Parallel Conductor - RHW and THHN

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ron

ron

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New York, 40.7514,-73.9925
310.4(B) says Conductor Characteristics. The paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall comply with all of the following: (1) Be the same length
(2) Have the same conductor material
(3) Be the same size in circular mil area
(4) Have the same insulation type
(5) Be terminated in the same manner

Existing feeder of 7 sets of 500's run underground with RHW insulation and is terminated in a SWBD on one end and an ATS on the other end.

Feeder will remain connected on the SWBD end, but needs to be extended to another SWBD elsewhere in the room overhead (disconnected from the ATS), and will use the gutter of the ATS as a raceway to go out the top. Plan is to butt splice (long barrel compression) same size / quantity THHN to the end of the RHW and run the THHN to the new SWBD.

Is the installation described above a violation of 310.4(B)(4)?
 
I'd say it meets the intent but definitely risks an overly-zealous literal interpretation as a violation.
 
Smart $ said:
I'd say it meets the intent but definitely risks an overly-zealous literal interpretation as a violation.
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+1
I don't see a problem but someone could get heartburn from there being 2 types of insulation.
Was there a reason for the RHW insulation?
 
qcroanoke said:
Was there a reason for the RHW insulation?
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It was underground and the client liked the more robust thick insulation of the RHW when it was originally installed 10 years ago.

The extension is a current project.

I guess the parrallel conductors do have the "same insulation type", albeit different within the run from beginning to end.

Just thinking of the risks an overly-zealous literal interpretation as a violation :)
 
+1

I don't see this as an issue either. I think if we look at he intent of the code it is to assure that each corresponding grounded/ungrounded of each set carries the same current. Using that logic each one of the conductor in one conduit matches the conductor in the next conduit and thus has the same characteristics. Along these same lines I think this would apply as well to a situation where, say, you decide to use 600 KCMIL to extend the 500's-the characteristics of each would be the same and thus carry the same current.
 
ron said:
310.4(B) says Conductor Characteristics. The paralleled conductors in each phase, polarity, neutral, grounded circuit conductor, or equipment grounding conductor shall comply with all of the following: (1) Be the same length
(2) Have the same conductor material
(3) Be the same size in circular mil area
(4) Have the same insulation type
(5) Be terminated in the same manner

Existing feeder of 7 sets of 500's run underground with RHW insulation and is terminated in a SWBD on one end and an ATS on the other end.

Feeder will remain connected on the SWBD end, but needs to be extended to another SWBD elsewhere in the room overhead (disconnected from the ATS), and will use the gutter of the ATS as a raceway to go out the top. Plan is to butt splice (long barrel compression) same size / quantity THHN to the end of the RHW and run the THHN to the new SWBD.

Is the installation described above a violation of 310.4(B)(4)?
Click to expand...

The rule clearly states that conductors have to have the same insulation type. RHW is a clearly a different type of insulation than THHN. What are the real life consequences of this violation other than possibly not passing inspection? Not very much at all. The thermal conductance for heat escaping the wire might be slightly different, but I don't think it would be different enough to even cause an ampere of current difference. You are much more likely to have differing currents in each set due to unintentional and unnoticed differences in lengths.

I can understand an inspector being more likely to pass a deviation from this rule, if you are using a modern wire equivalent parallel to an older version of the wire insulation. For instance, if you inherit TW wire, and you expand it with a new parallel set of THWN-2.
 
Carultch said:
The rule clearly states that conductors have to have the same insulation type. RHW is a clearly a different type of insulation than THHN. What are the real life consequences of this violation other than possibly not passing inspection? Not very much at all. The thermal conductance for heat escaping the wire might be slightly different, but I don't think it would be different enough to even cause an ampere of current difference. You are much more likely to have differing currents in each set due to unintentional and unnoticed differences in lengths.

I can understand an inspector being more likely to pass a deviation from this rule, if you are using a modern wire equivalent parallel to an older version of the wire insulation. For instance, if you inherit TW wire, and you expand it with a new parallel set of THWN-2.
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The RHW and THHN are not in parallel to one another, they are in series with each other.

Could have even extended the circuit with aluminum if he wanted - likely needed to increase size though.
 
kwired said:
The RHW and THHN are not in parallel to one another, they are in series with each other.

Could have even extended the circuit with aluminum if he wanted - likely needed to increase size though.
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Yep, similar to my example of increasing the size. But I see Ron's concern of having an inspector that can't think critically and look at the physics of this. I think I would make my case before execution of the work to be safe as this would be a costly redo.
 
qcroanoke said:
Let's not forget the raceways. Can they all be run the same way?
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Yes, they are all set.

But I see Ron's concern of having an inspector that can't think critically and look at the physics of this. I think I would make my case before execution of the work to be safe as this would be a costly redo.
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I draw pictures for a living, and so I was looking for other perspectives from folks that install for a living. :)

In reality, this is in HI and I will be the AHJ along with the client in this case, no permit.
 
texie said:
Yep, similar to my example of increasing the size. But I see Ron's concern of having an inspector that can't think critically and look at the physics of this. I think I would make my case before execution of the work to be safe as this would be a costly redo.
Click to expand...

Now you are thinking logically. While an argument can be made that if you extend each conductor (wire) identically and maintain the rules within 310.10(H) for each phase set, most may say that you have say for example that you have 90 feet RHW and say 10 feet THHN which are to be identical in every way ( splice length, etc) to all other conductors (wires) of that same phase set. So technically compliant with the wording but possibly not with the original intent of 310.10(H). Either way consulting with that the local AHJ will permit is your smart move....and get it written down if possible.
 
I think the proposed installation meets the intent and wording of the section in question. Only thing I'd check is that THW and THHN have an acceptable ampacity for the amperage (dont know the rating of THW w/o looking it up). If I were an inspector I'd allow it, fwiw...
 
What some are not looking at is the OP has an existing feeder that is made up of parallel conductors. The parallel conductor starts at the feeder overcurrent device and ends at a switchboard. The feeder itself continues onto the switchboard bus, and at some point the feeder also continues onto another parallel conductor set. All three components mentioned here are a part of same feeder but there is two separate parallel conductor sets.
 
texie said:
Yep, similar to my example of increasing the size. But I see Ron's concern of having an inspector that can't think critically and look at the physics of this. I think I would make my case before execution of the work to be safe as this would be a costly redo.
Click to expand...
If an inspector can't figure out the physics of this then maybe he needs to be beat with a 2 foot piece of the 500kcmil until it does come to him:happyyes:
 
ron said:
...
Feeder will remain connected on the SWBD end, but needs to be extended to another SWBD elsewhere in the room overhead (disconnected from the ATS), and will use the gutter of the ATS as a raceway to go out the top. Plan is to butt splice (long barrel compression) same size / quantity THHN to the end of the RHW and run the THHN to the new SWBD.
...
Click to expand...

kwired said:
What some are not looking at is the OP has an existing feeder that is made up of parallel conductors. The parallel conductor starts at the feeder overcurrent device and ends at a switchboard. The feeder itself continues onto the switchboard bus, and at some point the feeder also continues onto another parallel conductor set. All three components mentioned here are a part of same feeder but there is two separate parallel conductor sets.
Click to expand...
Seems to me the original parallel status is going to be defeated, extended, then reconnected as parallel. Only one set.
 
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