8.4 The Standard Dwelling Calculation — Article 220 Part III

The Standard Method Step by Step

1. General lighting        floor area x 3 VA/ft2
2. Small appliance         1500 VA x number of circuits (minimum 2)
3. Laundry                 1500 VA x 1
4. SUBTOTAL -> apply Table 220.42 demand factors
                           first 3000 VA at 100%
                           3001-120,000 VA at 35%
5. Fixed appliances        220.53 - 75% if four or more
6. Clothes dryer           220.54 - larger of 5000 VA or nameplate
7. Range / cooking         220.55 - Table demand
8. Heating or A/C          220.51 at 100%, larger only per 220.60
9. Motors                  largest motor at 125% per 220.50 / 430.24
10. TOTAL VA -> divide by 240 V for a 120/240 V single-phase service

Step 1 through 4 — The Lighting Group

220.52(A) Small-Appliance Circuit Load. In each dwelling unit, the load shall be calculated at 1500 volt-amperes for each 2-wire small-appliance branch circuit required by 210.11(C)(1). Where the load is subdivided through two or more feeders, the calculated load for each shall include not less than 1500 volt-amperes for each 2-wire small-appliance circuit.

220.52(B) Laundry Circuit Load. A load of not less than 1500 volt-amperes shall be included for each 2-wire laundry branch circuit installed as covered in 210.11(C)(2).

220.52 also states that these loads shall be permitted to be included with the general lighting load and subjected to the demand factors provided in Table 220.42.

Worked Example — Lighting Group for 2,000 Square Feet

General lighting  2,000 ft2 x 3 VA   =  6,000 VA
Small appliance   2 x 1,500 VA       =  3,000 VA
Laundry           1 x 1,500 VA       =  1,500 VA
                                        ---------
Subtotal                                10,500 VA

Table 220.42 demand:
  First 3,000 VA at 100%              =  3,000 VA
  Remaining 7,500 VA at 35%           =  2,625 VA
                                        ---------
Net lighting group demand               5,625 VA

Step 5 — Fixed Appliances, 220.53

It shall be permissible to apply a demand factor of 75 percent to the nameplate rating load of four or more appliances fastened in place, other than electric ranges, clothes dryers, space-heating equipment, or air-conditioning equipment, that are served by the same feeder or service in a one-family, two-family, or multifamily dwelling.

Count carefully. A dishwasher, disposal, water heater, and trash compactor is four appliances and qualifies for 75 percent. Three does not.

Step 6 — Clothes Dryer, 220.54

The load for household electric clothes dryers in a dwelling unit shall be either 5000 watts (volt-amperes) or the nameplate rating, whichever is larger, for each dryer served. The use of the demand factors in Table 220.54 shall be permitted.

For a single-family dwelling this reduces to: 5000 VA, or the nameplate if larger.

220.61(B)(1) later allows the feeder neutral for the dryer to be calculated at 70 percent of the ungrounded-conductor load.

Step 7 — Ranges and Cooking Appliances, 220.55

Table 220.55 gives demand loads for household electric ranges, wall-mounted ovens, counter-mounted cooking units, and other household cooking appliances over 1.75 kW rating.

Column C is used for ranges rated not over 12 kW:

Note 1 to Table 220.55. For ranges individually rated more than 12 kW but not more than 27 kW, and of the same rating, the maximum demand in Column C shall be increased 5 percent for each additional kilowatt of rating, or major fraction thereof, by which the rating of individual ranges exceeds 12 kW.

Note 2. For ranges individually rated more than 8.75 kW through 27 kW of unequal ratings, an average value shall be calculated by adding the ratings (using 12 kW for any range under 12 kW) and dividing by the number of ranges, then Note 1 applied.

Note 3. Over 1.75 kW through 8.75 kW, it shall be permissible to add the nameplate ratings of all household cooking appliances and multiply by the demand factors of Column A or B.

Note 4. The branch-circuit load for one range may be calculated in accordance with Table 220.55, and the branch-circuit load for one wall-mounted oven or one counter-mounted cooking unit shall be the nameplate rating.

