100% vs 80%: Choosing the right OCPD solution

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We make decisions every day, both personally and professionally, with our checkbook in mind.  When decisions are made to “value engineer” or cost out a design to save money on a project, attention is needed on the details; dot your i’s and cross your t’s. Sometimes you think you are saving money or time but in reality, the bottom line tells you different. We’re going to explore the topic of 100% versus 80% rated overcurrent protection and build a foundation for the next project you design, install or inspect. Remember the devil just could be in the details, but attention to the details will help ensure a safe, economical installation.

Overview

The basic process to select the right overcurrent protective device (OCPD) 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.  As we’ll see, when selecting an OCPD to be used at 100% of its current rating, consideration must be given to the enclosure / distribution equipment in which the breaker or fused switch is installed, as well as all associated listings.

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. Let’s do some math.

If the load on a branch circuit is a continuous load and calculated to be 100 A, NEC 210.20(A) requires the OCPD rating to be 125% of the calculated continuous load current.

“Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.”

The OCPD device ampere rating for this example is calculated as follows:

OCPD Amp Rating =

1.25 × Continuous Load  Amps =

1.25 ×100A =125 A

The 80% number is the percentage of the OCPD amp rating that is the continuous load amps, in this case, 100 Amps.  100 Amps is 80% of the 125 A rating of the OCPD as per the following equation:

% of OCPD Rating =

(Load Amps)/(OCPD Rating)×100% =

(100 A)/(125 A)  ×100% =80%

Applying an OCPD at 80% of its rating for continuous loads accounts for the resulting higher ambient temperatures found when an overcurrent device is contained within an enclosure. This also aligns with how an OCPD is tested by the standards which govern their performance.

For this above example, a 100% rated solution would have a 100 A breaker feeding this 100 A calculated continuous load.  Let’s explore this further.

The Load Calculation

The load calculation is where it all begins and where the decision is made as to how the system will be designed with regard to selecting equipment rated at 80% or 100%. In addition to the important contents of Article 220, Branch-Circuit, Feeder, and Service Calculations, which will be left to another article due to the simple fact that load calculations can be a book unto themselves, we need to understand some basic terminology.

Step back and think about what a continuous and non-continuous load is. Determining the difference between a continuous load and a non-continuous load 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. Here are a few examples of continuous loads specified in NEC 2014:

422.13 Storage-Type Water Heaters.  A fixed storage-type water heater that has a capacity of 450 L (120 gals) or less shall be considered a continuous load for the purposes of sizing branch circuits.

424.3 Branch Circuits. (B) Branch-Circuit Sizing.  Fixed electric space-heating equipment and motors shall be considered continuous load.

426.4 Continuous Load.  Fixed outdoor electric deicing and snow-melting equipment shall be considered as a continuous load.

427.4 Continuous Load.  Fixed electric heating equipment for pipelines and vessels shall be considered continuous load.

600.5 Branch Circuits.(B) Rating. Branch circuits that supply signs shall be rated in accordance with 600.5(B)(1) or (B)(2) and shall be considered to be continuous loads for the purposes of calculations.

625.41 Rating.   Electric vehicle supply equipment shall have sufficient rating to supply the load served. Electric vehicle charging loads shall be considered to be continuous loads for the purposes of this article. Where an automatic load management system is used, the maximum electric vehicle supply equipment load on a service and feeder shall be the maximum load permitted by the automatic load management system.

Now that a continuous load and non-continuous load are crystal clear, we take our journey to other appropriate Sections of the NEC for this discussion.  The sections include:

Article 210, Branch Circuits
Section 210.19, Minimum Ampacity and Size
Section 210.20, Overcurrent Protection

Article 215, Feeders
Section 215.2, Minimum Rating and Size
Section 215.3, Overcurrent Protection

Article 230, Services
Section 230.42, Minimum Size and Rating
VII. Service Equipment – Overcurrent Protection

As you can see, common to Services, Feeders and Branch Circuits is a section (Sections 210.19, 215.2 and 230.42) that focuses on sizing and rating of the portion of the circuit for which each article is responsible.  Article 210 is a good representative; the rest have similar language, and so we’ll begin here.  Section 210.20(A) states the following:

210.20 Overcurrent Protection. (A) Continuous and Noncontinuous Loads. Where a branch circuit supplies continuous loads or any combination of continuous and noncontinuous loads, the rating of the overcurrent device shall not be less than the noncontinuous load plus 125 percent of the continuous load.

The first step in our journey for a load calculation, per this requirement, must be to examine each load in the system and determine if each is continuous (three hours or more) or non-continuous.   From 210.20(A), we understand that the 125% factor applies only to continuous loads. The equation for calculating load current, which will drive the selection of our conductors and which ultimately will drive the selection of the OCPD, is as follows:

Load Current =

(Noncontinuous Load Amps) +

(1.25 × 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) reads as follows:

Exception: 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 noncontinuous 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 =

Noncontinuous 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.

Let’s move on in our journey to conductor selection.

