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.
- 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?
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.
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.
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:
- Determine the overcurrent protection required.
- Choose the circuit that matches the overcurrent protective device.
- 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.
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?
- Overcurrent protection: Calculate the overcurrent protection size: 20.8 amp x 1.25 = 26.0 amp.
- 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.
- 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.
- 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.
- Choose the circuit: The circuit in Table 5 next to the 800 amp overcurrent protective device is V3.
- 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
- 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).
- Choose the circuit: The circuit in Table 5 next to the 800 amp overcurrent protective device is V3, or two sets of 500 KCM.
- 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.
Because of this, we have to keep a column for our conductor ampacity with and without ampacity adjustment.
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:
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:
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:
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.
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