Note 5. The neutral load for a household electric range, wall-mounted oven, or counter-mounted cooking unit shall be permitted to be 70 percent of the load determined for the ungrounded conductors.

Worked Example — 14 kW Range

Column C for one range .................. 8 kW
Rating exceeds 12 kW by 2 kW
Increase = 2 x 5% = 10%
Demand = 8 kW x 1.10 = 8.8 kW = 8,800 VA
Neutral load (Note 5) = 8,800 x 0.70 = 6,160 VA

Step 8 — Heating and Air Conditioning

220.51 Fixed Electric Space Heating. Fixed electric space-heating loads shall be calculated at 100 percent of the total connected load. However, in no case shall a feeder or service load current rating be less than the rating of the largest branch circuit supplied. Exception: where reduced loading of the conductors results from units operating on duty cycle, intermittently, or from all units not operating at the same time, the authority having jurisdiction may grant permission for feeder and service conductors to have an ampacity less than 100 percent.

220.60 Non-coincident Loads allows the larger of heating and air conditioning to be used.

In Alaska the practical consequence is that dwellings with electric heat rarely benefit from 220.60, because there is usually no significant cooling load to displace. An electrically heated Alaska home carries its entire heating load at 100 percent, and that single line item often exceeds the whole rest of the calculation. This is a real design consideration and one reason many Alaska dwellings use fuel oil, natural gas, or wood as the primary heat source with electric heat only as backup.

Step 9 — Motors, 220.50

Motor loads shall be calculated in accordance with 430.24, 430.25, and 430.26 and with 440.6 for air-conditioning and refrigerating equipment. In practice, for a dwelling calculation this means adding 25 percent of the largest motor load to the total.

A Complete Worked Service Calculation

A 2,400-square-foot single-family dwelling in Anchorage with: two small-appliance circuits; one laundry circuit; a 12 kW range; a 5 kW clothes dryer; a 4.5 kW water heater; a 1.2 kW dishwasher; a 0.9 kW disposal; a 1.5 kW trash compactor; and 12 kW of electric baseboard heat. Calculate the service using the standard method at 120/240 V single phase.

1  General lighting  2,400 x 3           =  7,200 VA
2  Small appliance   2 x 1,500           =  3,000 VA
3  Laundry           1 x 1,500           =  1,500 VA
                                            ---------
   Subtotal                                 11,700 VA
   First 3,000 at 100%                   =   3,000 VA
   Remaining 8,700 at 35%                =   3,045 VA
   Net lighting group                    =   6,045 VA

5  Fixed appliances (4 units, 75%)
     Water heater 4,500
     Dishwasher   1,200
     Disposal       900
     Compactor    1,500
     Sum          8,100 x 0.75           =   6,075 VA

6  Dryer  larger of 5,000 or nameplate   =   5,000 VA

7  Range  Table 220.55 Column C, 12 kW   =   8,000 VA

8  Electric heat, 220.51 at 100%         =  12,000 VA
   (no A/C, so 220.60 offers no reduction)
                                            ---------
   TOTAL CALCULATED LOAD                    37,120 VA

   Service current = 37,120 / 240        =   154.7 A

A 200-ampere service is required — the next standard size above 154.7 A that satisfies 230.79(C) and the available equipment. Under 310.12(A), the service conductors may be sized at 83 percent of 200 A, or 166 A, which 2/0 AWG copper (175 A at 75 degrees C) satisfies.

Notice how much of the total is heat: 12,000 of 37,120 volt-amperes, nearly a third. Take the electric heat out and the calculation drops to 25,120 VA, or 104.7 A — a 125-ampere service. That single line item is what drives Alaska service sizes.

Neutral Sizing Preview

The grounded (neutral) conductor is sized separately under 220.61, covered in section 8.7 of this chapter. The 240-volt loads — the water heater and the electric baseboard heat, if they are straight 240-volt loads with no neutral — are omitted entirely from the neutral calculation, and the range and dryer are taken at 70 percent.