Conductor Selection

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.  But for now, our journey takes us to Article 310 for conductor selection, specifically Table 310.15(B)(16) of NEC 2014.  Because we have a calculated load current, whether it be based on an 80% or 100% of continuous load consideration, the selection process for the conductor is now as routine as it gets with all of the details around the environment and methods used to install the conductors.

Let’s use some examples to describe the process for selecting the conductor for the application.  As noted above, this is driven by the load calculation.  With this in mind, let’s use the following examples.

Example 1:  The load on the branch circuit is a continuous 300 Amp load. 

(80% Rated Design) 

Load Current =

(Noncontinuous Load Amps) +

(1.25 × Continuous Load Amps)

Load Current = 1.25 × 300 A = 375 A

The conductor size is selected from Table 310.15(B)(16).  Using the 75oC column of this table puts us into a 500 MCM conductor rated to carry 380 A.

A standard (80%-rated) circuit breaker rated at 400 amperes would be used.

(100% Rated Design) 

Load Current =

(Noncontinuous Load Amps) +

(Continuous Load Amps)

Load Current = 300 A

The conductor size is selected from Table 310.15(B)(16).  Using the 75oC column of this table puts us into a 350 MCM conductor rated to carry 310 A.

A 100%-rated circuit breaker rated at 300 amperes would be used.

Example 2:  The load on the feeder is comprised of a 200 Amp continuous load and 100 Amps of the noncontinuous load.

(80% Rated Design) 

Load Current =

(Noncontinuous Load Amps) +

(1.25 × Continuous Load Amps) Load Current =

100 A + (1.25 × 200 A) = 350 A

The conductor size is selected from Table 310.15(B)(16). Using the 75oC column of this table puts us into two 2/0 conductors or one 500 MCM conductor.

A standard (80%-rated) circuit breaker rated at 350 amperes would be used.

(100% Rated Design) 

Load Current =

(Noncontinuous Load Amps) +

(Continuous Load Amps) Load Current =

100 A + 200 A = 300 A

The conductor size is selected from Table 310.15(B)(16).  Using the 75oC column of this table puts us into two 1/0 conductors  or one 350 MCM conductor.

A 100%-rated circuit breaker rated at 300 amperes would be used.

OCPD Selection

Now that we have a conductor selected, the OCPD is selected to ensure the protection of the conductor. The exception that permits sizing the OCPD for 100% of the continuous load plus the noncontinuous load reads as follows:

“Exception: 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 noncontinuous load.”

These words, or some form thereof, can be found in each of the key articles mentioned above for branch circuits, feeders, and services. Notice that the exception refers to the OCPD and the assembly in which they are installed. Therefore, it is important to understand how an OCPD is tested per its UL listing.

The following text is taken from UL Standard 489, Molded-Case Circuit Breakers, Molded-Case Switches and Circuit-Breaker Enclosures.

“9.1.4.4 A circuit breaker, having a frame size of 250 A or greater, or a multi-pole type of any ampere rating rated over 250 V; 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.

  1. 100% rated solutions for a circuit breaker will have a frame size no smaller than 250 Amps at 250 volts and below, or any case size for multi-pole circuit breaker with a voltage greater than 250 V. Applications where the OCPD frame size is smaller than 250 Amps at 250 volts and less must utilize the circuit breaker at 80% of its amp rating (except for motor overload protection).
  2. The circuit breaker will be marked with a specific enclosure catalog number or minimum dimensions of the enclosure. This tells 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 has to be given to what 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 7.1.4.1.19 of UL 489 which states the following:

“7.1.4.1.19  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 1.55 A/mm2 (1000 A/in2) for ratings less than 1600 A.  For ratings of 1600 A and higher, the bus bar shall be in accordance with Table 7.1.4.1.3.  If the circuit breaker is intended only for use with wiring terminals, the test shall be conducted with insulated conductors, as specified in 7.1.4.1.15. The bus bars or cable shall be at least 1.219 m (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 not to violate the listing of the solution and, as always, the devil is in the details with this regard.

 

Closing Remarks

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 i’s must be dotted and t’s crossed. In this article, we’ve looked only at the source side of the circuit.  In order to complete the analysis, the equipment being supplied must also be investigated to determine if it can be supplied with the often smaller cables associated with the 100% rated circuit breakers or fused switches.

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ThomasADomitrovich@eaton.com'
Thomas Domitrovich, P.E. is a national application engineer with Eaton Corporation in Pittsburgh PA. He has more than 20 years of experience as an electrical engineer and is a LEED Accredited Professional. Thomas is active in various trade organizations on various levels with the Independent Electrical Contractors (IEC), International Association of Electrical Inspectors (IAEI), Institute of Electrical and Electronic Engineers (IEEE), National Electrical Manufacturer’s Association (NEMA) and the National Fire Protection Association (NFPA). Thomas is involved with, and chairs various committees for NEMA and IEEE and is an alternate member on NFPA 73. He is very active in the state-by-state adoption process of NFPA 70 working closely with review committees and other key organizations