How to Design a Circuit for Light-Load

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    • 1). Choose the wiring system based on the National Electric Code (NEC) and on the Authority Having Jurisdiction (AHJ) requirements of the local electrical inspector. The easiest wiring system to work with when designing an electrical system for a single- or two-family dwelling is Nonmetallic Clad (NMC) cable, commonly referred to as Romex. Familiarize yourself with Article 334 of the NEC that covers working with NMC cable.

    • 2). Choose the proper type of NMC cable for the type of installation you are doing. Select types NM or NMS for installation in normally dry locations or type NMC for installations in damp or corrosive environments, per Article 334.10

    • 3). Select the proper wire size based on NEC Article 240.4 and Article 310.16, Table 310.16. Table 30.16, known as an Ampacity Table, gives the maximum current that a wire can safely carry with two exceptions. The table gives 25, 30, and 40 amperes for AWG 14, 12 and 10 conductors, respectively, but Article 240.4 limits those conductors to be used on 15-, 20-, and 30-ampere branch circuits protected by an Over-current Protection Device (OCPD), a Circuit Breaker (CB), or fuse rated at 15, 20 and 30 amperes, respectively.

    • 4). Choose 12/2 w/Gr. NMC cable for a light load 20-ampere general-purpose receptacle branch circuit. The cable contains two insulated American Wire Gauge (AWG) size 12 copper conductors, one black and one white plus a bare AWG 12 copper grounding conductor.

    • 5). Choose a 120-Volt, 20-ampere circuit breaker per NEC Articles 220.19 and 240.6. General-purpose or light-duty branch circuits don't serve continuous duty loads, loads that are expected to pull maximum current continuously for three hours or more, so the OCPD can be loaded at 100 percent of its rating: Therefore, a 20-ampere meets code standards for this design.

    • 6). Choose 15- or 20-ampere-rated duplex receptacles to install on this circuit per NEC Table 210.21(B)(3).

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