End in Mind - Keep Your Eye on the Ball
Keep your eye on the ball. You may have heard your coach tell you that over and over until you realized that’s just what you needed to do in order to succeed. In business, the message is the same, and it doesn’t change when it comes to electrical safety. In order to keep our eye on the ball, we need a clear understanding of what that ball is. When it all comes together, our designs, plans, and actions ensure we achieve our goals.
All too often bare minimum requirements, whether it be the National Electrical Code (NEC) or others, drive design decisions, while they should instead be driven by our goals and objectives. We need to ensure our actions, decisions, and designs achieve our goals and do not violate bare minimum requirements. Not the other way around. One could argue that if safety is your goal, then not violating bare minimum codes comes along for the ride.
Often, decisions are made to value engineer or cost out designs to save money on projects. This activity is not an issue as long as attention to the details is maintained. We must dot the I’s and cross the T’s to ensure we don’t sacrifice achieving our goal of safety for the cost of getting the job done. Sometimes, we think we are saving money or time; but in reality, the bottom line tells a different story. Today, we’ll explore the topic of 80% vs. 100%-rated overcurrent protective devices (OCPD) and build a foundation for understanding the old phrase... the devil is in the details.
The basic process to select the right overcurrent protective device for this discussion of 80% rated versus 100% rated begins with a calculation of the load, includes a journey through conductor selection based on the calculated load current, and ends with the right OCPD to protect the conductor. A system that is engineered to be fully rated – an OCPD used at 100% of its rating – can result in a lower cost solution, but when we take this approach consideration must be given to details.
In general, for all but motor overload protection, when an overcurrent device, such as a molded case circuit breaker (MCCB) or fuse, is applied in an assembly, it must be sized at 125% of the continuous load. This results in an overcurrent device being applied at 80% of its nameplate rating. For example, if the load on a branch circuit is a continuous load and calculated to be 100A, NEC Section 210.20(A) requires the OCPD rating to be 125% of the calculated continuous load current, which would be 125A. The 100A continuous load is 80% of the 125A rating of the OCPD. This accounts for the resulting higher ambient temperatures found when an overcurrent device is contained within an enclosure and aligns with how an OCPD is tested by the standards that govern their performance. For this above example, a 100%-rated solution would have a 100A breaker feeding this 100A calculated continuous load.
The load calculation is where it all begins and where the basic decision is made as to how the system will be designed with regard to selecting equipment rated at 80% or 100%.
The difference between a continuous load and a non-continuous load is important, but it is not as simple as it sounds. To begin this discussion, open your Code book to Article 100 and review the definition of “continuous load.” NEC 2014 tells us that a continuous load is “a load where the maximum current is expected to continue for 3 hours or more.” For many loads, this will be a very subjective effort of load analysis; but for some, the NEC is specific with this regard (Reference NEC 2014 Sections 422.13“Storage-Type Water Heaters,” 424.3 “Branch Circuits,” 426.4 “Continuous Load,” and 427.4 “Continuous Load” as examples).
Common to services, feeders, and branch circuit requirements are a few sections (Sections 210.19, 215.2 and 230.42) that focus on the sizing and rating of the portion of the circuit for which each article is responsible. Article 210 is a good representative as the rest have similar language, so we’ll begin here. Section 210.20(A) states, “Where a branch circuit supplies continuous loads or any combination of continuous and non-continuous loads, the rating of the overcurrent device shall not be less than the non-continuous load plus 125 percent of the continuous load.”
The equation for calculating load current, which will drive the selection of conductors and ultimately drive the selection of the OCPD, is as follows:
Load Current = (Non-Continuous Load Amps) + (1.25 x Continuous Load Amps)
This equation changes slightly when the decision is made to have a 100%-rated system. A review of the exception to the parent text of 210.20(A) tells us, “Where the assembly, including the overcurrent devices protecting the branch circuit(s), is listed for operation at 100 percent of its rating, the ampere rating of the overcurrent device shall be permitted to be not less than the sum of the continuous load plus the non-continuous load.”
