Developing a circuit schedule
- 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.
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.
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.
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).
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. 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.
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.
For 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.
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.
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.
Ampacity 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.
For 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).
Because 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.
Starting 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.
Circuit adjustment: ground conductors
Note that, in the examples above for ampacity adjustment, we did not specify the ground conductor size. NEC 250.122(B) requires that "Where ungrounded conductors are increased in size from the minimum size that has sufficient ampacity for the intended installation, wire-type equipment-grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors."
Please note that 250.122(B) is not consistently applied across all jurisdictions. In many jurisdictions, you only need to adjust the equipment-grounding conductor if the ampacity, based on Article 240, is adjusted (typically voltage drop). In other jurisdictions, however, any adjustment to ampacity requires a proportional increase in the equipment-grounding conductor. For our purposes, we will assume the latter.
Using our example for calculating 4-wire circuits (Table 6), our 400-amp conductor increased in size from 500 to 600 kcmil; therefore, the size of our new ground conductor increased in size:
- 600/500 = 1.2.
Our existing ground was a #3 AWG, which per Chapter 9, Table 8, is 52,620 circular mils.
- 52,620 circular mils x 1.2 = 63,144 circular mils
Because a #2 AWG is 66,360 circular mils, our new ground conductor will be a #2 AWG.
Apply the circuit standard and protect the circuit
Now that we have developed a circuit schedule, we must 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 OCPD.
- Verify that the calculated ampacity of the circuit is greater than or equal to the required ampacity.
NEC Article 210 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).
Circuiting using modeling programs
As you can see from the examples, calculating a custom circuit legend for multiple conditions can get tedious. Recently, modeling programs, such as Revit, have included preloaded circuit legends. With these programs, you typically must select the conductor material, termination temperature rating, and the insulation type to use in your project. Based on that, preselected circuit legends are loaded into the project. That legend is then available for circuiting in the model. The circuit size is automatically selected from the legend based on the size of the upstream OCPD that is circuited.
However, there are a few shortcomings to some of these programs. For example, if you select a 400-amp OCPD and conductors with an ampacity of 380 amp, most programs will allow you to load the circuit to 381 amp with no errors. Many either do not apply ambient temperature correction or apply it to the entire model rather than a room or space. You may also disagree with the tables that are preloaded. For these cases, the true power of these programs is the ability to create custom legends similar to the circuit legends that we created above. For example, we can create a custom legend based on Table 8, 60°C Corrected Circuit Schedule, and then select circuits from that legend every time the circuit enters the 60°C ambient area.
When developing a circuit schedule, it must meet the requirements of the NEC, but it also must meet the objectives of the design. Circuit schedules can be developed to use 100%-rated breakers effectively, to account for voltage drop or to migrate easily into short-circuit, protection, and arc-flash-calculation software. By determining the important elements of the design early and developing circuit schedules around those elements, an engineer can enable design teams to quickly and consistently design a facility.
Nonlinear load example
In the simple example as shown in Figure 1, what circuit from the schedule in Table 5 is required for THHN conductors? To answer this question, use the steps outlined below:
1. Calculate the overcurrent protection size: 23.0 amp x 1.25 = 28.75 amp. NFPA 70: National Electrical Code (NEC) 210.20(A) requires that the overcurrent protection be greater than 28.8 amp, so when referring to Table 2, the next size up would be a 30-amp breaker.
2. Choose the circuit: The circuit for a 30-amp breaker is 30CU-3N.
3. Check the ampacity of the circuit:
- Check the terminations. Per NEC 210.19(A)(1)(a), the ampacity of the circuit must be 125% of the continuous load.
- 23 amp x 1.25 = 28.8 amp.
- The ampacity of our conductor using 60°C terminations is 30 amp, so our circuit is acceptable.
- Per NEC 210.19(A)(1)(b), the adjusted ampacity of the circuit must be greater than the load we are serving. Our load is 23 amp and, per Table 7, the adjusted ampacity is 32 amp, therefore circuit 30CU-3N is acceptable.
Ambient temperature correction
In Figure 2, assume that we need to feed a 286-amp continuous load in a space with an ambient temperature of 59°C and we are using THHN wire.
Applying the same methodology and Table 8 from our previous examples:
1. Calculate the overcurrent protection size: 286 amp x 1.25 = 357.5 amp. NFPA 70: National Electrical Code (NEC) 210.20(A) requires that the overcurrent protection be greater than 357.5 amp, so when referring to Table 8, the next size up would be a 400-amp breaker.
2. Choose the circuit: The circuit for a 400-amp breaker is 400CU-3_60.
3. Next, check the ampacity of the circuit:
- Check the terminations. Per NEC 210.19(A)(1)(a), the ampacity of the circuit must be 125% of the continuous load.
- 286 amp x 1.25 = 357.5 amp.
- The ampacity of our conductor at 75°C must be greater than 357.5 amp. In this case, our circuit ampacity is 525 amp, so our circuit is acceptable.
- Per NEC 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 369.2 amp and our load is 286 amp, thus our circuit is acceptable.
The requirements that were applied to branch circuits also apply to feeders, although those requirements are detailed in NFPA 70: National Electrical Code (NEC) Article 215.2(A)(1) and 215.3. In Figure 3, we have three continuous loads fed by THHN conductors.
Using the same procedure from the previous branch-circuit section, we determine the overcurrent protection (X) and the feeder circuit (Y):
1. Overcurrent protection: Per NEC 215.3, the overcurrent device must be not less than 125% of the continuous load.
X ≥ 1.25 x (286 amp + 310 amp + 23 amp) or
X ≥ 773.8 amp
Therefore, the overcurrent protection must be at least 774 amp. This is not a standard size; the next size up in Table 7 is 800 amp.
2. Choose the circuit: The circuit in Table 7 next to the 800-amp OCPD is 800CU-3N.
3. Next, check the ampacity of the circuit:
- Check the terminations. Per NEC 215.2(A)(1)(a), the ampacity of the circuit must be 125% of the continuous load.
- 1.25 x (286 amp + 310 amp + 23 amp) = 773.8 amp.
- The ampacity of our conductor at 75°C must be greater than 773.8 amp. In this case, our termination ampacity is 840 amp, so our circuit is acceptable.
- Per NEC 215.2(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 from Table 7 is 760 amp and our load is 286 amp + 310 amp + 23 amp = 619 amp, therefore our circuit is acceptable.
Brian Martin is a senior electrical engineer, buildings and places, with AECOM. Martin has more than 20 years of experience in engineering planning, project management, design, purchasing, and start-up of industrial projects. He is a member of the Consulting-Specifying Engineer editorial advisory board.