Selection of conductors: Do we have a problem with this?

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Apparently, we do. And there are numerous bits of information published on this subject that create further confusion. One such piece is published in this journal. It explains alternatives to the application of Rule 4-006 by using correction factors of Section 4. This referenced article also states that the ESA “Bulletin 4-12 permits and clarifies this alternative approach” to Rule 4-006.

As a former regulator who used to publish clarification bulletins for the jurisdiction in which I was authorized to administer the CE Code adopted for a regulatory purpose, I have come to appreciate that any clarification Bulletin should not change the legally adopted requirements of the Code, but should provide interpretation of these requirements. As far as I’m aware, Rule 4-006 has not been revised when the CE Code was adopted for use in the province of Ontario. So, a discussion of the alternatives by an AHJ to a very clearly spelled out Rule 4-006 (which does not provide for such alternatives) appears to be highly questionable. In fact, the reference to the NEC in this regard could be questionable as well, as while Article 110.14(C) of the NEC allows for use of “conductors with temperature ratings higher than specified for terminations” under particular conditions such provision does not exist in Rule 4-006. From a technical perspective, use of correction factors in Tables 5A, B, C or D, as stated in the article, appears to be questionable as well, as not one of these tables has any relation to the requirements of Rule 4-006.

So, let’s concentrate on Rule 4-006.
Why was the change made to the CE Code in this regard?

In order to follow clearly the sequence of events, we need to “unearth” a few relevant bits of history. A number of years ago, the NEC/CEC Ampacity Task Force was established.

This Task Force was charged with the responsibility to conduct a thorough review of the ampacity values of theNEC Tables 310.16 and 310.17 and the CEC Tables 1 – 4, and to harmonize these values in both Codes. The reason for such assignment was based on a need to re-calculate the ampacities used in both Codes in accordance with the objective scientific formula and to facilitate necessary changes to both Codes that would prevent excess of the maximum allowable operating temperature at any connected termination. There are many pieces of electrical equipment (fused switches, circuit breakers, panelboards, etc.) that have been tested, marked (and certified to the harmonized bi-national: UL/CSA) product standards based on the maximum permitted operating temperature at 75° C .

This fact has been captured by the NEC for quite some time. Article 110.14(C) of the NEC mandates temperature limitations at such termination and requires use of conductors being sized on the ampacity values in 75°C column of Tables 310.16 and 310.17.

Until 2012 edition of the CE Code, there was no similar requirement in the CEC, as the previous values of ampacity tables of the CEC were more conservative than in the NEC, and no temperature imitation at conductors terminations were deemed to be necessary. Therefore, users of the CE Code have been routinely selecting conductor sizes based on 90° C column from Tables 1 – 4 for termination on the equipment that is marked for the temperature restrictions up to 75° C.

However, after the assignment to correlate ampacity values have been completed by the NEC/CEC Ampacity Task Force, all values of the NEC Table 310.15(B)(16) [formerly Table 310.16] and 310.15(B)(17) [formerly Table 310.17], and the CEC Tables 1 – 4 have been harmonized.

The result of such harmonization is seen in both Codes. The 2011 edition of the NEC and 2012 edition of the CEC have absolutely identical ampacity values in these respective tables. If we now compare the ampacities values between 2009 and 2012 editions of the CEC, we’ll see that in the latest edition of the CE Code these ampacities (particularly in 90° C column of Tables 1 – 4) have been significantly raised.

It is obvious now that the restrictions mandated by Article 110.14(C) of the NEC have to be reflected in the CEC to prevent unsafe operating temperatures at the point of termination of the conductors to the equipment that is marked with the temperature limitation.

New Rule 4-006 of the CE Code states this requirement as follows:

4-006 Temperature limitations 
(see Appendix B)
(1) Where equipment is marked with a maximum conductor termination temperature, the maximum allowable ampacity of the conductor shall be based on the corresponding temperature column from Table 1, 2, 3, or 4.
(2) Where equipment is not marked with a maximum conductor termination temperature, 90° C shall be used by default.”

Appendix B Note on this Rule provides additional clarification on this requirement:

Appendix B Note on Rule 4-006
The intent of this Rule is to correlate the temperature rating of conductors where the ampacity is selected from Tables 1 to 4 with the lowest temperature rating of electrical equipment or any wire connector (terminal connector, lug, etc.). It is intended by this Rule that the ampacity of conductors be selected from the temperature column in Table 1, 2, 3, or 4 that corresponds to the temperature rating marked on the electrical equipment. As an example, where a conductor is terminated on a breaker with a 75° C rating, the maximum conductor ampacity would be based on the 75° C column of the Tables. It should be noted that the temperature rating of a wire connector (terminal connector, lug, etc.) that is connected to the equipment may be higher than that of the equipment itself; it is the equipment rating that determines the conductor size, not the lug.”

So, based on the requirement of Rule 4-006 and on the clarification Note on this Rule, it appears to be clear that the conductor ampacity should be selected from the temperature column that is consistent with the maximum allowable operating temperature of equipment to which that conductor is intended to be terminated.

