NEC 220.82 Optional Method800 Sq Ft • Accessory Dwelling Unit (ADU) / Backyard CottageCode Compliant
ADU / Guest House Subpanel Load Calculation (NEC 220)
Complete electrical demand load breakdown following NEC Article 220.82. Formally verified for electrical permit submissions, service disconnect sizing, and utility service drop requirements.
Calculated Demand77.9 Amps@ 240V Single Phase
Recommended Panel100 AmpsNEC Standard Rating
Service Feeder (Al)#2 AWGTable 310.12 (75°C)
Minimum Conduit1-1/4"EMT / PVC Sch 40
Connected Appliance Profile & Nameplate Loads
Specific mechanical equipment ratings evaluated for this residential dwelling unit
Cooking EquipmentElectric Cooktop & Convection Oven (8,000 VA)
Clothes DryerCompact 240V Electric Dryer (3,500 VA)
Water HeatingElectric Storage Water Heater (3,500 VA)
HVAC Primary Heating/CoolingMini-Split Heat Pump 1.5-Ton (2,500 VA)
Electric Vehicle Supply (EVSE)None Configured
Auxiliary Fixed AppliancesDishwasher (1,500 VA), Disposal (900 VA), Microwave (1,200 VA)
NEC 220.82 Step-by-Step Demand Worksheet
Official calculation table matching municipal electrical permit application format
| Calculation Step • NEC Section | Connected VA | Demand Factor | Demand VA |
|---|---|---|---|
| General Lighting & Receptacles3 VA/sq ft • NEC 220.82(B)(1) | 2,400 VA | Subtotal Group | — |
| Small Appliance Circuits2 Circuits @ 1,500 VA • NEC 220.82(B)(2) | 3,000 VA | Subtotal Group | — |
| Laundry Branch Circuit1 Circuit @ 1,500 VA • NEC 220.82(B)(2) | 1,500 VA | Subtotal Group | — |
| Fixed & Fastened-in-Place AppliancesRange, Dryer, WH, DW, Disposal • 220.82(B)(3) | 18,600 VA | Subtotal Group | — |
| General Connected Load Subtotal | 25,500 VA | — | — |
| • First 10,000 VA of General Load | 10,000 VA | 100% | 10,000 VA |
| • Remainder above 10,000 VA | 15,500 VA | 40% | 6,200 VA |
| General Demand Load Subtotal | — | 220.82(B) | 16,200 VA |
| Heating & Air Conditioning (HVAC)Largest Non-Coincident Load • NEC 220.82(C) | 2,500 VA | 100% | 2,500 VA |
| TOTAL CALCULATED DEMAND LOAD | — | Total / 240V | 18,700 VA • 77.9 A |
Service Conductor & Grounding Sizing Schedules
Minimum wire gauges and raceway sizes compliant with NEC Table 310.12 and Table 250.66
Service Entrance Conductors
Copper Conductor (THHN/XHHW):#4 AWG Copper THHN
Aluminum Conductor (XHHW-2):#2 AWG Aluminum XHHW-2
Minimum Raceway / Conduit:1-1/4" PVC Sch 40 Underground
Temperature Rating:75°C Terminations
Grounding & Bonding Electrode System
GEC to Water Pipe / Intersystem:#6 AWG Bare Copper (to 2 ground rods at ADU)
Ground Rod System:Dual 5/8" × 8ft Rods (6ft Spaced)
GEC to Ground Rods (Max):#6 AWG Copper
Main Bonding Jumper:Installed at Main Disconnect Only
Master Electrician Inspection & Installation Notes
- Calculated demand load for the 800 sq ft ADU is 77.9A at 240V.
- A 100A subpanel is the recommended standard for residential ADUs to provide adequate breaker spaces and capacity.
- NEC 225.30 Rule for detached structures: A detached ADU requires its own disconnecting means and its own grounding electrode system (2 ground rods 6 ft apart).
Frequently Asked Code Questions
Does a detached ADU require its own ground rods?
Yes. Per NEC 250.32, a detached structure supplied by a feeder requires its own grounding electrode system (typically two 5/8" ground rods driven 6 feet apart) with isolated neutral and equipment grounding conductors.
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