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 PROCESS

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

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