parallel 750 mcm Al

[RW31]

Hello
Can 750 Al. THWN (NEW) be paralleled with existing 750 Al. THW ?
Talked to a wire manufacturer and they said THWN would be fine to parallel with the old THW
Not sure the inspector will agree with the above
My concern is 310.4(B)-4 same insulation type.

Thanks for any views on this.

 

[qcroanoke]

Hello
Can 750 Al. THWN (NEW) be paralleled with existing 750 Al. THW ?
Talked to a wire manufacturer and they said THWN would be fine to parallel with the old THW
Not sure the inspector will agree with the above
My concern is 310.4(B)-4 same insulation type.

Thanks for any views on this.

If it is single phase (2 hots) they could be used to comprise 1 phase and you could use the thwn for the 2nd phase and neutral. Conductors of each phase must be the same but they don't all have to be the same.

 

[RW31]

Sorry I was not very clear on post .

Have 7- sets of 750 Kcmil Al. THW existing service.
Have 3-empty existing pipes for upgrade of service.

Cannot find any 750 Kcmil Al. THW to match existing.

Encore and South wire say they do not make it and have THWN or XHHW

Thanks

 

[RW31]

Sorry I was not very clear on post .

Have 7- sets of 750 Kcmil Al. THW existing service.
Have 3-empty existing pipes for upgrade of service.

Service is 3-phase 277/480 volt

Cannot find any 750 Kcmil Al. THW to match existing.

Encore and South wire say they do not make it and have THWN or XHHW

Thanks

 

[qcroanoke]

Sorry I was not very clear on post .

Have 7- sets of 750 Kcmil Al. THW existing service.
Have 3-empty existing pipes for upgrade of service.

Cannot find any 750 Kcmil Al. THW to match existing.

Encore and South wire say they do not make it and have THWN or XHHW

Thanks

In that case I don't see how you can make it work.
I'm sure somewhere out there someone may have the wire you need to do this (THW). Depends on how much you need.

 

[RW31]

3000'

Thanks for your help

 

[qcroanoke]

3000'

Thanks for your help

I foresee a customer that is not going to understand why you can't use the wire that is already there......

 

[Ragin Cajun]

Get the wire manuf's recommendation in writing and go to the AHJ, they may be ok with that. The alternative is simply outrageous! I assuime you will be loading everything to THW? Engineering wise I just don't see an issue.

RC

 

[jim dungar]

You should be able strip it all off and magically your THWN would become THW. But why bother, from what I have heard from inspectors and manufacturers, the nylon coating does nothing for the insulation.

And of course you could compare the reactance of your new run versus the reactance of an existing run to see if they are similar.

 

[RW31]

Thank you guys I appreciate your help on this.

 

[winnie]

What sort of pipe? What is the neutral load? Any chance you could change over to an isolated phase installation? (3 pipes for phase A, 3 for phase B, 3 for C, 1 for N)?

The reason for section 310.4 is to ensure equal division of the current amongst all of the individual wires that are paralleled to make up a 'single' conductor. A problem with section 310.4 is that there is no specification for allowed tolerance or deviation. For example all conductors are required to be the same length...but no two length measurements are ever _exactly_ parallel, and 310.4 doesn't provide any guidance on maximum tolerance.

Heck, 2 conductors of the same nominal KCMIL will have different actual cross section due to manufacturing variation.

My gut tells me that current distribution differences caused by mixing THWN-2 conductors with THW conductors will be smaller than those caused by the nature of being in different locations in the duct bank (magnetic or thermal gradient effects). I have not done any sort of engineering calculation to verify this gut feeling.

There will be a difference in the impedance of the conductors with the different insulation systems, because the dielectric constant for nylon is different from that of PVC. But I am betting that this difference will be small, and dominated by the resistance and inductance of the conductors themselves.

-Jon

 

[kwired]

What sort of pipe? What is the neutral load? Any chance you could change over to an isolated phase installation? (3 pipes for phase A, 3 for phase B, 3 for C, 1 for N)?

The reason for section 310.4 is to ensure equal division of the current amongst all of the individual wires that are paralleled to make up a 'single' conductor. A problem with section 310.4 is that there is no specification for allowed tolerance or deviation. For example all conductors are required to be the same length...but no two length measurements are ever _exactly_ parallel, and 310.4 doesn't provide any guidance on maximum tolerance.

Heck, 2 conductors of the same nominal KCMIL will have different actual cross section due to manufacturing variation.

