Section 8 circuit loading & demand factors • Code File, June 2017
June 28, 2017 - What do circuit loading and demand factors mean, and how are they different? A circuit load is a component or portion of the circuit that consumes power. A demand factor is a time-dependent fractional quantity that will always be less than, or equal to, one. Huh?
In layman’s terms, this means that when we are trying to determine conductor and overcurrent protection sizes, we need to know the load on the circuit, as well as how often and for how long that load will operate on the system. The more often a load operates over a defined period, the more it will affect the type and rating of the overcurrent device and the ampacity of the cables.
So, what is the difference between a calculated load and a continuous load? When we find the calculated load, we must remember it is also considered a continuous load unless it can be shown that some of it is non-continuous. This is determined by the operational factors of the load, the installation type, and the intended or proposed use of the equipment by the customer.
Consider a 15-ft conveyor belt used to move product into the back of a truck at a shipping facility versus a 200-ft conveyor moving product in a processing plant; this example helps illustrate the difference between equipment operation time, not to mention the length of the belt line and the type of work performed in the facility. CE Code Rule 8-104(3) provides guidelines for “normal operation” to determine whether a specific load is continuous or non-continuous.
What is meant by breaker rating? Breakers are built with an ampacity rating and continuous operation rating (expressed as a percentage). So when we talk about a 1000A breaker with an 80% rating, what we really mean is the breaker is rated for operation with a continuous load of only 800A, but may have a calculated load of 1000A. When we are designing a facility under this scenario, we need to be able to break out the non-continuous load from the total calculated load, or limit the calculated load to 800A.
How does circuit loading affect the operation of a breaker? Should the non-continuous load cause the ampacity of the circuit to exceed the breaker’s 1000A rating, then the excessive amount of heating should cause the bi-metal strip in the breaker to trip. However, should the continuous load on the breaker exceed 800A, then the same result could occur and, again, the breaker would trip. So when the system is being designed, we must ensure we are aware of the loading and demand factors to allow for a safe operation under both normal and abnormal operating conditions.
How does cable type affect breaker operation? Let’s first look at the ampacity of a circuit in terms of produced heat. The termination point on the breaker is assumed to be 60C or 75C (unless otherwise marked on the equipment, as per Rule 4-006). Similarly, we need to think about the function of the cables when they are terminated on the breaker or fused switch. When the conductors are selected in accordance with Section 4, they should be sized to satisfy ambient temperature, conduit fill and length of the run, as well as meet the requirements of Rule 14-104.
Once selected, then Rule 8-104(5)(6) places continuous load limits on these conductors. And, when they are single conductors, we are told heating at the point of termination can often be greater than that of multi-conductor cables because the breakers are tested with conductors sized to Tables 2 or 4 (see Appendix B Note to 4-006). So, the continuous load the conductors are permitted to carry needs to be reduced to ensure proper heat dissipation in the switchgear.
In the case of a 1000A 80% rated breaker, the continuous load applied to single conductors, as determined from the calculated load, shall not exceed 70% of ampacities chosen in accordance with Section 4.
Currently, these rules refer to single conductors in free air; however, a change to this Subrule (approved at the Part 1 level) now references cables selected in accordance with Section 4, which encompassed the D Tables for direct burial below 5kV and high-voltage cables above 5kV.
I continue to watch this subject with great interest, and I’m learning more about the historical reasoning behind the Rules. It will be interesting to watch how this all progresses, and to see whether more testing needs to occur or the calculations are deemed sufficient.
N.B. Always consult your AHJ for more specific interpretations.
* This article also appears in the June 2017 edition of Electrical Business Magazine. Check out our ARCHIVE page for back issues.
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