Home PDF Courses Electrical Courses Download Library Inspection Courses Quiz and Answer Electricity Today Branch Circuit Design Calculations – Part Three In the previous article " Branch Circuit Design Calculations – Part Two " in our new course " Course EE-3: Basic Electrical design course – Level II ", I explained some Additional definitions and features of Branch circuits. Also, I listed the (5) categories of Lighting Branch circuits, which were: 1. 2. 3. 4. 5. General lighting. Show-window lighting. Track lighting. Sign and outline lighting. Other lighting. Today I will explain the design calculations for the general lighting branch circuits as follows. You can review the following previous articles for more information: Branch Circuit Design Calculations – Part One 1- General lighting branch circuit 1.1 Definition: General lighting outlets are those Outlets intended for general use for fixed-in-place luminaires (lighting fixtures). They are only used for lighting for the normal use of the occupants and Its intensity should be adequate for any type of work performed in the area. 1.2 Lighting fixtures not included in this category: Specialized task lighting (Show-window lighting, Track lighting, accent, specialty, or display lighting). Any special lighting for workshops, photography labs, or studios that may be located in the dwelling. 1.3 Calculation Method Determining the general lighting load as per NEC will be based on the load per area method as follows: . the calculated floor area shall not include open porches.500 volt-amperes. the load is continuous and can only be 80% of the load supplied by the circuit.4 Notes for table 220.12. cellars. The circuit is designed to supply 109. However.12. hallways. there are normally several different types of areas like storage.5 VA/ft2 x 25.25 (the inverse of 80%) to determine the circuit requirements. The general lighting load is determined by multiplying this value by the square footage of the building: The general lighting load = 3. For dwelling units. and cafeterias. Example#1: A 25.The NEC cod introduce minimum general lighting loads (in VA/ft2) for various types of buildings in Table 220. dwelling unit. these areas must be considered separately if their (VA/ft2) values are available in table 220. office.25 to determine the circuit requirements: the circuit rating = 87. the calculated load is multiplied by 1.375 KVA 1.5 VA/ft2.500 VA So. and crawl spaces). (Please review the definition for the term “continuous loads” in Article) How to calculate the Floor area? The floor area for each floor shall be calculated from the outside dimensions of the building.12.25 = 109. What is the general lighting load and what load does the circuit need to supply? Solution: From Table 220.375 VA so. This value must be multiplied by 1. the unit load for an office building is 3.12: . If the load is continuous (as in Most commercial structures). or unused or unfinished spaces not adaptable for future use (like some attics.500 VA x 1.000 ft2 office building is being designed. the general lighting load is 87.000 ft2 = 87. garages. Within the same building. or other area involved. The general lighting load is calculated by multiplying the floor area (in ft2) of a building by its unit load (in VA/ft2) derived from the above table. The general lighting load unit values specified in table 220.12 includes the following loads: 1. Under any conditions. including receptacles connected to Bathroom Branch Circuits. 2. there are no exceptions. Calculate the total general lighting load for this unit and the Minimum Number of General . no need to add the above outlets in load calculations per NEC method. underfloor spaces. Example#2: A (2) floors (basement and main) dwelling unit have dimensions as show in below image. Wall lighting outlet used in Storage or Equipment Spaces (like attics. 4. All general-use receptacle outlets of 20-ampere rating or less. attached garages. Wall lighting outlet used in Habitable Rooms. 5. The unit values herein are based on minimum load conditions and 100 percent power factor (i. and detached garages.12 before reviewing the following notes. The outdoor receptacle outlets. So. and Accessory Buildings.12. Wall lighting outlet used in Guest Rooms or Guest Suites In hotels. the designer can choose a higher value based on the existing design conditions. stairways. Garages. Wall lighting outlets used in hallways.Important!!! Don’t apply the values of table 220.e. Load in VA = Load in Watt) and may not provide sufficient lighting for the installation contemplated. or similar occupancies. general-use receptacle Outlets used in Basements. don’t use values less than that specified in table 220.12 are applied for any Additions to Existing Installations for both dwelling and non-dwelling installations. 