Branch Circuit Design Calculations – Part One


In " Course 
EE-1: Beginners' Electrical Design Course " which is a preparation for beginners in electrical design, I explained the following subjects: 



1- The basic Concepts in electrical design (that every electrical designer must be familiar with it).

2- Electrical design philosophy for all building types.

3- Electrical load classification which include:

  1. Lighting loads. 
  2. Appliances loads. 
  3. Power loads: 
  • HVAC Loads, 
  • Elevator loads, 
  • Motor and pump loads. 

4- Electrical design stages.

5- Electrical design criteria. 


Also, I listed the steps for electrical design process in the following two articles:


Also, in " Course EE-2: Basic Electrical Design Course – Level I " which is the second level for beginners in electrical design, I explained the following subjects:


1- How to determine the electrical requirements, needs and characteristics for any building/project.

2- How to develop different schematic diagrams with different connection configurations.

3- How to select optimal power sources and distribution networks.

4- How to select the equipment from the manufacturer’s catalogs

5- Check list foe electrical distribution design.

6- Solved examples.

7- ID-Spec software.

8- Preliminary electrical load estimation methods:

  • Space by space method, 
  • Building method, 
  • Area method. 

9- Types of electrical diagrams.

10- How to draw and read the single line diagram



Important!!!
Before proceeding with this new course, it is highly recommended to review the previous two courses mentioned above.



Today, I will begin explaining the Third Course in electrical design series which is " Course EE-3: Basic Electrical design course – Level II ", you can review the list of contents of this course by click on its link.

I will begin with the first point; the branch circuit design calculations as follows.



Branch circuit design calculations



1. Introduction


In low voltage distribution system, the power is transferred in the following manner:

  1. From electrical power sources (Transformers, UDS, Generators and renewable energy sources) to service equipments ( MVSG, MLVSG or Main service Disconnect).
  2. Then to (MLVSG, MCC or distribution center).
  3. Then to (Switchboards and Panels).
  4. Then to Final User Loads (Lighting, Small Appliances, Power Outlets, HVAC equipments, Motors, etc.).

Regardless of the wiring methods used for transferring electrical power, the conductors carrying the power fall into one of two categories:

  1. Feeders, 
  2. Branch-circuit conductors. 



2. Difference between Service Conductors, feeders and Branch-circuit conductors

So, let’s know the difference between branch circuit and feeder as follows: 







A- Service Conductors:


The conductors between the terminals of the electrical utility service equipment and the main disconnect switch of the main distribution in the premises




The service conductor can be divided to (3) parts based on the service point as follows:

1- Service Lateral: The underground conductors between the electrical utility service equipment and the service point.

2- Underground Service Conductors: The underground conductors between the service point and the main disconnect switch of the main distribution in the premises. 

3- Service Drop: The overhead conductors between the utility electric supply system and the service point. 


Noting that The service point can be described as the point of demarcation between where the serving utility ends and the premises wiring begins. The serving utility generally specifies the location of the service point based on the conditions of service.


B- Feeder:

It is A conductor that originates at the main distribution or main disconnect device and terminates at another distribution center, panelboard, or load center.


C- Branch circuit:

It is the portion of the wiring system extending past the final overcurrent device. These circuits usually originate at a panel and transfer power to user load devices.

So, any circuit that extends beyond the final overcurrent protective device is called a branch circuit (Some branch circuits originate at safety switches (disconnects)). 

Example: 

Conductors from a 20A/1P circuit breaker in a panelboard connected to a receptacle in a room.




3. Essential definitions:


Some other definitions are essential to understanding branch circuits and feeders, which are:


3.1 Service conductors: These conductors extend from the power company terminals to the main service disconnect.


3.2 Sub-feeders: These conductors originate at distribution centers other than the main distribution center and extend to panelboards, load centers, and disconnect switches that supply branch circuits.


3.3 Panelboard: This can be a single panel or multiple panels containing switches, fuses, and circuit breakers for switching, controlling, and protecting circuits.



3.4 Demand and Diversity factors:

A- Demand factor: it is the ratio of the maximum demand of a system, or part of a system, to the total connected load on the system, or part of the system under consideration. 
So, Demand factor is always less than one. 


B- Diversity factor: it is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration.
So, Diversity factor is usually more than one.


The sum of the connected loads supplied by a feeder-circuit can be multiplied by the demand factor to determine the load used to size the components of the system.

