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.

By Brian P. Martin, PE, CH2M Hill, Portland, Ore. September 25, 2014
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. 

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.)

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.

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. 

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. 

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

Apply the circuit standard and protect the circuit 

Nothing that we have done above ensures that we end up with a safe, code compliant design. Now that we have developed a circuit schedule, we have to properly apply the rules governing branch circuits and feeders. For both branch circuits and feeders, we can apply this basic methodology:

  1. Determine the overcurrent protection required.
  2. Choose the circuit that matches the overcurrent protective device.
  3. Check the ampacity of the circuit. 

If all of the information about a circuit and its loads is known, it makes sense to check the ampacity of the circuit (step 3 above) before selecting the circuit. In our scenario, this is the early phase of the design. We most likely do not know the routing of the conductors, the ambient conditions, the amount of nonlinear load, or many other factors that impact the circuit ampacity. The ampacity of the circuits must be checked at the end of the design when all loads and routing issues have been confirmed. 

Branch circuits 

Article 210 of the NEC covers the protection of branch circuits. While there are many rules that govern the protection of branch circuits, the most important one to understand is the difference between the requirements for continuous and noncontinuous loads.

A continuous load is defined in NEC Article 100 as “a load where the maximum current is expected to continue for 3 hours or more.” For branch circuits, the rating of the overcurrent protective device must be at least the rating of 125% of the continuous load plus 100% of the noncontinuous load per NEC 210.20(A). 

Let’s look at a simple example. What circuit from our previous schedule would be required for THHN conductors carrying a 20.8 amp continuous load terminated on 60 C terminals? 

  1. Overcurrent protection: Calculate the overcurrent protection size: 20.8 amp x 1.25 = 26.0 amp. 
  2. Choose the circuit: NEC 210.20(A) requires that the overcurrent protection be greater than 26.0 amp, so referring to Table 1, the next size up would be a 30 amp breaker, and thus the circuit size is B2. 
  3. Next, check the ampacity of the circuit. Per NEC 210.19(A)(1)(a), the ampacity of the circuit must be 125% of the continuous load as well. In this case, the result is the same, 26.0 amp, and the ampacity of circuit B2 is 30 amp, so our answer is correct. 

Now that we have looked at a basic branch circuit calculation, how do you apply the circuit protection schedule to feeders? Our procedure for using our circuit schedule is nearly the same for feeders as it was for branch circuits. 

In Figure 1, we have three continuous loads fed by THHN conductors that are terminated on 75 C rated terminals. Using the same procedure from the previous branch circuit section, we determine: 
  1. Overcurrent protection: Per 215.3, the overcurrent device must be not less than 125% of the continuous load, so we determined that our overcurrent protection must be at least 755 amp. This is not a standard size; the next size up in Table 5 is 800 amp. 
  2. Choose the circuit: The circuit in Table 5 next to the 800 amp overcurrent protective device is V3. 
  3. Next, check the ampacity of the circuit: NEC 215.2(A)(1) states that the feeder conductor size shall have an allowable ampacity of not less than 125% of the continuous loads. We have already calculated that 125% of the continuous loads is 755A. Per Table 5, the allowable ampacity of circuit V3 is 760 amp, so circuit V3 is acceptable for these loads. 

Beyond the basics 

In the examples above, we developed and used basic circuit schedules for branch circuits and feeders, and this is enough for many designs. However, many designs will use 100% rated breakers or will consistently require ampacity adjustment/correction of conductors. Depending on the size and schedule requirements of the job, it may make sense to make additional circuit schedules to quickly determine the proper circuit size and protection for those circumstances. Additional common circuit protection schedules may include:

  • Four-wire circuits 
  • Motor circuits
  • Transformer primary and secondary conductors 
  • Conductors that have the same temperature correction applied consistently.

Creating new circuit protection standards for these conditions requires calculations and the proper application of additional sections of the NEC. 

100% rated breakers 

100% rated breakers may allow for additional loads to be connected to the same size feeder, but how do we properly apply them using a circuit schedule? If we weren’t using a 100% rated breaker, we would have to multiply continuous loads by 1.25. The use of a 100% rated breaker changes the math, but otherwise we can use the same methodology and circuit schedule that we developed previously 

  1. Overcurrent protection: Because we have a 100% rated breaker and assembly, per NEC 215.3 Exception 1, “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.” In Figure 1, we determined that our overcurrent protection must be at least 755 amp. This is not a standard size; the next size up in Table 5 is 800 amp (see Figure 2).
  2. Choose the circuit: The circuit in Table 5 next to the 800 amp overcurrent protective device is V3, or two sets of 500 KCM. 
  3. Next, check the ampacity of the circuit: Once again, we are able to use an exception. NEC 215.2(A)(1) Exception 1 states that the feeder conductors “shall be permitted to be not less than the sum of the continuous load plus the noncontinuous load” when we are using a 100% rated breaker and assembly. In Figure 2, we have already calculated that the sum of the continuous loads is 780 amp. However, per Table 5, the allowable ampacity of circuit V3 is 760 amp, so circuit V3 is not acceptable for these loads. We must increase the size of the conductor to a 600 KCM, which per NEC Table 310.15(B)(16) is rated for 420 amp at 75 C. 

