Lesson 2

BRANCH CIRCUITS

 

          A branch circuit is that portion between the over current device and utilization equipment.  For 120/240v single phase 3 wire circuits or 120/208 three phase 4 wire circuits must share the neutral of their grounded conductor. These multi wire circuits must be attached to an approved handle tie so when one phase is shut off the other is as well.  This purpose is to protect the electrician, the premises and equipment load on the other phase legs when disconnecting the neutral.  Remember the purpose of the grounded or neutral conductor.  This conductor carries the unbalanced load.  For single phase 120v/240v circuits, if one phase carries 20amps and the other phase is carrying 0 amps, the possible current flow on the neutral will be the difference of 20-0=20 amps.  The neutral or grounded conductors must have a marking to distinguish that circuit from the phase legs.  This will be by designated colors of white or gray.  If a flexible cord or lamps cord, notice the ridge or groove which designates that conductor as the grounded or neutral.

In order to calculate the load on the service entrance conductors or feeders using the appropriate demand factors we must first calculate the branch circuit loads of the area to be occupied.  Branch circuits must conform to various code requirements.  Receptacle locations, ground faults, arc faults, tamperproof, lighting locations, switches, appliances, motors, as well as minimum requirements for kitchens, bathrooms, laundry and other locations need to be addressed and the appropriate load considered by conforming to the code requirements before and during electrical design.  A detailed layout of branch circuits throughout the building must designed before the circuit makes its way back to its over current device located in its panel board.  That portion of the branch circuit between the panel board and its first splice point is known as a homerun.  The branch circuit homerun should be sized in order to keep the voltage drop to a minimum. 

·        Try drawing a sketch of the room that you are sitting in. 

·        Label the sketch with dimensions of the room. 

·        Can you identify the number and location of receptacles, switches, and luminaries needed within that room?

·        Probably yes but can you tell me if your local inspector would pass your installation practices?

·         Now can you list the exact article number of the code book to back your theory?

 

 So being a professional means not only being an electrician but also a lawyer now.  The cost of correcting your work is far less now by learning how to use the National Electrical Code book than correcting your installed work in the future.  Organization and studying is the key.  Passing your license doesn’t make you a Master Electrician only a good test taker.  Demonstrating that you are a Master Electrician is an everyday task through the use of the code. 

 

BRANCH CIRCUIT CALCULATIONS

 

          Branch circuit locations, receptacles and lighting whether AFCI or GFCI protected can be determined by article 210.  However the volt amp requirements per square foot of space, volt amps of a particular appliance or demand factors of the load (allowable percentage deduction of load or volt amps) can be determined by article 220. 

To sum it up

Branch Circuit locations = 210

And

Branch Circuit Calculations = 220

         

          Let’s say the room that you are sitting in is 12 ft x 14 ft.  Based upon article 210 a receptacle is required every 12 ft so that if you decide to plug in a piece of equipment anywhere along the wall, it will reach.  The cord attached to the piece of equipment which is 6 ft will reach the nearest receptacle to the left or to the right eliminating the use of an extension cord.  In order to determine the number of receptacle required, first calculate the linear feet of the room.  Adding up all the walls of the rectangular room in a dwelling unit would be 12ft + 14ft + 12f + 14 = 52 linear feet.  52 linear feet divided by 12 linear ft required = 4.3 receptacles required or 5 receptacles are most likely required. 

Since this is a dwelling unit we do not calculate 180 volt amps per receptacle.  To determine the volt amps or number of circuits required in a dwelling unit we calculate the square footage of the area and multiply by 3 volt amps per square ft.  This 3 volt amp per square foot combines the required lighting and receptacle load required by the NEC.  Therefore the 12 ft x 14 ft room has a square footage of 12x14= 168 square feet.  168 square ft x 3 volt amps per square ft = 504 volt amps.  One 15 amp 120 volt circuit should equal 1800 volt amps and one 20 amp 120 volt circuit can cover a load of 2400 volt amps.  Remember Power = E x I.  One 15 amp circuit should cover the load requirement for this room and then some. 

          Demand factors applied to the load calculated vary depending which calculation method is used (Standard or Optional). The demand factors take into consideration the average use that the load will be operating over a short period of time reducing the volt amps required on the service therefore reducing the needed ampacity in which the current carrying conductors must carry.  If P = E x I = volt x amps and I = P/E then reducing the size of the wire on the service saves labor, time, materials, money and the chance that your bid will be accepted.  

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