This article discusses Canadian Electrical Code Rule 8-104 Maximum Circuit Loading. The rule is significant, since it defines how electrical circuits and equipment must be rated and it provides limitations on the continuous loading of electrical equipment.
According to the CEC, loads that are ON for a long time are considered to be continuous. Examples are thermostatically controlled or other types of cyclical loads. Rule 8-104(3) defines a continuous load as one:
a) up to 225 amperes, that may continue for 1 hour or longer in any 2-hour period; or
b) that exceeds 225 amperes, that may continue for 3 hours or longer in any 6-hour period.
Why is a precise definition necessary and important? Because the suitability of electrical equipment for continuous loading is dependent upon testing, approval and marking for such use. We therefore need to recognize when the code considers an electrical load to be continuous to ensure that the right electrical equipment is selected. As discussed above, the CEC provides a “line in the sand” beyond which electrical loading is considered continuous and the following special rules apply.
According to Rule 8-104(4), when electrical equipment such as service boxes, fusible switches, circuit-breakers and panelboards is marked as suitable for continuous operation at 100% of its rating in amperes, the loads carried by such equipment must not exceed:
a) 100% of the equipment’s ampere rating when conductor sizes are based on Tables 2 or 4 (conductors in cable or conduit); or
b) 85% of the equipment’s ampere rating when conductor sizes are based on Tables 1 or 3 (single conductors in free air).
According to Rule 8-104(5), when any such equipment is marked as suitable for continuous operation at 80% of its rating in amperes, the loads carried by such equipment must not exceed:
a) 80% of the equipment’s ampere rating when conductor sizes are based on Tables 2 or 4; or
b) 70% of the equipment’s ampere rating when conductor sizes are based on Tables 1 or 3.
There is also an Appendix B note for Rule 8-104, which specifies that equipment not marked as suitable for continuous loading at either 80% or 100% is considered to be suitable for 80% continuous loading.
You may have wondered why electrical equipment rated for continuous operation at 100% or 80% continuous loading must be limited to loads of 85% and 70% when using conductors based on Tables 1 or 3. The answer, equipment standards require that testing and approval be carried using conductors selected on the basis of Tables 2 and 4.
Connecting the same equipment with conductors sized in accordance with Tables 1 or 3 (as permitted by the CEC) will result in smaller wire sizes and therefore higher termination and equipment temperatures. A higher than permitted temperature may build up within the equipment enclosure. As a result of overheating, equipment life may be reduced by insulation breakdown, or the result may be equipment failure, arcing and fire hazards.
Rules 8-105(6) and (7) are there to remind us that the derating factors for continuous loading that apply to electrical equipment also apply to its wiring. The minimum wire sizes are always based on Rule 8-104 plus any other derating factors such as voltage drop, which may be found in other rules of the CEC. Suggestion: when several derating factors apply, always use the one that results in a larger wire size.
Electrical equipment loading is often a combination of continuous and non-continuous loads. For example, a 2000-ampere switchboard is marked for continuous operation up to 80% of its ampere rating. In this example, the selected wiring method is parallel copper R90 single-conductors and wire sizes are based on Table 1.
Question: Does the CEC allow the above switchboard to supply a 1600-ampere load if one-half the load is determined in accordance with the CEC to be continuous and the other half to be non-continuous?
Answer: Since selection of the conductor sizes is based on Table 1, the switchboard must be derated to 70% for the continuous portion of the load and therefore one-half of the 1600-ampere load is recalculated to 800/.70 = 1143 amperes (the minimum required loading capacity of the switchboard). The noncontinuous portion remains at 800 amperes. The total permissible loading capacity then becomes 1143 + 800 = 1943 amperes. This is lower than 2000 amperes and therefore the equipment is acceptable.
As with previous articles, you should always consult the electrical inspection authority in each jurisdiction as applicable for a precise interpretation of any of the above.