MikeHolt_Section06_Dwelling Units

Unit 09: Dwelling (Single) Unit Calculations

NEC Article 220

  • NEC Example problems in Appendix, pg 70-791-799
  • 2 different calculation methods
    • Standard
    • Optional
  • Standard Method, Steps to determine Feeder or Service size, single Dwelling Unit (Article 220, Part III)
    1. Step 01:  Determine feeder demand factor load for General Lighting, General-use Receptacles, Small appliances, and Laundry circuits (220.42)
      1. Determine the total connected load (220.52)
        • General lighting & receptacles at 3 VA per sq ft (of interior floor plan)
          • 220.12
        • 2 Appliance circuits each at 1500 VA
          • 220.52(A)
        • 1 Laundry circuit at 1500 VA
          • 220.52(B)
      2. Apply table 220.42 (pg 70-72) demand factors to the total connected load
      3. Calculate 1st 3000 VA at 100% and remaining VA at 35% demand
        • NEC requires a minimum of 3 VA per Sq Ft to determine branch circuit feeder & service calculations
          • Includes all 15A & 20A general-use receptacles
          • Does not include small appliances & laundry circuit receptacles
        • Area calculated on building outside dimensions
          • Do not include open porches, garages, or spaces not adaptable for future use
    2. Step 02: Appliances
      • 220.53
      • Add nameplate ratings of all appliances fastened in place
      • Apply 75% demand factor for four or more permanent appliances
        • 3 appliances can be a bigger load than 4 appliances
        • Does not apply to electric loads in following steps below
      • Demand Load = Connected Load * Demand Factor (220.53)
    3. Step 03: Clothes Dryer
      • 220.54
      • Minimum demand load is whatever is greater, 5000W or nameplate data
      • Apply table 220.54 (pg 70-73) if ≥ 5 dryers
      • kVA is equivalent to kW for dryers
    4. Step 04: Cooking Equipment
      • 220.55 chart/table
        • Note 4: Permissible to use table 220.55-note 4 to calculate branch-circuit load sizing for 1 range
        • Branch-circuit load sizing for 1 wall-mounted oven or 1 counter-mounted cooking unit must be the appliance nameplate rating.
        • Branch-circuit load sizing for 1 counter-mounted cooking unit and 1 or 2 wall-mounted ovens is determined by adding nameplate ratings together & treating the resultant value as a single range.
      • Equipment rated over 1¾ kW calculated according to table 220.55 (pg 70-74)
        • Including Notes 1, 2, & 3 for determining demand loads
      • Demand Load = Column C Value * Multiplier
        • MH-pg 404, Table 220.55, Note 1
    5. Step 05: Air-conditioning VS Heat
      • 220.50
      • 220.51
      • 220.60
      • Air conditioning or heating load are calculated at 100%
      • If both not on at same time, smaller of 2 loads can be omitted
      • If both are on at same time, both are added together
    6. Step 06: Feeder & Service Conductor Size
      • 310.15(B)(7)(1) through (3)
        • Use this section to size the service & feeder conductors supplied by  1 Ø, 120/240V & 120/208V for a 1-family dwelling &/or single unit of 2-family/multi-family dwelling and size Service conductors to 83% of service over-current protection device rating.
          • 83%  rule can not be used for service conductors of 2-family and muilti-family units.
      • DO EXAMPLE MH-PG 407
