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Home > Home Wiring USA > Definitions and Calculations > General Dwelling Calculation Method (NEC 2002)

General Dwelling Calculation Method (NEC 2002)

By Warren Goodrich
General Dwelling Calculation Method (NEC 2002)

220.3.A: 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. = approximate watts)

220.17: List fastened-in-place appliance, and total the va. If 3 or less appliances, then enter at 100 percent. If there are four, or more appliances, on list of fastened-in-place appliances, enter at 75 percent 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. = approximate watts).

220.18: Enter clothes dryer at 5000 va. / or name plate rating whichever 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 and 430.24: Find the largest motor involved, and multiply that motor's full load current (flc) by 25 percent, and enter that figure.

Final Calculation 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.

Now refer to nec chart 310.15.b.6 to find the minimum service feeder conductor size. Then refer to nec article 240.6, to find the minimum breaker, or fuse, size required for the main service panel.

Special Notes: 240.4.B Allows that the main service overcurrent device be adjusted up to the next larger overcurrent device size if the conductor ampacity does not meet the overcurrent device size listed in 240.6, nor be a part of a multi-outlet branch circuit, nor when the overcurrent device is adjusted to the next larger size can it be 800 amps or larger.

Use (table 250.66) 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

REFERENCE ONLY

1,500 SQ., FEET X 3 Va.

4,500.0 Va.

SMALL APPLIANCE 2 X 1,500 Va.

3,000.0 Va.

LAUNDRY 1 X 1,500 Va.

1 ,500.0 Va

9,000.0 Va.

HOT
LINE CONDUCTORS

GROUNDED
NEUTRAL CONDUCTOR

LIGHTING DEMAND:
9000 Va.= 1st 3,000 Va. @ 100% 3,000.0 Va. 3,000.0 Va.
REMAINING 6,000 Va. @ 35% 2,100.0 Va 2,100.0 Va
5,100.0 Va . 5,100.0 Va .
5 H.P. A/C 28 AMP X 240 VOLT = 6,720 Va 0.0 Va 0.0 Va .

( SMALLER THAN 10 kW. HEAT ) OMIT A/C {SMALLER THAN HEAT}

10 kW. HEAT [ OMIT THE A/C, IT IS THE SMALLEST LOAD ] 10,000.0 Va. 0.0 Va .
3Kw. WATER HEATER

( 75 % AT FINAL TOTAL ) { MORE THAN 3 FASTENED APPLIANCES }

2,250.0 Va 0.0 Va.

SPECIAL NOTE ; { NORMALLY WATER HEATERS HEAT ONLY ONE ELEMENT AT A TIME }
2 kW. DISHWASHER
{MOTOR PLUS ELEMENT} { 120 VOLT }
1,500.0 Va. 1,500.0 Va.
COMPACTOR ( 9. 8 AMPS X 120 VOLTS = 1,176 Va. X '75% ) 882.0 Va. 882.0 Va.
DISPOSAL ( 7.2 AMPS X 120 VOLTS = 864 Va. X 75% ) 648.0 Va. 648.0 Va.
WATER PUMP ( 8.0 AMPS X 240 VOLTS = 1,920 Va. X 75% ) 1,440.0 Va. 0.0 Va.
4 kW. DRYER = ( 5 kW. MINIMUM ){ 240 / 120 VOLT } { MOTOR = 120 } 5,000.0 Va REDUCE 30% 3,500.0 Va.
l4 kW. RANGE = ( TABLE 220 - 19 NOTE 1 ) { 240 / 120 VOLT } {BULB/CLOCK120v} PER NEC ARTICLE 220.22 8,800.0 Va REDUCE 30% 6,160.0 Va.

LARGEST MOTOR ( 9.8 AMPS X 120 VOLTS = 1,176 X 25%) 294.0 Va. 294.0 Va.
35,914.0 Va. 18,084 Va.

TOTAL HOT PHASE LINE WATTAGE = 35,914.0 Va. / DIVIDED BY 240 VOLTS

EQUALS TOTAL DEMAND LOAD OF

146.5 AMPS TOTAL HOT PHASE LINE AMPS = 150 AMP MAIN SERVICE RATED PANEL

HOT LINE DEMAND LOAD IS CALCULATED TO DETERMINE MINIMUM MAIN SERVICE RATED PANEL SIZE SERVING YOUR DWELLING PER CHAPTER 2 AND NEC ARTICLE 240.6

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 nec article 310.15.b.6, using the total demand load that you just calculated. The main service breaker or main service disconnect fuse size is required to be sized equal to or exceeding that amp rating listed

FOR FUSES OR BREAKERS IN NEC ARTICLE 240.6

Therefore our minimum main service rated panel must a at least 150 amp rated then sizing our main service rated ungrounded [hot] conductors we should find in NEC Table 310.15.B.6 that we must use a minimum service conductor size of a 1 awg copper or 2/0 awg aluminum conductor to meet the 150 amp minimum service rated panel or disconnect considering our dwelling demand load calculations above.

