Rule 8-104 of the Canadian Electrical Code prescribes maximum permissible operating loads for electrical equipment, and maximum loads that may be carried by service, feeder and branch circuit wiring. This article looks into the requirements and limitations this rule imposes on the application of electrical equipment and wiring.
Rule 8-104(1) gives us some important information on maximum loading for services, feeders and branch circuits. The rule begins by defining the ampere rating of a circuit as the rating of the fuses or circuit-breaker protecting the circuit or the ampacity of the circuit conductors, whichever is less. Question: From this part of the rule, what would be the ampere rating of a motor branch circuit for a 10-ampere motor installed with 15-ampere wiring and protected by a 25-ampere circuit-breaker? Answer: Since the 15-ampere wire size is the lower, the ampere rating of this circuit would be 15 amperes.
Photo 1. Equipment required to be properly rated for the load served
Next, Rule 8-104(2) tells us that a calculated load must not exceed the ampere rating of the circuit that carries the load. This of course makes perfect sense, since overloading would surely cause fuses and breakers to blow or trip for no good reason and conductors may fail or deteriorate by overheating. This rule refers us to guidance provided by other rules that specify how maximum loads are calculated using their applied demand factors. The resultant loads must never exceed the ampere rating of the circuit in each case.
Rule 8-104(3) offers definitions for continuous and non-continuous loads by specifying that the calculated load on any service feeder or branch circuit must be considered continuous for loads:
- up to 225 amperes when they are on for more than one hour in any 2-hour period; or
- over 225 amperes when they are on for more than 3 hours in any 6-hour period.
To simplify this rule, a load is considered continuous when it is on for over 50 percent of the time. This could be a lighting load or an industrial heating load. Both of these loads could be considered continuous. Single-family dwellings, on the other hand, are not considered continuous loads.
Photo 2. Services and feeders must carry proper capacity ratings
Why should we be concerned with the need to define continuous and non-continuous loads? Rule 8-104(4) gives us our first clue. This rule tells us that when a service box, switch, circuit-breaker or panelboard is labeled to show that it is designed to be continuously loaded to up to 100 percent of its ampere rating, it may only be permitted to operate at a maximum load of:
- 8-104(4)(a)—100 percent when connected with conductors having insulation temperature ratings up to 90°C, and selected from either Table 2, for copper wiring or Table 4, aluminum wiring; or
- 8-104(4)(b)—85 percent when connected with conductors having insulation temperature ratings up to 90°C, and selected from either Table 1, for copper wiring or Table 3, aluminum wiring.
But wait, there is more. When the equipment is labeled for operation at up to 80 percent of its ampere rating, or if the equipment carries no information on its continuous loading capability, Rule 8-104(5) says the equipment may be permitted to be continuously loaded to:
- 8-104(5)(a)—80 percent when connected with conductors having insulation temperature ratings up to 90°C, and selected from either Table 2, for copper wiring or Table 4, for aluminum wiring; or
- 8-104(5)(b)—70 percent when connected with conductors having insulation temperature ratings up to 90°C, and selected from either Table 1, for copper wiring or Table 3, aluminum wiring.
Photo 3. Services and feeders must carry proper capacity ratings
Why does the CEC prescribe alternative maximum loading requirements when equipment is connected with either single or multiple conductors? CSA’s standard equipment temperature tests are done using wire sizes selected from Tables 2 and 4. Connecting such equipment with smaller conductors selected from Tables 1 and 3 will reduce the ability of the smaller conductors to absorb and dissipate excessive heating, resulting in higher operating temperatures. To correct this situation, the code derates equipment current ratings when the smaller Table 2 or 4 wire sizes are used.
Rules 8-104(6) and 8-104(7) provide some limitations on minimum wire sizes as well. Rule 8-104(6) tells us that when conductor derating factors from other code rules apply to an installation, we are obliged to use the lower of:
- conductors selected using the applied derating factors so applied; or
- the equipment derating factors specified in Rules 8-104(4) and (5).
For example, if we derate conductors to 80 percent based on Rule 8-104(5) and a lower rating also applies, from a different requirement of the code, the lower rating always applies.
Finally, Rule 8-104(7) tells us that underground conductor sizes must never exceed:
- 85 percent of their assigned current ratings when selected from Tables 1 and 3 for continuous loading up to 100 percent; or
- 70 percent of their assigned current ratings when selected from Tables 1 and 3 for continuous loading up to 80 percent.
Fortunately, these derating factors are already built into the underground conductor ampacity tables in Appendix D.
As with previous articles, you should consult your local electrical inspection authority for a precise interpretation of any of the above.