  • Optional Method, Steps to determine Feeder or Service size, single Dwelling Unit (Article 220, Part III)
    • Can only be used for dwelling units served by a single 120/240V or 120/208V, 3-wire set of service or feeder conductors with an ampacity of 100A or greater
      • 220.82
    • Step 01: General Loads
      • 220.82(B)
      • Demand Load
        • not less than 100% for the 1st 10 kVA, plus
        • 40% of the remainder of the following:
          • General Lighting & Receptacles
            • 3 VA per Sq. Ft
          • Small-appliance & Laundry
            • 1500 VA for each 20A circuit specified in 220.52
          • Appliances
            • Nameplate VA rating of all permanent appliances & Motors
              • except haeting & Air-conditioning
              • Ensure range & dryer calculations use nameplate rating
    • Step 02: Largest of Air-Conditioning VS Heating
      • 220.82(C)
      • Include the larger of (1) through (6)
        • Air-conditioning equipment
          • 100%
        • Heat-pump compressor without Supplemental Heating
          • 100%
        • Heat-pump compressor and Supplemental Heating
          • 100% of heat-pump compressors nameplate rating and,
          • 65% of supplemental electric heating for central electric space-heating
            • If heat-pump compressor is interlocked preventing simultaneous supplemental heat operation, omit it from calculation
        • Space-heating units
          • 3 or fewer units
            • 65%
        • Space-heating units
          • 4 or more units
            • 40%
        • Thermal Storage Heating
          • 100%
    • Step 03: Service Size
      • 310.15(B)(7)
      • 310.15(B)(7)(1) through (4) can size 1Ø, 120/240V service & feeder conductors for 1-family dwellings & individual units of multiple unit 9dwellings
  • Existing Dwelling Unit Optional Load Calculations
    • 220.83
      • Used to determine if existing service or feeder is sufficient capacity to serve additional loads using optional method
        • Existing installation feeder or service load calculations can also be based on maximum demand
          • 220.87
      • Step 01: General Loads
        • 220.83(B)
        • Demand Load
          • not less than 100% for the 1st 8 kVA, plus
          • 40% of the remainder of the following:
            • General Lighting & Receptacles
              • 3 VA per Sq. Ft
            • Small-appliance & Laundry
              • 1500 VA for each 20A circuit specified in 220.52
            • Appliances
              • Nameplate VA rating of all permanent appliances & Motors
                • except haeting & Air-conditioning
                • Ensure range & dryer calculations use nameplate rating
      • Step 02: Largest Air-conditioning VS Heating
        • 220.83(C)
        • Include the larger equipment
      • Step 03: Service Size
        • 310.15(B)(7)
        • 310.15(B)(7)(1) through (4) can size 1Ø, 120/240V service & feeder conductors for 1-family dwellings & individual units of multiple unit 9dwellings
  • Neutral Load Calculations
    • Cooking Equipment Neutral Load
      • 220.61(B)(1)
      • Calculated at 70% of the demand load determined by 220.55
    • Dryer Neutral Load
      • 220.61(B)(1)
      • Calculated at 70% of the demand load determined by 220.54
    • Neutral Amperes = Neutral Demand Load / System Voltage

