Receptacle Branch Circuit Design Calculations – Part One

In the previous article " 
Branch Circuit Design Calculations – Part One " in our new course Course EE-3: Basic Electrical design course – Level II ", I indicated that Branch-circuit are classified According To Type of Load into five categories: 

  1. Lighting Branch circuit, 
  2. Receptacle Branch circuit, 
  3. Equipment Branch circuit, 
  4. Heating and cooling loads Branch circuit, 
  5. Motor Branch circuit. 

I explained the design calculations for the first type: lighting branch circuits in the following Previous Articles:

Today I will explain the design calculations for the second type: Receptacle branch circuits as follows.

Second: Receptacles branch circuit

Receptacles are one of the most popular terms in the NEC code because of its importance especially in dwelling units and also in non-dwelling units as it is the device responsible for feeding power to most of equipments.

NEC code includes large number of rules that are assigning the receptacle requirements for special locations, special loads, special installation methods and etc.

So, I think we need to start with an introduction to receptacles before going to detailed explanation of NEC rules controlling the receptacles requirements and also, before explanation of design calculation steps for receptacle branch circuits.

1- Introduction

1.1 Essential Definitions 

Receptacle: A receptacle is a contact device installed at the outlet for the connection of an attachment plug.

Receptacle Outlet: An outlet where one or more receptacles are installed.

The number of receptacle Outlets my not equal to the number of receptacles installed on these outlets. For example, a duplex receptacle has two contact devices on the same receptacle outlet. In this case number of receptacles = 2 while the number of receptacle outlets = 1.

Attachment Plug (Plug Cap) (Plug): A device that, by insertion in a receptacle, establishes a connection between the conductors of the attached flexible cord and the conductors connected permanently to the receptacle.

Note that…
Attachment plug contact blades have specific shapes, sizes, and configurations so that a receptacle or cord connector will not accept an attachment plug of a voltage or current rating different from that for which the device is intended.

Multi-outlet Assembly: are metal or nonmetallic raceways that are usually surface, flush, or freestanding mounted designed to hold branch circuit conductors and receptacles, assembled in the field or at the factory.

The definition of multi-outlet assembly includes a reference to a freestanding assembly with multiple outlets, commonly called a power pole as shown in below image.

1.2 Types of Receptacles

There are many types of Receptacles that had been classified according to many factors such as:

These Receptacle classifications are made as per NEC code, there will be other receptacle classifications as per other codes like IEC and BS but in this course I will explain that based on the NEC code only.

1- According to Type of Receptacle load

a- Receptacle for dwelling loads: receptacles used for normal loads in dwelling units like general receptacles, Electric Dryers and Electric Cooking Appliances. These receptacles will have a maximum ampere of 30 A and voltage less than 600 V.

b- Receptacle for Non-dwelling loads: receptacles used for special loads for non dwelling units like welding machines, heavy lamp holders, sign and outline lighting, Show Windows and Fixed Multi-outlet Assemblies (power poles). These receptacles will have a maximum ampere of 60 A and voltage less than 600 V.

2- According to Installation location

a- Indoor Receptacles: Receptacles that will be used inside the building where there are no abnormal weather conditions, it is usually general purpose receptacles.

b- Outdoor Receptacles: Receptacles that will be used outside the building or in opening areas inside the buildings, it is usually weather proof receptacles and GFCI type.

c- Hazardous locations Receptacles: Receptacles that will be used in hazardous locations as defined by the NEC code, it is explosion proof receptacles type.

d- Damp and wet location Receptacles: Receptacles that will be used in damp and wet locations like in kitchen and indoor swimming pools, it is GFCI type.

3- According to Method of installation

a- Pendant: A cord connector that is supplied by a permanently connected cord pendant shall be considered a receptacle outlet. 

b- Wall mounted, flush Receptacles. 

c- Wall mounted, Surface Receptacles.

d- Wall mounted, Recessed Receptacles.

e- Floor mounted Receptacles: receptacles installed in under floor boxes as a separate power source or as a part of FCC system (Flat conductor cable).

4- According to grounding connection

a- Un-grounded Receptacles: These Receptacles are similar to standard duplex receptacles, in that they accommodate two-prong plugs, but they are missing the u-shaped grounding hole.

b- Grounded Receptacles: these receptacles have the U shaped grounding hole.

