Transformer Sizing Using NEC Article 220 Demand Factors

General Lighting Load and Table 220.12

NEC Table 220.12 assigns a unit load in volt-amperes per square foot based on building occupancy type. Dwelling units: 3 VA/ft². Office buildings: 3.5 VA/ft². Retail stores: 2.25 VA/ft². Warehouses: 0.25 VA/ft². Industrial commercial: 2 VA/ft². Schools: 3 VA/ft². These values cover general lighting and general-use receptacles.

Calculate the total general lighting load by multiplying the building area by the unit load value. For a 2,000 ft² dwelling: 2,000 × 3 = 6,000 VA. This load is then subject to the demand factors in Table 220.42.

Table 220.42 demand factors for dwelling units: first 3,000 VA at 100%, remainder from 3,001 to 120,000 VA at 35%. For the 6,000 VA example: 3,000 × 1.0 + 3,000 × 0.35 = 3,000 + 1,050 = 4,050 VA demand.

For non-dwelling occupancies, the demand factors differ. Hospital and hotel lighting uses the same tiers as dwellings. Warehouses get 100% for first 12,500 VA, 50% for the rest. All others get 100% of the total lighting load with no reduction.

Appliance, Range, and Dryer Demand Factors

Fixed appliances in dwelling units (dishwasher, disposal, water heater, etc.) get a demand factor under NEC 220.53. If there are four or more fastened-in-place appliances other than ranges, dryers, A/C, and space heating, the total nameplate rating of all such appliances may be reduced to 75%.

Ranges, cook-tops, and ovens use NEC Table 220.55. For a single household range rated 12 kW or less, the demand is 8 kW. For ranges over 12 kW, adjust per the table notes. For multiple ranges in apartments, the demand per unit drops dramatically. Twelve ranges at 12 kW each have a combined demand of only 27 kW versus 144 kW connected load.

Clothes dryers use NEC 220.54. The demand load is 5,000 watts or the nameplate rating, whichever is larger, for each dryer. For five or more dryers, Table 220.54 provides demand factors starting at 80% for 5 dryers and decreasing to 35% for 40 dryers.

Heating and air conditioning loads are computed at 100% of nameplate but you use only the larger of the two under NEC 220.60, since they do not operate simultaneously in most systems. This non-coincident load provision can significantly reduce the total demand.

Single-Phase vs Three-Phase Transformer Sizing

Single-phase transformers serve residential and small commercial loads. The demand load in VA divided by the secondary voltage gives the current. A 24,000 VA demand at 240V single-phase is 100 amps — a standard 100A residential service.

Three-phase transformers serve commercial and industrial loads. The demand in VA divided by (voltage × 1.732) gives the line current. A 150,000 VA demand at 208V three-phase draws 150,000 ÷ (208 × 1.732) = 416 amps.

Phase balance matters for three-phase sizing. If the load is heavily single-phase (common in apartments and offices with 120V loads), the transformer must be sized for the most heavily loaded phase multiplied by three, not the average of all phases. An unbalanced load of 50 kVA on phase A, 40 kVA on phase B, and 30 kVA on phase C requires a 150 kVA transformer (50 × 3), not 120 kVA.

Selecting Standard kVA Sizes and Growth Allowance

Transformers come in standard sizes. Single-phase: 10, 15, 25, 37.5, 50, 75, 100, 167, 250, 333, 500 kVA. Three-phase: 15, 30, 45, 75, 112.5, 150, 225, 300, 500, 750, 1000, 1500, 2000, 2500 kVA. Your calculated demand must be rounded up to the next standard size.

Allow 20% to 25% growth margin above calculated demand for future load additions. A calculated demand of 185 kVA should use a 225 kVA transformer, not a 200 kVA custom unit. The 225 kVA provides 22% growth capacity and is a readily available standard size with better lead time and lower cost than a non-standard unit.

Transformer efficiency peaks at about 50% to 70% of rated load. A transformer running at 30% load has higher percentage losses than one at 60% load because no-load core losses are constant regardless of loading. Extreme oversizing wastes energy 24/7 through elevated no-load losses.

For pad-mount utility transformers, the utility typically owns and sizes the transformer based on your submitted load data. For customer-owned dry-type transformers inside buildings, the electrical engineer specifies the size. In either case, accurate demand calculations are essential.

Impedance and Available Fault Current

Transformer impedance, expressed as a percentage (%Z), determines the maximum available fault current on the secondary side. A typical dry-type transformer has 4% to 6% impedance. Lower impedance means higher available fault current, which affects overcurrent protection device ratings.

Available fault current (amps) = Transformer FLA ÷ (%Z / 100). A 225 kVA, 208V three-phase transformer with 5.75% impedance has an FLA of 625A. Available fault current = 625 / 0.0575 = 10,870 amps. All downstream breakers and panels must have an interrupting rating equal to or greater than this value.

NEC 110.9 requires equipment to have adequate interrupting ratings. Standard residential panels are rated 10,000 AIC. Commercial panels are typically 14,000 to 65,000 AIC. If your available fault current exceeds the panel's rating, you need higher-rated equipment or a current-limiting device.