Technical Article

National Electrical Code Basics: Branch Circuits Part 2

May 31, 2022 by Lorenzo Mari

The NEC presents mandatory rules regarding voltage limitations and the use of ground-fault-circuit-interrupters and arc-fault-circuit-interrupters in branch circuits. Learn about voltage limitations, as well as GFCI and AFCI protection in branch circuits, according to the NEC, here in Part 2.

For additional insightful articles on the National Electrical Code and branch circuits, please follow these links:


The National Electrical Code® (NEC®) stipulates firm rules about voltage limitations in branch circuits. It also provides ground-fault-circuit-interrupter (GFCI) and arc-fault-circuit-interrupter (AFCI) requirements for the safety of people and the protection of properties.


Image courtesy of Pixabay.


Voltage limitations

Section 210.6 of the NEC copes with branch-circuit voltage limitations. The NEC classifies the voltage limitations into four categories

1. 120 V between conductors.

Notably, to lessen occupants’ exposure of dwelling units, hotels, motels, and similar residences to electric shock, the NEC limits the supply voltage to permanently mounted luminaires and receptacles for cord-and-plug-connected appliances and portable loads to 120 V.

The cord-and-plug-connected loads’ power limits are 1 440 VA or ¼ hp – a 15 A circuit may supply any of these loads.

In general – for all buildings or premises – circuits of a maximum of 120 V between conductors may supply.

   a. Lamp holders within their voltage ratings. A typical screw-shell type lamp base is medium-base or “Edison.” A popular name for a lamp holder is “socket.” See figure 1.


Figure 1. Medium-base screw-shell type (E26).


   b. Auxiliary equipment for starting the electric-discharge lamps – unique accessories to make the lights work – such as fluorescent and high-intensity discharge (HID).

      Three standard high-intensity discharge lamps are metal-halide, mercury-vapor, low-pressure sodium, and high-pressure sodium.

   c. Cord-and-plug-connected or fixed appliances.

There are three types of appliances:

  • Portable: equipped with a cord and a plug (cord-and-plug-connected). These appliances are transportable – e.g., toaster, iron, coffee maker, microwave.
  • Stationary: typically, they remain in their original location. They may also employ a cord and a plug to facilitate an eventual move – e.g., self-contained range, window air conditioner, cloth dryer.
  • Fixed: they have plumbing connections, are into the building structure, or are fastened by other means. They are difficult to remove – e.g., water heater, garbage disposer, central air conditioner, and wall-mounted oven.

Usual system arrangements to supply 120 V are

  • 120 V, 2-wire, single-phase.
  • 240/120 V, 3-wire, single-phase.
  • 208 Y/120 V, 4-wire, 3-phase.

2. 277 V to ground.

Covers permissions for circuits above 120 V (208 V, 240 V, 277 V, and 480 V) but not higher than 277 V to the ground. They may supply cord-and-plug-connected appliances, fixed appliances, or listed luminaires such as

a. Listed electric-discharge luminaires with integral ballasts.

b. LED luminaires with LED drivers between the branch circuit and the lamp holders.

c. Incandescent or LED luminaires equipped with medium-base or smaller screw-shell lamp holders. The lamp holders are supplied at 120 V or less from the output of a stepdown autotransformer, LED driver, or another power supply integral component of the luminaire.

Connect the outer shell terminal to the circuit’s grounded conductor.

d. Luminaire equipped with a mogul-base screw-shell type lamp holder. See Figure 2.


Figure 2. Mogul-base screw-shell type (E39).


e. Luminaires equipped with other standardized kinds and sizes of lamp bases applied within their voltage ratings.

f. Luminaires without lamp holders.

g. Auxiliary equipment of electric-discharge or LED-type lamps.

h. Luminaires converted with listed retrofit kits incorporating integral LED light sources or accepting LED lamps.

Listed denotes Underwriters Laboratories® (UL) – one of the most famous testing laboratories – listed materials and equipment. UL tests an item and records it in a book if it meets the safety standards. Their stamp on electric materials is synonymous with “proved good.”

Examples of cord-and-plug-connected appliances and fixed appliances are 277 V heaters (larger than 1 440 VA) in large buildings and fryers in restaurants.

Typical system arrangements are:

  • 277 V, 2-wire, single-phase.
  • 240/120 V, 3-wire, single-phase.
  • 240 V, single-phase (from a delta or corner-grounded delta).
  • 208 Y/120 V, 4-wire, 3-phase (grounded wye).
  • 480 Y/277 V, 4-wire, 3-phase (grounded wye).

3. 600 V between conductors.

Covers permissions for circuits above 277 V to the ground and not more than 600 V between conductors. They may supply:

   a. The auxiliary equipment of electric-discharge lamps mounted in permanently installed luminaires on poles or equivalent assemblies.

The height requirements are:

  • 6.70 m (22 ft) to illuminate large outdoor areas – e.g., highways, parking lots, and roads.
  • 5.5 m (18 ft) on other structures like tunnels.

   b. Utilization equipment, other than luminaires, permanently connected or cord-and-plug-connected.

   c. Luminaires powered by direct-current systems. The luminaires must contain a listed, dc-rated ballast. If the luminaires have provisions for changing lamps, the ballast must provide isolation between the dc power source and the lamp circuit and protection from electric shock.

Typical system arrangements are:

  • 480 V, 3-wire, ungrounded (either delta or wye).
  • 480 V, 3-wire, corner-grounded delta.

