Receptacle Branch Circuit Design Calculations – Part Two

In the previous article " Receptacle Branch Circuit Design Calculations – Part One " in
 our new course Course EE-3: Basic Electrical design course – Level II ", I explained the essential definitions for receptacles branch circuit and the different types of receptacles. 

Today I will explain some basic principles and features of Receptacle branch circuits before proceeding with the design calculations of such type of circuits. 

1- The difference between Multiple and Multiwire branch circuits

Multiple Branch Circuits: are two or more branch circuits supply devices or equipment on the same yoke.


Branch Circuit, Multiwire: A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system. 

What do you notice from the two definitions?

  • If we have two devices to be fed from two different receptacle outlets installed on the same yoke, so we will need two different branch circuits, then we have 4-wires(2 hot and 2 grounded) provided to this yoke. In this case we have a multiple branch circuit. 
  • But if we supply the same with a multiwire branch circuit, we will have 3 wires only (2 hot, one common grounded)

Don't Forget….
There is difference between receptacle device and receptacle outlet as explained before in the previous article.

1.1 Advantages of using multiwire instead of multiple branch circuit

A multiwire circuit shall be permitted to be considered as multiple circuits. But the multiwire branch circuit has some advantages more than the Multiple circuit as follows:

  • Using three wires to do the work of four (in place of two 2-wire circuits), 
  • Less raceway fill, 
  • Easier balancing and phasing of a system, 
  • Less voltage drop. 

1.2 Constraints for using multiwire branch circuits:

1- All conductors of a multiple branch circuit shall originate from the same panelboard or similar distribution equipment.

2- The simultaneous opening of all “hot” conductors at the panelboard effectively protects personnel from inadvertent contact with an energized conductor or device terminal during servicing.

For examples:
  • For a single-phase installation, the disconnecting means could be two single-pole circuit breakers with an identified handle tie or a two-pole circuit breaker, as shown in Fig.1 (top), or by a 2-pole switch, as shown in Fig.1 (bottom). 

Fig (1)

  • For a 3-phase installation, a 3-pole circuit breaker, three single-pole circuit breakers with an identified handle tie, or a three-pole switch provides the required simultaneous opening of the ungrounded conductors. 

3- It is not permitted that the wiring terminals of a device, such as a receptacle, to be the means of maintaining the continuity of the grounded conductor between devices in a multiwire branch circuit.

A splice is made and a jumper is connected to the terminal, unless the neutral is looped; that is, a receptacle or lampholder could be replaced without interrupting the continuity of energized downstream line-to-neutral loads

Opening the neutral could cause unbalanced voltages, and a considerably higher voltage would be impressed on one part of a multiwire branch circuit, especially if the downstream line to- neutral loads were appreciably unbalanced.

1.3 Uses of multiwire Branch circuits:

  • It can supply Appliances that have both line-to-line and line-to-neutral connected loads, such as electric ranges and clothes dryers.
  • It can supply Loads that are line-to-neutral connected only, such as the split-wired combination device as shown in Fig.1 (bottom). 
  • It can supply A device with a 250-volt receptacle (line-to-line) and a 125-volt receptacle (line-to-neutral), as shown in Fig.2, provided the branch-circuit overcurrent device simultaneously opens both of the ungrounded conductors. 

Fig (2)

2- GFCI protection basics

As I mentioned before in previous article 
 " Receptacle Branch Circuit Design Calculations – Part One " that Combined Receptacle are space-saving designs that provide two features in one device, such as an outlet with a guide light, a GFCI Receptacle with a switch or a switch with a Receptacle.

As per NEC 406.5(G), A receptacle shall not be combined in enclosures with other receptacles, snap switches, or similar devices, unless they are arranged so that the voltage between adjacent devices does not exceed 300 volts or unless they are installed in enclosures equipped with identified, securely installed barriers between adjacent devices.

