# Parallel Conductors



## charlie Bob (Jul 26, 2009)

The NEC 310.4 (A) states that conductors in parallel should be the same length and the same conductor material. . . i thought i knew that rule and how to aplly it, but doing some reading oer the weekend put some doubts in me. I read that the above mention rule aplies to a "_parallel set". . _now here's where the terminollogy gets in the way. . what do you call a parallel set?. . . are each individual phases and grounded conductor consider a "set" or you consider a set all the conductors in the same conduit a set?
I always thought that one set was the phases and grounded conductor in the same raceway. . .


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## brian john (Mar 11, 2007)

To me and the intent of the rule AS II UNDERSTAND IT, is the set should be all the A phase conductors same length or as nice to the same length as possible.
All the B phase conductors same length or as nice to the same length as possible.
All the C phase conductors same length or as nice to the same length as possible.

And the same goes for the neutral and GECs if pulled (I would think)


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## al13nw4r3LC76 (Apr 6, 2009)

I was on a job and we terminated a parallel run.

All A phase were the same length to their lug.

All B phase were the same to the lug (but slightly longer then a phase)

All C phase were the same to the lug. ( but slightly longer then b phase)

However B phase and C phase were longer then A (as the lugs are seperated by a couple inches)

The inspector interpreted this as a violation as all the wires were not the exact same length. He made us redo the work.

Hope that made sense.


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## BBQ (Nov 16, 2010)

I agree with Brian and will add you could have 100' long copper A, a 115' long aluminum B and and copper or aluminum of a different size for C.


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## BBQ (Nov 16, 2010)

al13nw4r3LC76 said:


> The inspector interpreted this as a violation as all the wires were not the exact same length. He made us redo the work.


Wow, that sucks and was the wrong call by the inspector.


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## charlie Bob (Jul 26, 2009)

brian john said:


> To me and the intent of the rule AS II UNDERSTAND IT, is the set should be all the A phase conductors same length or as nice to the same length as possible.
> All the B phase conductors same length or as nice to the same length as possible.
> All the C phase conductors same length or as nice to the same length as possible.
> 
> And the same goes for the neutral and GECs if pulled (I would think)


So if i have two raceways each having 4 conductors ,( phase A, phase B phase C ,and grounded conductor) i should aplly that rule to the phases and grounded conductors like for example :

Phase A - 2 250 Kcmil AL , 105 ft long
Phase B - 2 3/0 cu , 106 ft long
Phase C -2 3/0 cu, 103 ft long
Neutral Set - 2 1/0 AL 104 ft long

This set up should be fine as long as the rule is interpreted the same way. . .Right?


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## charlie Bob (Jul 26, 2009)

al13nw4r3LC76 said:


> I was on a job and we terminated a parallel run.
> 
> All A phase were the same length to their lug.
> 
> ...


so it's one of them rules that the last word belongs to the inspector and his interpretation.


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## charlie Bob (Jul 26, 2009)

BBQ said:


> I agree with Brian and will add you could have 100' long copper A, a 115' long aluminum B and and copper or aluminum of a different size for C.


Even if they are in different raceways?


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## BBQ (Nov 16, 2010)

charlie Bob said:


> so it's one of them rules that the last word belongs to the inspector and his interpretation.


Only if you don't fight it.


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## Dennis Alwon (May 9, 2009)

charlie Bob said:


> Even if they are in different raceways?


Sure but you would probably never do that. All 3 phase conductors must be in the same raceway. There are only a few instances when you can run phase "A" in one conduit, and the other phases in different conduits.


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## BBQ (Nov 16, 2010)

charlie Bob said:


> Even if they are in different raceways?


Yes.

The only conductors that must be the same are the conductors of the same phase, neutral or ground.


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## BBQ (Nov 16, 2010)

Here is the wording from the 2011




> 310.10(H)
> 
> (2) Conductor Characteristics. *The paralleled conductors
> in each phase, polarity, neutral, grounded circuit conductor,
> ...


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## charlie Bob (Jul 26, 2009)

Dennis Alwon said:


> Sure but you would probably never do that. All 3 phase conductors must be in the same raceway. There are only a few instances when you can run phase "A" in one conduit, and the other phases in different conduits.


