Playing matchmaker with conductors, overcurrent devices and loads • Nansy Hanna
November 9, 2016 - The main purpose behind matching the conductor with the correct overcurrent protection is to safeguard the downstream conductor from overload and/or short circuit. Based on that simple concept, the rating of overcurrent protective devices (OCPD) always needs to be equal to or lower than the ampacity of the protected conductor.
There are, however, specific exceptions where OCPDs are permitted to exceed conductor ampacity, such as Rule 28-200 for motors. Another general exception is found in Rule 14-104(1) Table 13.
Moreover, a conductor should be matched with the load, and should not feed a load that can draw more current than its ampacity. That said, the calculated load is permitted to slightly exceed conductor ampacity, and Rule 8-106(1)—a.k.a. the “5% Rule”—provides the parameters for this permission.
Let’s clarify the application of Rules 14-104(1) and 8-106(1), as some inconsistent practices within industry have led to conflicting approaches and confusion.
Rule 14-104(1) does not permit the OCPD’s rating to exceed the protected conductor’s ampacity. But when the conductor ampacity does not match a standard ampere rating of a fuse or circuit breaker, the Rule permits the use of the next-larger standard OCPD, as shown in Table 13, to a maximum rating of 600A.
Table 13 does not modify or change the allowable ampacity of the conductor. Table 13 stops at 600A, so when the OCPD exceeds 600A, its rating is always required to be equal to or lower than the ampacity of the conductor (except where permitted by amended sections of the code).
For example, a 500-kcmil copper conductor with an ampacity of 380A (Table 2 at 75°C) can be protected by a 400A OCPD as per Table 13 for conductors with an ampacity between 351A and 400A.
As circuit ampacity increases and, along with it, conductor size, it is common practice to run parallel sets of smaller conductors. For example, two parallel sets of 300-kcmil copper can be protected by a 600A breaker (Table 2 at 75°C = 285A•2 = 570A total ampacity). Table 13 permits conductors with an ampacity in the range of 501A to 600A to be protected by a 600A OCPD.
The premise behind the application of Table 13 is for the total conductor ampacity of the parallel runs to be protected by one OCPD. It is not permitted to add the OCPD values of Table 13. For example, a circuit protected by a 1200A OCPD is not permitted to be supplied by two parallel runs of conductors with a total ampacity of 1140A (2•570A), as it is not permitted to add the 600A OCPD value of Table 13 for each conductor run with 570A ampacity.
Interestingly, the OCPD rating as determined by Rule 14-104(1) and Table 13 is harmonized between the CE Code and the U.S. National Electrical Code (NEC). The only difference is NEC goes up to 800A. ESA has submitted a proposal to the CE Code to expand Table 13 to 800A.
Let’s now look at Rule 8-106(1), a.k.a. the 5% Rule. When the load is determined by calculations specified in Section 8, Rule 8-106(1) permits the use of the next-smaller standard size switch and/or conductor, provided it is not smaller by more than 5%.
There’s a popular misconception that the 5% Rule applies to all loads, but the 5% Rule is not applicable to fixed known loads that are not calculated. Furthermore, the 5% Rule does not permit the OCPD to exceed conductor ampacity (another misconception!).
For example, a demand calculation (in accordance with Section 8) results in a non-continuous load of 399A. When applying the 5% Rule, a conductor with 380A ampacity can be used. Rule 8-106 does not determine how the conductor is to be protected; that information comes from Rule 14-104. For this example, a 400A OCPD is permitted to be used, based on Table 13.
Matchmaking between conductors, overcurrent protection and loads is important for safety, so apply these Rules only within the specified parameters.
* This article also appears in the November 2016 edition of Electrical Business Magazine. Check out our ARCHIVE page for back issues.
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