Jim Dollard has an extensive background in codes and standards. Send questions about the National Electrical Code (NEC) to Jim at [email protected]. Answers are based on the 2020 NEC.

Baptistry pool

Our church is considering purchasing a portable baptistry pool or installing a permanent one. Are the installation requirements the same?

The NEC refers to this type of pool as an immersion pool. It is defined in 680.2 as a pool for ceremonial or ritual immersion of users, which is designed and intended to have its contents drained or discharged. Storable immersion pools are covered in Part III of Article 680. See Section 680.30, which requires storable immersion pools to be installed in accordance with Parts I and III of Article 680. Specific requirements for storable immersion pools are in Section 680.35. Permanently installed immersion pools are covered in Part IV of Article 680, and 680.40 requires installation in accordance with Parts I and IV of Article 680. Specific requirements for permanently installed immersion pools are in Section 680.45.

Service conductors to new garage

We submitted drawings for a new service to a new detached garage using PVC conduit underground with 4 AWG copper conductors as permitted in Article 310. The township required larger 3 AWG copper conductors. Is that correct? For dwelling unit installations, aren’t we permitted to derate service conductor ampacity to 83% of the service rating?

The permissive requirements in Section 310.12 apply to single-phase dwelling services and feeders. In this case, 310.12(A) would apply because these are service conductors, and permits a reduction of conductor ampacity to not less than 83%. Section 310.12(A) clearly states that this only applies to conductors supplying the entire load associated with the dwelling unit. The service referenced in this question was not supplying a dwelling unit; it was supplying a detached garage, so 310.12 would not apply.

Laundry area

The new definition of “laundry area” is a bit confusing. What is a laundry tray? Does this require a table of some sort?

This definition is new in Article 100. However, it is based on an existing definition from Article 550, Mobile Homes, Manufactured Homes, and Mobile Home Parks. This definition did not get much attention in Article 550 and most Code users did not know it existed. The NEC does not define “laundry tray.” This is a good example of when the Code user must reference a standard dictionary. Merriam-Webster defines a laundry tray as “a fixed tub (as of slate, earthenware, soapstone, enameled iron, or porcelain) with running water and drainpipe for washing clothes and other household linens.” A laundry tray is typically a utility tub used for more than just the laundry discharge. It could also be an assembly recessed in the wall that provides hot and cold water and a drain. A laundry tray is not a table.

1,200A service

A local contractor has proposed using a boring machine to go under a parking lot to supply a 1,200A, 208/120V service for commercial office space. The installation is for two parallel sets of 750-kcmil XHHW-2 aluminum (8 conductors) in two 6-inch conduits from the pad-mounted, utility-owned transformer to the service equipment. They want to avoid boring four times. Is this type of installation permitted?

Yes, the installation as described is permitted. All installations of enclosures or raceways underground are considered to be wet locations. See 300.5(B). Therefore, Type XHHW-2 conductors installed in a wet location are rated at 90°C (194°F) in accordance with Table 310.4(A). In accordance with 310.15(E)(3), it is possible that the major portion of the load is nonlinear and the neutral (grounded) conductor is considered as a current carrying conductor (CCC). Each 750-kcmil aluminum XHHW-2 conductor has a 90°C ampacity of 435A. There are eight CCCs in each raceway, and adjustments for the number of CCCs (Table 310.15(C)(1)) requires a reduction to 70% ampacity. 435 × 0.7 = 304.5A. The result is a total ampacity of 1,218A.

Aluminum dust area classification

In a location where aluminum dust can exist only under abnormal conditions, is the space considered Class II, Division I or Class II, Division II? We are replacing similar equipment that we installed in the past as Class II, Division II. Now, the drawings show it as Class II, Division I.

Section 500.6 contains requirements for material groups. See 500.6(B)(1), which categorizes an atmosphere containing combustible aluminum dust as a Group E. During the 2020 NEC revision process, the classification of spaces containing Group E combustible dust was modified for clarity. The previous requirements of 500.5(C)(1) for Class II, Division I included spaces where Group E combustible dust may be present in quantities “sufficient to be hazardous.” This required the Code user to determine if the quantity of aluminum dust was sufficient to be hazardous. In the 2020 NEC, the revised text of 500.5(C)(1) for Class II, Division I locations now includes spaces in which Group E combustible dust may be present in quantities sufficient to be hazardous in “normal or abnormal” operating conditions. If aluminum dust can exist under any situation in “quantities sufficient to be hazardous,” it is a Class II, Division I location. See the informational note that explains dust containing magnesium or aluminum is particularly hazardous, and using extreme precaution is necessary to avoid ignition and explosion.

OCPDs, temporary installations

Are all temporary service overcurrent protective devices (OCPDs) required to be of the current-limiting (CL) type? Notes on our drawings state that CL devices are required by the NEC for temporary service.

No, not all service OCPDs used for temporary installations are required to be CL. Section 590.8 contains requirements for OCPDs used in temporary installations, and this section requires that service OCPDs for solidly grounded wye electrical services of more than 150V to ground, but not exceeding 1,000V phase-to-phase, must be CL. Services rated at 208/120V would not require CL devices, but those rated at 480/277V would require CL devices. This can be achieved with current-limiting fuses or current-limiting circuit breakers. It is important to note that 590.8(A) requires that when previously used OCPDs are installed in a temporary installation, they must be examined to ensure they are properly installed and maintained and there is no evidence of impending failure.

Conductor ampacity

Where custom-size copper conductors are installed, such as a 450-kcmil conductor with 90°C insulation (which we saw on a job), how do we determine conductor ampacity?

Section 310.15 contains requirements for the ampacity tables. Where conductor sizes are not shown in the ampacity tables, conductors that meet Article 310’s general requirements are permitted to have a value of ampacity determined under engineering supervision. This is done through interpolation of the adjacent conductor sizes (those smaller and larger than the conductor) based on the conductor area. The term interpolation means that an engineer or other qualified person uses mathematical and statistical tools to determine conductor ampacity by comparing the area of the next-smaller and next-larger conductor and their associated table ampacities. Informational Note No. 2, following 310.15(A), sends the Code user to Chapter 9, Table 8 for the area of the adjacent conductors to be interpolated.

Mixing service conductors

Is it permissible to take service conductors into a wireway to two 200A service-rated transfer switches and have the load side of the transfer switches pass through the same wireway? Is a metal wireway a raceway?

No, see Section 230.7, which prohibits conductors other than service conductors to be installed in the same raceway. A raceway is defined in Article 100 as an enclosed channel designed expressly for holding wires, cables or busbars. The wireway is a raceway. Additionally, see the definition of metal wireway in 376.2 that refers to a wireway as a raceway.