I meant have one phase a conductor, one phase b conductor , one phase c conductor and one neutral conductor per raceway Dennis, . . that's where the terminology confused me, i consider that a "set". . i was wrong.:001_huh:,. . .


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## brian john (Mar 11, 2007)

Dennis Alwon said:


> Sure but you would probably never do that. All 3 phase conductors must be in the same raceway. There are only a few instances when you can run phase "A" in one conduit, and the other phases in different conduits.


The utility told an EC that all A had to be in one conduit, All B in one conduit......About 1-1/2 years later we got the emergency call. Was a nice repair 3000 amp service 600 kcmil 100' long. The customer operated on generator until the repair was made,


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## charlie Bob (Jul 26, 2009)

al13nw4r3LC76 said:


> I was on a job and we terminated a parallel run.
> 
> All A phase were the same length to their lug.
> 
> ...


Did you ever confront him about it ?


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## charlie Bob (Jul 26, 2009)

brian john said:


> The utility told an EC that all A had to be in one conduit, All B in one conduit......About 1-1/2 years later we got the emergency call. Was a nice repair 3000 amp service 600 kcmil 100' long. The customer operated on generator until the repair was made,


having that set up created that serice call? . . .where all the raceways the same materials?


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## Dennis Alwon (May 9, 2009)

brian john said:


> The utility told an EC that all A had to be in one conduit, All B in one conduit......About 1-1/2 years later we got the emergency call. Was a nice repair 3000 amp service 600 kcmil 100' long. The customer operated on generator until the repair was made,



What right did the utility have to do that? If they made me I would sure as hell got them to sign something. Now it can be done underground with PVC-- I believe Bob has an install done that way.300.3(B)(1) exception


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## al13nw4r3LC76 (Apr 6, 2009)

charlie Bob said:


> Did you ever confront him about it ?


My journeyman at the time told me I had done the job wrong. Telling me the inspector was right. I was a second year at the time. Having come from resi I didn't understand why I was wrong but was shown the code section and it seemed funky to me but the shop I worked for was happy to redo it. To the inspectors liking.


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## wildleg (Apr 12, 2009)

the way the 2011 is worded it sounds like all the conductors from all the sets have to be the same length. I don't have the handbook - anyone know if it clarifies it ?


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## brian john (Mar 11, 2007)

Dennis Alwon said:


> What right did the utility have to do that? If they made me I would sure as hell got them to sign something. Now it can be done underground with PVC-- I believe Bob has an install done that way.300.3(B)(1) exception


The EC asked for input, this facility had 3 buildings 3 services first two were PVC With all All A, all B and all C in separate conduits as the made the parallel tie in easy I guess? The third (the one that was damaged ) were in Rigid. Not sure how or why the utility was involved in the decision making.

And before anyone ask, all scrap went to the customer.


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## BBQ (Nov 16, 2010)

wildleg said:


> the way the 2011 is worded it sounds like all the conductors from all the sets have to be the same length. I don't have the handbook - anyone know if it clarifies it ?





> *The paralleled conductors*
> in each phase, polarity, neutral, grounded circuit conductor,
> equipment grounding conductor, or equipment bonding


Only the paralleled conductors.

The neutral conductors are not in parallel with phase B etc.




> shall be permitted to be connected in parallel (electrically
> joined at both ends)


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## charlie Bob (Jul 26, 2009)

al13nw4r3LC76 said:


> My journeyman at the time told me I had done the job wrong. Telling me the inspector was right. I was a second year at the time. Having come from resi I didn't understand why I was wrong but was shown the code section and it seemed funky to me but the shop I worked for was happy to redo it. To the inspectors liking.


Wow very surprised the money wasted didn't push them a little bit more.


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## Dennis Alwon (May 9, 2009)

brian john said:


> And before anyone ask, all scrap went to the customer.


Now that's too bad...:laughing:


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## dowmace (Jan 16, 2008)

brian john said:


> The utility told an EC that all A had to be in one conduit, All B in one conduit......About 1-1/2 years later we got the emergency call. Was a nice repair 3000 amp service 600 kcmil 100' long. The customer operated on generator until the repair was made,


I argued with my jman back when I was a 3rd year about this. It was a 1600 amp service with parallel 500's within a month it had melted all the insulation off the wire causing us to warranty the work and eat the cost. I had a hard time not giving him the business over it.


