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Home > Home Wiring USA Archive: NEC 1999 > Definitions and Calculations > General Dwelling Calculation Method (NEC 1999)

General Dwelling Calculation Method (NEC 1999)

By Warren Goodrich
General Dwelling Calculation Method (NEC 1999)













GENERAL METHOD OF DWELLING CALCULATION

      WRITTEN 7/1/94
LAST REVISE 7/9/01
BY WARREN GOODRICH

THIS DOCUMENT IS BASED ON THE 1999 NATIONAL ELECTRICAL CODE AND IS DESIGNED TO GIVE YOU AN OPTION, AS
A SELF HELP, THAT SHOULD PASS MINIMUM CODE REQUIREMENTS. WHILE EXTREME CARE HAS BEEN IMPLEMENTED
IN THE PREPARATION OF THIS SELF HELP DOCUMENT. THE AUTHOR AND / OR PROVIDER OF THIS DOCUMENT
ASSUMES NO RESPONSIBILITY FOR ERRORS, OR OMISSIONS, NOR IS ANY LIABILITY ASSUMED FROM THE USE OF
THE INFORMATION CONTAINED WITH THIS IiOCUMENT, BY THE AUTHOR, OR PROVIDER

220-3A LIGHTING LOAD [3 Va]. X OUTSIDE DIMENSIONS OF YOUR BUILDINGS ON THAT SERVICE. [REQUIRED]

220-16-A  ADD IN 3,000 Va FOR SMALL APPLIANCE BRANCH CIRCUITS FOR THE DWELLING. [REQUIRED]

220-16-B ADD IN 1,500 Va FOR LAUNDRY IN THE DWELLING WHETHER USED OR NOT. [REQUIRED]


             ADD FROM HERE DOWN WHEN FINDING THE TOTAL DEMAND LOAD OF THE DWELLING

220-11 APPLY GENERAL LIGHTING DEMAND FACTOR TO THE TOTAL OF THE ABOVE 3 STEPS. THIS WILL BE

YOUR STARTING POINT FROM HERE. [DO NOT ADD THE FIRST 3 STEPS TO THIS GENERAL LIGHTING

DEMAND FACTOR] THIS STEP IS THE STARTING OVER ADJUSTED CALCULATION OF THE

FIRST 3 STEPS ARRIVING AT THE ADJUSTED GENERAL LIGHTING DEMAND LOAD.

220-21 ENTER THE LARGER OF THE HEAT AND A/C IN Va.
[A/C - VERSUS - HEAT ] {Va. = APPROXIMAT WATTS}

220-17 LIST FASTENED IN PLACE APPLIANCE, AND TOTAL THE VA. /  IF 3 OR LESS APPLIANCES, THEN ENTER AT

AT 100 % / IF THERE ARE FOUR, OR MORE APPLIANCES, ON LIST OF FASTENED IN PLACE APPLIANCES,

ENTER AT 75 % OF THE TOTAL SUM OF APPLIANCES Va. THIS WILL, NOW BE, YOUR ADJUSTED DEMAND

LOAD, CONCERNING ALL OF THE FASTENED IN PLACE APPLIANCES Va. [Va. = APPROXIMAT WATTS]

220-18 ENTER CLOTHES DRYER AT 5000 Va. / OR NAME PLATE RATING WHICH EVER IS LARGER, REGARDLESS OF

THE ACTUAL LOAD OF LESS THAN 5000 VA. IF ONLY ONE DRYER IS USED.

5000 Va. IS THE MINIMUM LOAD CALCULATION ALLOWED.

220-19 APPLY HOUSEHOLD COOKING EQUIPMENT TOTAL NUMBER OF ALL COOKING EQUIPMENT TO

DEMAND FACTOR, IN Va., AND ENTER THE PRODUCT OF THAT CALCULATION PROVIDED IN THE CHART

220-14 & 430-24 FIND THE LARGEST MOTOR INVOLVED, AND MULTIPLY THAT MOTOR’S  FULL LOAD

CURRENT (FLC) BY 25 %, AND ENTER THAT FIGURE.


