WIRE SIZING AND LOADING BASICS AND TUTORIALS

The NEC establishes some very important fundamentals that weave their way through the decision making
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 Figure 3-1. 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 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 branch-circuit 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 not permitted if the circuit supplies receptacles where “plug-connected” appliances, and so on, could be used, because too many “plug-in” loads could result in an overload condition, NEC 240.4(B)(1).


You may go to the next standard size overcurrent device only when the circuit supplies other than receptacles for cord-and-plug-connected portable loads.

For instance, when a conductor having an allowable ampacity of 25 amperes [see NEC Table 310.15(B) (16 )] 14 AWG Type THHN is derated to 70%: 25 3 0.70 5 17.5 amperes It is permitted to use a 20-ampere overcurrent device if the circuit supplies only fixed lighting outlets or other fixed loads.

If the previous example were to supply receptacle outlets, then the rating of the overcurrent device would have to be dropped to 15 amperes; otherwise it is possible to overload the conductors by plugging
in more load than the conductors can safely carry.

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; see NEC Table 310.15(B)(3)(a).

Most general-use receptacle outlets in a residence are included in the general lighting load
calculations, NEC220.14(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. This is discussed later in this text. The minimum lighting load for dwellings is 3 volt-amperes per square foot. See NEC 220.12 and Table 220.12.

Continuous Loads
The NEC defines continuous load in Article 100 as a load where the maximum current is expected to continue for three hours or more.* Continuous loads shall not exceed 80% of the rating of the branch circuit.

General lighting outlets and receptacle outlets in residences are not considered to be continuous loads. Certain loads in homes are considered to be continuous and must be treated accordingly.

Examples are electric water heaters (422.13), central electric heating [424.3(B)], snow-melting cables (426.4), and airconditioning equipment (440.32). For these loads, the branch-circuit rating, the conductors, and the overcurrent device shall not be less than 125% of the rating of the equipment.

Mathematically, sizing the conductors and overcurrent device at 125% of the load is the same as loading the conductors and overcurrent device to 80%. For example, an electric furnace with a nameplate rating of 40 amperes would require the supply conductors and overcurrent protection to be not less than 40 x 1.25 = 50 amperes

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