4.4 Service Load Calculations

Why Service Load Calculations Are Tested

The Texas Journeyman Calculations part (26 questions, 110 minutes) is weighted toward load computations, and a single-family dwelling service-load problem is the most common scenario. You will be asked to compute total VA, convert to amps at 240 V single-phase, and select the conductor from Table 310.16. Speed matters: you have roughly 4 minutes per calculation question, so memorize 120.41 (2 VA/sq ft for dwellings) and the three-tier Table 120.45 demand factors.

Article 120 — Service Load

Step 1 — General Lighting Load (120.41)

For a single-family dwelling, the general lighting load is computed at 2 VA per square foot under 2026 NEC 120.41. Floor area comes from the outside dimensions (120.5(C)) and excludes open porches and unfinished areas not adaptable for future use, so an attached garage counts.

Formula: General Lighting VA = Floor Area (sq ft) × 2 VA/sq ft

Step 2 — Small-Appliance and Laundry Branch Circuits (120.52)

210.11(C)(1) requires at least two 20-amp small-appliance branch circuits for kitchen/dining receptacles, each computed at 1,500 VA → 3,000 VA total.

210.11(C)(2) requires one 20-amp laundry branch circuit, computed at 1,500 VA.

These are added to the general lighting load before applying the Table 120.45 demand factors.

Step 3 — Apply Demand Factors (Table 120.45)

For dwelling units, demand factors apply only to the combined general lighting + small-appliance + laundry load:

Step 4 — Fixed Appliance Loads

• Range (Table 120.55): A 12 kW range is demanded at 8 kW (Column C, "Not over 12 kW"). For ranges over 12 kW, increase Column C by 5% for each additional kW or major fraction.
• Dryer (120.54): Demand the larger of 5,000 VA or the nameplate VA. A 5 kW dryer demands 5,000 VA; a 14 kW dryer demands 14,000 VA.
• Water heater, dishwasher, disposal: add at nameplate VA (no demand factor).
• Space heating (120.51): add at nameplate; do not apply the 120.45 lighting demand.
• Air conditioning (120.60): compare A/C and heating loads and use the larger of the two (they are not simultaneous).

Step 5 — Neutral Load (120.61)

The neutral carries only the unbalanced load — the 120 V portion. For 240 V appliances (range, dryer, water heater), the 240 V portion does not contribute to the neutral.

120.61(B)(1) permits a 70% demand factor for the eligible household range and dryer load calculated under the referenced tables. It does not assert that 70% of the appliance physically operates at 120 V. Apply it only to the eligible neutral-load calculation.

Step 6 — Total Service Load and Conductor Size

Total VA ÷ 240 V = Service Amps

Select an adequate standard rating (including 100, 110, 125, 150, 175, 200, 225 and higher ratings), then size service-entrance conductors from Table 310.16 (75 °C column for service equipment rated 100 A or more).

Worked Example — 2,500 sq ft Single-Family Dwelling

A single-family dwelling has 2,500 sq ft of habitable floor area, a 12 kW electric range, a 14 kW electric dryer, two small-appliance branch circuits, and one laundry branch circuit. Compute the service load and size the service-entrance conductors (copper, 75 °C column).

General Lighting + Small Appliance + Laundry

General lighting: 2,500 sq ft × 2 VA = 5,000 VA
Small-appliance (2 circuits): 2 × 1,500 = 3,000 VA
Laundry (1 circuit): 1,500 VA
Subtotal = 9,500 VA

Apply Table 120.45 Demand

First 3,000 VA × 100% = 3,000 VA
Next 6,500 VA × 35%   = 2,275 VA
Demand load (lighting) = 5,275 VA

(The 9,500 VA subtotal falls entirely in Tiers 1 and 2; Tier 3 at 25% does not begin until 120,000 VA.)

Range and Dryer

Range (Table 120.55, 12 kW): 8,000 VA
Dryer (120.54, 14 kW nameplate): 14,000 VA

Total Service Load

5,275 + 8,000 + 14,000 = 27,275 VA
Service current = 27,275 ÷ 240 = 113.6 A

Round up to the next standard service rating → 125 A service.

Service-Entrance Conductor (Table 310.16, 75 °C Cu)

1 AWG Cu (130 A) exceeds the 125 A service rating → 1 AWG copper THWN-2 service-entrance conductors.

Cross-check with Table 310.12(A). Sizing straight from Table 310.16 as above is always defensible, but for a one-family dwelling service on a 120/240 V single-phase system, Table 310.12(A) permits a smaller conductor — it lists 2 AWG copper for a 125 A service (the 83% allowance). If an exam question names a one-family dwelling and a standard service rating, answer from Table 310.12(A); fall back to Table 310.16 when the dwelling allowance does not apply, such as when adjustment or correction factors are in play or the conductors do not carry the entire dwelling load. Section 13.3 works this table in full.

Neutral Calculation (120.61)

Lighting+SA+laundry demand (neutral) = 5,275 VA (all 120 V)
Range 120 V load: 8,000 × 70% = 5,600 VA
Dryer 120 V load: 14,000 × 70% = 9,800 VA
Neutral load = 5,275 + 5,600 + 9,800 = 20,675 VA
Neutral current = 20,675 ÷ 240 = 86.1 A

From Table 310.16, 75 °C Cu: a 3 AWG Cu conductor (100 A) carries the 86.1 A neutral load → 3 AWG copper neutral, subject also to 250.24(D) and the applicable minimum in Table 250.102(C)(1). The neutral may be smaller than the ungrounded conductors when the calculated neutral load is less.

Common Exam Errors

• Forgetting the 1,500 VA laundry circuit — it is required even if the laundry receptacle is shared.
• Applying Table 120.45 to appliances — demand factors apply only to lighting + small-appliance + laundry.
• Using 100% on a range — Table 120.55 demands 12 kW at 8 kW, not 12 kW.
• Forgetting the 25% tier — for very large dwellings, the remainder over 120,000 VA is at 25%, not 35%.
• Using 90 °C column — service equipment terminations for 100 A+ are rated 75 °C per 110.14(C)(1)(b); use the 75 °C column.
• Not rounding up to a standard service size — the conductor must meet the disconnect rating, not the raw calculated amps.

Standard Service Sizes and Typical Conductors

*When the exact conductor ampacity equals the service rating, the conductor is acceptable. When it is below, round up to the next size that meets the rating.

Exam Takeaways

• 2 VA/sq ft + 3,000 VA small-appliance + 1,500 VA laundry, then apply Table 120.45.
• 12 kW range → 8 kW demand. Dryer → larger of 5 kVA or nameplate.
• Total VA ÷ 240 = amps, round up to standard service size.
• Table 310.16, 75 °C column for service-entrance conductors on equipment rated 100 A or more.
• Neutral uses 120.61 and the 70% range/dryer factor — the neutral conductor can be smaller than the ungrounded conductors.