Load Calculations for Services and Feeders. Are we consistent on this subject?

0
215

Section 8 of the Canadian Electrical Code is absolutely critical to the Code users, as the accurate load calculations in accordance with this Section constitute the basis for selection of the appropriate conductors’ sizes in services, feeders and branch circuits, and for selection of the applicable rating or setting of overcurrent devices that protect these conductors.

Unfortunately, in addition to the imperfections in the requirements of 8-200 series rules (i.e., in rules that govern load calculations for selection of correct ampere rating for services and feeders in different types of occupancies), interpretation of these rules is highly inconsistent among the designers, installers, educators and regulators.

In fact, even the CSA Code Handbook (which clarifies the intent of each code rule), appears to be inconsistent with the written 8-200 series rules in the CE Code.

Currently (in 2015 edition of the CE Code), every 8-200 series rule (i.e., rule which deals with load calculations for services and feeders in any type of occupancy) starts with the following statement: “The minimum ampacity of service or feeder conductors…,” disregarding the fact that each such rule only applies to “calculated load for the service or feeder,“ and that the ampacity of the service or feeder conductors is selected in accordance with Section 4 of the CE Code, based on the criteria outlined in Subrules 8-104(5) or (6) of the CE Code.

Furthermore, (although each of these 8-200 series rules in the CE Code is limited right now only to “the minimum ampacity of service or feeders conductors,”) the examples in clarification of these rules in the CSA Handbook are extended to selection of overcurrent devices for such service or feeders conductors.

It should be noted that ampere rating of a consumer’s service, feeder, or branch circuit is defined by Rule 8-104(1) as “The ampere rating of a consumer’s service, feeder, or branch circuit shall be the ampere rating of the overcurrent device protecting the circuit or the ampacity of the conductors, whichever is less,“ and that the rating of the overcurrent device that must protect the circuit conductors is also governed by criteria Rule 8-104(5) or (6) in conjunction with the general provisions of Rule 14-104(1), which states the following:

“14-104 Rating of overcurrent devices (see Appendix B)

(1) The rating or setting of overcurrent devices shall not exceed the allowable ampacity of the conductors that they protect, except

(a) where a fuse or circuit breaker having a rating or setting of the same value as the ampacity of the conductor is not available, the ratings or settings given in Table 13 shall be permitted to be used within the maximum value of 600 A;

(b) in the case of equipment wire, flexible cord in sizes Nos. 16, 18, and 20 AWG copper, and tinsel cord, which are considered protected by 15 A overcurrent devices; or

(c) as provided for by other rules of this Code.”

Code users have indicated that the referenced 8-200 series rules fail to recognize the fact that they apply to calculated loads only, and that Rule 8-104, in fact, sets the fundamental criteria for ampere rating of a service or a feeder—based on the loads calculated in accordance with 8-200 series rules.  This fact created confusion among the code users, and the proposal has been submitted to Section 8 S/C to revise all 8-200 series rules as follows (underlined bold text indicates the newly proposed wording, and bold strikethrough text depicts the deleted wording):

“Revise title of the subsection that covers Rules 8-200 — 8-212 to read: Calculated load for services and feeders

Revise Rules 8-200 to 8-210 as shown (2015 Code text shown).

8-200 Single dwellings (see Appendix B)

(1) The minimum ampacity of service or feeder conductors Calculated load for the service or feeder supplying a single dwelling shall be based on the greater of Item (a) or (b): (no change to remainder)

8-202 Apartment and similar buildings (see Appendix B)

(1) The minimum ampacity of service or feeder conductors    Calculated load for the service or feeder from a main service supplying loads in dwelling units shall be the greater of Item (a) or (b): (no change to remainder)

8-204 Schools

(1) The minimum ampacity of service or feeder conductors   Calculated load for the service or feeder shall be based on the following: (no change to remainder)

8-206 Hospitals

(1) The minimum ampacity of service or feeder conductors    Calculated load for the service or feeder shall be based on the following: (no change to remainder)

8-208 Hotels, motels, dormitories, and buildings of similar occupancy (see Appendix B)        

 (1) The minimum ampacity of service or feeder conductors   Calculated load for the service or feeder shall be based on the following: (no change to remainder)

8-210 Other types of occupancy

The minimum ampacity of service or feeder conductors   Calculated load for the service or feeder for the types of occupancies listed in Table 14 shall be based on: (no change to remainder)”

The proposal was approved by the Technical Committee for the development of the CE Code at its June, 2016 meeting, and the revisions shown above will appear in the 2018 edition of the CE Code.

However, after the proposed changes have taken place, it was also noted by the Code users that the above stated proposal, approved by the Part I Committee, has overlooked the fact that the wording of Rule 8-200(2) and Rule 8-202(3) still references “the minimum ampacity of service and feeder conductors instead of using the adopted terminology Calculated load for service or feeder.”

It was also noted that for determination of a calculated load for service conductors supplying all units in a typical row housing/townhouse complex in accordance with the current wording of Rule 8-200 of the Code, applicable provisions of Subrule 8-200(1)(a) or Subrule 8-200(1)(b) must be used (see below):

(2) The minimum ampacity of service or feeder conductors from a main service supplying two or more dwelling units of row housing shall be based on

(a) Subrule (1), excluding any electric space-heating loads and any air-conditioning loads, with application of demand factors to the loads as required by Rule 8-202(3)(a)(i) to (v); plus

(b) the requirements of Rule 8-202(3)(b), (c), (d), and (e).“

This means that if, for example, the total calculated load in watts of each unit in such 4-unit townhouse complex exceeds 100 A (when it is divided over the supply voltage to the unit), this particular calculated load would have to be used—to determine the minimum ampacity of the service conductors (i.e., 100% of the heaviest load would have to be used plus 65% of the same or smaller load—in the next 2 units, and plus 40% of the same or small load in the unit 4).

