Boston ADU Electrical Rough-In: Complete Wiring Guide for Accessory Dwelling Units

Electrical rough-in represents a critical phase in Boston ADU construction, establishing the backbone of your accessory dwelling unit's electrical system. This phase occurs after framing completion but before insulation and drywall installation, requiring precise planning and execution to meet Massachusetts electrical codes and Boston's specific requirements. This comprehensive guide covers every aspect of ADU electrical rough-in, from service planning through final inspection preparation.

Boston Electrical Code Requirements for ADUs

Boston ADUs must comply with the Massachusetts Electrical Code (based on NEC 2020) plus local amendments specific to accessory dwelling units. Understanding these requirements is essential for successful rough-in planning and execution.

Service and Panel Requirements

Electrical Service Options

• Separate service: Required for ADUs over 600 sq ft or when main service inadequate
• Sub-panel feed: Acceptable for smaller ADUs with adequate main service capacity
• Service upgrade: Often required when adding ADU load to existing service
• Load calculations: Must include ADU demand in overall electrical load assessment

Panel Location and Access

• Indoor installation preferred in Boston climate
• Minimum 36-inch clearance in front of panel
• 6-foot minimum headroom at panel location
• Dedicated lighting for panel area
• Weatherproof enclosure if outdoor installation required

Circuit Requirements and Planning

Required Circuits for Boston ADUs

• General lighting: 15A circuits, maximum 12 outlets per circuit
• Small appliance: Minimum two 20A circuits for kitchen/dining areas
• Bathroom: Dedicated 20A circuit for each bathroom
• Laundry: Dedicated 20A circuit for washer, 240V for dryer
• HVAC: Dedicated circuits sized for equipment loads
• Hot water heater: Dedicated 240V circuit for electric units
• Smoke/CO detectors: Dedicated circuit with battery backup

GFCI Protection Requirements

• All bathroom receptacles
• Kitchen countertop receptacles
• Outdoor receptacles
• Garage and basement receptacles
• Receptacles within 6 feet of sinks
• All 15A and 20A receptacles in unfinished areas

Electrical Service Planning

Load Calculation and Service Sizing

Standard Load Calculations for ADUs

Basic Lighting and Receptacle Load:

• General lighting: 3 watts per square foot
• Small appliance circuits: 1,500 watts each (minimum 2 required)
• Laundry circuit: 1,500 watts
• Bathroom circuit: 1,500 watts

Fixed Appliance Loads:

• Electric range: 8,000-12,000 watts typical
• Electric water heater: 4,500-5,500 watts
• Heat pump system: 3,000-8,000 watts depending on size
• Electric dryer: 5,000 watts typical
• Dishwasher: 1,800 watts typical

Service Size Recommendations

• Studio ADUs (400-500 sq ft): 60-100 amp service typical
• One-bedroom ADUs (600-800 sq ft): 100-125 amp service
• Two-bedroom ADUs (900+ sq ft): 125-200 amp service
• All-electric ADUs: Add 40-60 amps for electric heating/cooling

Service Installation Coordination

Utility Company Coordination

• Eversource service availability assessment
• Service drop routing and clearance requirements
• Meter location approval and installation
• Transformer adequacy for additional load
• Underground vs. overhead service options

Permit and Inspection Requirements

• Electrical permit from Boston ISD before rough-in
• Service entrance inspection before energizing
• Rough-in inspection before covering wiring
• Final inspection before certificate of occupancy
• Utility inspection and connection authorization

Rough-In Installation Process

Cable Routing and Installation

Cable Types and Applications

• NM-B (Romex): Standard for dry indoor locations
• UF-B: Underground feeder for outdoor circuits
• THWN in conduit: Wet locations and underground
• MC cable: Commercial applications and garage areas

Installation Methods and Protection

• Route cables through framing members at center of studs
• Install nail plates where cables within 1.25" of edges
• Support cables every 4.5 feet and within 12" of boxes
• Maintain 6" separation from plumbing and HVAC ducts
• Use proper cable connectors at all junction boxes

Outlet and Switch Box Installation

Box Types and Sizing

• Plastic boxes: Adequate for most residential applications
• Metal boxes: Required for surface-mounted installations
• Old work boxes: For retrofit installations
• Box sizing: Minimum 16 cubic inches for basic switch/receptacle
• Deep boxes: Required for GFCI and dimmer switches

Installation Heights and Locations

• Receptacles: 12-18 inches above finished floor
• Switches: 44-48 inches above finished floor
• Kitchen counters: 18 inches above counter surface
• Bathroom vanities: 42 inches above floor minimum
• Outdoor receptacles: 6 feet minimum above grade

Specialized Systems and Requirements

Smoke and Carbon Monoxide Detection

Massachusetts Requirements

• Hardwired detectors with battery backup required
• Interconnected operation between all detectors
• Smoke detectors in each bedroom and common areas
• CO detectors within 10 feet of sleeping areas
• Heat detectors in kitchen areas recommended

Installation Specifications

• Dedicated 15A circuit for detection systems
• 14 AWG wire minimum for interconnection
• Junction boxes at each detector location
• Test/inspection prior to occupancy

Low-Voltage Systems

Communications and Data

• Category 6 cable for internet and phone
• Coaxial cable for cable TV/satellite
• Fiber optic preparation for future upgrades
• Central distribution panel in accessible location
• Separate pathways from power wiring

Security and Automation

• Doorbell transformer and wiring
• Security system pre-wiring
• Smart home automation infrastructure
• Outdoor lighting control systems
• Electric vehicle charging preparation

Code Compliance and Inspection Preparation

Common Code Violations to Avoid

Installation Errors

• Inadequate cable support and protection
• Improper box fill calculations
• Missing GFCI protection in required areas
• Incorrect wire sizes for circuit amperage
• Improper grounding and bonding connections

Planning Oversights

• Insufficient circuits for required loads
• Inadequate service size for total demand
• Missing dedicated circuits for fixed appliances
• Improper outdoor receptacle locations
• Inadequate lighting for stairways and exits

Inspection Process and Documentation

Pre-Inspection Checklist

• Verify all circuits properly identified at panel
• Test GFCI devices for proper operation
• Confirm proper grounding and bonding
• Check cable support and protection
• Verify box fill calculations
• Prepare load calculation documentation

Inspector Requirements

• Schedule inspection 24-48 hours in advance
• Ensure work area clean and accessible
• Have electrical permit posted at site
• Provide approved electrical plans if required
• Licensed electrician present during inspection

Energy Efficiency and Future-Proofing

LED Lighting Infrastructure

• Dimmer compatibility for LED fixtures
• Smart switch preparation for automation
• Adequate circuiting for LED efficiency
• Emergency lighting battery backup systems
• Daylight harvesting controls

Electric Vehicle and Renewable Energy Preparation

• 240V receptacle rough-in for EV charging
• Conduit and panel space for solar inverters
• Battery storage system preparation
• Smart meter integration capabilities
• Load management system infrastructure

Successful Boston ADU electrical rough-in requires careful planning, code compliance, and quality installation practices. Working with licensed electricians familiar with local requirements ensures your ADU electrical system provides safe, reliable service while meeting all regulatory requirements. Investment in proper electrical infrastructure supports both immediate functionality and future upgrade capabilities.

Proper electrical rough-in establishes the foundation for a modern, efficient ADU that serves residents safely and reliably for decades to come.