed the cable tray thought. I read "parallel runs of conductors" instead of "parallel runs of conduits" and missed the cable tray.
 
Yes, for this example a 4000A feeder in a single raceway (e.g. cable tray). The only reference I find is 310.4(E), which I'm not even sure is applicable. I think equivalent cross-sectional -total - grounding conductors (when paralelled) would suffice, I cannot find evidence otherwise.
Thank you.
 
I agee with Benaround.

Pull the minimum size, if seperate raceways are used, then pull the minimum size in each.
 
farmaped said:
Yes, for this example a 4000A feeder in a single raceway (e.g. cable tray). The only reference I find is 310.4(E), which I'm not even sure is applicable. I think equivalent cross-sectional -total - grounding conductors (when paralelled) would suffice, I cannot find evidence otherwise.
Thank you.
Click to expand...

Have you thought about how much fault current will divide between two or more smaller

conductors ? What will happen when one of the smaller paralelled conductors burns right

off it's termination point leaving only the other to trip the OCPD, that is if it lasts that long.

I would use what the NEC 250.122(A) and Table 250.122 require, knowing it has been

tested to serve the conditions at hand. jmo.
 
Frank,
Why would the current divide in a way to melt one of the paralleled EGCs? That is assuming that they are installed per the rules in 310.4 even though that section does not apply to EGCs. I don't see either a code or technical issue that would prevent the use of multiple EGCs in parallel.
 
benaround said:
Have you thought about how much fault current will divide between two or more smaller conductors ? What will happen when one of the smaller paralelled conductors burns right off it's termination point leaving only the other to trip the OCPD,
Click to expand...

And if there were one EGC and it burned off it's termination point then you would have no protection at all....;)
 
The argument that one EGC is better than two (smaller) EGC's is debatable. I am leaning toward two, 40 years from now, when there is a catastrophic fault, hoping that at least one of them has maintained a solid connection.

This question wasn't intended to discuss the "better way to do it", simply to find out if paralelling EGC's (in this application) is a "code violation". Most jobs I specify have a redundant ground, for these same reasons (in case one fails), not to mention the other benefits of a robust grounding system. This is not an actual installation I am involved with, simply a code quandary

I, also, cannot find a code or other technical reason to reject this installation.
 
Don,

The info on the current dividing is from the 2005 NECH 250.122(F)(1) , commentary . It is

mostly about the 'shortest' and ' lowest impedance' path getting more fault current than

the other, and what could happen.

I don't know how I'm seeing 310.4(A) so differently than you see it. IMO, that section is

telling you what uses are allowed to be run in parallel, phase, polarity,neutral, or grounded

circuit conductor. EGC is not on the list, to me that means EGC not allowed to be paralleled.

Last on this topic, I see 250.122(A) stating "EGC of the wire type 'shall not' be smaller than

shown in table 250.122" , agian I can only see this for what it says.
 
benaround said:
Don,

The info on the current dividing is from the 2005 NECH 250.122(F)(1) , commentary . It is

mostly about the 'shortest' and ' lowest impedance' path getting more fault current than

the other, and what could happen.

I don't know how I'm seeing 310.4(A) so differently than you see it. IMO, that section is

telling you what uses are allowed to be run in parallel, phase, polarity,neutral, or grounded

circuit conductor. EGC is not on the list, to me that means EGC not allowed to be paralleled.

Last on this topic, I see 250.122(A) stating "EGC of the wire type 'shall not' be smaller than

shown in table 250.122" , agian I can only see this for what it says.
Click to expand...
With a parallel conduits and a fault in the conduit a single EGC has to carry the fault current. I don't see it the same way with a cable tray installation.
As far as the EGC being paralleled, I see no way that the words "shall be permitted" can act to prohibit some thing else. Those words are intended to as to permit some alternate installation method and act as an exception. The problem in the case of 310.4 is that there is no main rule that says conductors shall not be paralleled. 310.4 needs a rewrite to make it say what the panel wants it to say.
 
Status
Not open for further replies.
Top