- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Can you run a pair of tens in lieu of an eight in a single run of conduit? Never thought of it before.
Parallel EGC
- Thread starter George Stolz
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
Presuming that you mean that the circuit needs an 8, my answer would be no. I offer two reasons. First, you can't parallel wires that small. Secondly, nothing gives us an option of creating equivalent sized EGCs from sets of smaller wires.
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
Charlie, what code prevents paralleling EGCs smaller than 1/0?
None really. Depending on how you read it, 250.122(F) could be interpretted as allowing parallel EGC's... but how do you get past the sizing requirement? All you'd end up doing is installing two or more of the minimum required size... not like your OP example using 2-10's instead of an 8.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
300.3(B) is what allows paralleling in the first place. It says to do it in accordance with 310.4. So we can't do it in any way that is not described in 310.4.
What 310.4 says is that it is acceptable to parallel (essentially) just phase and neutral conductors. It does not explicitly say this, but it is commonly understood that by paralleling two phase conductors, we get twice the ampacity. Then, when 310.4(E) mentions paralleling EGCs, it tells us to use a full size version in each conduit. So 310.4(E), at least, is not allowing us to combine smaller EGCs to get the value of a larger EGC. And that brings me back to 310.4. The absence of EGCs in a list that gives us the ability to use two smaller wires to gain the benefit of a single larger wire tells me that paralleling EGCs is not allowed.
What 310.4 says is that it is acceptable to parallel (essentially) just phase and neutral conductors. It does not explicitly say this, but it is commonly understood that by paralleling two phase conductors, we get twice the ampacity. Then, when 310.4(E) mentions paralleling EGCs, it tells us to use a full size version in each conduit. So 310.4(E), at least, is not allowing us to combine smaller EGCs to get the value of a larger EGC. And that brings me back to 310.4. The absence of EGCs in a list that gives us the ability to use two smaller wires to gain the benefit of a single larger wire tells me that paralleling EGCs is not allowed.
- Location
- Illinois
- Occupation
- retired electrician
How does the specific permission in 310.4 act to prohibit any other installation?
jumper
Senior Member
- Location
- 3 Hr 2 Min from Winged Horses
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
310.104H(1) seems to be permissive while (5) has both restrictive and permissive parts.
But the logic gets twisted when we have things like 250.122 which describe permissive uses which depend on meeting other specified permissive uses.
But it really comes down to the logic in H(1) itself:
The combination "shall be permitted ...only..." is actually restrictive. :happyyes:
Please define, give an example, elaborate or otherwise clarify what you mean by other installation.
310.4 general statement doesn't mention EGC's and uses the terminology "shall be permitted", yet under 310.4(E) paralleled EGC's "shall be" sized per 250.122, and only sectioned EGC's in multiconductor cable "shall be permitted" smaller than 1/0...
PS: I hereby retract my "None really" comment in post #4.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
It is an odd thing because 310.10(H)(1) states 1/0 for everything other than the equipment grounding conductor. So does that mean the equipment grounding conductor can be paralleled in smaller sizes??? 310.10(H)(3) specifically states that the equipment grounding conductor is allowed to be run in parallel.
So did they just forget to add the equipment grounding conductor in the first part or does it mean we can parallel equipment grounding conductor smaller than 1/0. Maybe if I understood why conductors of 1/0 and larger are only allowed to be in parallel I could better answer the question.
My take is that the NEC would prefer not having parallel wires so they limit it where it obviously isn't necessary to do so based on wire sizes. This would limit the margin of error that the parallel conductors may have.
If that is the case then I would suspect the equipment grounding conductor should not be paralleled smaller than 1/0.
So did they just forget to add the equipment grounding conductor in the first part or does it mean we can parallel equipment grounding conductor smaller than 1/0. Maybe if I understood why conductors of 1/0 and larger are only allowed to be in parallel I could better answer the question.
My take is that the NEC would prefer not having parallel wires so they limit it where it obviously isn't necessary to do so based on wire sizes. This would limit the margin of error that the parallel conductors may have.
If that is the case then I would suspect the equipment grounding conductor should not be paralleled smaller than 1/0.
I noted 310.4(E) above, but that's from 2008 NEC.
310.10(H)(5) from 2011 NEC below:
So only sectioned EGC's in multiconductor cable are permitted to be paralleled smaller than 1/0.
Ultimately, even when you do parallel 1/0 and larger, it is still a full sized EGC.
PS: FWIW, grammatically the highlighted text is written wrong if the intent was to only permit smaller than 1/0 for sectioned EGC's in multiconductor cable. As it is currently written, there is nothing which specifically prohibits non-sectioned EGC size when paralleled.
