6-41 Log #3178 NEC-P06 Final Action: Reject
(310.15(B)(3)(c), Exception (New) )
Submitter: Christel K. Hunter, Alcan Cable
Recommendation: Revise text to read as follows:
Exception: Conductors with thermoset insulation rated at 90C or higher are not subject to this ampacity adjustment.
Substantiation: The supposed risk posed to conductors on rooftops is that the insulation will melt when the wiring method is exposed to direct sunlight on rooftops. Melting can only occur with thermoplastic conductors (such as THWN-2, THW-2, etc.), not thermoset (such as XHHW-2, RHW-2, etc.).
Therefore, excepting thermoset insulation from the temperature correction required by this code requirement will pose no safety concerns, and it will provide a way for installers to avoid the difficulties associated with dramatically oversized conductors (such as incompatible terminations and exorbitant costs of installation) in many areas of the country.
As noted in this excerpt from a U.S. Nuclear Regulatory document discussing the characteristics of electrical cables, thermoset insulation is vastly superior to thermoplastic when resisting damage to heat.
A.2 Electrical Cable Construction
Cables come in a wide variety of configurations. The primary configuration features that define a given cable are the size of the individual conductors [expressed using the American Wire Gauge (AWG)], the number of conductors,shielding and/or armoring features, and the insulation/jacket materials used. Of the materials available for use as cable insulation and jacketing, the broadest categories are thermoplastic and thermoset. Thermoplastic materials melt when heated and solidify when cooled.
Thermoset materials do not melt, but do begin to smolder and burn if sufficiently heated. In general, thermoset materials are more robust, with failure temperatures of approximately 350 °C (662 °F) or higher.
Thermoplastic materials typically have failure temperatures much lower than 218 °C (425 °F), where failure is typically associated with melting of the material.
Panel Meeting Action: Reject
Panel Statement: See the panel statement on Proposal 6-22.
(Panel Statement (from proposal 6-22): Adequate technical substantiation has not been provided to support the proposal. Commercially available conductors are evaluated to specific maximum operating temperatures. 90°C rated conductor’s installations must be evaluated to assure the conductor’s insulation does not exceed its maximum rated temperature under load. Solar heating adjustment needs to be applied even when the wiring methods are 3.5 in. above the rooftop.)
Number Eligible to Vote: 10
Ballot Results: Affirmative: 6 Negative: 3 Abstain: 1
Explanation of Negative:
FRIEDMAN, S.: The Panel action on this proposal should have been Accept.
The information provided specifying that both IEEE and ICEA (Insulated Cable Engineers Association) standards permit thermosetting insulation to operate at 130°C for up to 100 hours in any 12 consecutive months should be sufficient to accept this proposal. The UL fact finding report recommends that a maximum increase of 33°C be added to ambient for ampacity adjustment when cable is on the roof in direct sunlight. The purpose of this addition is to insure that the cable does not exceed its rated operating temperature. Based on
temperatures recorded inside a closed conduit or cable by UL, there is very little chance that the temperature would exceed 130°C at any time during the life of the cable. This may not be true for thermoplastic insulation, since permitted overload temperature for thermoplastics is 105°C. It is agreed that the wire would need to be adjusted for outside ambient temperature, but the ability for thermoset materials to operate for an annual 100 hours at 130°C should satisfy the Panel’s concern in panel statement for 6-22 that wire “not exceed its maximum rated temperature under load”.
HUDDLESTON, JR., R.: Although this Panel member disagrees with the entire premise and basis for Table 310.15(B)(3)(c), this exception would have at least lended some credibility to addressing the real issue - that of insulation damage from heat from direct sunlight being reflected off of a roof. The proposal and its substantiation makes a lot of sense; yet, the Panel dismissed it summarily without addressing the points of issue. The use of thermosetting insulation would alleviate any concerns of damaged insulation due to sunlight heating. Thermosetting insulation has been accepted by IEEE and ICEA for operation at 130C for up to 100 hrs in any consecutive 12 months, which more that satisfies any rooftop sunlight situation. The Panel needs to vote ACCEPT on this proposal.
MADDOX, R.: The panel action on this proposal should have been Accept. In the absence of any testing of conductors or documented failures of conductors exposed to sunlight on roof tops, an exception for thermoplastic insulation is a viable alternative to increasing wire size. The use of thermosetting insulation is a sufficient alternative to address any safety concerns. Test data collected never provided anything other than temperature readings inside of conduits with the ends closed and no sweep up to an enclosure where heat could be dissipated. The methods used during the CDA testing were deliberate and created elevated temperatures within the conduits.
This exception gives the installer in high ambient temperature areas an option and a solution to avoid the termination problems associated with grossly oversized conductors.
Explanation of Abstention:
PICARD, P.: The Aluminum Association could not reach consensus.
Comment on Affirmative:
CLINE, S.: Degradation of insulation due to long-term heat is a decades-long “known” effect and is the basis of existing code. Insulation-flow is the condition which I have observed. If some modern insulations can be laboratory-proven to be more or completely resistant to the high temperatures involved including flow at pressure points, then by all means, please show us the documents. Then these insulation types can be sold in smaller sizes, and still provide a safe installation.
LAIDLER, W.: I ‘m voting to reject this proposal because the submitter makes reference in her substantiation to the fact that the temperature rating of thermoset conductor is higher than the calculated ambient temperature inside the raceway so there is no need to correct the conductors ampacity because the insulation will not be damaged. Article 310,15(B)(2) requires that the ampacity of the conductor be corrected when the ambient temperature surrounding the conductor exceeds the ambient temperature as listed in the table. This is done to insure that the conductor insulation will not be damaged and is also based on the fact that the resistance of an electrical conductor is directly proportional to its temperature. Resistance increases with an increase in temperature and decreases with a decrease in temperature. A larger conductor may have to be used if it is exposed to higher ambient temperatures than 30 degrees C/86 degree F.