My gut tells me that current distribution differences caused by mixing THWN-2 conductors with THW conductors will be smaller than those caused by the nature of being in different locations in the duct bank (magnetic or thermal gradient effects). I have not done any sort of engineering calculation to verify this gut feeling.

There will be a difference in the impedance of the conductors with the different insulation systems, because the dielectric constant for nylon is different from that of PVC. But I am betting that this difference will be small, and dominated by the resistance and inductance of the conductors themselves.

-Jon

You bring up some good points, the biggest is the NEC does not give us an acceptable tolerance level, and you can try hard and never get exactly the same impedance on each parallel path.

Something else to consider is the older conductor is likely a different aluminum alloy then the newer conductors and likely does have a different resistance per unit of length, also would watch out for compact / non compact conductors.

 

[kwired]

You should be able strip it all off and magically your THWN would become THW. But why bother, from what I have heard from inspectors and manufacturers, the nylon coating does nothing for the insulation.

And of course you could compare the reactance of your new run versus the reactance of an existing run to see if they are similar.

I understand the nylon coating gives it gasoline and oil resistance, does nothing for temperature rating.

 

[jim dungar]

There will be a difference in the impedance of the conductors with the different insulation systems, because the dielectric constant for nylon is different from that of PVC. But I am betting that this difference will be small, and dominated by the resistance and inductance of the conductors themselves.

The insulation would be PVC for both conductors, the newer one would just have the addition of a few mm of nylon for ease in handling. But, I wonder, have they changed the thichness of PVC insulation in the years since the first install?

 

[kwired]

The insulation would be PVC for both conductors, the newer one would just have the addition of a few mm of nylon for ease in handling. But, I wonder, have they changed the thichness of PVC insulation in the years since the first install?

THW is definitely thicker wall insulation then THHN/THWN commonly found now. Since temperature rating is different I would guess that even though both are PVC based material - they were made from different formulas, and that is going to change the capacitive properties of the conductor.

 

[jim dungar]

THW is definitely thicker wall insulation then THHN/THWN commonly found now. Since temperature rating is different I would guess that even though both are PVC based material - they were made from different formulas, and that is going to change the capacitive properties of the conductor.

The IEEE Red Book Table A4B-8, does not bother to break down reactance, by insulation, for 600V conductors, so it must consider it as inconsequential.
For identical raceways and number of conductors, SKM uses the same values of reactance for THHN, THWN, and TW.

Also, we never make accommodations for when one parallel raceway run is filled with water and another one isn't even though that impacts the capacitance.

 

[kwired]

The IEEE Red Book Table A4B-8, does not bother to break down reactance, by insulation, for 600V conductors, so it must consider it as inconsequential.
For identical raceways and number of conductors, SKM uses the same values of reactance for THHN, THWN, and TW.

Also, we never make accommodations for when one parallel raceway run is filled with water and another one isn't even though that impacts the capacitance.

All good information (I assume) but NEC still doesn't really tell us how much tolerance is permitted between the individual elements of a parallel set, yet is worded to almost indicate it is very critical to have all characteristics of each element be exactly the same.

 

[Fulthrotl]

Sorry I was not very clear on post .

Have 7- sets of 750 Kcmil Al. THW existing service.
Have 3-empty existing pipes for upgrade of service.

Cannot find any 750 Kcmil Al. THW to match existing.

Encore and South wire say they do not make it and have THWN or XHHW

Thanks

the only thing that the nylon is for is for pulling.
it's a thw insulation. i don't see a problem with it.

time to talk to the inspector. if he says no, then
you have bad news for the customer. i can't
imagine anyone having anything without a
nylon jacket any more... and with simpull or
it's varients replacing nylon jackets, nylon
is probably gonna become a thing of the past.

 

[tish53]

As an end user, I would spend a lot of time convincing the inspector and/or his supervisor that it as acceptable before I went the route of redoing all the existing. As the contributors above have stated, it is not an engineering concern and the differences would be too small to cause a problem.

 

[Julius Right]

In my opinion THW and THWN are equivalent and-outside of NEC jurisdiction-I could use it in parallel.
But, if you have to keep the NEC Art. 310.10(H)(2)(4) ?(4) Have the same insulation type?, it could be a problem.
First of all the ?type? of insulation is the same: PVC.
But, according to UL-83 Table 15.5 the insulation thickness [for THW] has to be from 99 to 110 mils and for THWN Table 15.8 only 63-70 mils.:angel: