Information Bulletin: Determining minimum service or feeder conductor size for single dwellings and single dwelling units of row housing or apartment and similar buildings

Persona

Technologies

Technologies
Reference number
IB-EL 2016-07

This Information Bulletin provides guidance on the application of rules pertaining to the 2015 BC Electrical Code Regulation. The requirements of local municipal authorities having jurisdiction may vary. Installers should consult with local authorities having jurisdiction, prior to undertaking work, to determine their requirements.

Topic: Use of Table 39 (T39)

Rule 4-004 Ampacity of wires and cables (see Appendix B)

(23) Notwithstanding Rule 4-006, 3-wire 120/240 V and 120/208 V service conductors for single dwellings and feeder conductors supplying single dwelling units of row housing of apartment and similar buildings and terminating on equipment having a conductor termination temperature of not less than 75 °C shall be permitted to be sized in accordance with Table 39.

When determining the size of service and feeder conductors for single dwellings and feeder conductors supplying single dwelling units of row housing of apartment and similar buildings, several rules throughout the code provide guidance in selecting the appropriate size. Specifically Rules 8-104 (1), 8-200 (1), and 8-202 (1), 4-006 and 4-004(23).

In accordance with 4-004(23), installers wishing to select conductors using the values in Table 39 must provide a load calculation. If the calculated load (A*) exceeds the limit shown in the table, the next larger size conductor shall be used as indicated in Note (3) of the table.

Example 1 – Single Family Dwelling with 105m2 total floor area supplied with Aluminum service conductors.

Minimum ampacity of service conductors shall be the greater of 8-200 (1)(a) or (1)(b). A load calculation determines the load as being 95A. In this example, the total floor area (>80m2) will require the minimum ampacity of the service to be 100A. When selecting the service conductor from Table 39, the rating of the service is the corresponding “Service or feeder rating, A” rating from the Table.

The Directive D-EL 2016-06 interprets the cable as satisfying Rule 8-104(1). From T39 a #2 AWG Al service conductor may be used to supply this installation.

A load calculation which exceeds the 95A threshold will require the next larger size conductor to be used in accordance with Note (3).

Example 2 – Single Family Dwelling with 105m2 total floor area supplied with Copper conductors.

Minimum ampacity of service conductors shall be the greater of 8-200 (1)(a) or (1)(b). A load calculation determines the load as being 180 A. In this example, the minimum service or feeder rating, (A), is 200A based on the load calculation. When Table 39 is used, the rating of the service is the rating of the overcurrent device protecting the conductor in accordance with Directive D-EL 2016-06.

From T39, a #2/0 AWG Cu service conductor may be used to supply this installation. If the load calculation had exceeded the 184 A threshold, the next larger size conductor must be used in accordance with Note (3).

Rationale:

Insulated conductors available in Canada are primarily supplied with a 90C insulation rating.

Rule 4-006 requires conductors to be sized using temperature columns in Tables 1-4 which correspond with the Termination Temperature Rating marked on equipment. This ensures the conductor operating temperature does not adversely affect the overcurrent device.

Subrule 4-004(23) allows the requirements of Rule 4-006 to be set aside, and the size of the service conductor to be selected from Table 39. The calculated loads provided in T39 are the maximum values within which the operating temperatures of the conductors will not affect an O/C device. Conductors with a 90C insulation rating and used in T39 have an ampacity which closely aligns with the rating of the overcurrent device.

Example:

A #2 RW90 Al conductor has an allowable ampacity of 100A, as listed in Table 4. In T39, for calculated loads up 95A, a #2 RW90 Al conductor may be used for service or feeders with a 100A rating. 95A is the ampacity provided in the 75C column of Table 4 increased by 5% in accordance with Rule 8-106(1). This current rating will result in the operating temperature not exceeding the 75C TTR rating of the O/C device.

 

 

References:

Bill 19 – 2003                Safety Standards Act

B.C. Reg. 100/2004       Electrical Safety Regulation

B.C. Reg. 105/2004       Safety Standards General Regulation D-EL 2016-06                                 Directive – Use of Table 39 (T39)