Basics Of Wire Sizing And Loading

The NEC establishes some very important fundamentals that weave their way through the decisionmaking process for an electrical installation. They are presented here in brief form and are covered in detail as required throughout this text.

The NEC defines a branch circuit as The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).* See the next Figure. In the residence discussed in this text, the wiring to wall outlets, the dryer, the range, and so on, are all examples of a branch circuit.

The branch circuit is that part of the wiring that runs from the final overcurrent device to the outlet.The rating of the overcurrent device, not the conductor size, determines the rating of the branch circuit. Feeder conductors are circuit conductors between service equipment and the final overcurrent device.

The NEC defines a feeder as All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branchcircuit overcurrent device.* In the residence discussed in this text, the wiring between Main Panel A and Subpanel B is a feeder.

The ampacity (current-carrying capacity) of a conductor must not be less than the rating of the overcurrent device protecting that conductor, NEC 210.19 and NEC 210.20. A common exception to this is a motor branch circuit, where it is quite common to have overcurrent devices (fuses or breakers) sized larger than the ampacity of the conductor. Motors and motor circuits are covered specifically in NEC Article 430. The ampere rating of the branch-circuit overcurrent protective device (fuse or circuit breaker) determines the rating of the branch circuit. For example, if a 20-ampere conductor is protected by a 15-ampere fuse, the circuit is considered to be a 15-ampere branch circuit, NEC 210.3.

Standard branch circuits that serve more than one receptacle outlet or more than one lighting outlet are rated 15, 20, 30, 40, and 50 amperes. A branch circuit that supplies an individual load can be of any ampere rating, NEC 210.3.

If the ampacity of the conductor does not match up with a standard rating of a fuse or breaker, the next higher standard size overcurrent device may be used, provided the overcurrent device does not exceed 800 amperes, NEC 240.4(B). This deviation is permitted if the circuit supplies fixed loads such as for lighting or not more than one receptacle for cord-and-plug-connected portable loads. Keep in mind a duplex receptacle is two receptacles on one strap or yoke according to the definition of “receptacle” in NEC Article 100. So, the round-up rule does not work for branch circuits that supply multiple receptacles, NEC 240.4(B)(1).

For example, a 6 AWG copper conductor has an allowable ampacity of 65 amperes in the 75°C column of NEC Table 310.15(B)(16). Because there is no standard overcurrent device rated 65 amperes, we are permitted to round up to the next standard rating of 70 amperes. However, the load on the conductor cannot exceed 65 amperes.

The allowable ampacity of conductors commonly used in residential occupancies is found in NEC Table 310.15(B)(16). This includes Type NM cable. It is required to be manufactured with 90°C insulated conductors. Typically, the insulation is Type THHN. As a result, the cable is limited to use in dry locations. See NEC 334.10(A)(1). NEC 334.80 allows the 90°C ampacity to be used for derating purposes so long as the final ampacity is selected from the 60°C column of NEC Table 310.15(B)(16).

The ampacities in Table 310.15(B)(16) are subject to correction factors that must be applied if high ambient temperatures are encountered—for example, in attics; see NEC Table 310.15(B)(2)(a).

Conductor ampacities are also subject to a derating factor if more than three current-carrying conductors are installed in a single raceway or cable or if these cables are installed without maintaining separation; see NEC Table 310.15(B)(3)(a). See Chapter 18 of this text for complete coverage of correction and derating factors.

Most general-use receptacle outlets in a residence are included in the general lighting load calculations, NEC 220.14(J).

Receptacle outlets connected to the 20-ampere small-appliance branch circuits in the kitchen, dining room, laundry, and workshop are not considered part of the general lighting load. Additional load values must be added into the calculations for these receptacle outlets.

The minimum lighting load for dwellings is 3 volt-amperes per square foot. See NEC 220.12 and Table 220.12.