Anatomy of a Conductor, Part 4: Branch Circuit Basics

This issue continues our series on the anatomy of a conductor. It is the first that focuses on the heart of the power distribution system: the branch circuit. And this article also returns to our analysis of sizing the conductor. In this case, the branch circuit conductor. 

NEC Article 100 "Definitions" defines branch circuit (BC) as "The circuit conductor between the final overcurrent protection device (OCPD) protecting said circuit and the outlet(s)."

The same article also defines outlet as "A point on the wiring system at which current is taken to supply utilization equipment."

Combining the base current value  previously calculated in Part 3 of this series, with the two definitions above, the complete design of the branch circuit requires the following:

  • An overcurrent protection device (OCPD) that protects
  • the circuit conductors supplying current to
  • the outlet or load.


A branch circuit is rated according to the trip setting of the OCPD that protects the circuit conductors (NEC Section 210.3), with some exceptions that will be discussed at length in future articles.

NEC Article 100 also tells us that there are four (4) types of branch circuits:

  1.  Branch Circuit:   appliances
  2.  Branch Circuit:   general purpose
  3.  Branch Circuit:   individual
  4.  Branch Circuit:   multi wires

The NEC distinguishes and defines these four (4) types of branch circuits because it limits the OCPD rating of any branch circuit with more than one outlet to 15, 20, 30, 40 and 50 Amps (See NEC 210.3). This requirement also applies to branch circuits other than individual branch circuits. As an example, if you have a 60 Amp or 25 Amp branch circuit with two (2) or more outlets, then this requirement is not applicable.

NEC Article 100 also includes in the definitions of receptacle (RCPT), that a duplex RCPT is considered to be two (2) outlets. Any branch circuit for a duplex RCPT must also conform to the requirements of NEC 210.3, and any outlet rated above 50 Amps should be connected on an individual branch circuit.

As a field note:  it is an observation that an indoor air handling unit (AHU) with heating strip and the associated outdoor condensing unit (CU) are  sometimes  observed to be designed on a single one (1) 60 Amp circuit. This is a direct violation of NEC 210.3.

The discussion of the complex requirement of the conductor will continue in future newsletters.

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