Based on the language in this exception, the load current is calculated for a 100%-rated system based on the following equation:
Load Current = Non Continuous Load Amps + Continuous Load Amps
You’ll note the missing 1.25 multiplying factor in the above equation. From this calculated load current through the selection of conductor and OCPD, the process is exactly the same as that for the 80%-rated system.
The selection of the conductor is based upon the calculated load current previously discussed. As always, Chapters 1–4 of the NEC apply generally, so we can’t forget about the details related to adjusting conductor ampacity and more. Table 310.15(B)(16) of NEC 2014 provides a resource to determine the correct conductor for the application based upon a calculated load current.
Once the conductor is selected, the OCPD is chosen to ensure protection of the conductor. The exception to 210.20(A) reminds us that it is important to understand that 100%-rated OCPD solutions rely on the enclosure in which they are installed. The following text is taken from UL Standard 489, “Molded- Case Circuit Breakers, Molded-Case Switches, and Circuit-Breaker Enclosures.”
“188.8.131.52: A circuit breaker, having a frame size of 250A or greater, or a multi-pole type of any ampere rating rated over 250V, and intended for continuous operation at 100 percent of rating shall be marked: Suitable for continuous operation at 100 percent of rating only if used in a circuit breaker enclosure Type (Cat. No.) ____ or in a cubicle space ___ by ___ by ___ mm (inches). Equivalent wording shall be permitted. Location Category C. The blanks are to be filled in with the minimum dimensions.”
This paragraph enlightens us to some important details:
- 100%-rated solutions for a circuit will have a frame size no smaller than 250A at 250V or below, or any case size for multi-pole circuit breaker with a voltage greater than 250V. Applications where the OCPD frame size is smaller than 250A at 250V or less must utilize the circuit breaker at 80% of its amp rating (except for motor overload protection).
- The circuit breaker will be marked with a specific enclosure catalog number or minimum dimensions of the enclosure.
These details tell us that we can’t just swap out a circuit breaker with one that is rated to handle 100% of its rating for continuous loads. Consideration must be given to the enclosure in which the device is installed. It is not always possible to replace a standard-rated circuit breaker with a 100%-rated circuit breaker and obtain a 100% rating for the application.
There are also requirements that pertain to the enclosure for 100%-rated applications, as demonstrated by Section 184.108.40.206.19 of UL 489, which states the following:
“220.127.116.11.19: For the 100 percent rated test, a circuit breaker shall be connected with copper bus bars if the circuit breaker is intended for use with both bus bars and wiring terminals. Unless the circuit breaker is marked to indicate otherwise, the bus bars shall have a cross section of 1.55A/mm2 (1000 A/in2) for ratings less than 1600A. For ratings of 1600A and higher, the bus bar shall be in accordance with Table 18.104.22.168.3. If the circuit breaker is intended only for use with wiring terminals, the test shall be conducted with insulated conductors, as specified in 22.214.171.124.15. The bus bars or cable shall be at least 1.219m (4 feet) long. The test shall be permitted to be repeated using insulated cable for a circuit breaker intended for use with both bus bars and wiring terminals.”
Not only the material of bus bars but also the dimensions are specific for these applications. Manufacturers will help with what can and cannot be achieved with their equipment. It is important to not violate the listing of the solution and as always, the devil is in the details with this regard.
The use of circuit breakers and fused switches is strictly controlled by the NEC and the UL standards governing the circuit breakers, fused switches, and equipment into which they are installed. There are times when it might be economically advantageous to utilize the devices at 100% of their ratings, but all of the I’s must be dotted and T’s crossed first.
Thomas Domitrovich, P.E. manages a team of Application Engineers for the Circuit Protection Division of Eaton Corporation, specifically Eaton’s Bussmann business. Thomas is based out of St. Louis, Missouri, and has more than 25 years of experience as an Electrical Engineer. He is a LEED Accredited Professional and a licensed Professional Engineer. Thomas is active in various trade organizations and chairs committees for NEMA and IEEE. Thomas is a principal member of Code Making Panel 2 for the National Electrical Code (NFPA 70) and an Alternate member on NFPA 73, both representing NEMA. Thomas is also active in state by state adoption of NFPA 70 and other building codes, working closely with review committees and other key organizations to increase safety.