Feeder and Service Load Calculations

This article describes how to set up your project to calculate feeder and service loads correctly.

The Setting the Load Calculation Method section explains the different load calculation methods available in Design Master Electrical, where they are set, and how they impact your load calculations.

The Modeling Loads for NEC 220, 550, & 551 section describes how to properly set up your model for different calculation methods with relevant NEC code references.

References will be made to fixed loads and load types throughout this article. To assign fixed loads to a circuit, in the Circuiting command, press the Set Circuit Information button.

Setting the Load Calculation Method

How the service load is calculated can be set using the Load Calculation Method ▾ field in the Feeder and Service Calculation Settings dialog box. The chosen method impacts how the various load types assigned to devices are converted to calculated loads. Each option is described below.

General Commercial or Dwelling Unit, Part III

This option calculates a commercial or dwelling unit service load according to NEC 220 Part III.

• Use the Occupancy Areas section to specify the area served by the panel and the general lighting load.

  Set the Demand Factor ▾ to the appropriate row from NEC Table 220.45 to calculate the demand for the lighting load. You can have different demand factors for different areas in the building.

• For small appliance and laundry loads, use the Dwelling Unit Small Appliance and Dwelling Unit Laundry load types.

  The specific load on the device does not matter. If any devices with this load type are on a circuit, the load on the circuit will be 1.5 kVA.

  These loads will be included in the general lighting load for demand purposes. In the Circuiting command, set the General Lighting Circuit ▾ value for these circuits to No.

• For kitchen loads, use the Kitchen load type. Use the Demand Factor or Multiplier field to specify the number of pieces of equipment the load represents. The demand factor will be based upon NEC Table 220.55. In the Circuiting command, set the General Lighting Circuit ▾ value for these circuits to No.

• For clothes dryers, use the Dwelling Unit Electric Dryer load type. Use the Demand Factor or Multiplier field to specify the number of dryers the load represents.

  You are responsible for setting the load correctly for the dryer, particularly using 5 kVA as a minimum value when the nameplate rating of the dryer is less than that.

• For any appliances that use NEC 220.53 for their demand factor, use the appropriate Dwelling Unit Appliance load type.

• It does not matter which heating and cooling load types you use. All heating load types will be added together for the total heating load. All cooling load types will be added together for the total cooling load.

• The Marina / Mobile Home / RV load type is not intended to be used for this type of panel. If it is used, the load is added to the Dwelling Unit Appliance load types.

New Restaurant, All Electric / New Restaurant, Not All Electric

The restaurant calculation should only be used for the main service of a new restaurant. A demand factor is applied based upon the total load. The different types of loads do not matter.

There are two restaurant options, depending upon whether the kitchen equipment is all electric or not.

School

All loads connected to the panel except for heating and cooling loads are combined into a single general load. The total load is the general load plus the greater of the heating or cooling load.

The demand factor is based upon the power density of the total load. The Occupancy Area values must be set to the correct total area of the school to calculate the correct power density.

Dwelling Unit

This option calculates a dwelling unit service load according to NEC 220.82.

• Set the area of the dwelling unit in the Occupancy Areas section. Set the Demand Factor ▾ to Dwelling Unit. Areas that are set to other Demand Factor ▾ values will be calculated using Part III.

• • For circuits serving light fixtures or general-use receptacles, in the Circuiting command, set the General Lighting Circuit ▾ value for these circuits to Yes.

• For small appliance and laundry loads, use the Dwelling Unit Small Appliance and Dwelling Unit Laundry load types.

  The specific load on the device does not matter. If any devices with this load type are on a circuit, the load on the circuit will be 1.5 kVA.

  In the Circuiting command, set the General Lighting Circuit ▾ value for these circuits to No.

• The heating and air-conditioning loads are based upon NEC 220.82(C).

  The cooling load is the sum of the cooling load types.

  For the heating load, you need to decide which heating options are applicable to your design. Only set the heating load types that apply to your design. If you specify more than one heating load type, the software will attempt to determine the largest heating load, but there are some ambiguous situations where it might assume a larger load than necessary.

• If the dwelling unit uses a heat pump, see NEC 220.82(C)(2) and (3).