5- According to polarization feature

a- Polarized Receptacles: Receptacles that have wider neutral slot and narrower hot slot, the wider prong on the polarized plug will permit it to be plugged in only with the correct polarity. The narrower prong is the "hot" lead and the switch to the appliance is placed in that lead, guaranteeing that no voltage will reach the appliance when it is switched off. A non-polarized plug may have the switch in the neutral leg and thus be a shock hazard even when it is switched off as shown in the below image. 

b- Non-polarized Receptacles: These outlets have two slots (hot and neutral) of the same width.

6- According to locking feature

a- Locking Receptacles: Receptacles designed to lock an inserted plug with a matching blade configuration when the plug is rotated in a clockwise direction. The plug can only be removed by first turning it in a counter-clockwise direction.

b- Non-locking Receptacles: Receptacles without a locking feature, the plug can be removed by drawing it in the straight direction from the receptacle.

7- According to Mechanism of inserting and removing Plugs

a- Twisted-Locking Receptacles: Twist-locking Receptacles types are used for heavy industrial and commercial equipment, where increased protection against accidental disconnection is required. Numbers prefixed by L are curved-blade, twist-locking connectors; others are straight blade and non-locking

b- Straight blade Receptacles: A non-locking receptacle into which mating plugs are inserted at a right angle to the plane of the receptacle face.

8- According to Number of gangs on the same yoke

a- Single Receptacle: A single receptacle is a single contact device with no other contact device on the same yoke.

b- Multiple Receptacle: A multiple receptacle is two or more contact devices on the same yoke.

c- Multiple Receptacle, Duplex: Two receptacles built with a common body and mounting means on the same yoke and accept two plugs.

d- Multiple Receptacle, Triplex: A receptacle with a common mounting means on the same yoke which accepts three plugs.

Multiple Receptacles, Quad (four-plex-type): A receptacle in a common housing that accepts up to four plugs. Four-In-One receptacles can be installed in place of duplex receptacles mounted in a single-gang box, providing a convenient means of adding receptacles without rewiring.

Don’t be confused between duplex and twins receptacles, the duplex receptacles are two receptacles installed on the same outlet/yoke but the twins receptacles are two receptacles installed on two different outlets/yokes i.e. twin duplex receptacles = double duplex receptacle.

9- According to Ampere rating

15 A- Receptacles

20 A- Receptacles

30 A- Receptacles

50 A- Receptacles

60 A- Receptacles

In dwelling units, the maximum rating of Receptacles will be 30 A while in Non-dwelling it will be 60A.

10- According to Number of Receptacle poles

2- Poles Receptacle
3- Poles Receptacle

4- Poles Receptacle

11- According to Number of phases

Single phase Receptacle

Three phase Receptacle

12- According to Number of wires connected to the Receptacle

2- Wire Receptacle

3- Wire Receptacle

4- Wire Receptacle

5- Wire Receptacle

13- According to Receptacle Voltage rating

Single phase Receptacle
Three phase Receptacle
Single/Three phase Receptacle



14- According to Receptacle Configurations (NEMA Configurations)

As per the following two images:

Note: the above two images summarize also the categories # 9 to # 13 explained in above.

The NEMA Nomenclature is explained in the following image:

15- According to Receptacle Combination with other devices

a- Non-Combined Receptacle: Receptacles that will be the only device installed on a yoke i.e. it will not share the same yoke with any other devices.

b- Combined Receptacle: combined Receptacles 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. 

Combined Receptacles will include the following types: 

1- GFCI Receptacle: A receptacle with a built in circuit that will detect leakage current to ground on the load side of the device. When the GFCI detects leakage current to ground, it will interrupt power to the load side of the device, preventing a hazardous ground fault condition. They can be used in indoor and outdoor locations. 

GFCI Receptacle

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.

2- 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. 

AFCI Receptacle must be installed to be the first receptacle outlet of the branch circuit.

3- TVSS (SPD) Surge suppression Receptacle: A receptacle with built-in circuitry designed to protect its load side from high-voltage transients and surges like surge protectors. The circuitry will limit transient voltage peaks to help protect sensitive electronic equipment such as PC’s, modems, audio/video equipment, etc. without the need for power strips. 

4- Switched Receptacle: A receptacle with built-in switch designed to turn on and off the power for the connected equipment to that Receptacle.