 Ampacity adjustment 

Ampacity adjustment and correction, or “de-rating,” is one of the most frequently misapplied and confused items in the NEC. For illustrative purposes, let’s examine a four-wire branch circuit legend. In this case, assume that our four-wire circuit feeds a nonlinear load. Therefore, per NEC Article 310.15(A)(5)(c), we must count the neutral conductor as a current carrying conductor. As a result, we now have four current carrying conductors. The adjustment factor is given in NEC Table 310.15(B)(3)(a), and in our case is 0.8. Now let’s convert our three-wire branch circuit table to a four-wire table. 
1. Calculate ampacity adjustment: The ampacity adjustment of each circuit is calculated in Table 7. Note that for the ampacity adjustment we use the 90 C column of Table 310.15(B))(16). NEC Article 110.14(C) states that “Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.” So as long as our conductor is rated for 90 C, we may use the 90 C rating for this adjustment. 
2. Compare the allowed conductor ampacity and the adjusted ampacity: NEC Article 210.19(A)(1) requires that the conductors must be sized to carry the larger of: 
a) The sum of the noncontinuous load plus 125% of the continuous load, or
b) An allowable ampacity not less than the maximum load to be served after ampacity adjustment. 

Because of this, we have to keep a column for our conductor ampacity with and without ampacity adjustment. 

3. Determine the breaker size: Per NEC Article 240.4(A), we can select the next higher standard overcurrent device. For circuit K4, the next highest is a 150 amp breaker, so this size did not change from our three-wire example. However, for circuit R4 our adjusted ampacity is now 344 amp, so our maximum overcurrent protective device size is 350 amp. This is a reduction from the 400 amp breaker that we used in our three-wire example. 

Now let’s apply Table 8 to a simple example. Assume that we have a four-wire branch circuit feeding a 275 amp continuous load. Our terminals are rated 75 C and we are using THHN wire. Applying the same methodology from our previous examples:

1. Overcurrent protection: Calculate the overcurrent protection size: 275 amp x 1.25 = 343.8 amp. 
2. Choose the circuit: NEC 210.20(A) requires that the overcurrent protection be greater than 343.8 amp, so referring to Table 8, the next size up would be a 350 amp breaker, and thus the circuit size is R4. 
3. Next, check the ampacity of the circuit.
a. Per 210.19(A)(1)(a), the ampacity of the circuit must be 125% of the continuous load.
275 amp x 1.25 = 343.8 amp
The ampacity of our conductor at 75 C must be greater than 343.8 amp. In this case, our circuit ampacity is 380 amp, so circuit R4 is acceptable. 
b. Per 210.19(A)(1)(b), the adjusted ampacity of the circuit must be greater than the load we are serving. In this case, our adjusted circuit ampacity is 344 amp and our load is 275 amp, so circuit R4 is acceptable. 

The method that we used to perform ampacity adjustments for four conductors would work equally well for ambient temperature correction and other adjustments to ampacity. Additionally, the requirements that were applied to branch circuits also apply to feeders, although those requirements are detailed in NEC Article 215.2(A)(1) and 215.3. 

Additional protection, coordination considerations 

Several sections of the NEC have a heavy influence on the protective devices that are selected during design. NEC 240.87 now requires that there be a method to reduce the clearing time of the protective device when the overcurrent device is rated or can be adjusted to 1200 amp or higher. NEC Articles 700 and 701 require selectivity between overcurrent protective devices. 

Electronic trip units (see Figure 3) provide a great deal of flexibility and help the design team meet these requirements. However, they have a dizzying array of settings and options. Using a consistent circuit schedule allows the design engineer to determine the protective device settings for a given circuit and then apply them throughout the design. This can save considerable time during the protection and coordination study and saves time during commissioning. 

Beyond the NEC

It is critically important to remember that the NEC is a minimum safety standard, not a design guide. Just because a design is code-compliant does not mean that it will be easy to construct, commission, and operate. When developing a circuit schedule, the objectives of the design must influence the circuit schedule. The circuit schedule must be influenced by the relative importance of the schedule requirements of design and construction, first cost, lifecycle cost, and the future expansion plans of the facility while maintaining code compliance. 

For example, Figure 2 uses a 1600 amp frame breaker and an 800 amp trip unit to protect feeder V3 (2 parallel sets of 500 KCM). However, if there is a future plan to increase the trip unit size to 1200 amp, the choice of 500 KCM conductors for circuit V3, while code compliant, is a poor one. If we add a third run of 500 KCM, the allowed ampacity of our circuit would be:

380 amp (from 310.15(B)(16)) x 3 = 1140 amp

Applying a circuit with an ampacity of 1120 amp to a 1200 amp breaker is a clear violation of 240.4(C). A better choice would be to create a different circuit for this instance that uses parallel 600 KCM conductors. When a third set of 600 KCM is added, the allowed ampacity of the circuit would be:

420 amp (from 310.15(B)(16)) x 3 = 1260 amp

Using parallel runs of 500 KCM is legal and has the lowest first cost, but is a poor choice for the future expansion plans of the facility. 

The proper design and protection of circuits is a fundamental design task for all facility designs. For a large design, there will be thousands of circuits, all required to meet the NEC, meet the design objectives of the client, and be affordable. Developing a circuit protection standard can enable a large design team to quickly and consistently design a facility on incomplete information. The proper application of the relevant codes and standards, as well as development of a circuit protection standard for the design, can enable the design engineer to rapidly design a facility while meeting all of these objectives. 

Brian P. Martin is PDX electrical discipline manager at CH2M Hill. He is a member of the Consulting-Specifying Engineer editorial advisory board.