Back to basics: Sizing low-voltage power conductors for a lift station

NEC insights

• Electrical engineers should pay particular attention to NFPA 70: National Electrical Code Article 310: Conductors for General Wiring.
• A code-compliant design minimizes the risk of overheating and fire due to excessive electrical resistance.

Ensuring proper wire selection is an important part of electrical design. While NFPA 70: National Electrical Code (NEC) is not meant to be used as a design book, it does provide minimum requirements to prevent overheating and fire. The minimum requirements for conductors rated up to 2,000 volts (V), per NEC Article 310, includes information regarding the type of conductors, type of insulation, ampacity ratings and application.

NEC conductor differences

When it comes to applications pertaining to NEC Article 310, conductors are required to be aluminum, copper-clad aluminum or copper, unless otherwise specified. Both aluminum and copper are great industrial conductors, but they do have some differences. The primary differences are the current carrying capacity and resistance of the wire.

Because aluminum conductor has a lower capacity for carrying current and greater resistance than copper conductor, a larger aluminum conductor will be required for the same application as a copper wire. Additionally, aluminum will corrode, requiring the use of special compounds at termination points or for the termination lug to be constructed of copper-clad aluminum.

While copper is the better conductor, it is nearly three times heavier than aluminum and much more expensive. It is the designer’s responsibility to determine which material is appropriate for an installation.

Figure 1: This image shows a motor control center (left), switches (front) and switchgear (right) in an electrical room for a wastewater facility located on the West Coast. Courtesy: CDM Smith

Insulation required by NEC

After determining the wire material, it is necessary to determine which type of insulation to use. This is important because the wire insulation prevents the current in the wire from encountering other conductors, resisting electrical leakage and preserving the wire material. All bare conductors must be insulated unless specifically permitted elsewhere in the NEC.

The type of insulation to select will depend on the application and maximum operating temperature as there are a wide array of insulation types and materials used in the industry. Applications may require the insulation to be flame-retardant, waterproof, resistant to direct sunlight exposure, high heat resistant, corrosion resistant or contain some combination of these qualities. See Table 1 for a few common insulation types used during industrial design. For a complete list of conductor insulations included in the NEC and where they are permitted to be used, refer to Tables 310.4(A) and 310.10.

Table 1: Excerpt from conductor applications and insulations rated 600 V table from NFPA 70, Table 310.4(A). Courtesy of CDM Smith

Conductor sizes

Once the wire material and insulation type has been chosen, the size will need to be determined. The NEC uses two metrics for denoting the size of conductors: AWG and kcmil. American Wire Gauge (AWG) is a standard method for measuring and identifying cable thickness in the United States.

In the NEC, the sizes go from 18 AWG to 4/0 AWG, with the smaller numbers being larger conductors. The 4/0 AWG in this case represents the size 0000, while 3/0 AWG would be 000, working up to size 1 AWG and above.

For conductors larger than 4/0 AWG, the NEC uses the kcmil method of measuring. This method provides a more direct label of size to describe the area of the wire. The “k” represents kilo for 1,000, the “c” represents circular and “mil” represents 1/1,000 of an inch. Therefore, 1 kcmil is 1,000 circular mils, which corresponds to a cross sectional area of approximately 0.5067 mm2.

It should be noted that the NEC requires the minimum size of power conductors for voltage ratings up to and including 2,000 V to be 14 AWG when the wire is copper and 12 AWG if the wire is aluminum or copper-clad aluminum, except as permitted elsewhere in the NEC (NEC 310.3(A)).

Additionally, if conductors are being installed within a raceway, conductors that are 8 AWG or larger must be stranded (NEC Article 310.3(C)). The reason for this requirement is stranded conductors are made of multiple small strands grouped together to form a single conductor, allowing greater flexibility over a solid conductor. This becomes important as conductors become larger and need to be bent and maneuvered into place. See Figure 2 for a cross-section illustration of a solid wire and a stranded wire.

Figure 2: Cross sections of solid and stranded wires. Courtesy: CDM Smith

How to reference conductor sizing in NEC

Now that the conductor material and insulation has been chosen and sizing units are understood, it’s time to size the conductor. For low-voltage conductors, there are several tables that can be used. The table used depends on the type of insulation and whether the conductors are in free air, in a raceway, in a cable, on a messenger or directly buried in earth.

These tables include Tables 310.16 through 310.20. An additional table, Table 310.21, is included for bare copper and bare aluminum conductors in free air. Refer to Table 2 for an excerpt from the NEC Table 310.16.

