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Adding central A C to an existing dwelling service.

Mar 1, 1995 12:00 PM, Stallcup, James G.

Can a central air-conditioner be added to existing service conductors without upgrading the elements of the service equipment?

Designers and installers always seem to use the standard or optional calculation procedure pertaining to existing dwelling units to determine if special appliance loads can be added without upgrading the elements of the service equipment (service entrance conductors and load center).

For many years, it's been permissible to add special appliance loads to existing dwelling units without applying the more complicated standard calculation.

Existing dwelling units per Sec. 220-31 of the 1993 NEC may use the optional calculation method to size the components for the service equipment to determine if additional loads can be added to the service. To apply the optional procedure, a 120/240V or 208/120V, 3-wire system is required to supply the service. Two columns are used in separating the loads in the dwelling unit. General lighting and general purpose receptacle loads are computed at 3VA per sq ft per Sec. 220-31. All other loads a re computed at 100 % of their name-plate ratings. Heating and A/C loads are calculated at 100% and the smaller load is dropped per Sec. 220-31.

Other loads: Column 1

For our first column, we'll use existing loads and generate this column by adding the 3VA per sq ft lighting load to the small appliance loads figured at 1500VA for each 20A small appliance branch circuit. We'll also add in the nameplate ratings of each appliance load that is permanently connected or fastened in place, with the exception of space heating or A/C equipment. The demand factors in Sec. 220-31 are used in reducing this VA rating for the existing load and this value is then added to the second column, which contains the appliance load to be added.

Added air-conditioning load: Column 2

We'll use the added appliance loads for the second column. This column is determined by adding the total VA rating of these loads at 100%. The added load usually consist of a heating unit, air-conditioner, dryer, or some other type of special appliance load. The second column then is added to the first to derive the total VA rating of the dwelling unit.

Calculation example

From the following loads in an existing dwelling unit, we can determine if an A/C unit, with a VA rating of 5040, can be added to the service conductors. (The existing conductors are No. 3 THWN copper conductors rated at 100A per Table 310-16).

* 1800 sq ft dwelling unit

* Two small appliance circuits

* One laundry circuit

* 12kW range; 240V

* 5kW dryer; 240V

* 1000VA compactor; 120V

* 900VA disposal; 120V

* 1200VA dishwasher; 120V

Step 1. Column 1, existing load, Table 220-31.

* General lighting load:

1800 sq ft at 3VA/sqft = 5400VA

* Small appliance circuits:

1500VA at 3VA/sq ft = 4500VA

* Other loads:

Range load = 12,000VA Dryer load = 5000VA Disposal load = 900VA Compactor load = 1000VA Dishwasher load = 1200VA

Total load = 30,000VA

Step 2. Column 1, Table 220-31.

* Applying demand factor:

First 8000VA at 100% = 8000VA Remainder VA at 40% = 8800VA

Total load = 16,800VA

Step 3. Column 2, Table 220-31.

* Added appliance load:

A/C unit = 5040VA X 100% = 5040VA

Step 4. Total VA, Table 220-31.

Column 1 = 16,800VA Column 2 = 5040VA

Total load = 21,840VA

Step 5. Total amps, Table 220-31.

A = VA/V A = 21,840VA/240V A = 91A

Solution: The calculation verities that the A/C unit load can be added because the 91A computed load is less than the 100A No. 3 THWN copper conductors utilized for the existing service.

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