4. Over 600 V between conductors.

Systems with voltages above 600 V between conductors require attendance from qualified personnel. The NEC allows their use when only qualified personnel supervise and maintain such systems. Typical voltages are 2 300 V, 4 160 V, and 13 800 V.


Ground-Fault Circuit-Interrupter (GFCI) Protection for Personnel

NEC Section 210.8 requires and rules the installation of ground-fault circuit-interrupter protection for personnel in several locations.

The GFCI is safety protection, mainly when operating electrical equipment outdoors and in wet spots while standing on the ground.

In a 2-wire system, currents in the ungrounded and grounded conductors are identical. If the wiring, appliance, or tool insulation fails, it will allow some current to leak into the ground. When it happens, a GFCI will detect the difference in current in the two wires and will trip the circuit if the fault current exceeds the trip level – between 4 mA and 6 mA. The disconnecting time is about 1/40 s.

Most likely, the fault current will not be enough to trip a circuit breaker or blow a fuse, exposing personnel to an electric shock. The GFCI will not prevent a person, part of a ground-fault circuit, from receiving a shock or causing a derived accident – like a fall – yet it will inhibit damaging current levels through the body.

A GFCI disconnects a circuit in the event of a ground fault. However, it does not protect against phase-to-neutral or phase-to-phase contacts.

There are three types of GFCIs:

  • Receptacle/Outlet. Protects an appliance or other downstream outlets or devices.
  • Circuit breaker. Replaces a standard circuit breaker, protecting through the branch circuit.
  • Portable. Primarily used for mobile applications.

Figure 3 shows a typical Receptacle/Outlet GFCI.


Figure 3. 15 Amp, 125 Volt Receptacle/Outlet GFCI. Image used courtesy of Leviton.


A Receptacle/Outlet GFCI incorporates RESET and TEST buttons. The RESET button arms the GFCI, after clearing the fault, for future defects. The TEST button simulates a fault to check that the GFCI is working right.

A GFCI does not require an equipment grounding conductor (EGC).

The NEC requires GFCIs in the following locations:

  • Dwelling units.
  • Other than dwelling units.
  • Crawl space lighting outlets.
  • Specific appliances.
  • Equipment requiring servicing.
  • Outdoor outlets.

Some particular locations are bathrooms, garages, outdoors, crawl spaces, basements, kitchens, sinks, boathouses, bathtubs or shower stalls, laundry areas, indoor wet spots, and rooftops.

One critical requirement is to locate the GFCI in a readily accessible location.


Arc-fault Circuit-interrupter (AFCI) Protection

NEC Section 210.12 requires and rules the installation of arc-fault circuit-interrupter protection in several locations.

The AFCI detects arcing (sparks) between and along damaged wires or contacts – the sparks may cause fires.

When an AFCI detects a current imbalance of about 30 mA or a waveform anomaly indicative of an arcing fault, it opens the circuit – arcing faults have identifiable waveform characteristics.

There are two types of arcing failures:

  • Parallel arc. It arises between wires of different voltages, such as line-to-line, line-to-ground, or line-to-neutral – in a grounded neutral system, line-to-ground and line-to-neutral failures are equivalent. These failures are in parallel with the load, and the maximum current will be the short-circuit current available at the point of failure.
  • Series arc. It develops along one wire. Examples are the arcs between two disengaged ends of the same wire or a poor connection to the screw of a terminal. These failures are in series to the load, and the maximum current will be the load current.

There are three types of AFCIs:

  • Branch/feeder breaker installed on the panelboard.
  • Combination breaker. It fuses the protective features of series and parallel protective devices, detecting series and parallel arcing failures. The series arc detection senses lower-level arcing in branch circuits and cords.
  • AFCI and GFCI protection (dual function). It provides shock protection and arc-fault mitigation simultaneously.

Figure 4 shows an AFCI.


Figure 4. 15 Amp, 125 Volt, Receptacle/Outlet AFCI. Image used courtesy of Leviton.


Figure 5 shows a dual function AFCI/GFCI.


Figure 5. 15 Amp, 125 Volt, Dual Function AFCI/GFCI Receptacle. Image used courtesy of Leviton.


Though they look similar, GFCIs and AFCIs have different tasks. AFCIs complement the protection offered by GFCIs against electrical shock, electrocution, and property loss. In essence, AFCIs prevent fires, and GFCIs avoid electrocution.

AFCIs and GFCIs work independently – it is viable to install an AFCI breaker on a branch circuit having GFCI receptacles.

The NEC requires AFCIs installed on most branch circuits that supply outlets or fixtures in new constructions and revamp.

The following locations require an AFCI:

  • Dwelling units.
  • Dormitory units.
  • Guest rooms, guest suites, patient sleeping rooms, and limited-care facilities.
  • Branch circuit extensions or modifications.

One essential requirement is to locate the AFCI in a readily accessible location.



The NEC presents mandatory rules regarding voltage limitations and the use of GFCIs and AFCIs in branch circuits.

The NEC limits to 120 V the voltage rating of branch circuits supplying lamp holders, fixtures, and cord-and-plug connected loads lesser than or equal to 1 440 VA or motor loads rated ¼ hp or less. It also gives rules for 120 V between conductors, 277 V to ground, 600 V between conductors, and over 600 V between conductors.

AFCIs and GFCIs complement themselves to offer protection to people and property. AFCIs prevent fires, and GFCIs avoid electrical shock and electrocution.


Feature image used courtesy of Pixabay