Electrical Load Guide: NEC Article 220 Calculations
Why Load Calculations Matter
An electrical load calculation determines how much power a building needs. It sizes the service entrance, main breaker, panel, and feeders. An undersized service causes tripped breakers and voltage drop. An oversized service wastes money.
NEC 220.12 — General Lighting Load
| Occupancy | VA per sq ft | NEC Reference |
|---|---|---|
| Dwelling unit | 3 VA/sq ft | 220.12 |
| Office building | 3.5 VA/sq ft | 220.12 |
| Restaurant | 2.0 VA/sq ft | Table 220.12 |
| Retail store | 3.0 VA/sq ft | Table 220.12 |
| Warehouse | 0.25 VA/sq ft | Table 220.12 |
| Hospital | 2.0 VA/sq ft | Table 220.12 |
| Hotel/Motel | 2.0 VA/sq ft | Table 220.12 |
NEC 220.82 — Optional Residential Method
The simplified method for single-family dwellings:
- General loads: 3 VA/sq ft for lighting + 1,500 VA per small-appliance circuit (min 2 required) + 1,500 VA laundry circuit
- Appliance loads: Nameplate VA for dryer (5,000 VA min), range/oven, water heater, AC, etc.
- Demand factors:
- First 10 kVA at 100%
- Remaining at 40%
Worked Example: 2,000 sq ft Home
Step 1: General loads
- Lighting: 2,000 × 3 = 6,000 VA
- Small appliance: 2 × 1,500 = 3,000 VA
- Laundry: 1,500 VA
- Subtotal general: 10,500 VA
Step 2: Appliance loads
- Dryer: 5,000 VA
- Range: 8,000 VA
- Dishwasher: 1,500 VA
- Water heater: 4,500 VA
- AC: 5,000 VA
- Subtotal appliances: 24,000 VA
Step 3: Apply demand factors (NEC 220.82(B))
- Total: 10,500 + 24,000 = 34,500 VA
- First 10,000 VA × 100% = 10,000 VA
- Remaining 24,500 VA × 40% = 9,800 VA
- Total calculated load: 19,800 VA
Step 4: Service size
I = 19,800 / 240 = 82.5A → 100A service minimum (next standard breaker up)
Note: Most new homes install 200A service for future expansion (EV chargers, heat pumps, induction cooktops).
Common Dedicated Circuit Requirements (NEC 210)
| Circuit | Rating | Wire | NEC Reference |
|---|---|---|---|
| Small appliance (kitchen) | 20A, 120V | 12 AWG | 210.11(C)(1) |
| Laundry | 20A, 120V | 12 AWG | 210.11(C)(2) |
| Bathroom | 20A, 120V | 12 AWG | 210.11(C)(3) |
| Refrigerator | 20A, 120V | 12 AWG | 210.52(B)(1) |
| Dishwasher | 20A, 120V (dedicated) | 12 AWG | 422.16 |
| Dryer | 30A, 120/240V | 10 AWG | 210.52(C) |
| Range | 40-50A, 120/240V | 8-6 AWG | 210.52(C) |
Common Load Calculation Mistakes
- Adding all nameplate loads without demand factors: Not all loads run simultaneously. NEC demand factors account for diversity.
- Using VA instead of watts for motors: Motor nameplate may say 1,500W but draw 1,875VA at 0.8 PF. Use VA for load calculations.
- Forgetting future loads: Install 200A service even if calculated load is 100A — future EV chargers, heat pumps, and induction ranges will need it.
Standards Reference
- NEC Article 220 — Branch Circuit and Feeder Calculations
- NEC Article 210 — Branch Circuits
- NEC Article 230 — Services
- NEC 220.82 — Optional Residential Calculation
Frequently Asked Questions
How do I calculate electrical load for a house?
Use NEC 220.82 optional method: 3 VA/sq ft for lighting + 1,500 VA per small-appliance circuit (min 2) + 1,500 VA laundry + all appliance nameplate VA. Apply demand factor: first 10 kVA at 100%, remainder at 40%. Divide total VA by 240V for service amps.
What size service do I need for a 2,000 sq ft house?
A typical 2,000 sq ft house with standard appliances needs approximately 80–100A calculated load. NEC allows rounding up to 100A service. However, most new homes install 200A service to accommodate future loads like EV chargers, heat pumps, and induction cooktops.
What are NEC demand factors?
Demand factors recognize that not all loads operate simultaneously. A building with 30 kVA of connected load doesn't need 30 kVA of service — NEC Table 220.42 and 220.82 provide standard reduction factors based on building type and load diversity.
Do I need a load calculation to add a subpanel?
Yes, if the existing panel is near capacity. Calculate the additional load, verify the existing service can handle it, then size the subpanel feeder accordingly. If the main breaker is already at 80% utilization, you may need a service upgrade.
How many circuits can I put in a 200A panel?
A 200A, 120/240V panel has 40,000VA capacity. The number of circuits depends on loads, not slots. A 40-slot panel can have 40+ circuits using tandem breakers. The limit is the total connected load vs the 200A main breaker, with NEC demand factors applied.