Circuit protection in facilities

Circuit protection, as defined by NFPA 70, can be interpreted in many ways. Understand the codes and standards to create a circuit protection protocol that can be followed in all buildings.

09/25/2014


This article has been peer-reviewed. Learning objectives

  • Understand the basics of circuit protection in nonresidential buildings.
  • Learn how to create a circuit protection standard at your firm.
  • Know the applicable codes and standards for circuit protection. 

You have to complete a big design, with miles of conductors, thousands of circuits, and plenty of new staff to help you. Now you are faced with several dilemmas:

  • How do you make sure that everyone on the team sizes circuits the same way?
  • Since NFPA 70: National Electrical Code (NEC) is the minimum safety code, not a design standard, how do you make sure that the initial design decisions are followed by the entire team? 
  • What do you do when you won’t know the final loads or ambient conditions in a room for months? 

Many articles concentrate on doing the calculations for every branch circuit and feeder and then properly sizing the conductor and overcurrent protection. This works well when you have all of the information needed for these calculations. When you don’t, a well-designed circuit protection standard that incorporates the requirements of the NEC as well as the design objectives, can enable a design team to move forward. The design team can proceed with the design using incomplete information and meet the schedule and budget.

Develop the circuit standard 

Branch circuits: Developing a circuit protection standard (circuit schedule) for branch circuits is relatively easy. For this example, we have assumed that all terminations are rated 60 C because the circuits are all smaller than 100 amp and all conductors are copper. We will also deal with ampacity correction/adjustment in a later section of the article. Once those assumptions are made, we can develop a single- and 3-phase circuit legend using NEC Table 310.15(B)16 as the starting basis. 

Our first step is to create table with the standard overcurrent protection sizes from NEC Article 240 and to assign a circuit designator. An example of this is shown in Table 1. 

Table 1: This is a basic single-phase circuit legend, partially complete. Courtesy: CH2M Hill

Next, we look up the ampacity of conductors in the 60 C column of NEC Table 310.15(B)16. For example, 12 AWG is rated 20 amp in the 60 C column. 12 AWG also has a ** next to it referring to NEC 240.4(D), which limits the overcurrent device size for 12 AWG conductors to 20 amp. Therefore, we can fill in circuit A2 in Table 2 with an allowed ampacity of 20 amp and 3-12 AWG conductors. Circuits B2 and C2 follow using the same logic. (Note: ground conductors should be installed in all conduits. While the NEC permits the use of metallic conduit as a ground path, it is too high an impedance for speedy circuit tripping.)

Table 2: The basic single-phase circuit legend is shown as completed. Courtesy: CH2M Hill

Do not worry about how to apply the “allowed ampacity” column for now; that will be addressed later. 

We can apply a similar methodology to build a 3-phase, three-wire circuit legend (Table 3). In this case, even three-wire delta circuits should have a ground conductor.

Table 3: Shown is a basic 3-phase branch circuit legend example. Courtesy: CH2M Hill

Our phase branch circuit legend is only partially complete. The circuit legend will need circuits rated larger than 100 amp. NEC 110.14(C)(1)(b) allows us to use the 75 C column of NEC Table 310.15(B)16 for circuits larger than 100 amp. An example of a few of these circuits is shown in Table 4. 

Table 4: This is a basic 3-phase branch circuit legend example, continued (75 C). Courtesy: CH2M Hill

Circuit R3 in Table 4 requires further explanation. For circuit R3, the allowed ampacity of the circuit is 380 amp while the breaker size is 400 amp. For overcurrent devices rated 800 amp or less, NEC 240.4(B) allows the “next higher standard overcurrent device rating (above the ampacity of the conductors being protected)” to be used. 

Feeders:The same basic strategy can be employed to complete the schedule so that it includes feeder circuits. While there are code differences between how the loads are calculated for branch circuits and for feeders, we can combine the branch circuit and feeder schedules into one schedule. Later in the design when we select circuits from our schedule, we can adjust the way to calculate the circuit required based on the type of circuit. Examples of larger feeder circuits are shown in Table 5. 

Table 5: Here is a basic 3-phase feeder legend example, continued (75 C).

Circuits V3 and Y3 require further explanation. First, the circuit ampacity requires some simple calculations as shown in Table 6. Courtesy: CH2M Hill

Table 6: This shows 3-phase circuit ampacity calculations. Courtesy: CH2M Hill

For circuit V3, just like circuit R3 in the branch circuit example, we are allowed to round up to the next higher standard overcurrent protective device rating per NEC 240.4(B). For circuit Y3, because the overcurrent protective device is rated over 800 amp, we are unable to round up per NEC 240.4(C). To meet this requirement, the conductor size increased from 500 KCM to 600 KCM. 


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JACOB , FL, United States, 10/30/14 09:21 AM:

Thanks for sharing your methodology. A few comments follow:
1. The building block for load calculations is the branch-ckt load, according to Art. 220 of the NEC. In your methodology, you start from a given load, but do not mention how you arrived at it (except for the continuous / noncontinuous exmple).
2. The service / feeder calculation for the grounded (neutral) conductor was not mentioned.
3. The methodology shown is a manual time-consuming procedure. The answer for an efficient result would be the using of an appropriate professional software program, based on the NEC, where you simply input the pertinent data and the program automatically displays / print the results. This will assure precise and uniform results for all of the engineers working on a given project.
, , 10/31/14 09:40 AM:

Excellent information. Clear, concise and to the point yet very comprehensive at the same time.
EDWARD , OH, United States, 11/04/14 07:15 AM:

For Jacob: Where can I get professional software as a free download? Using this software would make sense and be a useful tool after someone fully understands the design steps that the software would present. Brian Martin described these steps in detail and that is always the first step in understanding. The tables at the end of this article are unreadable.
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