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- Location
- Illinois
- Occupation
- retired electrician
My point is, that prior to the 2011 code, the code did not actually prohibit the paralleling of conductors smaller than 1/0.(yes, we all know that was the intent, but the code language did not reflect that intent, there is new wording triggered by a proposal of mine in the 2011 code that added to word only... it comes closer to reflecting the intent, but am not convinced it actually does)
There was no rule that said you could not do that. Telling me that I can do something, does not create a restriction on doing something else. The term "shall be permitted" is intended to act like an exception to some rule.
kwired
Electron manager
- Location
- NE Nebraska
Example: we have a 400 amp circuit that consists of parallel 3/0 conductors in separate raceways. If we run an EGC we must run one in each raceway, but 250.122 says 3AWG is sufficient for 400 amp OCPD - each raceway needs full sized EGC which is 3 AWG minimum. Or must the EGC's be 1/0 because they are parallel to one another? (I think exceptions allow the 3 AWG here)
Example: we have two or more raceways between two enclosures, none of the contained circuits are over 30 amps so 250.122 says 10 AWG for 30 amps OCPD. Can we run 10 AWG in these raceways for EGC or must they be 1/0 since they are parallel to one another? Big difference in size of raceway needed to accommodate the EGC if we only needed 1/2 or 3/4 before determining the EGC must be 1/0. (I think exceptions allow the 10 AWG here)
I understand and agree. In order for ""shall be permitted" to act like an exception, there has to be rule which otherwise prohibits what the exception permits.
I agree, essentially... but there are no "Exceptions" regarding paralleled EGC's. There is no requirement for parallel EGC's to be 1/0 or larger.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
as i read it you can parallel EGCs but they have to be of the minimum size.
It would be impossible to prevent paralleling EGCs anyway. Think about a green wire inside a piece of EMT.
It would be impossible to prevent paralleling EGCs anyway. Think about a green wire inside a piece of EMT.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
I believe that article 300 differs from others, in that 300.2(A), 300.3(A), and for the present discussion 300.3(B)(1) state requirements that tend to invalidate the notion of, "that which is not prohibited is (tacitly) permitted. They carry the message of, "wiring shall use the methods described in this chapter." What that means is that if it is not expressly permitted, it is forbidden. That is why I interpret the absence of the EGC from the list of allowable parallel conductors as constituting a prohibition against it.
Keep in mind that the issue under discussion is not whether you can connect two wires to each other at both ends. There are circumstances in which you can do that to wires that are serving as EGCs. The issue is what happens when you do connect them in parallel. Allow me to point out that nowhere in the NEC, and I mean nowhere, are we told that putting two 1/0 THHWs in parallel will give us an ampacity of 300 amps. We tend to take for granted that we can add ampacities. Here again, nothing tells us that the performance of an EGC under fault conditions is purely a matter of cross-sectional area. For that matter, nothing tells us that the performance of an EGC under fault conditions is a matter of the ampacity the same wire would have, if it had been serving as a phase conductor instead of as an EGC.
I will say that last part again, but with specific examples. Suppose I am designing a new 800 amp feeder. Table 250.122 tells me I need a 1/0 copper EGC. Which (if either) of the following two methods could I use, in order to replace the 1/0 with smaller parallel EGCs?
Keep in mind that the issue under discussion is not whether you can connect two wires to each other at both ends. There are circumstances in which you can do that to wires that are serving as EGCs. The issue is what happens when you do connect them in parallel. Allow me to point out that nowhere in the NEC, and I mean nowhere, are we told that putting two 1/0 THHWs in parallel will give us an ampacity of 300 amps. We tend to take for granted that we can add ampacities. Here again, nothing tells us that the performance of an EGC under fault conditions is purely a matter of cross-sectional area. For that matter, nothing tells us that the performance of an EGC under fault conditions is a matter of the ampacity the same wire would have, if it had been serving as a phase conductor instead of as an EGC.
I will say that last part again, but with specific examples. Suppose I am designing a new 800 amp feeder. Table 250.122 tells me I need a 1/0 copper EGC. Which (if either) of the following two methods could I use, in order to replace the 1/0 with smaller parallel EGCs?
- Method 1: A 1/0 has a cross-sectional area of 105,600 circular mils. To get the same (or higher) area with two smaller wires, each wire would need an area of 52,800 circular mils. The smallest wire that fits that description is a #3. So I will use two #3s in parallel.
- Method 2: A 1/0 has an ampacity of 150 amps. To get the same (or higher) ampacity with two smaller wires, each wire would need an ampacity of 75 amps. The smallest wire that fits that description is a #4. So I will use two #4s in parallel.
- Location
- Illinois
- Occupation
- retired electrician
In the 2008 and earlier codes, there is no requirement for any paralleled conductor to be 1/0 or larger.
- Status