However, if the total calculated load in watts of each unit in such 4-unit townhouse complex is less than 100 A (when it is divided over the supply voltage to the unit), and each unit area is not less than 80 sq. m, then in accordance with Subrule 8-200(1)(b)(i)—the minimum ampacity of the feeder conductors supplying each such unit must be assigned at 100 A, regardless whether the total calculated load in the unit is significantly less than 100 A (when it is divided over the supply voltage to the unit).

If, for example, the load of a 1000 sq. ft. unit calculated in accordance with Subrule 8-200(1)(a) was 15000 W, such load would yield 72.12 A for 208 V single-phase supply to the unit, or will yield 62.5 A – for 240 V single-phase supply to the unit.

In light of this requirement, in order to comply with provisions of Rule 8-202(3)(a), calculation of loads in other units of such townhouse complex appears to be performed in a variety of ways, as this fact creates confusion among the Code users.

For the purpose of our example, if the townhouse complex consists of 4 identical units, each with area of 100 sq. m. (or 1000 sq. ft.), despite the fact that Rule 8-200(2)(a) requires to use calculated loads of a unit, (and not the assigned ampacity of feeder conductors supplying the unit), some Code users take 100 A (assigned as the minimum ampacity of the feeder conductors supplying each unit) for the purpose of determination of the minimum ampacity of service conductors in conformance with Subrule 8-202(3)(a). In this particular case, the minimum ampacity of service conductors supplying a 4-unit townhouse building will be: 100 A plus 2 x 100A x 65% plus 100A x 40% = 270A.

Other Code users take the assigned ampacity of 100 A and multiply it by the supply voltage to the unit, in order to use calculations required by Rule 8-202(3)(a). Thus, if the supply voltage to a unit in a townhouse complex is 208 V, such Code users determine that the load in watts of the unit is 208V x 100A = 20,800 W. After that, such Code users take 20,800 W of one unit and add the “loads” of other units as follow: 20,800 W plus 2 x 20,800W x 65% plus 20,800W x 40% = 56160W. Under this calculation, the minimum ampacity of service conductors supplying the townhouse complex with 120/208V 3-phase, 4 conductors service will be 156.1 A.

If the service to such townhouse complex is provided from 120/240 V single-phase system, the exercise shown above looks completely different, as such Code users determine that the load in watts of the unit is 240V x 100A = 24000 W. After that, such Code users take 24000 W of one unit and add the “loads of other units as follow: 24000W plus 2 x 24000 W x 65% plus 24000 W x 40% = 64800 W. Under this calculation, the minimum ampacity of service conductors supplying the townhouse complex with 120/240V 1-phase, 3 conductors service will be 270 A.

Finally, there are Code users who determine the minimum ampacity of service conductors of such 4- unit townhouse complex based on truly calculated loads of each unit (in our example – based on 15000 W), and the result of such determination will be completely different from the values shown above, depending on whether the townhouse complex is supplied by120/208 V 3-phase, 4 conductors service or by 120/240 V 1-phase, 3 conductors service.

Thus, a new proposal was submitted to Section 8 S/C – to remove confusion on this subject, ensure consistency in calculations, and to improve safety. This proposal was intended to correlate wording of Subrules 8-200(2) and 8-202(3) with the wording of every other rule in 8-200 series rules.

Wording of this proposal is as follows (underlined bold text indicates the newly proposed wording, and underlined bold strikethrough text depicts the deleted wording):

“(A) Revise Subrule 8-200(2) to read:

(2) The minimum ampacity of service or Calculated load for the feeder conductors from a main service supplying two or more dwelling units of row housing shall be based on

(a) calculated load in the dwelling unit, as determined in accordance with Subrule (1)(a), excluding any electric space-heating loads and any air-conditioning loads, with application of demand factors to the calculated loads as required by Rule 8-202(3)(a)(i) to (v); plus

(b) the requirements of Rule 8-202(3)(b), (c), (d), and (e)”.

(B)  Revise Subrule 8-202(3) to read:

(2)  The minimum ampacity of service or feeder conductors    Calculated load for the service or feeder from a main service supplying two or more dwelling units shall be based on the calculated load obtained from Subrule (1)(a) and the following: “

This proposal has been unanimously accepted by the members of Section 8 S/C and has been sent to the voting members of the TC for the CE Code, Part I, for their approval.

It is intended that the proposed changes will appear in the 2018 edition of the Code.

So, as illustrated in this article, the changes to the Code represent a continuous improvement process of the

  1. clarity of the language,
  2. consistency between the Code provisions on the relevant subjects on the relevant subjects, and
  3. betterment of electrical safety.

Meanwhile, as usual, the local AHJ administrating the Code, should be consulted on all specific issues of interpretation of the Code technical requirements.

Views All Time
Views All Time
200
Views Today
Views Today
1
SHARE
Previous articleESFI Addresses Common Causes of Shock and Electrocution this National Electrical Safety Month
Next articleCode Hunter – Signs, Part I, 2017 NEC
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

LEAVE A REPLY

Please enter your comment!
Please enter your name here