Paralleling Conductors

[amgar]

I have read 310.10(H) regarding paralleling of conductors larger than 1/0 AWG. However, I am unsure about the resulting ampacity of paralleled conductors. For example, if two 1/0 AWG THHN copper conductors are in parallel, does this mean the total ampacity is simply doubled when taken from the tables in 310.15(B)? So it is now 340 amps instead of 170 amps?

I understand that each parallel conductor must be derated as per per 310.15(B)(3)(a). (more than 3 current-carrying conductors in a raceway or cable.)

 

[fmtjfw]

That's correct you multiply the ampacity of one conductor by the number of parallel conductors. If they are in the same raceway you reduce their ampacity by the appropriate factor.

 

[charlie b]

I would have to agree. But I would offer a challenge for anyone to produce a code article that proves it. :happyno:

 

[Dennis Alwon]

I would have to agree. But I would offer a challenge for anyone to produce a code article that proves it. :happyno:

Okay I'll bite. Why would one need a code article. We know the ampacity of one so it would stand to reason that 2 conductors of the same size would each carry the know ampacity or in this case double. I would bet a proposal to add that info would get squashed.

 

[charlie b]

All the code says is that we can use the ampacities in the table. Since when does "stands to reason" mean anything to the NEC? :happyno:

 

[texie]

I would have to agree. But I would offer a challenge for anyone to produce a code article that proves it. :happyno:

Interesting thought. It never occurred to me that it is not specifically spelled out, but now that you mention it, I guess you're right. Just another of those NEC things that make it difficult to master the NEC.

 

[kingpb]

Something of interest - If you look at the explanation (Handbook) under 310.10 (H) (1), first it says the NEC handles parallel conductors as a single conductor with total cross-sectional area of all conductors in parallel.

But then, and here's the irony, the explanation goes on to say that it is economical to use parallel because an increase in cross sectional area of a single conductor, even 100% for type THW of 1000KCMIL to 2000KCMIL would only increase current by 120A (2%).

But using 2 you can double it? Why? The first part says the NEC handles it as a single conductor.

I would LOVE to see an explanation of the explanationhmy: They couldn't make it more confusing.

Better explanation -
Doing the math it is easy to see why the doubling of CMIL does not get you a doubling of the current. Current is related to surface area of conductors, or combined surface areas of all the strands in a conductor. Going from 1000kcmil to 2000kcmil only increase the diameter from 1-in to 1.4-in. If you assume a solid conductor (not practical I know) but the circumference is then 3.145-in for a 1000kcmil, and 4.4in for a 200kcmil.

So the current increase is not going to double for a doubling of the kcmil size of the conductor. What you have to double is the diameter, and the easiest way to do that is parallel the same size conductor.

As far as cost savings goes, yes, it would be cheaper to use 2 x 1000KCMIL instead of having a custom made 1 x 4000kcmil conductor (I'd actually like to see that).

 

[Dennis Alwon]

All the code says is that we can use the ampacities in the table. Since when does "stands to reason" mean anything to the NEC? :happyno:

Now I did not say it was in the NEC. My point was it isn't necessary.

 

[Besoeker]

Something of interest - If you look at the explanation (Handbook) under 310.10 (H) (1), first it says the NEC handles parallel conductors as a single conductor with total cross-sectional area of all conductors in parallel.

But then, and here's the irony, the explanation goes on to say that it is economical to use parallel because an increase in cross sectional area of a single conductor, even 100% for type THW of 1000KCMIL to 2000KCMIL would only increase current by 120A (2%).

But using 2 you can double it? Why? The first part says the NEC handles it as a single conductor.

I would LOVE to see an explanation of the explanationhmy: They couldn't make it more confusing.

Better explanation -
Doing the math it is easy to see why the doubling of CMIL does not get you a doubling of the current. Current is related to surface area of conductors, or combined surface areas of all the strands in a conductor. Going from 1000kcmil to 2000kcmil only increase the diameter from 1-in to 1.4-in. If you assume a solid conductor (not practical I know) but the circumference is then 3.145-in for a 1000kcmil, and 4.4in for a 200kcmil.

So the current increase is not going to double for a doubling of the kcmil size of the conductor. What you have to double is the diameter, and the easiest way to do that is parallel the same size conductor.

As far as cost savings goes, yes, it would be cheaper to use 2 x 1000KCMIL instead of having a custom made 1 x 4000kcmil conductor (I'd actually like to see that).

Good practical points in my opinion. Surface area. I've taken a few different conductor sizes and compared published current ratings for the same operating conditions. Roughly speaking, doubling the conductor cross sectional area results in about a 50% increase in current rating. Paralleling conductors makes more effective use of the copper in terms of current carrying capacity. Busbars are paralleled for that same reason.
One thing I noted in our regulations is that you are not permitted to have separate switching devices for the conductors which makes sense.

 

[jumper]

Good practical points in my opinion. Surface area. I've taken a few different conductor sizes and compared published current ratings for the same operating conditions. Roughly speaking, doubling the conductor cross sectional area results in about a 50% increase in current rating. Paralleling conductors makes more effective use of the copper in terms of current carrying capacity. Busbars are paralleled for that same reason.
One thing I noted in our regulations is that you are not permitted to have separate switching devices for the conductors which makes sense.