Once the correct table is selected, the column with the desired insulation type should be used to determine the correct conductor size based on the amperage needed. For example, using Table 310.16 for no more than three current-carrying conductors in a raceway, cable or earth, copper conductors insulated with THWN rated at 75°C is rated at 50 amperes (A) for size 8 AWG. Conductor amperage ratings must be determined by the design professional and meet the minimum NEC requirements, which vary based on the desired application.

There may be times when the necessary conductor amperage is greater than the rated capacity of the largest conductor included in the NEC or in installations where it is not preferrable to use a given size conductor due to the difficulty of installing certain conductor sizes. Often conductors larger than 600 kcmil will not be used because they are difficult to bend and thus more difficult to install. For these instances, it can be a good idea to install multiple sets of conductors in parallel.

Installing conductors in parallel increases the amperage capacity while reducing the size and difficulty of installing the conductors. To install conductors in parallel the conductors must be at least 1/0 AWG or larger and comply with all the requirements of 310.10(G)(2) through (G)(6). Some of these requirements include the conductors being the same length, the same material, the same kcmil size and being terminated in the same manner.

Adjustment factors

As stated in NEC Table 310.16, which was used in the previous example, the table is meant for no more than three current-carrying conductors in a raceway, cable or directly buried. If it is desired to have more than three current-carrying conductors within a raceway or cable, then it will require “derating” of the conductors. The process of derating conductors involves reducing the maximum amperage capacity from the rated ampacity defined in Tables 310.16 through 310.19.

The reason for this adjustment is because increasing the number of conductors within a raceway or cable will increase the heat generation within the raceway. To prevent an excess of heat buildup, an adjustment factor is used to lower the conductor resistance and thereby lower the heat production.

For example, if eight current-carrying conductors were placed within a single conduit, the maximum amperage rating of the conductors would be reduced to 70% of the current rated in the ampacity tables. This adjustment may require upsizing conductors larger than usual. The complete table for adjustment factors for more than three current-carrying conductors can be found in Table 310.15(C)(1).

To ensure conductors and cables can be identified, the NEC requires that the maximum rated voltage, type of wire or cable, manufacturer name or trademark and size in AWG or kcmil are marked on the conductors and cables. Depending on the type of conductors and cables this can take the form of surface marking, marker tape or tag marking. See Figure 3 for a photo of conductor cables with appropriate markings per the NEC.

Figure 3: Conductor cables with markings for a transformer. Courtesy: CDM Smith

Learn how to size low-voltage power conductors for a wastewater lift station

This case study presents the process for sizing low-voltage insulated power conductors at a wastewater lift station. This does not include the sizing of the equipment grounding conductor, which is in NEC Article 250.122. Although power conductor sizing works hand in hand with various aspects of design such as calculation of loads, sizing of protective devices, size/number of conduits, etc., these topics will not be fully detailed in this case study.

However, it is important to remember that the size of the load (i.e., the demand) is what drives the size of the conductors. Before the type of wire can be selected or sized, it is important to understand the location where the wire will be installed.

What is a wastewater lift station?

A wastewater lift station is commonly used to provide the additional pressure boost to the raw sewage in the wastewater transportation system. Wastewater lift stations are typically located outdoors, in a residential neighborhood or in locations convenient for the piping system.

The loads commonly seen at a wastewater lift station are:

• 120 V loads (i.e., lights, receptacles, sump pumps, instruments, etc.).

Wastewater lift stations require utility power and a secondary standby power. Depending on the criticality of the lift station, the standby power can be in the form of a portable generator connection or a standby generator.

In other words, a wastewater lift station is a critical industrial facility typically located outdoors (i.e., wet, corrosive, hazardous location) that needs reliable and sturdy connections for motor loads in a small space. This also means that wires must be housed in a raceway to allow for extra protection against big equipment and corrosive materials.

In this case study, a standby generator will be considered in tandem with a single utility source. The two power sources feed an automatic transfer switch (ATS) that is normally fed from the utility source and switches over to the standby power source during a utility outage. The ATS then feeds a pump control panel that powers two pumps, a grinder control panel and a lighting transformer that feeds the 120 V loads. The one-line power diagram is shown in Figure 4.

Figure 4: This is the one-line diagram for a typical wastewater lift station, which will be used for the case study. Courtesy: CDM Smith

Wire type

Copper is the most common choice for many wiring applications due to its durability and conductivity. On the other hand, aluminum is much cheaper and a lightweight material that is more malleable and easier to install. Every material has its advantages and disadvantages. For this case study, copper conductor is the best option because this application requires durable connections that doesn’t take up a lot of space.

