How to Wire in Generator to House: Inlet & Interlock Guide

Figuring out how to wire in generator to house electrical systems safely and legally comes down to two critical components: an exterior generator inlet box and an interior mechanical interlock kit (or manual transfer switch). You cannot simply hardwire a generator directly into your main panel or backfeed through a dryer outlet. Doing so bypasses vital safety mechanisms, risks electrocuting utility line workers, and violates the National Electrical Code (NEC). This guide walks through the exact wire gauges, terminal landings, and testing procedures for a standard 30-amp, 240-volt portable generator setup.

Critical Safety & Code Requirements

According to NFPA generator safety guidelines, portable generators must never be connected to house wiring without a listed transfer equipment device that prevents the inadvertent interconnection of the utility source and the generator source. A mechanical interlock kit achieves this by physically blocking the main breaker and the generator backfeed breaker from being turned on at the same time.

Generator Inlet and Breaker Sizing Matrix

Selecting the correct wire gauge and breaker size depends entirely on the maximum continuous output of your generator. The table below outlines the standard configurations for residential portable generators. Always size the breaker to protect the wire, and size the wire to handle the generator's maximum rated amperity.

*Note: 6 AWG copper THHN is rated for 65A at 75°C, which is sufficient for a 60A breaker in most residential applications, but always verify the 75°C column in NEC Table 310.16 and check your generator manufacturer's specific derating requirements.

Tools and Materials Checklist

For this guide, we are using the most common residential setup: an 8,000W running / 10,000W peak generator requiring a 30-amp circuit. Do not substitute NM-B (Romex) for the exterior run; UV exposure and physical damage require conduit and individual THHN conductors.

• Inlet Box: Reliance Controls PB30 (30-Amp, 120/240V, NEMA L14-30 configuration)
• Interlock Kit: Panel-specific (e.g., Siemens ECSBPK01 for Siemens/Murray panels, Eaton MECHINT for Eaton/Cutler-Hammer)
• Backfeed Breaker: 30-Amp, 2-pole (matching your panel brand, e.g., Siemens Q230)
• Wire: Four strands of 10 AWG THHN (Black, Red, White, Green)
• Conduit: 3/4-inch Liquid-Tight Flexible Metal Conduit (LFMC) or PVC schedule 40 with outdoor fittings
• Generator Cord: 10 AWG, 4-prong L14-30P to L14-30R twist-lock cord (SOOW or STW outdoor-rated jacket)
• Tools: Torque screwdriver (calibrated to in-lbs), wire strippers, NCVT, digital multimeter, drill with hole saw (matching conduit hub size)

Step-by-Step: Wiring the Inlet Box and Panel Interlock

1. Mount the Inlet Box: Install the Reliance PB30 inlet box on the exterior wall, ideally within 5 feet of the main panel to minimize voltage drop and conduit bends. Drill a hole through the siding and sill plate to route the conduit directly into the panel knockout.
2. Pull the Conductors: Route the four 10 AWG THHN wires (Black, Red, White, Green) through the 3/4-inch conduit from the inlet box to the main panel. Leave at least 8 inches of slack inside the panel and 6 inches inside the inlet box.
3. Terminate the Inlet Box: Strip 3/4 inch of insulation from the wire ends. Land the wires on the inlet box terminals as follows:
   • Black Wire (Line 1): Lands on the X terminal (Brass screw).
   • Red Wire (Line 2): Lands on the Y terminal (Brass screw).
   • White Wire (Neutral): Lands on the W terminal (Silver screw).
   • Green Wire (Ground): Lands on the G terminal (Green screw, bonded to the metal box).
   Torque all terminal screws to 20 in-lbs using a calibrated torque screwdriver to prevent loose connections that cause arcing under load.
4. Install the Interlock and Breaker: Inside the main panel, install the 30-Amp 2-pole backfeed breaker in the lowest available slots. Route the Black and Red wires to the breaker terminals (torque to 25 in-lbs). Land the White neutral wire on the neutral bus bar (torque to 20 in-lbs). Land the Green ground wire on the equipment grounding bus bar. Finally, mount the mechanical interlock plate over the main breaker and the new generator breaker according to the kit's specific drilling template.
5. Verify the Mechanical Interlock: With the panel cover temporarily off and power still dead, test the physical slide plate. You must not be able to turn on the generator breaker unless the main utility breaker is OFF. Conversely, you must not be able to turn on the main breaker unless the generator breaker is OFF.

Verify and Test: Expected Meter Readings

Before plugging in the generator, perform a dead-front test. Once the panel cover is replaced, move outside to the inlet box with your digital multimeter set to AC Voltage.

Start the generator and let it warm up for 60 seconds. Plug the L14-30 twist-lock cord into the generator and the inlet box. Turn the generator breaker ON inside the house (ensuring the main breaker is OFF). Return to the inlet box and remove the weatherproof cover to access the test points, or measure at the panel's backfeed breaker terminals if accessible.

• Line 1 to Neutral (X to W): Expect 120V AC (Acceptable range: 114V - 126V)
• Line 2 to Neutral (Y to W): Expect 120V AC (Acceptable range: 114V - 126V)
• Line 1 to Line 2 (X to Y): Expect 240V AC (Acceptable range: 228V - 252V)
• Ground to Neutral (G to W): Expect 0V AC (Any reading above 2V indicates a floating neutral or a broken ground path).

If your readings are outside these parameters, shut down the generator immediately. Low voltage on 240V appliances (like well pumps or HVAC compressors) will cause motors to draw excess amperage and burn out.

The Most Common Botch: Neutral-Ground Bonding Errors

The most frequent and dangerous mistake DIYers make when learning how to wire in generator to house systems is misunderstanding the neutral-to-ground bond. This botch manifests with a very specific symptom: every GFCI and AFCI breaker in your main panel trips immediately when you switch to generator power, or you feel a slight electrical tingle when touching the generator's metal frame.

The Cause: In a standard home panel, the neutral and ground bars are bonded together at the main service disconnect. However, portable generators fall into two categories: bonded neutral (neutral and ground are tied together inside the generator) and floating neutral (they are separated). If you use a generator with a bonded neutral and connect it to a house panel that also has a bonded neutral, you create a parallel path. The neutral return current splits, sending electricity back down the ground wire. GFCI breakers detect this current imbalance on the ground wire and trip instantly to protect you.

The Fix: Check your generator's manual. If it has a bonded neutral, you must either use a Manual Transfer Switch (MTS) that switches the neutral pole (breaking the parallel path), or physically remove the neutral-ground bonding strap inside the generator's alternator terminal box (converting it to a floating neutral, which is the preferred method for interlock setups). As noted by FEMA and Ready.gov safety protocols, ensuring proper grounding and avoiding backfeed paths is critical not just for your home's electronics, but for preventing the generator frame from becoming energized.

Always use a 4-prong cord and inlet. Never use a 3-prong adapter to bypass the ground pin; this removes the equipment grounding path entirely, leaving the generator frame as the sole path for fault currents.