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Calculated Demand and Underground Ampacities

Posted By Robert Edwards, Sunday, July 01, 2007
Updated: Sunday, February 10, 2013

Synopsis

The main thrust of this article is to establish that Section 8 of the Canadian Electrical Code Part I does not have adequate rules to address derating factors for continuous loads, or to calculate the minimum ampacity requirements for the safe sizing of conductors and electrical equipment, when the underground ampacity calculation is performed to size conductors, and to propose how solutions to these issues should be developed. The current derating factors for continuous loads apply strictly to installations above ground.

It is shown that the continuous load derating factors in circuits installed underground, and the correction factors to be applied to the calculated demand to obtain the minimum required ampacity, can only be derived from derating factors currently in Section 8. However, their application is not intuitively clear and lead to increased complexity compared to circuits installed above ground. Accordingly, the proposals for accommodation of new rules applicable to continuous loads in underground installations need to be simplified as far as possible. The application of appropriate correction factors to the calculated demand is recommended to this end, in place of the more obscure format of derating factors applied to the ampacity, which presently exist for installations above ground.

Introduction

Most code users are generally familiar with the provisions of Section 8, which addresses circuit loading and demand factors. In general terms the rules provide a means of properly sizing conductors and electrical equipment so that they will adequately and safely support the anticipated load currents demanded of the circuits of electrical installations, whether in consumer services, feeders, or branch circuits.

The purpose of most of the rules from 8-200 onward relate to the calculation of the expected electrical loads based on a number of defined attributes of the premises. They include such attributes as their area, the type of use and occupancy, and the types of electrical load anticipated on those premises. Having estimated these load currents by calculation as provided for in the 8-200 to 8-400 series rules, the calculated loads are then used in turn for the sizing of conductors and equipment in each of the circuits.

Rules for the sizing of conductors are contained mostly in Section 4 of the Code, with some further applicable rules appearing in Section 12. However, the provisions of Rule 8-104 must also be observed, and this rule plays an important role in the safe and proper sizing of conductors and electrical equipment. Rule 8-104 has been around for several code cycles in various incarnations, and its provisions merit some in-depth scrutiny to examine whether it continues to meet the needs of code users and regulators.

Rule 8-104

If the purpose of Section 8 is to determine the required circuit ampacity as a basis for establishing the appropriate conductor sizes and equipment ratings for the circuits of the installation, Rule 8-104 has to be examined thoroughly to find any clues as to how this goal is furthered. The title Maximum Circuit Loading suggests limitations or constraints on the loading of the circuits rather than a basis to choose conductor sizes and equipment ratings. In fact, Rule 8-106 appears to give the appropriate signal as to how to obtain conductor sizing, particularly subrule (6). The maximum circuit loading theme is picked up in subrules (4) and (5) of 8-104. In both subrules the key phrase "….the continuous load…..shall not exceed (some prescribed percentage of) the rating of the circuit…” is employed. We can conclude that one major reason for subrules (4) and (5) to exist is to limit the continuous load on the circuit, based upon its ampere rating.

The basis for limiting the continuous load is found in CSA and UL standards for certain common types of electrical equipment, which are described in the first few words of subrules (4) and (5). The product standards provide for equipment to be marked either as suitable to carry 100% or its nameplate rating, or, more commonly, as suitable to carry 80% of its nameplate rating. In order to prevent the overheating of electrical equipment marked as suitable to carry 80% of its nameplate rating, the equipment is required to be derated. When the conductor size is determined in accordance with Tables 2 or 4 of the code (for wiring in raceways, or multiple conductor cables) the continuous current is limited to 80% of the circuit ampere rating. In the case of conductors selected from Tables 1 or 3 of the code (for single conductors in free air), the applicable derating factor is 70%. A derating factor of 85% is also applicable to equipment marked as suitable to carry 100% of its nameplate rating when conductors are chosen from Tables 1 or 3.

Influence of Conductor Size on Electrical Equipment

The application of the specified derating factor is essential to avoid the overheating of the electrical equipment. The conductors are derated along with the equipment, not because the conductors cannot carry the continuous load without overheating in their installed location (which they can), but because the selection of the smaller conductor sizes which are permissible for non-continuous loads would potentially contribute to equipment overheating. Smaller conductors simply cannot conduct heat away from the terminals of the equipment as efficiently. For many years there was no special consideration given to the sizing of equipment in the case of conductors rated according to the single conductor free air ratings of Tables 1 and 3. Rule 8-104 only used to make provision for a derating to 80% of the circuit ampere rating, no matter which tables were used as a basis for conductor selection. The growing application of the single conductor free air ampacities raised concerns that the smaller conductors permitted in Tables 1 and 3, compared with Tables 2 and 4, might give rise to a greater incidence of equipment overheating, particularly in the case of continuous loads. In the 1970s a manufacturers’ task group, with representation from the electrical equipment and wire and cable manufacturers, recommended the derating factors which are present today in subrules (4) and (5). The higher derating factors to be applied when conductors are sized in accordance with the free air ratings of Tables 1 and 3 were supported by test programs carried out by electrical equipment manufacturers.