  Use the Dwelling Unit Heating and Cooling Motor load type for the heat pump load. If there is supplemental electric heating, use the Dwelling Unit Heating load type for that.

• If the dwelling unit uses electric space heating, see NEC 220.82(C)(4) and (5).

  Use the Dwelling Unit Heating load type. Use the Demand Factor or Multiplier field to specify the number of separately controlled units.

• If you are using electric thermal storage or other heating systems where the load is expected to be continuous, see NEC 220.82(C)(6).

  Use the Heating load type for the load. Do not use the Dwelling Unit Heating load type.

• For all other loads, use the appropriate Dwelling Unit Appliance load types.

Existing Dwelling Unit

This option calculates a dwelling unit service load according to NEC 220.83.

Existing dwelling units should be modeled in the same way as new dwelling units that use the Dwelling Unit calculation method.

The calculation is slightly different (the demand factor changes at 8 kVA rather than 10 kVA), but the setup is the same.

Multifamily Dwelling (Single units use Part III / Part IV / Existing)

This option calculates a multifamily dwelling service load according to NEC 220.84 and 220.85. Separate options are available to calculate single units in the dwelling based upon NEC 220 Part III, NEC 220.82, or NEC 220.83.

When using this calculation method, house loads should be modeled as general load types and dwelling unit loads should be modeled as dwelling unit load types.

• Set the Multifamily Dwelling Units field to the number of dwelling units served by the panel. This value is used to calculate the diversity.

  warning

  Only count dwelling units immediately served by the panel. Values set for downstream panels will be included automatically in the load calculations.

• For each panel serving a dwelling unit, set the area of the dwelling unit in the Occupancy Areas section. Set the Demand Factor ▾ to Dwelling Unit. Areas that are set to other Demand Factor ▾ values will be calculated as house loads using NEC 220 Part III.

• For circuits serving light fixtures or general-use receptacles, in the Circuiting command, set the General Lighting Circuit ▾ value for these circuits to Yes.

• For small appliance and laundry loads, use the Dwelling Unit Small Appliance and Dwelling Unit Laundry load types.

  The specific load on the device does not matter. If any devices with this load type are on a circuit, the load on the circuit will be 1.5 kVA.

• The heating and cooling loads will be split between the house load and the dwelling unit load. The dwelling unit heating load is the sum of the Dwelling Unit Heating and Dwelling Unit Heating and Cooling Motor load types.

• For other house loads, use the appropriate General Loads types.

• For all other loads, use the appropriate Dwelling Unit Appliance load types.

Marina / Marina with Sub-Meters

Panels calculated using the Marina method are calculated the same as the General Commercial or Dwelling Unit, Part III method. The one difference is that the Marina / Mobile Home / RV load type is used for the load from shore power receptacles.

• If multiple shore power receptacles are provided, but only the largest should be used for the service load, set the demand on the largest receptacle to 1 and the demand on all the others to 0.

• Set the Shore Power Receptacles field to the number of shore power receptacles served by the panel. This value is used to calculate the diversity.

  warning

  Only count shore power receptacles immediately served by the panel. Values set for downstream panels will be included automatically in the load calculations.

Mobile Home Park

Panels calculated using the Mobile Home Park method are calculated the same as the General Commercial or Dwelling Unit, Part III method. The one difference is that the Marina / Mobile Home / RV load type is used for the load from mobile home lots.

You are responsible for determining the load from each lot and setting it correctly according to NEC 550.31.

• If you need to model a mobile home to determine the load, use the Dwelling Unit or Existing Dwelling Unit method.

• Set the Mobile Homes field to the number of mobile homes served by the panel. This value is used to calculate the diversity.

  warning

  Only count mobile homes immediately served by the panel. Values set for downstream panels will be included automatically in the load calculations.

RV Park

Panels calculated using the RV Park method are calculated the same as the General Commercial or Dwelling Unit, Part III method. The one difference is that the Marina / Mobile Home / RV load type is used for the load from the RV sites.

You are responsible for determining the load from each site and setting it correctly according to NEC 551.73.

• Set the Recreational Vehicle Sites field to the number of sites served by the panel. This value is used to calculate the diversity.

  warning

  Only count sites immediately served by the panel. Values set for downstream panels will be included automatically in the load calculations.