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## charlie Bob (Jul 26, 2009)

brian john said:


> The EC asked for input, this facility had 3 buildings 3 services first two were PVC With all All A, all B and all C in separate conduits as the made the parallel tie in easy I guess? The third (the one that was damaged ) were in Rigid. Not sure how or why the utility was involved in the decision making.
> .


I take that being in the metal conduit created the problem. . .inducted heat?. . .


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## RandyM (Apr 5, 2012)

charlie Bob said:


> Did you ever confront him about it ?


If you carried out the inspectors flawed logic, you would have to make all the conductors the same length, even when not in parallel for every feeder!


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## dowmace (Jan 16, 2008)

charlie Bob said:


> I take that being in the metal conduit created the problem. . .inducted heat?. . .


Exactly the phases can be separated when in pvc without much issue but not emt, or grc. It would still be a violation though.


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## chicken steve (Mar 22, 2011)

it's virtually impossible to terminate _'parrallel'_ down to the inch in switchgear and xformers

yet i have been privy to debates where inches mathematically count

~CS~


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## BBQ (Nov 16, 2010)

RandyM said:


> If you carried out the inspectors flawed logic, you would have to make all the conductors the same length, even when not in parallel for every feeder!


Exactly!


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## hardworkingstiff (Jan 22, 2007)

chicken steve said:


> it's virtually impossible to terminate _'parrallel'_ down to the inch in switchgear and xformers
> 
> yet i have been privy to debates where inches mathematically count
> 
> ~CS~


Those must have been short runs.


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## hardworkingstiff (Jan 22, 2007)

No one posted the reason why this needs to be done and maybe if we discuss it anyone reading that doesn't quite understand why might benefit. 

Electricity takes all paths for current, but more current will flow through a path with less resistance. Resistance in a wires of the same size will be determined by the length of the conductor. Therefore, it is important that the conductors be the same length (the PARALLEL CONDUCTORS) so that the resistance will be the same in each conductor.

Now, what happens if this rule is not followed? More current will flow in the shortest wire than in the longer one/s. That will put more heat stresses on that conductor and when the service is fully loaded could easily overheat the conductor which in turn will overheat the lugs the conductor terminates in.

The 2nd issue that comes into play (which was mentioned above) is when someone decides it's easiest to keep the conductors the same length by putting all of the same phase conductors in the same conduit. Then you get magnetic fields not be canceled as well as they would if one conductor of each phase was in each conduit. This can cause heating issues which are detrimental to the life of the electrical system.


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## chicken steve (Mar 22, 2011)

hardworkingstiff said:


> Electricity takes all paths for current, but more current will flow through a path with less resistance. Resistance in a wires of the same size will be determined by the length of the conductor. Therefore, it is important that the conductors be the same length (the PARALLEL CONDUCTORS) so that the resistance will be the same in each conductor.
> 
> Now, what happens if this rule is not followed? More current will flow in the shortest wire than in the longer one/s. That will put more heat stresses on that conductor and when the service is fully loaded could easily overheat the conductor which in turn will overheat the lugs the conductor terminates in.
> 
> .


agreed....

but what math are we using HWS?

are we held to normal operating currents?, or AIC currents?

~CS~


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## hardworkingstiff (Jan 22, 2007)

Just one more thing from a technical perspective (not sure if it is practically important).

Let's say you parallel 3 sets of conductors and have one conductor of each phase in each conduit. To do the job "perfect" (and it's impossible) not only do the conductors of the same phase need to be the same length, any difference in the length of the conductors should be proportionally the same in each conduit.

The reason is you want to have the current divided equally between the conduits in order to minimize heating from magnetic fields. So, if conduit 1 had A-phase conductor .5% shorter than conduit 2 A-phase conductor, conduit 1 should have B & C-phase conductors .5% shorter than conduit 2 B&C-phase conductors. Practically I'm not sure this makes a difference, but from a theory perspective, it would contribute to a better installation.


