Developing a circuit schedule

Circuit protection, as defined by NFPA 70: National Electrical Code, can be interpreted in many ways, depending on building load and use. Electrical engineers should design a circuit schedule that can be used on the project.

08/30/2017


Learning objectives

  • Learn how to create a circuit sizing standard.
  • Know the applicable codes and standards for circuit sizing and protection.
  • Understand how to develop a circuit legend that can be applied to electrical modeling software. 

To efficiently, quickly, and cost-effectively design electrical systems, developing a circuit schedule that all engineers and designers can use on the project is required. With advance planning, circuit schedules can be created that handle the overwhelming majority of design conditions and ensure continuity across the design. Now that many programs are evolving from simple CAD programs to modeling programs, development of a circuit schedule can help engineers fully use their modeling capabilities.

Developing the circuit standard can be broken down into a few key steps, typically completed in sequence: 

  • Determine the design conditions for which to build a circuit legend.
  • Decide upon a naming standard.
  • Determine standard circuit sizes.
  • Determine standard ground sizes.
  • Develop circuit legends for special conditions.

Following these basic steps, we can develop a legend for our most common design conditions.

Design conditions

The circuit legend is only good for the design conditions for which it is built. Typically, we need to know the ambient conditions, quantity of current-carrying conductors, temperature rating of the terminations, conductor material, and the conductor type and temperature rating. Unless otherwise noted, the examples below will assume: 

  • 30°C—the default condition from NFPA 70: National Electrical Code (NEC) Table 310.15(B)(16).
  • Three current-carrying conductors.
  • Termination provisions of the default condition as per NEC Article 110.14(C)(1):

    • 60°C for 100 amp and lower.
    • 75°C for higher than 100 amp.

  • Thermoplastic high heat-resistant nylon-coated (THHN) wiring (90°C).
  • Copper conductors.
  • A maximum conductor size of 600 kcmil. This size of conductor is widely accepted on terminations for large size breakers while 750 kcmil is less widely accepted.

Table 1: This is a basic single-phase circuit legend, partially complete. This will be expanded upon as the calculations progress. Courtesy: AECOMNaming standard

There are many different schools of thought when deciding upon a circuit-naming standard. Many legends choose to either use sequential naming, alpha-numeric combinations, or circuit-conductor ampacity. The naming in this article will be based on the overcurrent protective device (OCPD), conductor material (e.g. CU), and an indication of phase quantity (-1 for single, -3 for three).

Naming the circuit with the OCPD will enable us to easily select trip sizes during one-line drawing development and allow us to closely align our circuit legend with programs like Autodesk Revit, which uses the OCPD to determine the circuit conductors.

Single-phase legend

Developing a circuit schedule for single-phase circuits is relatively easy. The first steps are to create a table with the standard overcurrent protection sizes listed from NEC Article 240 and to assign a circuit designator (see Table 1).

Table 2: This is the single-phase circuit legend without grounding conductor sizing completed. Courtesy: AECOMNext, 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. There are also two asterisk (**) next to #12 AWG, 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 20CU-1 in Table 2 with an allowable ampacity of 20 amp and 2- to 12-AWG conductors. Circuits 30CU-1 and 40CU-1 use the same logic.

3-phase legend

We can apply a similar methodology to build a 3-phase, 3-wire circuit legend (Table 3). The circuit legend will need circuits rated higher 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 rated higher than 100 amp. An example of a few of these circuits is shown in Table 3.

Circuits 300CU-3, 400CU-3, 800CU-3, and 1200CU-3 in Table 3 require explanation. First, for circuits 800CU-3 and 1200CU-3, the circuit ampacity requires some simple calculations as shown in Table 4.

Table 3: The beginning step to developing the 3-phase legend closely follows the method used for the single-phase legend. This is the 3-phase circuit legend without grounding conductor sizing completed. Courtesy: AECOMFor circuits 300CU-3, 400CU-3, and 800CU-3, the allowable ampacity of the circuit is less than the size of trip. For overcurrent devices rated 800 amp or lower, NEC 240.4(B) allows the "next higher standard overcurrent device rating (above the ampacity of the conductors being protected)" to be used.

Lastly, NEC 240.4(C) states that for overcurrent devices rated more than 800 amp, the "ampacity of the conductors must be equal to or greater than the rating of the overcurrent device." For circuit 1200CU-3, the overcurrent device is more than 800 amp, therefore the ampacity of the conductors must at least be equal to the rating of the overcurrent device.

Ground-circuit sizing

Now that we have the current-carrying conductor sizing determined, we need to size the ground wires. For our example circuit schedule, NEC Table 250.122 describes the required sizes of grounding conductors based on the rating of the circuit's overcurrent device. Using this table, we can fill out our circuit schedule as shown in Table 5. 