Continuous and Non-Continuous Loads

Note that subrules (4) and (5) only apply in the case of continuous loads. The case of non-continuous loads is adequately provided for in subrule (2). The question is, how do we differentiate between continuous and non-continuous loads? The best answer to this is found in subrule (3). The adjective "best” is used because subrule (3) provides a practical "rule of thumb,” rather than a precise means of differentiation. In practice, equipment can be installed in a wide variety of possible locations, with different rates of heating and cooling depending on a wide range of factors. These factors include those such as physical dimensions, proximity of other heating and cooling sources, loading factors, and many others besides. Consequently, there is a wide variety of possibilities of the temperatures on the equipment terminals, but only two options are available for the decision as to whether the load is continuous or non-continuous, as outlined in subrule (3)(a) and(b). [See also Rule 8-302(2)]. In addition, the onus of proof for the decision as to whether the load is continuous or non-continuous generally rests with the installer or designer, although some authorities having jurisdiction take this decision out of their hands by declaring that all loads are to be considered continuous. In some "rough and ready” way, subrule (3) provides a means to differentiate between what is considered to be a continuous load, and one which is not. By the wording of the different criteria [either (3)(a) or (3)(b)], there is also some recognition that equipment of lesser ampere rating, and therefore of generally lesser physical bulk, will achieve its steady state temperature more quickly than equipment of higher ampere rating, and greater bulk.

A further observation is pertinent to the issue of continuous and non-continuous loads. The "rule of thumb” of subrule (3) has been around in the Code for many years, and precedes the inclusion of underground ampacities in Section 4. In an underground environment, the time for the conductors to reach a steady state temperature will take far longer than the period of 6 hours, which is the longest period mentioned in subrule (3). When conductors are run underground, whether directly buried or in raceways, the ultimate steady state temperature of the hottest conductor underground will not be achieved within a matter of hours, rather of days, perhaps even many days in the case of high ampacity circuits with many conductors connected in parallel. (A more detailed discussion of this point is provided separately elsewhere). It is clear that subrule (3) is inadequate to describe the characteristics of the conductors installed in the greater (underground) part of the circuit. Of course, the ends of these conductors are connected to the equipment outside of the underground portion of the conductors, because the equipment is installed almost invariably in air, even though the conductors may run underground for much the greater part of their length. For this reason, installations run underground may still quite properly be addressed by the criteria of subrule (3), even if the steady state conductor temperatures in the underground portion of the circuit take much longer to achieve their steady state temperature than does the electrical equipment connected at their ends.

Determination of Circuit Ampacity

We still have to address the question of how the Section 8 rules provide for the progression from the calculated demand to the required ampacity and equipment rating. Subrules (4) and (5) are designed primarily, as we have just seen, to limit the temperature of connections on electrical equipment to safe levels through the use of derating factors. How does this help with the sizing of conductors and equipment? There is perhaps a clue in the use of the phrase "…….the continuous load as determined from the calculated load” in both subrules (4) and (5). The phrase may be considered as ambiguous upon first reading, in fact. It does not say how the continuous load is to be determined from the calculated load. What the wording in italic print is intended to mean is "….the continuous load which for, the purpose of this rule, is to be read as the calculated demand load……” In practice, this phrase actually intends to equate the maximum permitted continuous load with the calculated load, and therefore creates a tie to the circuit ampacity through the derating factors specified. Neither subrule, in fact, explicitly tells the code user to calculate the circuit ampacity from the calculated demand and the derating factors. However, it is implicit, provided that it is agreed that the continuous load mentioned in subrules (4) and (5) is the calculated load. Most code users have learned to apply the derating factors of subrules (4) and (5) inversely, correcting from percentage values to per unit values. For example, a derating factor of 80% would equate to a per unit factor of 0.80, used as a divisor to the calculated demand load to determine the ampacity. This would require a calculation to divide the calculated demand load by 0.80, which is the same as multiplying by 1.25. This is logical, if a little convoluted. It does raise the question, however, as to whether a subrule intended primarily as a means of preventing equipment overheating is the best vehicle for the determination of ampacity from the calculated demand load. Why does such an important calculation not deserve explicit rules of its own?