FINAL CALCULATON AS FOLLOWS;


THE TOTAL OF ALL OF THE ABOVE CALCULATIONS, WITH THE EXCEPTIONS OF THE FIRST 3 STEPS,

WILL GIVE YOU THE TOTAL Va. [APPROXIMATE WATTS] DEMAND OF THE HOME, THEN DIVIDE THE

TOTAL Va. [APPROXIMATE WATTS] BY THE SUPPLY VOLTAGE TO FIND THE DEMAND LOAD, IN AMPS.

THEN REFER TO ARTICLE 240 / 6, IN THE NEC, TO FIND THE MINIMUM BREAKER, OR FUSE, SIZE

REQUIRED FOR THE MAIN SERVICE PANEL. NOW REFER TO CHART 310/16 NOTE # 3 TO FIND THE

MINIMUM SERVICE FEEDER CONDUCTOR SIZE.


USE [TABLE 250-94] FOR SIZING THE GROUNDING ELECTRODE CONDUCTOR.


GENERAL METHOD OF DWELLING CALCULATION

            FOR NEUTRAL CONDUCTOR SIZING

220-3A LIGHTING LOAD [3 Va]. X OUTSIDE DIMENSIONS OF YOUR BUILDINGS ON THAT SERVICE. [REQUIRED]

220-16-A  ADD IN 3,000 Va FOR SMALL APPLIANCE BRANCH CIRCUITS FOR THE DWELLING. [REQUIRED]

220-16-B ADD IN 1,500 Va FOR LAUNDRY IN THE DWELLING WHETHER USED OR NOT. [REQUIRED]



             ADD FROM HERE DOWN WHEN FINDING THE TOTAL DEMAND LOAD OF THE DWELLING

220-11 APPLY GENERAL LIGHTING DEMAND FACTOR TO THE TOTAL OF THE ABOVE 3 STEPS. THIS WILL BE

YOUR STARTING POINT FROM HERE. [DO NOT ADD THE FIRST 3 STEPS TO THIS GENERAL LIGHTING

DEMAND FACTOR] THIS STEP IS THE STARTING OVER ADJUSTED CALCULATION OF THE

FIRST 3 STEPS ARRIVING AT THE ADJUSTED GENERAL LIGHTING DEMAND LOAD.

INSTALL THE HEAT OR AIR CONDITIONER/USING ONLY THE PART OF THE LOAD THAT IS CARRIED ON THE NEUTRAL / 120 V LOAD
220-21 ENTER THE LARGER OF THE HEAT AND A/C IN Va.
[A/C - VERSUS - HEAT ] {Va. = APPROXIMAT WATTS}

IF EITHER, THE HEAT OR A/C USES THE NEUTRAL, OR BOTH, THEN USE THE LARGEST NEUTRAL LOAD ONLY.
220-17 LIST FASTENED IN PLACE APPLIANCE, AND TOTAL THE VA. /  IF 3 OR LESS APPLIANCES, THEN ENTER AT

AT 100 % / IF THERE ARE FOUR, OR MORE APPLIANCES, ON LIST OF FASTENED IN PLACE APPLIANCES,

ENTER AT 75 % OF THE TOTAL SUM OF APPLIANCES Va. THIS WILL, NOW BE, YOUR ADJUSTED DEMAND

LOAD, CONCERNING ALL OF THE FASTENED IN PLACE APPLIANCES Va. [Va. = APPROXIMAT WATTS]
THIS 25% REDUCTION IS USED, ONLY IF, MORE THAN 3 FASTENED IN PLACE APPLIANCE USES THE NEUTRAL CONDUCTOR

USING ONLY THOSE LOADS THAT ARE ON THE NEUTRAL. /  ADJUSTED BY 25% LESS, ONLY IF, THERE IS MORE THAN

3 FIXED APPL.  LOADS ON THE NEUTRAL
220-18 ENTER CLOTHES DRYER AT 5000 Va. / OR NAME PLATE RATING WHICH EVER IS LARGER, REGARDLESS OF

THE ACTUAL LOAD OF LESS THAN 5000 VA. IF ONLY ONE DRYER IS USED.

