October 24, 2023 – What is the difference between circuit loading and conductor protection? What is meant by the continuous rating of overcurrent (OC) devices, and does that change the ampacity of the circuit?

CE Code Rule 8-104(1) has a lot to say on the matter! The ampere rating of a consumer’s service, feeder or branch circuit shall be the ampere rating of the OC device protecting the circuit, or the ampacity of the conductors—whichever is less.

Regardless of the continuous operation rating of the OC device, however, the circuit rating is based on the ampere rating, so an 80%-rated 600A breaker has an ampere rating of 600A (just as a 100%-rated 600A breaker also has an ampere rating of 600A).

Rule 8-104(2) then states, “the calculated load in a circuit shall not exceed the ampere rating of the circuit”. Now we get into the realm of circuit loading of overcurrent protection, where the per cent rating of the OC protection is used to determine the maximum continuous load the OC device can sustain.

The first step is to perform a service calculation using CE Code Section 8 to ensure we design a service with sufficient capacity to operate the facility. Next, we determine whether the load is continuous or non-continuous.

At this point, we move over to Section 4 to figure out how the conductors are to be installed (e.g. conduit, free air), and to identify and apply any relevant correction factors to maintain the ampere rating of our conductors. (This information is required in the Section 8 calculations for making our final determination, as per CE Code 8-104[5] or [6].)

We now have our load calculation, which is considered continuous unless it meets Rules 8-104(3), 8-202(3) or 8-202(2), and we know how our conductors are being installed (Rule 4-004) and their ampere rating… but we haven’t yet looked at how to protect them.

The OC device’s ampacity is supposed to protect our conductors, but the second part of Rule 8-104(1) alludes to the ampere rating of the conductor as being circuit ampacity.

(As a best practice, you should always read all the relevant rules every time you use the CE Code-Part I and, yes, this often involves flipping back and forth throughout your code book to find them all.)

In our case, Rule 8-104 is only about circuit loading, not circuit protection. That’s the job of Section 14, but for Section 14 to do its job, we need to know our circuit limits.

We’ve already determined our load and conductor requirements. Now, do we need 100%-rated OC devices, or are standard 80%-rated breakers or fuses sufficient? Also, how do we determine which of the two—the OC device or the conductor—is going to define the circuit?

Ask yourself: is this a maximum calculated or known load? If so, we can look at Rule 14-104 to size the OC device. Rule 14-104(1) states that the rating or setting of the overcurrent device shall not exceed the allowable ampacity of the conductors it protects except 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 words “not available” above do not mean “hard to find” or “delayed in shipping”. Instead, not available refers to non-existent equipment and often leads to requests for exemptions—which are not an automatic “gimme”.)

Also, when the maximum calculated load or known load is in accordance with the rules of Section 8, the ratings or settings given in Table 13 shall be permitted to be used within the maximum value of 800A.

Finally, in accordance with Rule 2-100, you are also required to mark the maximum calculated load, and that no other loading may be added without first re-evaluating system calculations.

David Pilon is manager, Electrical Inspections, at Technical Safety Authority, Saskatchewan (TSASK). He also serves as vice-chair of the Canadian Certified Electrical Inspector (CCEI) committee of the International Association of Electrical Inspectors (IAEI), Canadian Section. He can be reached at david.pilon@tsask.ca.

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