Per NEC Article 310.10, some of the types of conductors permitted for use in wet locations are as follows: Types RHW, RHW-2, THHW, THWN, THWN-2, TW, XHHW, XHHW-2, XHWN, XHWN-2, etc. All conductors will be protected by a raceway, therefore, thicker wire types that can be used for direct burial applications such as type USE will not be used in this application.

Due to the wet location type and criticality of the lift station, the service entrance feeder wire will be Type RHW-2 because RHW-2 is rated 90°C for dry and wet locations and has one of the thicker insulations in addition to a moisture-resistant, flame-retardant, nonmetallic covering. Type XHHW-2 will be used for the feeders and branch circuits as it is rated 90°C for dry and wet locations but has an insulation thickness that is slightly smaller and cheaper than Type RHW-2.

In addition, type cross-linked polyethylene insulation is lighter than other rubber insulations, which makes it easier to install.

Sizing power conductors

NEC Table 310.16 will be used as a reference because the wires for this wastewater lift station are insulated conductors with no more than three current-carrying conductors in a raceway. The conductor, as mentioned previously, is copper so the column section for copper in Table 310.16 will be used.

Per NEC Article 110.14(C), unless otherwise indicated on the equipment, termination provisions of equipment for circuits rated 100 A or less or marked as 14 AWG through 1 AWG conductors, shall be used only for conductors rated 60°C along with the other conditions mentioned in Article 110.14(1)(a).

In other words, when sizing conductors using Table 310.16, the ampacity of conductors sized 1 AWG or smaller shall be determined using the column for 60°C in Table 310.16 or as shown underlined in Table 2. Per Article 110.14(C)(1)(b), termination provisions of equipment for circuits greater than 100 A or marked for conductors larger than 1 AWG, shall be used for conductors rated 75°C along with other conditions mentioned (i.e., conductors sized larger than 1 AWG shall be determined using the column for 75 ° C or as shown underlined in Table 2). The following details the design decisions for sizing the lines in Figure 4.

Table 2: Excerpt of ampacities of insulated conductors with not more than three current-carrying conductors in raceway, cable or earth (directly buried) from NEC Table 310.16. Courtesy: CDM Smith

Utility transformer to service entrance disconnect

NEC Article 230.42 covers the conditions for selecting the ampacity of the service entrance conductors. In this case study, the service entrance conductors terminate in an overcurrent device where both the overcurrent device and its assembly are listed for operation at 100% of their rating.

Therefore, the service entrance conductor (shown in Line 1) must have an ampacity of 400 A or more. Referring to Table 310.16, column 75°C, the wire would be 600 kcmil (420 A). However, 600 kcmil will not be used because 600 kcmil wires can be difficult to bend and pull in certain installations. Therefore, paralleling conductors will be considered and per Article 310.10(G)(1), only sizes 1/0 AWG and larger can be paralleled. Size 4/0 AWG wires (230 A per Table 310.16, 75°C) will be paralleled to get over 400 A, thus resulting in two sets of three #4/0 (i.e., six 4/0 AWG wires total, three in each conduit).

The 3/0 wire can be used but because the protective device is 100% rated (carries 400 A continuously instead of 320 A), it is recommended to provide conductors with ampacity higher than the protective device ampacity.

Service entrance, generator to ATS to pump control panel

All these wires are essentially sized the same because their overcurrent protection device upstream of the wire are the same ratings. Per Article 240.4, conductors, other than flexible cords, flexible cables and fixture wires, shall be protected against overcurrent. Therefore, the conductor must be sized equal to or bigger than the overcurrent protective device, which is 400 A. Paralleling 4/0 AWG wires will result in 460 A; thus, resulting in two sets of three #4/0 AWG.

Additionally, NEC Article 445.13 requires the ampacity of conductors from the output terminals of a generator to the first distribution with overcurrent protection must be 115% of the generator’s nameplate current rating or greater. If the generator design and operation prevent overloading, the conductor’s ampacity must be 100% of the generator’s nameplate current rating or greater. Because the current rating of the example generator is approximately 376 A, at 115% this would require approximately 432 A.

Pump control panel to pump

The size of the load determines the size of the conductors and indirectly the size of the protective device. In this case study, the protective device has already been sized to protect the wire and account for the variable frequency drive (VFD) load. The wire is protected by a 200 A circuit breaker, meaning the wires from the pump control panel to the VFD must be rated for 200 A, which is three #3/0 AWG.

Line 6 would essentially be of the same size. However, let’s assume the pump and its respective pump terminal cabinet is around 800 feet away from the VFD. Because of the distance, voltage drop needs to be considered. Due to the resistance of the wire, which correlates to the size of the wire, the voltage can drop down to 3%.