I have looked at our charts and have noticed the similarities, charlie b's point was that nowhere in our codes does it state that the parallel ampacities of two conductors of a size can be doubled for a single ampacity. We just assume that if x=yA, 2x =2yA. Common sense, yes, but we do not seem to have specific permission. We assume this, but I cannot find anything in our NEC that states it.

 

[Besoeker]

I have looked at our charts and have noticed the similarities, charlie b's point was that nowhere in our codes does it state that the parallel ampacities of two conductors of a size can be doubled for a single ampacity. We just assume that if x=yA, 2x =2yA. Common sense, yes, but we do not seem to have specific permission. We assume this, but I cannot find anything in our NEC that states it.

Nor anything that contradicts it?

 

[jumper]

Nor anything that contradicts it?

Nope. Good point.

 

[Dennis Alwon]

Common sense, yes, but we do not seem to have specific permission. We assume this, but I cannot find anything in our NEC that states it.

I don't think it is necessary as we do have permission to parallel wires. Given that permission I think we do the math and go with it. The code is permissive. Where does it state that we cannot.

 

[jumper]

I don't think it is necessary as we do have permission to parallel wires. Given that permission I think we do the math and go with it. The code is permissive. Where does it state that we cannot.

Okay, I got permission to parallel conductors- 310.10, I got permission to use 310.16 for a single conductor ampacity, and I have permission to combine the two in...................?

charlie b has a point IMO.

 

[Besoeker]

Nope. Good point.

TY.
I was just approaching it from the practical perspective.
As I'm sure you know, we sometimes do some fairly high current stuff that requires me to size the copper for the application.

No prizes for second place.

 

[charlie b]

Okay, I got permission to parallel conductors- 310.10, I got permission to use 310.16 for a single conductor ampacity, and I have permission to combine the two in...................? charlie b has a point IMO.

And in this case, I think it is only a minor, academic point. I really have no objection to someone using a pair of parallel 1/0 for a 300 amp feeder. What I do object to is the attempt to apply the same reasoning to the use of a pair of 1/0 for a 350 amp feeder to an individual dwelling unit, by doubling the "ampacity" value in Table 310.15(B)(6).

 

[jumper]

TY.
I was just approaching it from the practical perspective.
As I'm sure you know, we sometimes do some fairly high current stuff that requires me to size the copper for the application.

No prizes for second place.

You certainly get to design some cool stuff.:thumbsup:

I have never designed anything more special than a lighting contactor panel-boring.

 

[david luchini]

Okay, I got permission to parallel conductors- 310.10, I got permission to use 310.16 for a single conductor ampacity, and I have permission to combine the two in...................?

There is no need for "permission" to combine the ampacities of parallel conductors. Imagine if you had two parallel sets of 3#1/0 (3 ccc's) in separate raceways. 310.16 tells me that my six conductors each have an amapcity of 150. As long as my load on any of those conductors doesn't exceed 150, then I'm fine. 310.4(B) tells me that the parallel conductors have to have the same length, insulation, conductor material, cm size, termination method, etc. If my load current is 300A, then my parallel conductors, having the same electrical properties, will both carry 150A. Nothing is "combined," I still have individual conductors carrying current within their allowable ampacities.

What I do object to is the attempt to apply the same reasoning to the use of a pair of 1/0 for a 350 amp feeder to an individual dwelling unit, by doubling the "ampacity" value in Table 310.15(B)(6).

This is a good point, but your use of parenthesis around "ampacity" indicates that you are clear that T310.15(B)(6) doesn't not indicate the ampacities of any conductors. This table only gives you the allowable conductor "types and size" for feeders or services for 120/240 dwelling units, based on various Service of Feeder ratings.

 

[jumper]

And in this case, I think it is only a minor, academic point. I really have no objection to someone using a pair of parallel 1/0 for a 300 amp feeder. What I do object to is the attempt to apply the same reasoning to the use of a pair of 1/0 for a 350 amp feeder to an individual dwelling unit, by doubling the "ampacity" value in Table 310.15(B)(6).

Agree.

 

[jumper]

There is no need for "permission" to combine the ampacities of parallel conductors. Imagine if you had two parallel sets of 3#1/0 (3 ccc's) in separate raceways. 310.16 tells me that my six conductors each have an amapcity of 150. As long as my load on any of those conductors doesn't exceed 150, then I'm fine. 310.4(B) tells me that the parallel conductors have to have the same length, insulation, conductor material, cm size, termination method, etc. If my load current is 300A, then my parallel conductors, having the same electrical properties, will both carry 150A. Nothing is "combined," I still have individual conductors carrying current within their allowable ampacities.



This is a good point, but your use of parenthesis around "ampacity" indicates that you are clear that T310.15(B)(6) doesn't not indicate the ampacities of any conductors. This table only gives you the allowable conductor "types and size" for feeders or services for 120/240 dwelling units, based on various Service of Feeder ratings.

I like this. Thanks.