NEC Article 220 Defined
NEC Article 220 (in NFPA 70 National Electrical Code) is titled 'Branch-Circuit, Feeder, and Service Calculations.' It provides the methodology for calculating the minimum electrical service required for any dwelling unit based on actual loads + applied demand factors.
The methodology recognizes that not all electrical loads run simultaneously. A house with a 12 kW range, 5 kW dryer, 4 kW water heater, and 36 kW of small appliance receptacles never actually draws 57 kW (240A at 240V) — the demand factors model real-world usage to size service rating practically.
Two Methods: Standard vs Optional
NEC offers two calculation approaches:
Standard Method (Article 220.42-50)
Sums each load category with its specific demand factor. More conservative result. Used for unusual configurations.
Optional Method (Article 220.82)
Simpler formula: 100% of first 10 kVA + 40% of remainder (general lighting + appliance) + 100% of largest of [heating, A/C, heat pump]. Standard for most residential calculations. Pro Build uses optional method on 95% of MA single-family calculations.
Demand Factors: The Heart of the Calc
Each load type has a specific demand factor:
| Load | Demand Factor |
|---|---|
| General lighting + small appliance (first 10 kVA) | 100% |
| General lighting + small appliance (remainder) | 40% (optional method) |
| Cooking equipment (range, oven) | 40% (per Table 220.55) |
| Dryer | 5,000 VA OR nameplate, greater. 100% on first dryer. |
| Heat pump heating (continuous) | 100% of nameplate |
| Cooling (AC or heat pump cooling) | 100% |
| Water heater (electric) | 100% |
| EV charger (continuous) | 100% |
| Other fixed appliances (4+) | 75% per Article 220.53 |
Continuous loads (operate 3+ hours at a time): no demand factor allowed. Heat pumps, EV chargers, and water heaters all qualify as continuous.
Worked Example: 2,200 Sq Ft MA Home with EV + Heat Pump
Optional method calculation:
| Load Category | VA | Demand | Adjusted VA |
|---|---|---|---|
| General lighting (3 VA × 2,200 sq ft) | 6,600 | — | 6,600 |
| Small appliance (2 × 1,500 VA) | 3,000 | — | 3,000 |
| Laundry (1 × 1,500) | 1,500 | — | 1,500 |
| Range (12 kVA) | 12,000 | 40% | 4,800 |
| Dryer (5 kVA) | 5,000 | 100% first | 5,000 |
| HPWH (water heater) | 5,500 | 100% | 5,500 |
| Heat pump (3-ton, 36 kBTU heating) | 9,600 | 100% | 9,600 |
| EV charger (40A continuous) | 9,600 | 100% | 9,600 |
| Subtotal Sum | 52,800 | — | 45,600 |
| First 10 kVA at 100% | — | — | 10,000 |
| Remainder (35,600) × 40% | — | — | 14,240 |
| Plus larger of heating/AC at 100% | — | — | 9,600 (heat pump) |
| Total Demand | — | — | 33,840 VA |
33,840 VA ÷ 240V = 141 amps. 200A service is sufficient with 30% headroom. Same calc with 2 EVs + sauna: ~210 amps → 225A or 400A required.
When NEC Article 220 Calc Is Required
527 CMR + NEC require formal load calculation:
- Any new electrical service installation
- Any service upgrade (changing main breaker rating)
- Any addition of major continuous load (EV charger, heat pump, electric vehicle, electric oven)
- Any new circuit installation when total panel load approaches 80% of main breaker rating
- Per Mass Save rebate requirement: heat pump installs document Article 220 calculation showing service is adequate
Pro Build runs Article 220 calc on every panel quote, every heat pump install, every EV charger install — free, with printed output for the homeowner.
Frequently Asked Questions
Can a homeowner run an Article 220 calculation themselves?
What's the difference between Article 220 standard and optional methods?
Does Article 220 account for solar PV generation?
How accurate is Article 220 in practice?
Why is heat pump heating at 100% but cooking at 40%?
Does an HPWH add significant load?
How does Article 220 handle multiple appliances on one circuit?
What if my Article 220 calc shows I need 225A?
References & Sources
- NFPA 70 National Electrical Code Article 220. https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=70
- 527 CMR Massachusetts Electrical Code. https://www.mass.gov/regulations/527-CMR-12-massachusetts-electrical-code-amendments
- Massachusetts Board of State Examiners of Electricians. https://www.mass.gov/orgs/board-of-state-examiners-of-electricians
