Paralleling Different Insulation Types

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Pdan

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Hoping someone can help.

Existing conditions:
2000 ft ductbank
old feeder 4-500 kcmil in one duct
not so old feeder 4-500 kcmil in another duct
cable insulation type of feeders may not match
existing load has been demolished

New load is 150kVA, 480V, 3PH

I would like to parallel the two sets of existing feeders in order to reduce the voltage drop from about 5% (if one set were used) to about 2.5% (using both sets). Since the ampacity of one set of cables is not a limiting factor, I don't see why 310.10(H)(2)(4) would apply. Would paralleling the two feeders with different insulation types violate the code in this case? If so, why?

Thanks in advance.
 
Not sure what the ampacity of the cables has to do with the rules for parallel conductors. This clearly violates 310.10(H)(2)(4).

310.10(H)(2) Conductor and Installation Characteristics. The par-
alleled conductors in each phase, polarity, neutral, grounded
circuit conductor, equipment grounding conductor, or equip-
ment bonding jumper shall comply with all of the following:
(1) Be the same length.
(2) Consist of 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.
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5% voltage is fine for most applications.

Is 2.5% a spec, request from customer, or personal preference?

Only place the NEC mandates a minimum VD is fire pumps and 647 installs, sensitive electronic equipment.
 
While the code does not agree, there is no technical reason to follow the requirements of 310.10(H) where each set of conductors, on its own, has an ampacity equal to or greater than the supply side OCPD.
 
don_resqcapt19 said:
While the code does not agree, there is no technical reason to follow the requirements of 310.10(H) where each set of conductors, on its own, has an ampacity equal to or greater than the supply side OCPD.
Click to expand...

Would that be the same if the issue was about length, size, or material; instead of just insulation?
 
MAC702 said:
Would that be the same if the issue was about length, size, or material; instead of just insulation?
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Yes. Note that the condition stated was if all single sets individually have sufficient ampacity to be protected by the upstreamOCPD.
In other words, if the parallel sets were used only for redundancy or to reduce VD.

The conditions on identical construction and routing for the parallel sets are based on the need to insure an equal (almost exact in some cases) division of current among the sets to avoid overloading any one set. This is a safety concern.

If any set used in the hypothetical situation had a rated ampacity lower than the upstream OCPD rating, then there would be a potential safely issue. As hypothesized, and as was stated, it would be a Code violation even though not a safety issue.
 
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Thanks for the responses. Regarding the special case wherethe ampacity of one set of paralleled feeder cables has not been exceeded, isit likely that the NEC has simply not addressed this condition (even though theyhave for small cables in 310.10(H)(1) Exception 1(c)),or could there be another reason why insulation types should not be mixed forany condition?
 
Pdan said:
Thanks for the responses. Regarding the special case wherethe ampacity of one set of paralleled feeder cables has not been exceeded, isit likely that the NEC has simply not addressed this condition (even though theyhave for small cables in 310.10(H)(1) Exception 1(c)),or could there be another reason why insulation types should not be mixed forany condition?
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My guess is that the NEC wants the parallel conductors to be identical which is including the insulation type. What the insulation has to do with it I don't really know.
 
I hear what Pdan is saying. If just one set of conductors is enough for the load then why would the different insulation matter. It probably doesn't matter, however, the NEC does not give exception to that.
 
If you are adding parallel sets to get a lower voltage drop, are you also upsizing the EGC in the existing parallel sets?
 
Dennis Alwon said:
I hear what Pdan is saying. If just one set of conductors is enough for the load then why would the different insulation matter. It probably doesn't matter, however, the NEC does not give exception to that.
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It is a very small factor, but different insulation type could result in a different conductor temperature under high load, which in turn could cause a small change in resistance. But the result would be to increase the resistance and so decrease the current in that set. Any effect would be reduced by that feedback. If the current is held constant, the increased temperature would increase the resistance which would increase the heat dissipation and thus further increase the temperature. Small positive feedback, but no chance of runaway with normal copper or aluminum wires.
 
tkb said:
If you are adding parallel sets to get a lower voltage drop, are you also upsizing the EGC in the existing parallel sets?
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Yes. And that also calls in to question an unspecified detail in the algorithm for how you determine the EGC upsizing.

Example:
350A circuit, 500kcmil Cu default size, #3 Cu default EGC.

Is adding another set in parallel to curtail voltage drop, equivalent to doubling it in size?
Or do I then have to consider the "minimum size that has sufficient ampacity for the intended installation" to be two sets of 2/0, which would mean an upsize ratio of 3.76?
 
Carultch said:
Yes. And that also calls in to question an unspecified detail in the algorithm for how you determine the EGC upsizing.

Example:
350A circuit, 500kcmil Cu default size, #3 Cu default EGC.

Is adding another set in parallel to curtail voltage drop, equivalent to doubling it in size?
Or do I then have to consider the "minimum size that has sufficient ampacity for the intended installation" to be two sets of 2/0, which would mean an upsize ratio of 3.76?
Click to expand...

The latter. You must consider the total cross sectional area of your conductors compared to the minimum that ampacity requires and upsize your EGC from its minimum size (a la Table 250.122) by at least that ratio.
 
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