5000 Va. IS THE MINIMUM LOAD CALCULATION ALLOWED.
YOUR MAY USE A 30% REDUCTION FOR THE NEUTRAL LOAD, REGARDLESS OF THE ACTUAL LOAD CARRIED.
220-19 / 220-22  APPLY HOUSEHOLD COOKING EQUIPMENT TOTAL NUMBER OF ALL COOKING EQUIPMENT TO

DEMAND FACTOR, IN Va., AND ENTER THE PRODUCT OF THAT CALCULATION PROVIDED IN THE CHART
YOUR MAY USE A 30% REDUCTION FOR THE NEUTRAL LOAD, REGARDLESS OF THE ACTUAL LOAD CARRIED.
220-14 & 430-24 FIND THE LARGEST MOTOR INVOLVED, AND MULTIPLY THAT MOTOR’S  FULL LOAD

CURRENT (FLC) BY 25 %, AND ENTER THAT FIGURE.
APPLY THE NEUTRAL LOAD, ONLY IF, THERE ARE MOTOR LOADS ON THE FEEDER THAT USES A NEUTRAL


FINAL CALCULATON AS FOLLOWS;

      THE TOTAL OF ALL OF THE ABOVE CALCULATIONS, WITH THE EXCEPTIONS OF THE FIRST 3 STEPS, WILL GIVE YOU
     THE TOTAL Va. [APPROXIMATE WATTS]     NEUTRAL DEMAND OF THE DWELLING, THEN DIVIDE THE TOTAL Va.
[APPROXIMATE WATTS]      BY THE SUPPLY VOLTAGE TO FIND THE NEUTRAL DEMAND LOAD, IN AMPS. THEN REFER
   TO ARTICLE 220-22, IN THE NEC, TO ALLOW A FURTHER REDUCTION OF 30 % OF THE REMAINING NEUTRAL LOAD THAT
    IS OVER 200 AMPS.       NOW REFER TO TABLE 310-16 TO SIZE THE NEUTRAL CONDUCTOR TO THE PRODUCT OF THE
     NEUTRAL DEMAND LOAD, THAT YOU HAVE JUST CALCULATED.

REMEMBER THAT YOU MAY NOT SIZE THE NEUTRAL SMALLER THAN THE GROUNDING ELECTRODE CONDUCTOR
SIZED USING TABLE 250-66. YOU CAN FIND THIS MINIMUM SIZE NEUTRAL CONDUCTOR REQUIREMENT, REGARDING
THE GROUNDING CONDUCTOR IN ARTICLE 250-24-B-1. ALSO THE NEUTRAL CONDUCTOR IS NOT ALLOWED MORE
THAN 12 1/2% IN CIRCULAR MILL IN AREA THAN THE CIRCULAR MILL IN AREA OF THE UNGROUNDED {HOT} CONDUCTOR.


USE [TABLE 250-94] FOR SIZING THE GROUNDING ELECTRODE CONDUCTOR.


EXAMPLE DWELLING CALCULATION

LIVING AREA 25' X 60' [OUTSIDE DIMENSIONS]
5 HP 1 PH 240 VOLT A/C MOTOR
10 KW ELECTRIC SPACE HEAT 240 VOLT
14 KW RANGE 240 VOLT / 120 VOLT
4 KW WATER HEATER
4 KW CLOTHES DRYER 240 VOLT / 120 VOLT
2 KW DISHWASHER 120 VOLT
1/2 H.P. COMPACTOR 120 VOLT
1/3 H,P. DISPOSAL 120 VOLT
1 H.P. POOL PUMP 240V
LINE CONDUCTORS NEUTRAL CONDUCTOR

1,500 SQ., FEET X '3 va
        4,500 va.         4,500 va.
SMALL APPLIANCE 2 X 1,500 va.
        3,000 va.         3,000 va.
LAUNDRY 1 X 1,500 va.
        1,500 va.         1,500 va.

        9,000 va.         9,000 va.
LIGHTING DEMAND:





1st 3,000 va. @ 100%
        3,000 va.         3,000 va.