Unit 10: Multi-Family Dwelling Calculations

NEC Article 220

  • NEC Example problems in Appendix, pg 70-791-799
  • 2 different calculation methods
    • Standard
    • Optional
  • Standard Method, Steps to determine Feeder or Service size for a Dwelling Unit (Article 220, Part III)
    1. Step 01:  Determine feeder demand factor load for General Lighting, General-use Receptacles, Small appliances, and Laundry circuits (220.42)
      1. Determine the total connected load (220.52)
        • General lighting & receptacles at 3 VA per sq ft (of interior floor plan)
          • 220.12
        • 2 Appliance circuits each at 1500 VA
          • 220.52(A)
        • 1 Laundry circuit at 1500 VA
          • 220.52(B)
        • NEC requires a minimum of 3 VA per Sq Ft to determine branch circuit feeder & service calculations
          • Includes all 15A & 20A general-use receptacles
          • Does not include small appliances & laundry circuit receptacles
      1. Apply table 220.42 (pg 70-72) demand factors to the total connected load
      2. Calculate 1st 3000 VA at 100% and remaining VA at 35% demand
    1. Step 02: Appliances
      • 220.53
      • Apply 75% demand factor for four or more permanent appliances
        • Include all appliances in all dwelling units
        • Does not apply to electric loads in following steps below
    2. Step 03: Clothes Dryer
      • 220.54
      • Minimum demand load is whatever is greater, 5000W or nameplate data
      • Adjust according to table 220.54 (pg 70-73)
      • Dryers in common laundry rooms do not do load calculations using this method
    3. Step 04: Cooking Equipment
      • 220.55 chart/table
      • Equipment rated over 1¾ kW calculate feeder & services according to table 220.55 (pg 70-74)
        • Including Notes 1, 2, & 3 for determining demand loads
          • Note 1: Ranges rated more than 12 kW, the maximum demand in Column C is increased 5%for each kW, or major fraction, over 12kW
          • Note 2: Ranges of different ratings, use a minimum of 12 kW for ranges rated less than 12 kW and use average kW rating across all ranges to determine the column C demand load
      • 1Ø Cooking equipment supplied by 3Ø , 4-wire Service
        • 220.55
        • Total load is based on twice the maximum number of cooking equipment connected between any 2Ø
      • Demand Load = Connected Load * Table 220.55 Demand Factor
    4. Step 05: Air-conditioning VS Heat
      • 220.50
      • 220.51
      • 220.60
      • Air conditioning and/or heating load are calculated at 100%
      • If both not on at same time, smaller of 2 loads can be omitted
      • If both are on at same time, both are added together
      • Air-Conditioning VA = E*I (Volts * Amps)
    5. Step 06: Feeder & Service Conductor Size
      • 310.15(B)(16)
        • Size service & feeder conductors to this table, based on Article 220 calculated load demand
      •  1Ø
        • Service Size = Demand Load VA / System Volts
        • Service Size = Demand Load / (System Volts * 1.732)
      • Note: 83% rule is only for 1 dwelling
      • *DO EXAMPLE MH-PG 451-456
  • Optional Method, to determine size of Multi-dwelling unit Service
    • Article 220, Part IV
    • 3 or more dwelling units
    • Each unit must be equiped with electric cooking equipment, electric heating &/or air-conditioning, and supplied by only 1 feeder
    • Calculate according to demand factors of Table 220.84
      • based on number of dwelling units
        • 220.84(A)(1), (2), & (3)
    • Feeder/Service Neutral demand load determined according to 220.61
    • Common area House loads
      • Calculated according to Article 220, Part III, then
      • Added to Table 220.84 demand load
      • 220.84(B)
    • Step 01: Total Connected Load
      • 220.84(C)
      • Demand Load determined by adding the following connected loads from all dwelling units, and then the Table 220.84 demand factor is applied
        • 3 VA per SqFt for general lighting & receptacles
        • 1500 VA for each small-appliance circuit with minimum of 2
        • 1500 VA for laundry circuit
          • Not required if common laundry area exists
        • Nameplate rating of all appliances
        • Nameplate rating of all motors
        • Larger load of air-conditioning or heating
    • Step 02: Demand Load
      • Apply the demand load from Table 220.84 to the Total Connected Load from Step 01
    • Step 03: Service & Service Conductor Size
      • Hot conductors sized according to Table 310.15(B)(16) based on the demand load
    • Total Demand Load = Connected Load * Table 220.84 Demand Factor
    • Service Size = Demand Load VA / System Volts
  • 2-Family Dwelling Units
    • When supplied by a single feeder/service
      • Calculate the load using 220-Part III based on 3 identical units, then
      • calculate again using 220.84, then
      • select the lesser of the 2 calculations
    • DO EXAMPLE-MH-PG 459-461
      • DONT UNDERSTAND THIS EXAMPLE
        • Why is answer (c) 250A Service, with 250 kcmil Conductors?
          • Isn’t 225A Service, with 4/0 AWG the “lesser” of 250A/250 kcmil?
          • MH-pg 461
  • Neutral Load Calculations
    • Dryers and Cooking Equipment Neutral Load
      • 220.61(B)(1)
      • 220.54
      • 220.55
      • Calculated at 70% of the demand load
      • Neutral Demand Load = Service Demand Load * 70%
      • Service Demand Load = Connected Load * Table 220.54 Demand Factor
    • Service Neutral
      • Demand factor of 70% can be applied to portion of unbalanced load exceeding 200 amperes
        • Neutral Load is maximum unbalanced calculated load between neutral conductor and any ungrounded conductor,
          • as determined by Article 220, Part III, and
          • adjusted with 220.61(B)
      • DO MH EXAMPLES, PG 463-465