In retrospect, it is considered that the rules related to limiting the continuous current on electrical equipment for safety reasons would be better separated from the rules related to the determination of minimum required ampere rating from the calculated demand. It can even be argued that, if the code had satisfactory rules to determine the required circuit ampacity from the calculated demand, the necessity to consider what the maximum circuit loading should be would become a redundant exercise. If, in the example given in the previous paragraph, the rules were rewritten to say something such as "Multiply the calculated demand by 1.25 to obtain the minimum required circuit ampere rating,” the step from the calculated demand to the ampere rating of the circuit would be clear. The need to consider what the maximum circuit loading should be also becomes redundant, since the rules would provide the correct ampacity, and the equipment would be automatically protected from overheating without the need to address it. The need for a "determination of minimum circuit rating” rule, instead of a "maximum circuit loading” rule, becomes clearer in considering the particular circumstances associated with underground ampacities and the determination of the appropriate derating factors and correction factors to be applied.

Derating Factors When Conductors Are Installed Underground

A further look at Rule 8-104 brings out a point which is even more important, in that there is a serious deficiency concerning the requirements for conductors sized to underground ampacities. Several years ago, when Rules 4-004(1)(d) and (2)(d) were added to the Code to better accommodate ampacities of conductors installed underground, a further subrule was added to Rule 8-104 in recognition of the possible effects of the conductor sizing on equipment overheating. [See 8-104 subrule (7)]. Calculated underground ampacities can yield conductor sizes which are smaller even than those provided in Tables 1 and 3 for single conductors in free air in some cases. Subrule (7) was added to ensure that conductors used in underground installations cannot in any case be smaller than would be obtained from Tables 1 and 3 for the same ampacity, particularly in the case of continuous loads. However, for conductors sized according to Tables 1 and 3, a derating factor is required according to subrules (4)(b) and (5)(b). As we have seen, this would lead to an increase in the required ampacity and conductor size for continuous loads. How does Rule 8-104 address derating factors for conductor sizes calculated from underground ampacity rules of Section 4? Are derating factors required, or not required, when conductors are sized according to Rules 4-004(1)(d) and (2)(d)? Scrutiny of 8-104(4) and (5) shows that the rules are silent on this issue. Rule 8-104 subrules (4) and (5) only apply to cases when the conductors are installed above ground, whether according to Tables 1 and 3 (for single conductors in free air) or according to Tables 2 and 4 (for wiring in raceways or multiple conductor cables). Not in Rule 8-104, nor anywhere else in the code, are derating factors required when the conductors are sized according to the calculated underground ampacities. This is a serious omission which needs correction. In fact some code users and regulators may sometimes intuitively turn to established derating factors intended for conductors installed above ground, commonly 80%, even without code rules for underground circuits to justify this. However, it will be shown that the technically correct choice may not be readily apparent, and some thought needs to be applied in order to obtain the correct derating factor when conductors are installed underground.

From the point of view of the equipment, the terminals do not "know” whether the conductors connected to them are installed underground or above ground outside the equipment. Common sense dictates that the equipment should be protected from overheating in equal fashion, whether the conductors run underground, or above ground outside the equipment. We have seen, however, that we apply different derating factors for single conductor free air installations than we do for conductors installed in raceways above ground. For conductors installed underground, would the derating factors for single conductor ratings or those for conductors in raceways be the most appropriate choice? Or should there be a separate set of derating factors when the conductor sizes are based on the underground ampacity calculation?

Development of Derating Factors for Underground Installations

It has been shown elsewhere the calculated ampacity of an underground circuit conductor can be significantly different than the ampacity of the same conductor run in free air, and than that run in a raceway above ground. In lower ampacity circuits, the calculated underground ampacity may be considerably higher than that of the same conductor run in raceways above ground, and even exceed the free air ampacity of that same conductor. However, in higher ampacity circuits, the calculated ampacity of the underground conductor may be lower even than the ampacity of that same conductor run in raceways above ground. In circuits of intermediate ampacity, the calculated underground ampacity of the conductor will usually fall between the single conductor free air value and the value of that same conductor run in a raceway above ground. Various cases will be considered for the determination of which derating factor is most appropriate, when the load is continuous. For explanatory purposes, only the case of copper conductors connected to electrical equipment marked as suitable to carry 80% of its nameplate ampere rating will be considered.
The following notes refer to figure 1.

Note 1. When the conductor size calculated from underground ampacity rules equals that provided in Table 2
Given that the equipment is connected to the same size conductor, whether the conductors run above ground in a raceway or below ground, it stands to reason that the derating factor to be applied to the continuous load should be the same in either case, i.e., 80%. Conversely, a correction factor of 1.25 should be applied to the calculated demand load in order to determine the appropriate minimum circuit ampacity.

Note 2. When the conductor size calculated from underground ampacity rules equals that provided in Table 1
By similar reasoning to Note 1, the derating factor to be applied to the continuous load should be the same in either case, i.e., 70%. Conversely, a correction factor of 1.43 should be applied to the calculated demand load in order to determine the appropriate minimum circuit ampacity.

