Branch Circuit Design Calculations – Part Four


In the previous article "
 Branch Circuit Design Calculations – Part Two " in our new course " Course EE-3: Basic Electrical design course – Level II " , I listed the (5) categories of Lighting Branch circuits, which were:

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

Also, in the previous Article " Branch Circuit Design Calculations – Part Three ", I explained the following points:

  • The design calculation method for general lighting, 
  • The calculation method for determination of the Minimum Number of General Lighting Branch Circuits in a building, 
  • The calculation method for Determination of maximum Permissible number of lighting fixtures on a general lighting branch circuit. 

Today I will explain the design calculations for other categories of lighting branch circuits as follows.

You can review the following previous articles for more information: 






2- Show-window lighting Branch Circuit





2.1 Definition

From NEC article 100 the definition for Show-window is as follows:

Show Window: Any window used or designed to be used for the display of goods or advertising material, whether it is fully or partly enclosed or entirely open at the rear and whether or not it has a platform raised higher than the street floor level.


The Show-window lighting branch circuits are often used in commercial buildings.



2.2 NEC rules applied for Show-window lighting

The Code includes several sections applicable to show-window lighting:

  • Section 210.62 Show Windows 
  • Section 220.14(G) Load computation 
  • Section 220.43(A) Show-window lighting. 


2.3 Design calculations for show window Load

A- Determination of number of receptacles need for a show window:

As per section 210.62, At least one receptacle outlet shall be installed within 450 mm (18 in.) of the top of a show window for each 3.7 linear m (12 linear ft) or major fraction thereof of show window area measured horizontally at its maximum width.



Notes to section 210.62: 


1- To reduce the use of extension cords and floor receptacles which may cause physical injury, the required receptacle outlet(s) installed within 18 in. of the top of the show window.
2- Meaning of (12 linear ft) or major fraction is as follows: 

  • If we have a show window with total length = 12 feet then, we will need one receptacle, 
  • If we have show window with total length = 13 feet (> 12) = 12 +1 then, we will need (2) receptacles 
  • If we have show window with total length = 5 feet (<12) then, we will need one receptacle. 
  • If we have show window with total length = 25 feet (> 12) = 2 x 12 +1 then, we will need (3) receptacles 


B- Determination of show window load:


As per Sections 220.14(G) and 220.43(A), two options are permitted for the load calculations for branch circuits serving show windows:

Option#1:

If the receptacle(s) required according to 210.62 supplies the show window lighting load then the show window lighting load = 180 volt-amperes x number of receptacles


Option#2:

If the receptacle(s) required according to 210.62 don’t supplies the show window lighting load then the show window lighting load = 200 volt-amperes x show-window length (in feet) as shown in below image. And in this case:
The show window total load = load of receptacles + load of lighting = (180 VA X number of receptacles) + (200 VA x show-window length (in feet) 



Important!!!
If the maximum volt-ampere rating of the equipment and lights used for show window lighting is known, then, calculate the show widow load as explained in the above two options. then, you must select the greater load value to be used for design of branch circuits and feeder.


Don’t forget …
The show window load is a continuous load and its value must be multiplied by 1.25 for determination of the circuit load requirements.



Important!!
If you need to determine the minimum number of feet of show window allowed on a single branch circuit or/and maximum number of fixtures for a show window, please follow the same procedure explained in previous article " Branch Circuit Design Calculations – Part Two " for general lighting branch circuit.





Example#1:


A department store has two lighted show windows, one 25 feet long and the other 20 feet long. What are the branch-circuit requirements for the show window load?


Solution:
Compute the load based on linear feet of show window:

Total length = 25 + 20 = 45 feet

Show-window load = 45 × 200 VA/ft = 9000 VA
The lighting is a continuous load, so the show window load is multiplied by 1.25 to determine the circuit load requirements: 
Show-window load = 9000 VA × 1.25 = 11,250 VA 

The circuits supplying power for the show-window lighting must have a minimum capacity of 11,250 volt-amperes.


In addition, receptacles are required for every 12 feet of show window. A total of five receptacles (two for the 20 feet window and three for the 25 feet window) are needed.
The receptacle load = 5 × 180 VA = 900 VA 

The show window total load = 11,250 VA + 900 VA = 12,150 VA



3- Track Lighting






3.1 Definition:


From NEC section 410.2 the definition for track lighting is as follows:
Lighting track is a manufactured assembly designed to support and energize luminaires (lighting fixtures) that are capable of being readily re-positioned on the track. Its length can be altered by addition or subtraction of sections of track. 

Track lighting is often used in commercial buildings for accent lighting.

3.2 NEC rules applied for Track lighting:


The Code includes several sections applicable to Track lighting:

  • Section 220.43(B) Track Lighting 
  • Section 410.151 (B) Connected Load. 


3.3 Design calculations for Track Lighting Load

As per NEC section 220.43(B), an additional load of 150 volt-amperes shall be included for every 600 mm (2 ft) of lighting track or fraction thereof.



