Branch Circuit Design Calculations – Part Two

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

Today I will list additional types of branch circuits and explain the basic features for any branch circuit as follows.

4.A Additional Branch circuits types

1- According to location

A- Indoor branch circuit

It includes All branch circuits inside the premises for lighting, receptacles, equipments, HVAC or other outlets.

B- Outdoor Branch Circuits

It is the branch circuits run on or between buildings, structures, or poles on the premises; and electrical equipment and wiring for the supply of utilization equipment that is located on or attached to the outside of buildings, structures, or poles. Examples for Outdoor Branch Circuits are listed in table 225.3 in below. 

2- According to purpose of the Load

A- General-Purpose Branch Circuits 

It included in Chapters 1, 2, 3, and 4 and not listed under Specific-Purpose Branch Circuits tables 210.2, 220.3 and chapters 5, 6&7.

B- Specific-Purpose Branch Circuits 

The provisions for branch circuits supplying equipment listed in Table 210.2 amend or supplement the provisions in this article and shall apply to branch circuits referred to therein.

Also table 220.3 list the calculation of loads in specialized applications that are in addition to, or modifications of, those Of general rules.

NEC code Chapters 5, 6, and 7 apply to special occupancies, special equipment, or other special conditions. These chapters supplement or modify the general rules.

3- According to type of building

A- Dwelling Building

A building providing complete and independent living facilities for one or more persons, including permanent provisions for living, sleeping, cooking, and sanitation.

Dwelling buildings include the following types:

  • Dwelling, One-Family: A building that consists solely of one dwelling unit. 
  • Dwelling, Two-Family: A building that consists solely of two dwelling units. 
  • Dwelling, Multifamily: A building that contains three or more dwelling units. 

B- Non-dwelling buildings

other buildings than dwelling like schools, hotels, factories, etc.

4- According to building’s construction status

A- Branch circuits for Existing building.

B- Branch circuits for New building.

5- According to Branch circuit voltage

A- Branch Circuits Not More Than 600 Volts

It is divided to (4) categories as follows:

a- Branch Circuits not exceed 120 volts
In residences and hotel rooms, circuits supplying lighting fixtures and small receptacle loads.

b- Branch Circuits 120 volts and less
it may be used to supply lampholders, auxiliary equipment of electric-discharge lamps, receptacles, and permanently wired equipment.

c- Branch circuits 120 volts - 277 volts
It may supply mogul-base screw-shell lampholders, ballasts for fluorescent lighting, ballasts for electric-discharge lighting, plug-connected appliances, and hard-wired appliances. Incandescent lighting operating over 150 volts is permitted in commercial construction.

d- Branch circuits 277 volts - 600 volts
it can supply mercury-vapor and fluorescent lighting, provided the lighting units are installed at heights not less that 22 feet above grade and in tunnels at heights no less than 18 feet.

B- Branch Circuits over 600 Volts

It used for some special uses in non-dwelling buildings.

6- According to Branch circuit rating:

A- 15 and 20 Ampere Branch Circuits
It shall be permitted to supply lighting units or other utilization equipment, or a combination of both.

B- 30 Ampere Branch Circuits
It shall be permitted to supply fixed lighting units with heavy-duty lampholders in other than a dwelling unit(s) or utilization equipment in any occupancy.

C- 40 and 50 Ampere Branch Circuits
It shall be permitted to supply cooking appliances that are fastened in place in any occupancy.
In other than dwelling units, such circuits shall be permitted to supply fixed lighting units with heavy-duty lampholders, infrared heating units, or other utilization equipment. 

D- Branch Circuits Larger Than 50 Amperes
It supply only non-lighting outlet loads.

5. Branch circuit features

5.1 Branch circuit Voltages

  • The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected. 
  • Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used. 
  • Branch-circuit voltage limits are provided by NEC Code. These limits are based on the equipment being supplied by the circuit.

5.2 Branch Circuit Conductors

Conductors normally used to carry current shall be of copper unless otherwise noted.

Conductors size unit:
The American Wire Gage (AWG) for size identification, which is the same as the Brown and Sharpe (BS) Gage. 

The notation circular mils is used for Conductors larger than 4/0 AWG which will be equal to  250,000 circular mils. 

the notation MCM is used instead of 
circular mils, where MCM = 1000 circular mils. so, a 250,000-circular-mil conductor was labeled 250 MCM.

the notation 
kcmil  is used by both UL standards and IEEE standards instead of MCM. where kcmil = MCM = 1000 circular mils.

The circular mil area of a conductor is equal to its diameter in mils squared (1 in. = 1000 mils). What is the circular mil area of an 8 AWG solid conductor that has a 0.1285-in. diameter?


0.1285 in. x 1000 = 128.5 mils

128.5 x 128.5 = 16,512.25 circular mils

5.3 Branch Circuit Rating

  • The rating of any branch circuit will be the maximum permitted ampere rating or setting of the overcurrent device protecting this branch circuit. 
  • Branch circuits serving only one device can have any rating, while a circuit supplying more that one load is limited to ratings of 15, 20, 30, 40, or 50 amps. 

A branch circuit wired with 10 AWG copper conductors has an allowable ampacity of at least 30 amperes, 
If this branch-circuit overcurrent protective device is a 20-ampere circuit breaker or fuse, what is the rating of this branch circuit?.


the rating of this branch circuit is 20 amperes, based on the size or rating of the overcurrent protective device. 

5.4 Number of Branch Circuits

The minimum number of branch circuits shall be determined from the total calculated load and the size or rating of the circuits used. In all installations, the number of circuits shall be sufficient to supply the load served.

I will explain the branch circuit design calculations as per the classification of branch circuit according to type of load mentioned in Previous Article 
Branch Circuit Design Calculations – Part Two "

First: Lighting Branch circuits

In the broad sense, lighting loads may be categorized as follows:

  1. General lighting. 
  2. Show-window lighting. 
  3. Track lighting. 
  4. Sign and outline lighting. 
  5. Other lighting. 

Each lighting load is computed separately and then combined to determine the total lighting load.

In the next Article, I will explain the design calculations for general lighting branch circuit. Please keep, following.

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