None

Panels are calculated the same as the General Commercial or Dwelling Unit, Part III method, but the load calculations will not be displayed in the footer on the panel schedule.

This option is intended to be used when you perform the load calculations outside of Design Master Electrical.

Modeling Loads for NEC 220, 550, & 551

Design Master Electrical can calculate feeder and service loads based upon NEC 220 Part III and Part IV. This section lists each code reference and how to properly set up your model for that calculation.

This article is intended to be used as a reference together with the NEC and is based upon the assumption that you know with which section in the NEC you need to comply. You can then look up the appropriate section below and see how to model it.

References will be made to occupancy areas and types, as well as load calculation methods. Those are set in the Feeder and Service Calculation Settings dialog box.

Part III. Feeder and Service Load Calculations

• 220.45 General Lighting. To use the demand factors for general lighting in Table 220.45, you must also use the general lighting calculations. Otherwise, all lighting has a calculated load of 125%.

  Use the Occupancy Areas section to set the occupancy areas associated with the panel. Set the areas on the panels furthest from the utility; those areas will be carried up to the upstream panels. The area on a panel is the sum of the areas specified in the Occupancy Areas section for that panel and all of the areas of the panels connected to it.

  • Dwelling units: Set the Type of Occupancy ▾ to Dwelling Unit. Use the Area field to set the area. Set the Demand Factor ▾ to Dwelling Unit.

  • Hotels and motels: Set the Type of Occupancy ▾ to Hotel or Motel. Use the Area field to set the area. Set the Demand Factor ▾ to Hotel or Motel.

  • Warehouses: Set the Type of Occupancy ▾ to Warehouse. Use the Area field to set the area. Set the Demand Factor ▾ to Warehouse.

  • All others: Set the Type of Occupancy ▾ to the most appropriate type based upon NEC Table 220.42(A). If none of the types is applicable, select Custom and use the Description and Unit Load fields to define the occupancy type. Use the Area field to set the area. Set the Demand Factor ▾ to Other (Continuous) or Other (Noncontinuous).

• 220.46 Show-Window and Track Lighting. Either create a fixture to represent the load or use fixed loads.

  • Create a Fixture: Create a fixture in the Light Fixture Project Schedule to represent the load. For show windows, each fixture will represent 1 linear foot of show window. Set the Load to 0.2 kVA. For track lighting, each fixture will represent 2 feet of track. Set the Load to 0.15 kVA. Insert fixtures on the drawing to represent the appropriate length of show window or track. Circuit the fixtures normally.

  • Fixed Loads: Use the Lighting load type for show-window or track lighting. Draft the lighting by either inserting light fixtures that are not circuited or have 0 load, or using standard CAD drafting commands to insert light fixtures that are not connected to the Design Master Electrical database.

• 220.47 Receptacle Loads — Other Than Dwelling Units. Use the Receptacles load type for receptacles that should be calculated based upon this section. Typically, the load for each receptacle will be 0.18 kVA. The load on the panel will be based upon NEC Table 220.47.

• 220.50 Motors. See the Motor Load Calculations article for a description of how to use the Single Motor and Multiple Motor load types to specify motors.

• 220.51 Fixed Electrical Space Heating. Use the Heating load type for devices that should be calculated based upon this section.

• 220.52 Small-Appliance and Laundry Loads — Dwelling Unit. Set the Load Calculation Method ▾ to General Commercial or Dwelling Unit, Part III to use this code section.

  • (A) Small-Appliance Circuit Load. Use the Dwelling Unit Small Appliance load type for devices that are on a small appliance circuit.

    The specific load on the device does not matter. If any devices with this load type are on a circuit, the load on the circuit will be 1.5 kVA.

  • (B) Laundry Circuit Load. Use the Dwelling Unit Laundry load type for devices on a laundry circuit.

    The load on the circuit will be at least 1.5 kVA. If the load on the devices is greater than 1.5 kVA, the actual device load is used.

• 220.53 Appliance Load — Dwelling Unit(s). Set the Load Calculation Method ▾ to General Commercial or Dwelling Unit, Part III to use this code section.

