How to Wire a Generator Transfer Switch | PES Supply

How to Wire a Generator Transfer Switch

Complete step-by-step guide for installing manual and automatic transfer switches

What is a Transfer Switch?

A transfer switch safely connects your generator to your home's electrical panel, allowing you to power essential circuits during an outage without the danger of backfeeding the grid. It's required by the National Electrical Code (NEC Article 702) for any permanent generator connection.

Types of Transfer Switches

Type Cost Best For Installation
Manual Transfer Switch $200-$600 Portable generators DIY or electrician
Automatic Transfer Switch (ATS) $400-$2,500 Standby generators Professional only
Interlock Kit $50-$150 Budget portable setup DIY (check local code)
Inlet Box + Cord $100-$300 Simple portable connection DIY or electrician

Tools Required

  • Voltage tester (non-contact and multimeter)
  • Wire strippers and cable cutters
  • Screwdrivers (Phillips and flathead)
  • Conduit benders (if running conduit)
  • Fish tape (for pulling wire through walls)
  • Drill with hole saw bits
  • Torque screwdriver (for breaker terminals)

Sizing Your Transfer Switch

The transfer switch amperage must match or exceed your generator's output and your panel's capacity:

Generator Size Switch Size Wire Size Breaker
5,000W (120V) 30A 10 AWG 30A single-pole
7,500W (240V) 50A 6 AWG 50A double-pole
10,000W (240V) 60A 4 AWG 60A double-pole
15,000W+ (240V) 100A 2 AWG 100A double-pole
22,000W+ (240V) 200A 2/0 AWG 200A (whole panel)

Step-by-Step: Manual Transfer Switch Installation

WARNING: This guide is for reference. Transfer switch installation requires working inside your main electrical panel. If you're not comfortable working with live electrical, hire a licensed electrician. Permit and inspection may be required.

Step 1: Turn Off Main Power

  1. Turn off the main breaker in your electrical panel
  2. Verify with a voltage tester that power is OFF
  3. Notify family members not to turn power back on

Step 2: Select Circuits to Back Up

Choose 6-10 essential circuits to power during an outage:

  • Refrigerator / freezer
  • Lighting circuits
  • Well pump
  • Furnace / heating
  • Medical equipment
  • Garage door opener
  • Sump pump

Step 3: Mount the Transfer Switch

  1. Mount transfer switch within 18 inches of main panel
  2. Use appropriate anchors for drywall or studs
  3. Ensure the switch is level and secure

Step 4: Run Conduit and Wire

  1. Install conduit between panel and transfer switch
  2. Pull appropriate gauge wire through conduit
  3. Connect wiring to transfer switch terminals

Step 5: Connect Circuit Wires

  1. Remove selected circuit breakers from main panel
  2. Disconnect hot wires from breakers
  3. Route wires through transfer switch
  4. Connect to transfer switch breaker positions
  5. Connect neutral and ground wires properly

Step 6: Install Generator Inlet Box

  1. Mount inlet box on exterior wall near generator location
  2. Run appropriate gauge wire from inlet to transfer switch
  3. Connect wires to transfer switch generator input
  4. Install weatherproof cover

Step 7: Test the System

  1. Turn on main power
  2. Verify utility power flows through transfer switch to circuits
  3. Switch to generator mode (with generator OFF)
  4. Start generator and connect to inlet box
  5. Flip transfer switch to generator position
  6. Verify all backed-up circuits have power

Automatic Transfer Switch (ATS) Installation

ATS installation is similar but includes:

  • Control wiring between ATS and generator (for auto-start signal)
  • Time delay settings (configurable cool-down period)
  • Load shedding capabilities (priority circuit management)
  • Utility monitoring sensors

ATS installation must be performed by a licensed electrician and typically requires a permit and inspection.

Common Mistakes to Avoid

  • Backfeeding through a dryer outlet — illegal and can kill utility workers
  • Oversizing the generator — causes frequency/voltage issues at low loads
  • Undersizing the wire — causes voltage drop and fire risk
  • Not grounding the generator — required by NEC 250.34
  • Skipping the transfer switch — backfeeding is dangerous and illegal