Though not required by the NEC, the NEC recommends that the voltage drop be 3% or less at the farthest load and the maximum total voltage drop on both feeders and branch circuits to the farthest load does not exceed 5% for reasonable efficiency of operation. Other state building codes and standards may require a voltage drop of around 3%-5%. A bigger wire means more copper, which means less resistance, resulting in a lower voltage drop. In this instance, if the three #3/0 is increased to three #4/0, the 800-foot distance would result in a voltage drop of less than 3%.

Line 7 is cables typically provided by the submersible pump manufacturer and each pump manufacturer recommends different sizes and types that is most suitable for their pumps and application. Therefore, it is crucial to coordinate with the submersible pump manufacturer when it comes to providing the correct cable from the pump terminal cabinet to the submersible pump.

Pump control panel to transformer

Line 8 is the wires from the pump control panel to the 15 kilovolt-ampere (kVA) lighting transformer, TX-1. The protective device for TX-1 is rated 30 A. Per Table 310.16, 60°C column, three #10 (30 A) wires should be protected by the protective device.

Figure 5: Electrical equipment at a wastewater lift station. Courtesy: CDM Smith

Transformer to lighting panel

Line 9 is the wires from the transformer to 120/208 V lighting panelboard, LP-1. Wires on the secondary side of a transformer, per Article 240.4(F), are not considered to be protected by the primary overcurrent protective device if the transformer is a single-phase and multiphase except for two-wire, delta-delta or three-wire transformers.

Therefore, the wires on the secondary side shall not be sized based on the primary overcurrent protective device. In this case, the transformer is located within 10 feet of LP-1; thus, the transformer secondary conductor can be sized per NEC Article 240.21(C)(2). The ampacity of the secondary conductors must not be less than the combined calculated loads on the circuits supplied by the secondary conductors, not less than the rating of the equipment containing an overcurrent device(s) supplied by the secondary conductors or not less than the rating of the overcurrent protective device at the termination of the secondary conductors.

In addition to this, Article 240.21(C)(2) details other requirements for the secondary conductors concerning location, protection and installation, which will not be discussed in this article.

As the one-line in Figure 4 does not detail the size of the LP-1, the maximum capacity a 15 kVA, three-phase, four-wire transformer can support can be calculated with the following equation:

A = kVA / (kV x sq rt(3)) = 15 kVA/(0.208 kV x sq rt(3)) = 41.63 APer Table 450.3(B), the maximum secondary protection shall be 125% of the transformer-rated current. Therefore, the secondary conductor is 125% of 41.63 A, which is 52.04 A. This is the maximum protection, therefore 50 A protective device can be used.

However, as 52.04 A does not correspond to a standard rating of a fuse or nonadjustable circuit breaker, a higher rating that does not exceed the next higher standard rating shall be permitted. In other words, a 60 A protective device can also be used as a secondary protective device.

However, for this case study, a 50 A protective device will be assumed for LP-1. This would essentially mean four #6 (55 A per Table 310.16, column 60°C) is the size of the secondary conductors.

Lighting panel to generator loads

Line 10 is not shown on the one-line in Figure 4 but the wires fed from LP-1 would typically be shown on a panel schedule. Line 10 includes the wires from LP-1 to the generator enclosure.

Panel LP-1 is providing power to the 120 V loads in the generator enclosure, which may include the following:

• Oil level control.

• Jacket water heater.

• Battery charger.

• FfGenerator space heater.

For simplicity’s sake, it is assumed these six loads are essentially all 120 V, 20 A circuits that would typically require two #12 (20 A per Table 310.16, column 60°C). Because the loads are in the same location, all the wires can be put in the same raceway.

This is 12 current-carrying conductors. Per NEC Table 310.15(C)(1), 10-20 current-carrying conductors in a raceway or cable have an adjustment factor of 50%. This essentially means that the ampacity of each of the #12 wires has 10 A capacity as opposed to 20 A. For the wires to have a 20 A capacity, the #12 wires will have to be upsized to a #8 wire (40 A per Table 310.16, column 60 ° C). This would not be recommended because the wire size increased by two sizes and although this may work per code, the load may not be able to receive such oversized conductors.

Therefore, the six circuits can be split into two raceways (i.e., six current-carrying conductors or three circuits per raceway). Per Table 310.15(C)(1), 4-6 current-carrying conductors in a raceway have an adjustment factor of 80%. Therefore, #10 wires would be sufficient for all six of the loads.

The size for the wires labeled in the one-lines in Figure 4 are shown in Table 3.

Table 3: Legend for current-carrying conductor in Figure 4. Courtesy: CDM Smith

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