  Use the Dwelling Unit Appliance, Continuous and Dwelling Unit Appliance, Noncontinuous load types for devices that are appliances. If an appliance is also a motor, use the Dwelling Unit Appliance Motor load type. Use the Multiplier field to specify the number of appliances the load represents.

• 220.54 Electrical Clothes Dryers — Dwelling Unit(s). Set the Load Calculation Method ▾ to General Commercial or Dwelling Unit, Part III to use this code section.

  Use the Dwelling Unit Electric Dryer load type for electric dryers. You must specify a value of at least 5 kVA for the load on the dryer. Use the Multiplier field to specify the number of appliances the load represents.

• 220.55 Electrical Cooking Appliances in Dwelling Units and Household Cooking Appliances Used in Instructional Programs. Use the Dwelling Unit Electric Cooking load type for electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances. Use the Multiplier field to specify the number of appliances the load represents.

• 220.56 Kitchen Equipment — Other Than Dwelling Unit(s). Use the Kitchen load type for commercial kitchen equipment. Use the Multiplier field to specify the number of appliances the load represents.

• 220.60 Noncoincident Loads. When both noncoincident loads are connected to the same panel, use the Diverse load type for the smaller of the two loads. Set the Multiplier field to 0.

  For heating and cooling loads on the same panel or different panels, use the Heating, Cooling, Largest Motor, Cooling, Other Motors, and Heating and Cooling Motor load types. The larger of the heating or cooling load will be used for the feeder load total.

  warning

  If noncoincident loads that are not heating or cooling are connected to separate panels, the software does not have a way to model them. If you specify the load normally, panels that have both loads on them will have feeder load totals that are too high. If you specify the load using the Diverse load type, the panel with the diversified device on it will have a feeder load total that is too low.

Part IV. Optional Feeder and Service Load Calculations

• 220.82 Dwelling Unit. Set the Load Calculation Method ▾ to Dwelling Unit to calculate the feeder load total using this section.

  • (A) Feeder and Service Load. You are responsible for specifying a 100A+ feeder.

  • (B) General Loads.

    • (1) Set the Type of Occupancy ▾ to Dwelling Unit. Use the Area field to set the area. Set the Demand Factor ▾ to Dwelling Unit.

    • (2) Use the Dwelling Unit Small Appliance and Dwelling Unit Laundry load types for devices on these circuits.

    • (3) Use the Dwelling Unit Appliance load type for a, c, and d. Use the Dwelling Unit Electric Cooking load type for b.

    • (4) Use the Dwelling Unit Appliance Motor load type for these motors.

  • (C) Heating and Air-Conditioning Load.

    • (1) Use the Dwelling Unit Cooling, Largest Motor, Dwelling Unit Cooling, Other Motors, and Dwelling Unit Heating and Cooling Motor load types for this load.

    • (2) Use the Dwelling Unit Heating and Cooling Motor load type for the heat pump load.

    • (3) Use the Dwelling Unit Heating and Cooling Motor load type for the heat pump compressor. Use the Dwelling Unit Heating load type for supplementary heating.

      warning

      If the heat pump compressor will not run at the same time as supplementary heating, do not use this load type. Use the Dwelling Unit Cooling, Largest Motor or Dwelling Unit Cooling, Single Motor load type instead.

    • (4) Use the Dwelling Unit Heating load type for the electric space heaters. Use the Multiplier field to specify the number of heaters the load represents.

    • (5) Use the Dwelling Unit Heating load type for the electric space heaters. Use the Multiplier field to specify the number of heaters the load represents.

    • (6) Use the Heating load type for continuous heating.

      Do not use any of the Dwelling Unit heating or cooling load types if you need to use this code section.

• 220.83 Existing Dwelling Unit. Set the Load Calculation Method ▾ to Existing Dwelling Unit to calculate the feeder load total using this section.

  • (A) Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is Not to Be Installed.

    • (1) Set the Type of Occupancy ▾ to Dwelling Unit. Use the Area field to set the area. Set the Demand Factor ▾ to Dwelling Unit.

    • (2) Use the Dwelling Unit Small Appliance and Dwelling Unit Laundry load types for devices on these circuits.