Note 3. When the conductor size calculated from underground ampacity rules lies between the sizes obtained from Table 1 and Table 2

If the calculated load according to Section 8 rules leads to a conductor size in this range, i.e., smaller than that provided by Table 2, but larger than that provided by Table 1, a derating factor of 80% would not be appropriate, as it would not protect the equipment adequately. The smaller underground conductor size than that obtained from Table 2 translates into poorer heat dissipation from the terminals of the equipment, and higher terminal temperatures. Clearly the risk of overheated terminals would exist, although less severe than the degree of overheating which would result from choosing the even smaller conductors which would be obtained from Table 1. Logically, there would be a theoretical derating factor somewhere between 70% and 80% which would be safe for any particular calculated load, but there is no way of knowing what that value would be without extensive testing. In the absence of any other appropriate derating factors to be applied to the continuous load, other than 80% or 70%, the 70% value must be used, even though it would be conservative in many cases. Conversely, a correction factor of 1.43 would be appropriate to apply to the calculated demand load in order to determine the appropriate minimum circuit ampacity.

Note 4. When the conductor size calculated from underground ampacity rules is smaller than that obtained from Table 1
It has already been shown that the effect of 8-104(7) is to limit the ampacity of conductors installed underground so that they cannot exceed the free air ampacity, in this case, of Table 1. The appropriate derating factor is the same as in the case of item 2, i.e., 70%, since the Table 1 ampacity overrules the calculated underground ampacity. Applied to the continuous load, the 70% derating would ensure safety from overheating. Conversely, a correction factor of 1.43 would be appropriate to apply to the calculated demand load in order to determine the appropriate minimum circuit ampacity.

Note 5. When the conductor size calculated from underground ampacity rules is larger than that obtained from Table 2
The larger conductors would lead to more efficient heat dissipation at the equipment terminals than the conductors selected from Table 2, and this is clearly a safer condition than in the case of Note 1. Such a condition would be similar to the case of oversized conductors required from voltage drop considerations. It would also be expected that the equipment would run cooler than would be the case under certification testing, in which the conductors are effectively the same sizes as in provided in Table 2. A derating factor of 80% applied to the continuous load would be appropriate to ensure safety from overheating. Conversely, a correction factor of 1.25 would be appropriate to apply to the calculated demand load in order to determine the appropriate minimum circuit ampacity.

Note 6. When the conductor size calculated from underground ampacity rules is as large or larger than obtained from Table 2, when the calculated demand is increased by 125%

A special case exists of that described in Note 5 when the calculated underground ampacity is less than 80% of the Table 2 ampacity of the same conductor. In that case, the conductor size is clearly adequate without any derating to the underground ampacity, for the conductor is as large or larger than would be the case of the choice from Table 2 even after the application of a 125% factor to the calculated demand. The conductor size would be as large or larger than would apply for certification testing, and would therefore be considered safe. On the other hand, the equipment would still require to be derated to 80% of its rating, for this is also the test current for certification testing. In this case, a factor of 125% would be applied to the calculated demand in order to give the minimum equipment rating, but the conductors could safely be sized with an underground ampacity of 100% of the calculated demand.

Although in the examples chosen, we have considered only copper conductors and electrical equipment marked as rated to carry only 80% of its nameplate ampere rating continuously, it is considered a simple task to extend the logic of items 1 to 5 above to those installations having aluminum conductors, and to equipment marked as rated to carry 100% of its nameplate ampere rating continuously.

It is beyond the purpose of this discussion to propose new code rules to address the points that have been raised. This is the prerogative of the Part I Committee and the Section 8 Subcommittee. It is hoped that the nature of the perceived deficiencies of the Part 1 rules in addressing derating factors, and the required factors to obtain the minimum circuit ampacity from the calculated demand, are better understood. It is expected that the rules addressing the needs of deratings and determination of ampacities may turn out to be more complex in their wording than the wording of the existing 8-104 subrules (4) and (5). This is because the determination of conductor size according to underground ampacity rules leads inevitably to a comparison against what the conductor size would be if the same conductors were to be installed in free air, and if they were to be installed in a raceway above ground. Only then can the appropriate derating factor and factor appropriate to determine the circuit ampacity from the calculated demand be established.

A significant part of this exercise is expected to be the development of a flow chart, intended to be located in Appendix B, associated with rule 8-104. The flow chart will complement the code rules and provide a step-by-step basis to work through the natural progression from the calculated demand to the minimum circuit ampacity. The flow chart has been under development for some time. Because the determination of the minimum ampacity from the calculated load in underground installations is tied closely to the method required for single conductors in free air, and conductors installed in raceways above ground, each of these conditions is included in the flow chart. It should be understood that the flow chart does not apply under current code rules in Section 8, but under Section 8 rules which still require development.


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Tags:  Featured  July-August 2007 

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