REMAINING 6,000 va. @ 35%
        2,100 va.         2,100 va.

        5,100 va.         5,100 va.
5 H.P. A/C 28 AMP X 240 VOLT = 6,720 va. ( SMALLER THAN 10 Kw. HEAT ) OMIT A/C {SMALLER THAN HEAT
10 Kw. HEAT [ OMIT THE A/C BECAUSE IT IS THE SMALLEST LOAD ]        10,000 va.              -   va.
3Kw. WATER HEATER ( 75 % AT FINAL TOTAL ) { MORE THAN 3 FASTENED APPL. }         3,000 va.              -   va.
                     SPECAIL NOTE { NORMALLY WATER HEATERS HEAT ONLY ONE ELEMENT AT A TIME } va.

2 Kw. DISHWASHER {MOTOR PLUS ELEMENT} { 120 VOLT }         1,500 va.         1,500 va.
COMPACTOR ( 9. 8 AMPS  X 120 VOLTS  = 1,176 va. X '75% )
           882 va.            882 va.
DISPOSAL
( 7.2 AMPS  X 120 VOLTS  =  864 va. X 75% )
           648 va.            648 va.
WATER PUMP ( 8.0 AMPS X 240 VOLTS = 1,920 va. X 75% )
        1,440 va.              -   va.
4 Kw. DRYER = ( 5 Kw. MINIMUM ) { 240 / 120 VOLT } { MOTOR = 120 }         5,000 REDUCE 30% -->         3,500 va.
l4 Kw. RANGE ( TABLE 220 - 19 NOTE 1 ) { 240 / 120 VOLT } {BULB/CLOCK120v}         8,800 REDUCE 30% -->         6,160 va.
LARGEST MOTOR ( 9.8 AMPS  X 120 VOLTS  = 1,176 X 25%)
           294 va.            294 va.

       36,664 va.        18,084 va.
LINE =     36,664 va. 152.77 AMPS { EXTRA 30% REDUCTION ALLOWED ON NEUTRAL FEEDER LOAD IN EXCESS OF 200 AMPS }

240 volts 310/16 NOTE 3 = 1/0 CU. FEEDER (REMAINING OVER 200 AMPS) X 70 %
         USING 240/6 TO NEXT LARGEST OVERCURRENT DEVICE {BREAKER OR FUSE}
NO REDUCTION
        ADJUSTED TO MEET NEC MINIMUMS  = 175 amps {UNGROUNDED CONDUCTOR} (HOT)     18,084 va.
NEUTRAL =     18,084 va.

240 volts       = 75.35 AMPS 310/16 -250/94 = # 4 THHN CU. NEUTRAL FEEDER

{ NEUTRAL IS NOT ALLOWED TO BE SMALLER THAN THE GROUNDING CONDUCTOR }
GROUNDING ELECTRODE CONDUCTOR
SPECIAL NOTES:

     YOU MUST SIZE THE DWELLING SERVICE ENTRANCE UNGROUNDED CONDUCTOR { HOT }

BY USING THE TABLE FOUND IN THE NEC. THIS TABLE CAN BE FOUND IN ARTICLE

310/15-B-6, USING THE TOTAL DEMAND LOAD YOU JUST CALCULATED.

AND ARTICLE 240/6 TO SIZE THE MAIN SERVICE OVERCURRENT DEVICE BREAKER / FUSE

     YOU MUST SIZE THE DWELLING SERVICE ENTRANCE GROUNDED CONDUCTOR
{ NEUTRAL }

BY USING THE TABLE FOUND IN THE NEC. THIS TABLE CAN BE FOUND IN ARTICLE

310/16, AND THE TOTAL NEUTRAL  DEMAND LOAD YOU JUST CALCULATED. {BOLD PRINT }
ATTENTION:

   THE NEUTRAL MAY NOT BE SMALLER THAN THE GROUNDING CONDUCTOR {BARE OR GREEN}

   FOR SIZING THE SERVICE ENTRANCE NEUTRAL CONDUCTOR.












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