But, what if a RW90 conductor is readily available and is intended to be used by a contractor? Could we use the ampacity value from the 90° C column and apply some de-rating factors of Tables 5A – 5C? From the discussions on this issue between the designers, electrical contractors, and regulators, it appears that there is some confusion in relation to this particular point.

Nobody would argue that a typical RW90 conductor could not be selected, but the ampacity of this conductor would now have to be selected from 75° C column of Tables 1 – 4 of the CE Code. So, why can’t we select ampacity for a RW90 conductor from the 90° C column of a respective ampacity Table and apply one of the correction factors of Tables 5A – 5C?

Correction factors of Table 5A, 5B, and 5C are intended only for the allowable ampacity values obtained from Tables 1, 2, 3, and 4.
It should be noted that for equipment with a conductor termination rating of 75° C, Rule 4-006 requires the allowable ampacities to be “based” (i.e., to be obtained) on the 75° C column of Table 1, 2, 3, or 4. The Rule does not permit to base the allowable ampacity on any other value than the value obtained from the 75° C column for equipment with a required conductor termination rating of 75° C.

The rationale for Rule 4-006 was to recognize how the equipment was tested during the certification process.

If we’ll review the CSA standards C22.2 No 4 Enclosed and Dead-Front Switches; C22.2 No 5 Molded-Case Circuit Breakers; Molded-Case Switches and Circuit-Breaker Enclosures; C22.2 No 29 Panelboards and Enclosed Panelboards and C22.2 No 244 Switchboards, we’ll find that all these standards require the conductor used during the temperature testing to be based on a 75° C ampacity.

The maximum allowable operating temperature of any circuit should be always limited to the lowestpermissible value of such temperature for any component of the circuit.

In a practical sense, a conductor ampacity is the operating temperature for a conductor that is fully loaded to the applicable values in Tables 1, 2, 3, and 4, and this ampacity would be equal to the temperature rating of the conductor.

For example, if 4X 350 kcmil copper conductors rated at 75° C are installed in four raceways at an ambient temperature of 30° C and loaded to 1200 A, the operating temperature of these conductors would be 75° C. If we would select the 90° C rated conductor for the same example, the operating temperature of the conductor would be also 75° C.

If, in another example, 3X#3 RW90 conductors in a raceway are loaded to 115 A in accordance with Table 2 of the CEC, then such conductor loading would result in a conductor operating temperature of 90° C. When the number of #3 RW90 conductors is increased (let’s say to eight conductors), Table 5C would require a correction factor of 0.7.  In this case, 115 A x 0.7 = 80.5 A. Therefore, each of these 8X#3 RW90 conductors loaded to 80.5 A will again operate at the temperature of 90° C.

To limit the operating temperature of the conductor to 75° C (the maximum allowable temperature rating of the equipment) the ampacity would need to be selected from the 75° C column, and then the correction factor of Table 5C should be applied to the selected ampacity. In the example of 8X#3 RW90 conductors in the raceway (from the 75° C column of Table 2 and from Table 5C) 100 A x 0.7 = 70 Amp. Therefore, each of these 8X#3 RW90 conductors loaded to 70 amps will again operate at the temperature of 75° C.

The main issue here is the effect that the operating temperature of the conductor has on the equipment to which this conductor is connected. The conductor can handle the ampacity, but the equipment was not tested with terminations above 75° C.

If, in addition to the discussion above, the selected 3X#3AWG RW90 conductors are terminated at the equipment rated at 75° C, and these conductors are operating in the ambient temperature exceeding 30° C, then the appropriate de-rating factor of Table 5A should be applied to the ampacity already selected from the 75° C column. If, for example, the ambient temperature is 40° C, then 100-A ampacity must be multiplied by the correction factor 0.91 from Table 5A, as the actual conductor used in this example is RW90 (and not RW75). Therefore, each of these 3X#3 RW90 conductors would be now loaded to 100 A x 0.91 = 91 A, and these RW90 conductors will again operate at the temperature of 75° C.

And, of course, regardless what correction factors are used for the selected ampacities, the resulting (decreased) ampacity of conductor should be checked for compliance with Rule 8-104 to ensure that it is not less than 125% of the calculated load.

Therefore, in my view, unless Rule 4-006 is revised so as to allow for some alternatives, compliance with this Rule should be consistently applied by all Code users.

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Ark Tsisserev is president of EFS Engineering Solutions, Ltd., an electrical and fire safety consulting company, and is a registered professional engineer with a master’s degree in Electrical Engineering. Prior to becoming a consultant, Ark was an electrical safety regulator for the city of Vancouver. He is currently the chair of the Technical Committee for the Canadian Electrical Code and represents the CE Code Committee on the CMP-1 of the National Electrical Code. Ark can be reached by e-mail at: ark.tsisserev@efsengineering.ca His company web site is: http://www.efsengineering.ca