While, the sum of the maximum demand loads for two or more feeders is divided by the diversity factor for the feeders to derive the Maximum Demand Load.
Example: 

Consider four individual feeder-circuits with connected loads of 250 kVA, 200 kVA, 150 kVA and 400 kVA and demand factors of 90%, 80%, 75% and 85% respectively.
Use a diversity factor of 1.5. calculate the Max demand load for the supply transformer. 



Solution:
Calculating demand for feeder-circuits

250 kVA x 90% = 225 kVA

200 kVA x 80% = 160 kVA

150 kVA x 75% = 112.5 kVA

400 kVA x 85% = 340 kVA


The sum of the individual demands = 837.5 kVA
If the main feeder-circuit were sized at unity diversity: 

kVA = 837.5 kVA ÷ 1.00 = 837.5 kVA

The main feeder-circuit would have to be supplied by an 850 kVA transformer.
However, using the diversity factor of 1.5, the kVA = 837.5 kVA ÷ 1.5 = 558 kVA for the main feeder.

For diversity factor of 1.5, a 600 kVA transformer could be used.
This means that a 600 kVA transformer can be used instead of an 850 kVA when applying the 1.5 diversity factor. 



3.5 Electrical Load:


it is the power drawn from the supply circuit by a particular piece of equipment, appliance, or luminaire (lighting fixture). A load can best be depicted as voltage drop. Wherever there is voltage drop in an electrical circuit, there is a load being served.

The electrical load for and device/equipment can be expressed by (3) different ways as follows:

  1. Connected Load,
  2. Calculated/Estimated Load,
  3. Demand Load.


A- Connected Load:


The load, measured in amperes or watts (volt-amperes), that is actually required by a load to operate properly. Connected load is the nameplate load rating of an appliance or other type of electrical utilization equipment.


Example:
A 60-watt light bulb connected to a 120- volt circuit will draw 1⁄2 ampere from the voltage source.

This is the actual level of current flow obtained by dividing the 60 watts by the 120 volts

(60 watts/120 volts = .5 ampere) as required by the power formula. There is a load of 60 watts, or 1⁄2 ampere, on this circuit due to this one load.


The Total Connected Load: Many and various types of loads may be connected to the same circuit and the total load for this circuit is determined by adding together all of the connected loads.



B- Calculated/Estimated Load:

It is The load of utilization equipment or outlets as determined by procedures allowed or required to be employed by the NEC. General lighting and general-use receptacle loads are examples of calculated loads.

Examples: 

  1. The general lighting load is calculated from the total floor area of a building. 
  2. A motor load is determined to be 125% of the motor’s FLA rating. 

C- Demand Load:

The load remaining after the demand factors allowed by the NEC for a number of like or similar loads have been employed.


Example: 

A demand load reduction is allowed for the total load for the ranges of an apartment building on the theory that not all ranges will be in use at the same time.

Demand loads apply when a load that is available to many areas of the structure will not be fully loaded at the same time.

Example:

It is recognized by the NEC that not all of the outlets on all of the general use circuits will be in use at the same time. Therefore, for dealing with feeders, the NEC allows for de-rating of those loads to some lower level i.e. demand factors will apply only for a group of loads originate from same distribution center.



Also, Electrical Loads can be classified according duty time into: 

A- Continuous loads: loads where the maximum current is expected to continue for 3 hours or more. 

B- Non-continuous loads: other loads than Continuous Loads.



For more essential definitions, please view our Course " EC-1: Understanding NFPA 70 (National electrical code) course ".




4. Branch circuits types


1- According To Number of Loads:




A- Individual branch circuit: A branch circuit that supplies a single load. If this load is an appliance, it will be called Appliance branch circuit.

B- Multi outlet branch circuit: A branch circuit with multiple loads which may be one of two cases as follows:

  • If these loads are appliances and lighting loads, it will be called General purpose branch circuit. 
  • If these loads are appliances only, it will be called appliance branch circuit. 

So, Appliance branch circuit: A branch circuit that supplies energy to one or more outlets to which appliances are to be connected and that has no permanently connected luminaires that are not a part of an appliance.

And, General purpose branch circuit: A branch circuit that supplies two or more receptacles or outlets for lighting and appliances. 



2- According To Number of Ungrounded Wires:


A- Two wire branch circuit: A branch circuit with one ungrounded conductors and one grounded conductor.

B- Multi-wire branch circuit: A branch circuit with two or more ungrounded conductors and one grounded conductor.

A multi-wire circuit can be an individual circuit or a multi-outlet circuit.



3- According To Type of Load

Branch-circuit loads are classified According To Type of Load into five categories:

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


In the next Article, I will explain the Branch Circuit Design Calculations Rules. Please, keep following.


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