Important!!!
As per Section 220.43(B) ,This method for calculations for Track Lighting Load is applied for commercial buildings but it will not be applied for dwelling units or guest rooms or guest suites of hotels or motels) because in these buildings the track lighting load will not be added to the service load.



Example#2:


28 feet of track lighting is shown on the floor plans of a one-family dwelling. How much additional load will this track lighting add to the service? 


Solution:
In accordance with 220.43(B), no additional load is required for track lighting installed in a dwelling unit.



Notes to section 220.43(B):


  1. Where multi-circuit track is installed, the load shall be considered to be divided equally between the track circuits. 
  2. Meaning of mm (2 ft) of lighting track or fraction is that if there is a faction of track lighting the load computed for this faction is the same as a complete one. 


Notes to section 410.151(B):

  1. The maximum load on the track cannot exceed the rating of the branch circuit supplying the track. 
  2. Also, the track must be supplied by a branch circuit that has a rating not exceeding the rating of the track. (Please, review our previous article " Branch Circuit Design Calculations – Part One " for definition of branch circuit rating).

Design calculations steps:


  • Step#1: Determine the total length of track lighting, 
  • Step#2: Divide the total length of the track by two, 
  • Step#3: If, there is a faction, Round up the faction to one, 
  • Step#4: Multiply the result from step#2 or 3(if there is a faction) by 150 volt-amperes to get the total track load. 



Don’t forget …
If The Track lighting load is considered as a continuous load then its value must be multiplied by 1.25 for determination of the circuit load requirements.


Important!!!
The actual track lighting fixtures are owner supplied, so, neither the quantity of track lighting fixtures nor the lamp wattage is specified.

If the quantity of track lighting fixtures and the lamp wattage are known, then calculate the total load of the track = quantity of track lighting fixtures x number of lamps/fixture x lamp wattage, then, you must select the greater load value to be used for design of branch circuits and feeder.


Important!!
The above design calculation method as per Section 220.43(B) is not intended to determine the number of feet of track allowed on a single branch circuit and/or the maximum number of fixtures on an individual track. If you need to do this, please follow the same procedure explained in previous article " Branch Circuit Design Calculations – Part Two " for general lighting branch circuit.




Example#3:

A lighting plan shows 62.5 linear ft of single-circuit track lighting for a small department store featuring clothing. What is the minimum calculated load associated with the track lighting that must be added to the service or feeder supplying this store?



Solution:

According to 220.43(B), the minimum calculated load to be added to the service or feeder supplying this track light installation is calculated as follows:

Step#1: the total length of track lighting = 62.5 feet


Step#2: 62.5 ft / 2 ft = 31.25,


Step#3: Round up the length fraction, than rounded up to 32


Step#4: the total track load = 32 x 150 VA = 4800 VA

The minimum load for the lighting track that is added to the service and/or feeder calculation is 4800 volt-amperes.



Special case of calculations for Track Lighting Load


As per 220.43(B) exception, If the track lighting is supplied through a device that limits the current to the track (a supplementary over-current protective device that may be fuse or circuit breaker), the load shall be permitted to be calculated based on the rating of the device used to limit the current.

I.e. the track lighting total load = voltage x device rating (Amps)

Example#4:

For department store in example#2, the entire length of the used track will be supplied by a single 20-ampere, 120-volt branch circuit.

What is the minimum calculated load associated with the track lighting that must be added to the service or feeder supplying this store?



Solution:
According to the exception to 220.43(B), the minimum calculated load to be added to the service or feeder supplying this track light installation is calculated as follows:
The track lighting total load = 120 V x 20 A= 2400 VA 
The minimum load for the lighting track that is added to the service and/or feeder calculation is 2400 volt-amperes. 



In the next article, I will explain design calculations for other categories of Lighting Branch circuits. So, please keep following.





3 comments:

  1. Hello Eng. Ali,
    There's a doubt w.r.t show window lighting load. It is explained there are two types of load calculation.
    1) As per NEC 210.62, there will be no lighting load taken separately.
    i.e = 180 VA x No. of receptacles.

    2) If not complying NEC 210.62,
    Total load = Lighting load + Receptacle load.

    Where as in the example-1, when the calculations are being made as per NEC 210.62 are carried out, then why lighting loads are considered? Pls help.

    Thanks in advance.


    Naveen

    ReplyDelete
    Replies
    1. both options #1 & #2 are according to 210.62, the difference between them is considering receptacles as a lighting supply or not?

      the two options refer to 220.14(G) and 230.43(A)and in example#1, the option#2 must be applied to calculate the total branch load of the show lighting window (i.e. receptacles+lighting)

      Delete
    2. Thanks a lot for clearing the doubt.

      There's one query, it's been explained:

      Option#1:

      If the receptacle(s) required according to 210.62 supplies the show window lighting load.....

      Option#2:

      If the receptacle(s) required according to 210.62 don’t supplies the show window lighting load..

      In option#2, receptacle is not supplying show window lighting load according to 210.62. Does this mean, there is another clause in NEC 210.62 which provides additional option, if the load is not a lighting load for show window?

      It will be very helpful, pls help.

      Thanks in advance.

      Delete

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