6. motels. So. utility rooms. Energy saving–type calculations (which used to reduce the connected lighting load and actual power consumption) are not permitted to be used to determine the minimum calculated lighting load if they produce loads less than the load calculated according to 220. 3. The NEC method and table 220.12. and basements). 7. ) = 20 FT 2 IN. + 11 FT 0 IN. X 14 FT 8 IN. X 28 FT 1 IN. + 3 FT 8 IN.) = 17 FT 9 IN. = 295. X (6 FT 3 IN. = 730.5 FT2 Total main floor area (IN FT2) = 1304. the crawl space and garage areas are not considered as they are included in the NEC method calculation. X (3 FT 8 IN.) = 6 FT 0 IN.0 FT2 AREA “C”= 4 FT 4 IN. + 17 FT 1 IN.) = 18 FT 6 IN. X (3 FT 8 IN. X 31 FT 9 IN. X (11 FT 0 IN. = 512. X 38 FT 0 IN. + 11 FT 0 IN.Basement area: area “A” = 17 FT 9 IN.Main floor area: AREA “A”= 6 FT 0 IN. + 11 FT 0 IN.Lighting Branch Circuits.) = 4 FT 4 IN. + 17 FT 1 IN.5 FT2 AREA “B”= 18 FT 6 IN.5 FT2 AREA “D”= 20 FT 2 IN. = 168. = 137. + 6 FT 0 IN. X (4 FT 4 IN. Solution: Step#1: Calculate the total area of the dwelling unit as follows: a. + 17 FT 1 IN. + 18 FT 6 IN.12. b. X 28 FT 10 IN.0 FT2 Note that as mentioned in notes for table 220.5 FT2 . Methods for determining the actual full load for every individual lighting fixture in the circuit is explained in our course " Advanced Course for Lighting Design . autotransformers. etc. you can calculate the actual load for the lighting branch circuit by summing of actual full load for its individual lighting fixtures.) so. you need to compare the values obtained from NEC method with that obtained from actual load method and select the greater load value to be used in the design. internal wiring. The general lighting load for the dwelling unit = 1816. Section 220. In this case.5 Notes for NEC method for calculation of lighting branch circuit load Important!!! The NEC doesn’t introduce a procedure for calculating the actual full load for the individual lighting fixtures in a general lighting branch circuit.5 ft2 Step#2: The unit load listed in Table 220.0 FT2 + 1304.So.5 watts 1. or LED drivers. the calculated load shall be based on the total ampere ratings of such units and not on the total watts of the lamps. But actually. .Level I " and I recommend reviewing these methods very well. Total area of the dwelling unit = 512.18 (b) states that For circuits supplying lighting units that have ballasts.5 FT2 = 1816. This means that we must take into account the losses in light fixture switchgear (ballast.12 shows 3 volt-amperes per ft2 for dwelling units. you must use the current rating of the ballast.5 ft2 x 3 watts per ft2 = 5449. If the required information for calculating the actual full load for every individual lighting fixture in the circuit is available. The NEC method for calculation of lighting load is not required if the actual full load for every individual lighting fixture in the circuit is determined. not the tube wattage. transformers. in this time. Solution: From example#2.25 to determine the circuits requirements: the Load Rating = 5449.9 watts The total ampere = 6811.77/ 15 = 3. If we design 20-ampere circuits: .Example#3: A fluorescent lighting fixtures with 4 numbers 2 feet lamps. the general lighting load for the dwelling unit = 5449.6 Determining the Minimum Number of General Lighting Branch Circuits Example#4: For the dwelling unit in example# 2 above. we will use the same equation in another form The actual total load of fixture = 4 ballast x watt/ballast = 4 x 20 w = 80 watt 1. NEC method assumed that PF=1) If we design 15-ampere circuits: The number of the general lighting circuits = 56. the load is continuous and can only be 80% of the total load. we can’t know the actual losses.9 watts / 120 V = 56.5 watts x 1.77 A (note that .25 = 6811. This value must be multiplied by 1.5 watts However. 