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Old 05-10-2011, 01:47 AM   #1
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Demand Load Calculations


I'm looking at Rex Cauldwell's Wiring A House, and I have a question about his Demand Factors for lighting. Reference to pp. 52 & 54, if anyone has this book.

In his exampled steps, (for a 2000 SQ FT house) he first determines the General Lighting Load (SF @ 3va. He then adds 3000va for the 2 required small appliance circuit in the kitchen, and then the 1500 Laundry circuit. He then calculates all other appliances (and, excluding the AC and Htg.), and arrives at 30,000W. Totaling all of these together, we come up with 40,000 watts.

Now, he states that "[b]ecause all loads do not operate at the same time, take the first 10,000 at face value, then take 40% off the rest:10,000 +[.4(30,000)] = 10,000 +12,000 = 22,000."

Question: Is Mr. Cauldwell mistaken here, or am I missing another detail? Table 220.11 says to take the first 3,000 @100%, the rest (up to 120K) at 35%, and the final (over 120K) at 25%.

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Old 05-10-2011, 06:27 AM   #2
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Demand Load Calculations


This long but here how it is done
As you can see, Remainder from 3001watts to 120,000 watts: @ 35% = watts


RESIDENTIAL ELECTRICAL LOAD WORKSHEET

LIGHTING LOAD
Article 220-3(a)
Total square footage of habitable living area: __________ @3 watts per sq. foot = _________ watts
Article 220-16(a)
Two small appliance branch circuits @ 1500 watts each: 2 @ 1500 watts each = 3000 watts
Article 220-16(a)
Additional small appliance circuits each: @ 1500 watts each = _____ watts
Article 220 -16(b)
Laundry circuit @ 1500 watts each: @ 1500 watts each = _____ watts
Lighting Load Subtotal = _____ watts
Table 220-11
First 3000 watts of lighting load: __________ @ 100% = 3000 watts
Remainder from 3001watts to 120,000 watts: __________ @ 35% = __________ watts
Remainder over 120,000 watts: __________ @ 25% = __________ watts
Lighting Load Total = ___________watts
APPLIANCE LOAD
Article 220-17
Garbage disposal @ 600 watts each: ____________ @ 600 watts each = __________ watts
Microwave @ 1500 watts each: ____________ @ 1500 watts each = __________watts
Trash compactor @ 1200 watts each: ____________@ 1200 watts each = __________watts
Dishwasher @ 1200 watts each: ____________@ 1200 watts each = __________watts
Refrigerator @ 600 watts each: ____________@ 600 watts each = ___________watts
Name Plate Rating of Miscellaneous Appliances:
___________________________ @______watts each = __________watts
___________________________ @______watts each = __________watts
Appliance Subtotal = ______________watts
Appliance subtotal: ____________watts x ____________ = _____________watts
(Less than 4 appliances @ 100%; 4 or more appliances @ 75%)
ELECTRIC CLOTHES DRYER

Article 220-18 5000 watts or name plate rating: (Whichever is Larger)
Dryer Total = _________watts
WATER HEATER


(If Electric)

Article 220-3(b) @ Nameplate Rating:


Water Heater Load = _________watts
HOUSEHOLD COOKING EQUIPMENT
Table 220-19 Cooking Units-Includes ranges, wall mounted ovens, countertop units, and other household cooking units.
Number of Units- ONE unit use= 8000 watts
TWO unit use = 11,000 watts
THREE unit use = 14,000 watts
FOUR unit use = 17,000 watts
FIVE unit use = 20,000 watts
COOKING TOTAL= _________________watts
SPACE HEATING/AIR CONDITIONING
Article 220-4(a) Air conditioner nameplate rating @ 125%: ________watts x 125% = _________watts

OR
Article 220-15 Electric Heat nameplate rating @ 100%: ________watts x 100% = _________watts
(Whichever is Larger)
HEAT PUMP
Article 220-4(a) Largest condensing unit nameplate @ 125%:________ watts @ 125% = ________watts
Article 440-33


AND
Article 220-15 Supplementary heat (resistance) @ 100%: __________watts @ 100% = ________watts
ADDITIONAL HVAC EQUIPMENT
Article 440-33 Condensing units or A/C @ 100%: __________watts @ 100% = ________watts
SPACE HEATING/AIR CONDITIONING TOTAL= ___________watts
OPTIONAL EQUIPMENT
Article 220-3(b) Swimming Pool and/ or
Spa and Article 430-24 Largest motor nameplate: ____________watts @ 125% = __________watts
Remaining motor(s) nameplate(s): ____________watts @ 100% = __________watts
Lights and other miscellaneous equipment: ____________watts @ 100% = __________watts
POOL/SPA TOTAL = ______________watts
Article 220-3(b) Welders, Kilns, Etc.
Name or description of equipment @ rated nameplate: ______________watts x 100% =________watts
Name or description of equipment @ rated nameplate: ______________watts x 100% =________watts
Name or description of equipment @ rated nameplate: ______________watts x 100% =________watts
MISCELLANEOUS EQUIPMENT TOTAL = _____________watts
TOTAL DEMAND ON SYSTEM
Article 220-10 Sum of all totals: LIGHTING LOAD TOTAL = _______________WATTS
APPLIANCE TOTAL = _______________WATTS
DRYER TOTAL = _______________WATTS
WATER HEATER TOTAL = _______________WATTS
COOKING TOTAL = _______________WATTS
SPACE HEATING/AIR CONDITIONING TOTAL = _______________WATTS
POOL/SPA TOTAL = _______________WATTS

MISCELLANEOUS EQUIPMENT TOTAL = _______________WATTS


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Last edited by NJMarine; 05-10-2011 at 06:30 AM.
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Old 05-10-2011, 10:12 AM   #3
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Demand Load Calculations


Caldwell is using the optional calculation method for a dwelling (see Section III of Article 220), so his numbers are correct.
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Old 05-10-2011, 06:31 PM   #4
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Demand Load Calculations


Well, I've looked at Section III, and I don't find anything about the first 10000 @ 100%
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Old 05-10-2011, 06:33 PM   #5
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Demand Load Calculations


III. Feeder and Service Load Calculations


220.40 General.
The calculated load of a feeder or service
shall not be less than the sum of the loads on the branch
circuits supplied, as determined by Part II of this article,
after any applicable demand factors permitted by Parts III
or IV or required by Part V have been applied.

FPN: See Examples D1(a) through D10 in Annex D. See
220.18(B) for the maximum load in amperes permitted for
lighting units operating at less than 100 percent power
factor.

220.42 General Lighting.


The demand factors specified in
Table 220.42 shall apply to that portion of the total branchcircuit
load calculated for general illumination. They shall
not be applied in determining the number of branch circuits
for general illumination.


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Old 05-10-2011, 06:40 PM   #6
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Demand Load Calculations


I'm trying to attach an image of table 220.42. I hope it uploaded
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Old 05-10-2011, 06:42 PM   #7
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Demand Load Calculations


The above is showing that only the first 3000va is allowed/required to go at 100%.??

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