2.1 Definition:

Ground-Fault Circuit Interrupter (GFCI) is A device intended for the protection of personnel that functions to de-energize a circuit or portion thereof within an established period of time when a current to ground exceeds the values established for a Class A device.

2.2 Theory of operation of GFCI Device:

As in image below that shows a typical circuit arrangement of a GFCI which operates in the following procedure:

  • The line conductors are passed through a sensor and are connected to a shunt-trip device. 
  • As long as the current in the conductors is equal, the device remains in a closed position. 
  • If one of the conductors comes in contact with a grounded object, either directly or through a person’s body, some of the current returns by an alternative path, resulting in an unbalanced current. 
  • The toroidal coil senses the unbalanced current, and a circuit is established to the shunt-trip mechanism that reacts and opens the circuit. 

The use of GFCI Receptacles can be used as replacements for Un-grounded receptacles where a grounding means does not exist.

A receptacle supplying only a permanently installed fire alarm or burglar alarm system shall not be required to have ground-fault circuit-interrupter protection.

GFCIs do not protect persons from shock hazards where contact is between phase and neutral or between phase-to-phase conductors.

2.3 Types of GFCI devices:

A variety of GFCIs are available, including:

  1. portable types, 
  2. plug-in types, 
  3. circuit-breaker types, 
  4. types built into attachment plug caps, 
  5. receptacle types. 

Each type has a test switch so that units can be checked periodically to ensure proper operation. 

All combined GFCI with receptacles must be readily accessible to be periodically tested, by pressing the test button on the device and verify that all receptacles protected through that GFCI device are de-energized.

3- AFCI Protection Basics

3.1 Definitions:

Arc-Fault Circuit Interrupter (AFCI) is a device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected.

AFCI Receptacle: A receptacle with a built in device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected.

3.2 AFCI device uses:

From the above definition, the uses for such devices are for:
  1. De-energizing the branch circuit when an arc fault is detected. 

Also, AFCI devices may have the capability to perform other functions such as:

  1. Overcurrent protection, 
  2. Ground-fault circuit interruption 
  3. Surge suppression. 

AFCI devices must be tested to verify that arc detection is not unduly inhibited by the presence of loads and circuit characteristics that may mask the hazardous arcing condition.

AFCI devices must be tested for unwanted tripping due to the presence of arcing that occurs in control and utilization equipment under normal operating conditions or to a loading condition that closely like to an arcing fault, such as a solid-state electronic ballast or a dimmed load.

3.3 AFCI device types:

UL 1699 currently recognizes five types of arc fault circuit interrupters:

  1. Branch/feeder AFCI, 
  2. Combination AFCI, 
  3. Cord AFCI, 
  4. Outlet AFCI, 
  5. Portable AFCI. 

The product standard requires specific marking on AFCI devices to indicate the type of protection provided. 

To provide the full range of AFCI protection covered in the UL standard for the branch-circuit conductors and at the outlets supplied by the branch circuit, use the combination- type AFCI devices not the Cord AFCI devices.

NEC Rules for using AFCI devices:

  • AS per NEC section 210.12(A), AFCI can be used with the following branch circuits: 

  1. For all 15- and 20-ampere, 120-volt branch circuits that supply outlets including receptacle, lighting, and other outlets located throughout a dwelling unit except those outlets installed in kitchens, bathrooms, unfinished basements, garages, and outdoors (in this case GFCI will be used). 
  2. For shared circuits like circuit that feed a bedroom and other areas such as closets and hallways.

There is no prohibition against using AFCI protection on other circuits or locations other than those specified in 210.12(A).

  • As per 210.12(B), the branch circuit shall be protected by one of the following: 

  1. A listed combination-type AFCI located at the origin of the branch circuit.
  2. A listed outlet branch-circuit type AFCI located at the first receptacle outlet of the existing branch circuit. 

In the next Article, I will explain the design calculations for branch circuits in dwelling and Non-dwelling buildings. Please, keep following.

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