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Feeder and Service Calculations

Monday, November 25th, 2013

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

The way the service load is calculated can be chosen 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 in a section below.

General Commercial or Dwelling Unit, Section III

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

Use the Occupancy Areas section to specify the area served by the panel and the general lighting load. Set the Demand Factor to choose the row from NEC Table 220.42 used to calculate the demand for the lighting load. You can have different demand factors for different areas in the building.

Small appliance and laundry loads need to be created as individual circuits on the panel. You are responsible for setting the load on the circuit to 1.5 kVA, either using device loads or fixed loads. Use the Small Appliance and Laundry load type for these loads. It will be included in the general lighting load for demand purposes. Set the General Lighting Circuit value for these circuits to No.

Use the Dwelling Unit Kitchen load type for kitchen loads. Use the Demand Factor or Multiplier fields to specify the number of pieces of equipment the load represents. The demand factor will be based upon NEC Table 220.55. Set the General Lighting Circuit value for these circuits to No.

Use the Dwelling Unit Electric Dryer load type for clothes dryers. Use the Demand Factor or Multiplier fields 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.

Use the Dwelling Unit Appliance load type for any appliances that use NEC 220.53 for their demand factor.

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 type.

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. Calculating the power density requires that the Occupancy Area values be set to the correct total area for the school.

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 Section III. Make sure that any circuits serving light fixtures or general-use receptacles have the General Lighting Circuit value set to Yes.

Small appliance and laundry loads need to be created as individual circuits on the panel. You are responsible for setting the load on the circuit to 1.5 kVA, either using device loads or fixed loads. Use the Small Appliance and Laundry load type for these loads. Do not set the connected circuits for these loads as general lighting circuits.

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, you are using NEC 220.82(C)(2) or (3). Use the Heat Pump 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, you are using NEC 220.82(C)(4) and (5). Use the Dwelling Unit Heating load type and use the Demand Factor and 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, you are using NEC 220.82(C)(6). Use the Heating load type for the load, not the Dwelling Unit Heating load type.

All other loads should use the Dwelling Unit Appliance load type. The General Residential 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 type.

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 Dwelling Unit.

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 Section III / Section IV / Existing)

This option calculates a multifamily dwelling service load according to NEC 220.84 and 85. Calculations for single units in the dwelling are based upon NEC 220 Section IIINEC 220.82, or NEC 220.83, respectively. 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 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 Section III. Make sure that any circuits serving light fixtures or general-use receptacles have the General Lighting Circuit value set to Yes.

Small appliance and laundry loads need to be created as individual circuits on the panel. You are responsible for setting the load on the circuit to 1.5 kVA, either using device loads or fixed loads. Use the Small Appliance and Laundry load type for these loads.

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 Heat Pump load types.

All other loads should use the Dwelling Unit Appliance load type. The General Residential 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 type.

All other house loads should be specified using the general load types.

The Multifamily Dwelling Units field in the Feeder and Service Calculation Settings dialog box sets the number of dwelling units used to calculate the diversity. The number of dwelling units on a panel will include the total number on downstream panels.

Marina / Marina with Sub-Meters

Panels calculated using the Marina method are calculated the same as the General Commercial or Dwelling Unit, Section 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.

The Shore Power Receptacles field in the Feeder and Service Calculation Settings dialog box sets the number of shore power receptacles used to calculate the diversity.

Mobile Home Park

Panels calculated using the Mobile Home Park method are calculated the same as the General Commercial or Dwelling Unit, Section 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 one of the Dwelling Unit methods.

The Mobile Homes field in the Feeder and Service Calculation Settings dialog box sets the number of mobile homes used to calculate the diversity.

RV Park

Panels calculated using the RV Park method are calculated the same as the General Commercial or Dwelling Unit, Section 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.

The Recreational Vehicle Sites field in the Feeder and Service Calculation Settings dialog box sets the number of sites used to calculate the diversity.

None

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

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

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