18 watt/ lamp. Solution: The actual total load of fixture = 4 lamps x 18 watt/lamp + losses So. Calculate the actual load for this lighting fixture.78 circuits. Calculate the Minimum Number of General Lighting Branch Circuits. The number of the general lighting circuits = 56.84 circuits rounded up to 3 circuits). In no case shall the load exceed the branch-circuit ampere rating.23 states that: 1. Note: according to 210. An individual branch circuit shall be permitted to supply any load (any number of outlets) for which it is rated. dining room. the total number of branch circuits with use of 15-ampere circuits is (4) circuits (3. so the current used by the lights can only be 80% of the circuit current rating: Allowable current = 20 A x 0. 2. such fastened-in-place equipment is not permitted on the small-appliance branch circuits required in a kitchen.77/ 20 = 2. Example#5: A 4 feet long.7 Determination of maximum Permissible number of lighting fixtures on a general lighting branch circuit You need to review the definition for branch circuit rating explained in the previous article " Branch Circuit Design Calculations – Part Two ". and because these are minimum units. Under the conditions specified in this requirement.23(A)(2) permits a 15. two-lamp fluorescent fixture ballast draws 0.5 amperes on a 15ampere circuit and 10 amperes on a 20-ampere circuit). Rules to be applied are as follows: Section 210. 1. and so on. Section 210.78 circuits rounded up to 4 circuits) and (3) circuits with use of 20-ampere branch circuits (2. How many of these fixtures can be connected on a 20-amp circuit? Solution: This is a continuous load.7 amps at 120 volts.or 20-ampere branch circuit supplying lighting outlets to also supply utilization equipment fastened in place.52(B).80 = 16 A . such as appliances or an air conditioner.84 circuits Because it is not possible to have a partially powered electrical circuit that operates properly. the utilization equipment load must not exceed 50 % of the branch-circuit ampere rating (7. the Allowable current = 16 A x 50% = 8 A By dividing the allowable load by the load of each lamp. .23(A)(2). Example#6: In example#5. the total number of Fixtures is determined: the total number of Fixtures = 16 A /0.80 = 16 A By following section 210.8 fixtures So.7 A = 22.4 fixtures So. If some utilization equipment fastened in place will be added to the circuit How many of these fixtures can be connected to the circuit? Solution: This is a continuous load. so the current used by the lights can only be 80% of the circuit current rating: Allowable current = 20 A x 0. the maximum number of fixtures on the circuit is 22.7 A = 11. the maximum number of fixtures on the circuit is 11.By dividing the allowable load by the load of each lamp. Special Rules In non-dwelling buildings: Lighting branch circuits that supply fixed lighting units with heavy-duty Lampholders can have 30 A or 40 A ratings. Important!! For good design. the total number of lamps is determined: the total number of Fixtures = 8 A /0. usually assume that the general lighting branch circuits will have some utilization equipment fastened in place and must be derated to 50% of its rating. thanks for the priceless contribution of information. . 2) In Main Floor area calculation for Area-B. please keep following. Email ThisBlogThis!Share to TwitterShare to FacebookShare to Pinterest 5 comments: 1. Lastly. this is being pointed out: 1) Dimension in the pictorial representation has 10 FT 6 IN in the place of 18 FT 6 IN. computed total area would be 703 FT2 in the place of 730 FT2 (Total Area has no mismatch though). Once Naveen Reply Replies again. 2013 at 6:14 AM Hello Eng.Lighting Outlets for heavy-duty Lampholders shall be calculated at a minimum of 600 volt-amperes. An industrial information has been conveyed with every minute detail. In the next article. Heads up. Use The above rules when it is applicable. Ali. so. Hoping this would clear for the other readers. not trying to point out the mistake here but there was confusion in the mismatch of data provided in the Example-2. I will explain design calculations for other categories of Lighting Branch circuits. NAVEEN SJune 16. Contents are mind blowing for any practicing professional. i think it was a typing error. thanks for your cooperation for highlighting any mistakes. but your note#2 is accurate . Ali HassanJune 17. Thanks for responding. 2013 at 1:32 PM Hi Naveen. I may be wrong though. and i think that your note#1 is not accurate the two lengths are identical in the figure of example#2. 2013 at 2:28 AM I downloaded the image & viewed in PICASA. 2. thanks again. NAVEEN SJune 19. Reply . the area "B" must equal 703 ft2.1. 2014 at 4:24 PM nice Reply .2. 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