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## hardworkingstiff (Jan 22, 2007)

chicken steve said:


> agreed....
> 
> but what math are we using HWS?
> 
> ...


I'm not sure what you are getting at Steve, but does it matter?

BTW, thanks for changing your fonts.


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## chicken steve (Mar 22, 2011)

hardworkingstiff said:


> I'm not sure what you are getting at Steve, but does it matter?
> 
> .


i'm unsure HWS, but i do know there can be a big dif bettween the two, and those inches are then relevant

~CS~


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## hardworkingstiff (Jan 22, 2007)

chicken steve said:


> i'm unsure HWS, but i do know there can be a big dif bettween the two, and those inches are then relevant
> 
> ~CS~


Of course it depends on the overall length of the conductor as to how much of an impact 2" has, but you bring up a great point that under fault conditions it matters even more.


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## chicken steve (Mar 22, 2011)

maybe we need a hypothetical HWS?

~CS~


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## BBQ (Nov 16, 2010)

hardworkingstiff said:


> I'm not sure what you are getting at Steve, but does it matter?


Code wise they 'shall be the same length' as written it is impossible to comply with.

It should be 'shall be the same length +/- 1% of overall length'.

I picked 1% out of the air, I don't know what % would really be correct.


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## hardworkingstiff (Jan 22, 2007)

BBQ said:


> Code wise they 'shall be the same length' as written it is impossible to comply with.
> 
> It should be 'shall be the same length +/- 1% of overall length'.
> 
> I picked 1% out of the air, I don't know what % would really be correct.


I agree with your logic but might consider 1/2% (like you, picked it out of the air).


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## BBQ (Nov 16, 2010)

hardworkingstiff said:


> I agree with your logic but might consider 1/2% (like you, picked it out of the air).


Mine is a 1' per 100', yours is 1' per 200'.

I am willing to bet both are overly conservative and are both routinely violated.


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## don_resqcapt19 (Jul 18, 2010)

If you have a 600 amp circuit and use two sets of parallel conductors with one set 100' and the other 105', there will be 307.3 amps on the 100' conductors and 292.7 amps on the 105'. (assuming 350kcmil, neither set exceeds the 75°c ampacity)

If you use 3 sets of conductors and the lengths are 100' 105' and 110', there will be 209.7 amps on the 100', 199.7 amps on the 105' and 190.6 on the 110' conductor. (assuming 3/0, the short run exceeds the 75°C ampacity)

If you have two runs of 10 and 11', the currents would be 314.3 and 285.7.

If you have 3 runs of 10, 11, and 12' the currents would be 218.8, 198.9, and 182.3.

This assumes that the only difference in the impedance of the sets is caused by the change in length.


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## brian john (Mar 11, 2007)

don_resqcapt19 said:


> This assumes that the only difference in the impedance of the sets is caused by the change in length.


Terminations and routing of conductors play into this as well.


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## don_resqcapt19 (Jul 18, 2010)

brian john said:


> Terminations and routing of conductors play into this as well.


 Yes, they do, and a poor termination can make a lot more difference than a 10% change in length, but without knowing the actual impedance of the installed conductors there is no way to determine the current in each set. My numbers just show the current division based on change in length with everything else being identical.


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## Wag 2.0 (Sep 9, 2012)

I recently worked on a job where the customer was having a problem with the main breaker on an MCC that was tripping periodically. We checked everything (megged wire and bus) and nothing strange was going on. We took amp readings at the transformer and found one set on the secondary side was pulling considerably more. The gear and transformer were only about 10' apart with the transformer located outside the building. The poco required no more than 10% difference in length between parallel conductors. This situation was a matter of inches. Repulled the two sets and never have had another trip since. Made a believer out of me.


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## Cl906um (Jul 21, 2012)

BBQ said:


> Here is the wording from the 2011


I don't know if it is still the same, but I thought they were supposed to be same manufacturer as well. Have to dust off the book. No I guess they don't now that I have looked. Not now at least.


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## greenman (Apr 20, 2012)

Here is a good link
http://www.cable.alcan.com/NR/rdonlyres/14D284B4-23D6-4D82-9B8E-A08FAB0ED764/0/ENInstallHandbook.pdf


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