16- According to Receptacle Usage

a- General purpose/Use Receptacles: These receptacles are normally used to supply lighting and general-purpose electrical equipment inside the buildings or outside the dwelling buildings only. 

general use - outdoor receptacle outlets are not required for commercial, institutional, and industrial occupancies, but as exception, outdoor receptacle outlets shall be installed at an accessible location for the servicing of heating, air-conditioning, and refrigeration equipment as per NEC 210.63 or at the discretion of the designer or owner. This receptacle is required to be provided with ground-fault circuit-interrupter protection. While an outdoor receptacle outlet shall not be required at one and two-family dwellings for the service of evaporative coolers.

b- Special purpose Receptacles: It includes the following types: 

1- Pin and sleeve (industrial) Receptacle: A Receptacle with round pin-type contacts intended to mate with a connector having hollow cylindrical female contacts.

2- Hospital grade Receptacle: A receptacle designed to meet the performance requirements of high-abuse areas typically found in health care facilities. These receptacles are tested to the Hospital Grade requirements of Underwriters Laboratories Inc. Standard 498. 

3- Isolated ground Receptacle: Receptacles intended for use in an Isolated Grounding system where the ground path is isolated from the facility grounding system. The grounding connection on these receptacles is isolated from the mounting strap. 

4- Factory installed Receptacles: Receptacles installed on the equipment like electric baseboard heaters and provided as a separate assembly by the manufacturer.

5- Manufactured wiring system receptacles: special receptacles used only with Manufactured wiring system as shown in below image. 

6- Shore power receptacles: Single locking- and grounding-type Receptacles intended to supply shore power to boats, it shall be housed in marine power enclosure listed as marina power enclosure or listed for set locations as shown in below image.

7- Tamper resistant Receptacle: A receptacle specially constructed so that access to its energized contacts is limited. It is designed to help protect children from electrical injury, they have a built-in shutter mechanism that blocks insertion of most small objects. The shutters only open when a properly rated plug is inserted. Once installed, they are permanent, offering continuous protection unlike plastic outlet caps that can be removed. 

8- Weather-resistant Receptacle: These outlets are required by the NEC in damp or wet locations, such as patios, decks and pool areas, or any other residential outdoor location. The 15-amp and 20-amp weather-resistant outlets are built with UV stabilized thermoplastic and corrosion-resistant metals for superior performance outdoors, including cold impact resistance. You can choose from combined weather/tamper-resistant outlets or weather-resistant GFCIs with or without tamper-resistance. 

Note: Outlets in damp or wet locations should always be installed with weather-resistant covers.

9- Rotating receptacles: These Receptacles can be positioned to accommodate more than one of the large transformer-type plugs from cell phone chargers, hairdryers, cordless appliances, MP3 players, night lights and more, eliminating the need for power strips.

10- Split circuit receptacles: A split receptacle has two outlets with each wired on a different circuit or with one outlet live and the other switched. With a split receptacle you can have a wall switch to turn a light on or off or remotely control one plug-in location but not the other. Most duplex receptacles provide break-off tabs that allow them to be converted into split-circuit receptacles. 

11- Explosion Proof Receptacle: A receptacle constructed to meet the requirements of hazardous locations as defined by the NEC Code.

17- According to Receptacle’s conductor marking

a- Receptacles having no conductor marking: these receptacles will be used only with copper conductors or copper-clad conductors.

b- Receptacles marked “AL-CU”: these receptacles will be used only with copper conductors or copper-clad conductors.

c- Receptacles marked “CO/ALR”: these receptacles will be used with UL-labeled aluminum, copper or copper-clad aluminum conductors.

18- According to Type of Receptacle branch circuit

a- Branch Circuit, General-Purpose: A branch circuit that supplies two or more receptacles or outlets for lighting and appliances.

b- Branch Circuit, Individual: A branch circuit that supplies only one utilization equipment via a single receptacle.

A branch circuit supplying one duplex receptacle that supplies two cord-and-plug-connected appliances or similar equipment is not an individual branch circuit.

c- 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.

In the next Article, I will explain the basics of Multiwire Branch circuits and design calculations for the Receptacle branch circuits. Please, keep following.

1 comment:

  1. Great informative article. I enjoyed reading it. Thanks a lot for collecting data and making us available on a single page.