Neutral Conductor Sizing Using Your Demand Load Calculation Performed Above
18,084 volt amps was calculated as your neutral service conductor demand load calculation performed above in the far right column. During your neutral service conductor demand load calculation you were allowed to use the adjusted general lighting load then added to that you were to add all the 120 volt fastened in place loads which also allowed you to take a credit calculating only 75 % of that fastened in place loads if you had 4 or more fastened in place appliances that were rated 120 volts only. Then you were to add to that only 70% of the demand load per nec article 220.19 for ranges and nec article 220.18 for dryers allowing this credit reduction on the neutral service conductor load because of the minor loads existing in a range and dryer that are exclusive to 120 volt rated components of that range and dryer. All the above was added together to obtain your neutral service conductor demand load calculation of 18,084 volt amps. Then if you hot main service conductor required a main service panel that was rated at or larger than 200 amps main service rated panel size then you could have taken another credit reduction of 70 % of your total neutral service conductor demand load to size your neutral service conductor.

In this case your calculated demand load dictating your main service rated panel required a minimum main service rated panel size of 150 amps. Therefore considering this particular demand load calculation sizing your minimum service panel size in amps is less that the 200 amps so you were not able to take that reduction credit of 70% because your minimum service rated panel size was only 150 amps on this demand load calculation.

Minimum Neutral Size Conductor

With the above considerations in mind your minimum neutral service conductor must be size no smaller than the 18,084 volt amps. If we divided that 18,084 volt amps by the apparent voltage serving that dwelling of 240 volts we should find that your minimum neutral service conductor size must be at least with an ampacity of 76 amps.

Before we size our minimum neutral service conductor size we must compare that neutral service conductor size found in NEC Table 310.15.B.6 which calls for a neutral conductor size of 4 awg copper or 2 awg aluminum. We might want to compare the conductor ampacity ratings found in NEC Table 310.16 to make sure we don’t have a better deal considering that NEC Table 310.15.B.6 does not list conductor ampacity ratings of less than 100 amps. In NEC Table 310.16 using the 75 degree column due to the 75 degree ratings of your lugs of your breakers and fuses we find that we could use a minimum size to meet the ampacity requirements of 76 amps to call for a 4 awg copper conductor in the 75 degree column or a 2 awg aluminum conductor in the 75 degree column. In comparing for the best deal between the two NEC tables 310.15.B.6 and 310.16 we find our best deal to still be the same no matter which NEC Table we used with both the NEC Table 310.15.B.6 and the NEC Table 310.16 both calling for a minimum neutral service conductor size of a 4 awg copper or 2 awg aluminum neutral service conductor thus meeting your neutral service conductor minimum demand load calculated as the maximum unbalanced load expected on that neutral service rated conductor.

One More Comparison

We must then make one more comparison to ensure our calculated neutral service rated conductor size in wire gauge is not smaller than the minimum grounding electrode conductor size as required in NEC Article 250.24.B.1. We must size our grounding electrode conductor by NEC Table 250.66. We should find in that table that we must use our minimum hot service conductor found in NEC Table 310.15.B.6 calling for a 1 awg copper or a 2/0 awg aluminum service conductor size in amps to equal our 150 amp minimum main service rated panel or disconnect dictated by the NEC Chapter 2. Checking with the NEC Table 250.66 dictating our minimum grounding electrode conductor size we should find we must serve that main dwelling’s main service with a minimum grounding electrode conductor size of 6 awg copper or 4 awg aluminum grounding electrode conductor.

If we compare that grounding electrode conductor minimum size required to our minimum neutral main service conductor size of 4 awg copper or 2 awg aluminum we find that we are meeting the requirement in NEC Article 250.24.B.1 requiring our neutral main service conductor not to be sized smaller than our grounding electrode conductor size requiredto serve that 150 amp rated panel that NEC Chapter 2 dictates as a minimum service amp rated size main service rated panel or disconnect.

This document is based on the 2002 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 providers of this document assumes no responsibility for errors or omissions, nor is any liability assumed from the use of the information, contained in this document, by the author and / or provider.

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