    • (3) Use the Dwelling Unit Appliance load type for a, c, and d. Use the Dwelling Unit Electric Cooking load type for b.

  • (B) Where Additional Air-Conditioning Equipment or Electric Space-Heating Equipment Is to Be Installed. Use the Dwelling Unit Cooling, Largest Motor and Dwelling Unit Cooling, Single Motor load types for air-conditioning equipment. Use the Dwelling Unit Heating load type for central electric space heating and separately controlled space-heating units.

    • (1) Set the Type of Occupancy ▾ to Dwelling Unit. Use the Area field to set the area. Set the Demand Factor ▾ to Dwelling Unit.

    • (2) Use the Dwelling Unit Small Appliance and Dwelling Unit Laundry load types for devices on these circuits.

    • (3) Use the Dwelling Unit Appliance load type for a, c, and d. Use the Dwelling Unit Electric Cooking load type for b.

• 220.84 Multifamily Dwelling. Set the Load Calculation Method ▾ to Multifamily Dwelling to calculate the feeder load total using this section.

  • (A) Feeder or Service Load. This calculation method is for three or more dwelling units. For calculations involving a single dwelling unit, use the Dwelling Unit or Existing Dwelling Unit method instead.

    This calculation method also requires 8 kVA of electric cooking load if there is no electric cooking, which must be manually specified on the panel.

  • (B) House Loads. Enter house loads as you would for NEC 220 Part III.

  • (C) Connected Loads.

    • (1) Set the Type of Occupancy ▾ to Dwelling Unit. Use the Area field to set the area. Set the Demand Factor ▾ to Dwelling Unit.

    • (2) Use the Dwelling Unit Small Appliance and Dwelling Unit Laundry load types for devices on these circuits.

    • (3) Use the Dwelling Unit Appliance load type for a, c, and d. Use the Dwelling Unit Electric Cooking load type for b.

    • (4) Use the Dwelling Unit Appliance Motor load type for these motors.

    • (5) Use the Dwelling Unit Heating, Dwelling Unit Cooling, Largest Motor, Dwelling Unit Other Motors, and Dwelling Unit Heating and Cooling Motor load types for the air-conditioning and electric space-heating loads.

• 220.85 Two Dwelling Units. If two dwelling units are calculated using the Multifamily Dwelling load calculation method, the software assumes they are identical. The total load for both dwelling units is multiplied by 1.5 to increase it from 2 to 3 units, then a demand factor of 45% is applied.

• 220.86 Schools. Set the Load Calculation Method ▾ to School to use this code section.

• 220.87 Determining Existing Loads. Use the Metered Demand field to specify the existing load.

• 220.88 New Restaurants. Set the Load Calculation Method ▾ to New Restaurant, All Electric or New Restaurant, Not All Electric to use this code section.

Part V. Farm Load Calculations

Farm load calculations are not included in Design Master Electrical.

Part VI. Health Care Facilities

Health care facility calculations are not included in Design Master Electrical.

Part VII. Marinas, Boatyards, Floating Buildings, and Commercial and Noncommercial Docking Facilities

• 220.120 Receptacle Loads. Set the Load Calculation Method ▾ to Marina or Marina with Sub-Meters to use this code section.

  Set the number of receptacles using the Shore Power Receptacles field.

  When circuiting, use the Marina / Mobile Home / RV load type for the load on the shore power receptacles. If multiple shore power receptacles are provided for an individual boat slip, set the Demand Factor to 1 for the largest receptacle load and set it to 0 for all the others.

Article 550 — Mobile Homes, Manufactured Homes, and Mobile Home Parks

• 550.31 Allowable Demand Factors. Set the Load Calculation Method ▾ to Mobile Home Park to use this code section.

  Set the number of mobile homes using the Mobile Homes field.

  When circuiting, use the Marina / Mobile Home / RV load type for the load on each mobile home lot.

Article 551 — Recreational Vehicles and Recreational Vehicle Parks

• 551.73 Calculated Load. Set the Load Calculation Method ▾ to RV Park to use this code section.

  Set the number of recreational vehicle sites using the Recreational Vehicle Sites field.

  When circuiting, use the Marina / Mobile Home / RV load type for the load on each recreational vehicle site.