Table 4: In order to complete the 3-phase legend, we must multiply by the number of conductors per phase to determine the total circuit ampacity. Courtesy: AECOMSpecial conditions

Many different conditions may impact the basic circuit schedule that we have developed. Design conditions that can impact the application of our circuit schedule include: 

  • More than three current-carrying conductors in a raceway.
  • Ambient temperature.
  • Adjustments due to voltage drop.
  • Adjusting ground conductor sizing.

Creating new circuit-protection standards for these conditions requires calculations and the proper application of additional sections of the NEC. If these conditions happen commonly enough in the design, it saves time to make additional circuit schedules to quickly determine the proper circuit size for those circumstances.

Table 5: The next step in completing the circuit legend is determining the proper ground conductor sizing. This table includes the 3-phase circuits with the ground sizes determined. Courtesy: AECOMAmpacity adjustment and correction, or "derating," is frequently required and can be confusing. In this case, assume that we have several 4-wire circuits that feed nonlinear loads. Per NEC Article 310.15(B)(5)(c), we must count the neutral conductor as a current-carrying conductor. The adjustment factor for more than three current-carrying conductors in a raceway is given in NEC Table 310.15(B)(3)(a). The adjustment required for four to six current-carrying conductors is 0.8. Now let's convert our 3-wire branch-circuit table to a 4-wire table.

To calculate the ampacity adjustment, as long as our conductor is rated 90°C, we can 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."

First, let's try and use a few of the conductors from Table 5 and see if the circuit is still properly protected. For the 300CU-3 circuit, the conductor ampacity per Table 310.15(B)(16), 90°C column, is 320 amp.

  • 320 amp x 0.8 = 256 amp

Because the next highest trip rating is 300 amp, as long as the load current does not exceed 256 amp, this circuit can remain the same.

Table 6: To properly size our circuits, ampacity adjustments must be made for four wire circuits. The table includes the results of the ampacity adjustments using Table 5 as the basis. Courtesy: AECOMFor the 400CU circuit, the conductor ampacity per Table 310.15(B)(16), 90°C column, is 430 amp.

  • 430 amp x 0.8 = 344 amp

Because the next higher trip rating is 350 amp, we need to increase the size of the current-carrying conductors for this circuit so it can be protected by a 400-amp breaker. The results are shown in Table 6.

For these circuits, we need to keep a table of the allowable ampacities for terminations as well as adjusted ampacities. NEC Article 215.2(A)(1) and 210.19(A)(1) require that the conductors must be sized to carry the larger of either:

  • The sum of the noncontinuous load plus 125% of the continuous load.
  • An allowable ampacity not less than the maximum load to be served after ampacity adjustment (refer to Example 3 for the application of continuous and noncontinuous loads).

Table 7: Now that the conductor ampacities have been adjusted, the 3-phase legend including grounded conductors can be completed. Courtesy: AECOMBecause of this, our conductors must be calculated for the ampacity of the conductor at its normal temperature rating in accordance with NEC 110.14(C) and the ampacity of the conductors after derating. In Tables 7 and 8, these will be referred to as termination ampacity and adjusted ampacity, respectively. The 3-phase circuit legend in Table 5 has been recalculated with current-carrying grounded conductors in Table 7. 

Ampacity adjustment: ambient temperature correction

Assume that we plan to route conductors to a 400-amp panelboard through a building with an ambient temperature of 60°C. We can apply the same basic methodology to create a circuit schedule for those conditions. NEC Table 310.15(B)(2)(a) describes derating factors for conductors operating in ambient temperatures higher than 30°C. For a 60°C ambient, the correction factor is 0.71.

Table 8: Using the same basic methodology as was shown in Table 7, we can develop a legend for an ambient temperature adjustment that is frequently encountered in our design. The table shows the ampacity adjustment for an ambient temperature of 60°C. Courtesy: AECOMStarting with the 400CU circuit in Table 5, the conductor ampacity per Table 310.15(B)(16), 90°C column, is 475 amp.

  • 475 amp x 0.71 = 337.3 amp

Because the next highest trip rating per NEC 240.6 is 350 amp, we cannot apply this circuit with a 400-amp breaker in this application.

Because 750- kcmil conductors are unlikely to terminate on a 400-amp panelboard, two conductors per phase are necessary. Using two 4/0 conductors per phase:

  • 520 amp x 0.71 = 369.2 amp

This is acceptable to use on a 400-amp breaker. Repeating this method, the results for a 60°C compensated circuit legend are shown in Table 8. 


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