Title 24: Housing and Urban Development
PART 3280—MANUFACTURED HOME CONSTRUCTION AND SAFETY STANDARDS
Subpart I—Electrical Systems
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§ 3280.811 Calculations.
(a) The following method shall be employed in computing the supply cord and distribution-panelboard load for each feeder assembly for each manufactured home and shall be based on a 3-wire, 120/240 volt supply with 120 volt loads balanced between the two legs of the 3-wire system. The total load for determining power supply by this method is the summation of:
(1) Lighting and small appliance load as calculated below:
(i) Lighting volt-amperes: Length time width of manufactured home (outside dimensions exclusive of coupler) times 3 volt-amperes per square foot; e.g. Length × width × 3=lighting volt-amperes.
(ii) Small appliance volt-amperes: Number of circuits time 1,500 volt-amperes for each 20-ampere appliance receptacle circuit (see definition of “Appliance Portable” with Note): e.g. Number of circuits × 1,500=small appliance volt-amperes.
(iii) Total volts-amperes: Lighting volts-amperes plus small appliance=total volt-amperes.
(iv) First 3,000 total volts-amperes at 100 percent plus remainder at 35 percent=watts to be divided by 240 volts to obtain current (amperes) per leg.
(2) Nameplate amperes for motors and heater loads (exhaust fans, air conditioners, electric, gas, or oil heating). Omit smaller of air conditioning and heating except include blower motor if used as air conditioner evaporator motor. When an air conditioner is not installed and a 40-ampere power supply cord is provided, allow 15 amperes per leg for air conditioning.
(3) 25 percent of current of largest motor in paragraph (a)(2) of this section.
(4) Total of nameplate amperes for: Disposal, dishwasher, water heater, clothes dryer, wall-mounted oven, cooking units. Where number of these appliances exceeds three, use 75 percent of total.
(5) Derive amperes for free-standing range (as distinguished from separate ovens and cooking units) by dividing values below by 240 volts.
------------------------------------------------------------------------ Nameplate rating (in watts) Use (in watts)------------------------------------------------------------------------10,000 or less............................ 80 percent of rating.10,001 to 12,500.......................... 8,000.12,501 to 13,500.......................... 8,400.13,501 to 14,500.......................... 8,800.14,501 to 15,500.......................... 9,200.15,501 to 16,500.......................... 9,600.16,501 to 17,500.......................... 10,000.------------------------------------------------------------------------
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(6) If outlets or circuits are provided for other than factory-installed appliances, include the anticipated load. The following example is given to illustrate the application of this Method of Calculation:
Example A manufactured home is 70×10 feet and has two portable appliance circuits, a 1000 volt-ampere 240 volt heater, a 200 volt-ampere 120 volt exhaust fan, a 400 volts-ampere 120 volt dishwasher and a 7000 volt-ampere electric range.
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------------------------------------------------------------------------ Volt- Lighting and small appliance load ampheres------------------------------------------------------------------------Lighting 70x10x3.............................................. 2,100Small Appliance............................................... 3,000 --------- Total................................................... 5,1001st. 3,000 Volt-Ampheres at 100%.............................. 3,000Remainder (5,100 -3,000 =2,100, at 35%........................ 735 --------- Total................................................... 3,735------------------------------------------------------------------------
------------------------------------------------------------------------ Amperes Amperes per leg A per leg B------------------------------------------------------------------------Lighting and small Appliance...................... 15.5 15.5Heater 240 volt................................... 4.1 4.1Fan 120 volt...................................... 1.7 .........Dishwasher 120 volt............................... ......... 3.3Range............................................. 23.3 23.3 --------------------- Total....................................... 44.6 46.2------------------------------------------------------------------------Note: Based on the higher current calculated for either leg, use one 50- A supply cord.
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(b) The following is an optional method of calculation for lighting and appliance loads for manufactured homes served by single 3-wire 120/240 volt set of feeder conductors with an ampacity of 100 or greater. The total load for determining the feeder ampacity may be computed in accordance with the following table instead of the method previously specified. Feeder conductors whose demand load is determined by this optional calculation are permitted to have the neutral load determined by Article 220.61 of the National Electrical Code, NFPA No. 70–2005. The loads identified in the table as “other load” and as “Remainder of other load” must include the following:
(1) 1500 volt-amperes for each 2-wire, 20-ampere small appliance branch circuit and each laundry branch circuit specified.
(2) 3 volt-amperes per square foot for general lighting and general-use receptacles.
(3) The nameplate rating of all fixed appliances, ranges, wall-mounted ovens, counter-mounted cooking units, and including 4 or more separately controlled space heating loads.
(4) The nameplate ampere or kVA rating of all motors and of all low-power-factor loads.
(5) The largest of the following:
(i) Air conditioning load;
(ii) The 65 percent diversified demand of the central electric space heating load;
(iii) The 65 percent diversified demand of the load of less than four separately-controlled electric space heating units.
(iv) The connected load of four or more separately-controlled electric space heating units.
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Optional Calculation for Manufactured Homes With 110-Ampere or Larger Service------------------------------------------------------------------------ Demand Load (in kilowatt or kilovoltampere) factor (percent)------------------------------------------------------------------------Air-conditioning and cooling including heat pump compressors 100Central electric space heating.............................. 65Less than 4 separately controlled electric space heating 65 units......................................................1st 10 kW of all other load................................. 100Remainder of other load..................................... 40------------------------------------------------------------------------
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[40 FR 58752, Dec. 18, 1975. Redesignated at 44 FR 20679, Apr. 6, 1979, as amended at 58 FR 55021, Oct. 25, 1993; 70 FR 72052, Nov. 30, 2005]
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