To wire a portable generator into a house safely and legally, you must install a 30A NEMA L14-30R inlet box outside, run 10/3 NM-B cable to your main service panel, terminate it on a 30A 2-pole breaker, and install a mechanical interlock kit. This prevents backfeeding the utility grid and protects line workers. For a standard 5,000W to 7,500W portable generator, this 30A, 240V setup is the definitive standard.

Before pulling any wire, you need to choose the right switching mechanism. Below is the exact framework to decide between an interlock kit and a manual transfer switch, followed by the step-by-step installation procedure.

The Decision Path: Interlock Kit vs. Manual Transfer Switch

When figuring out how to wire a generator into a house, your first decision is how to isolate the generator power from the utility grid. NEC Article 702 (Optional Standby Systems) strictly prohibits connecting a generator to a home's wiring without a physical, mechanical means of preventing simultaneous connection to the utility.

Criteria Interlock Kit + Inlet Box Manual Transfer Switch (MTS)
Panel Space Required 2 adjacent breaker slots in main panel None (mounts next to panel, feeds sub-circuits)
Approximate Cost $60 - $120 (Kit + Inlet) $300 - $600 (Pre-wired MTS box)
Circuit Selection Unlimited (use any existing breaker in panel) Limited to 6-10 pre-wired circuits
Installation Complexity Moderate (requires main panel termination) High (requires splicing existing branch circuits)
The Concrete Pick: If your main service panel has at least two adjacent empty slots and you want the flexibility to power any circuit in your home (like a well pump or electric range during an emergency), choose the Interlock Kit method. It is cheaper, faster, and vastly superior for modern 200A panels. Only choose an MTS (like the Reliance Controls 31410CRK) if your main panel is completely full or if local code explicitly forbids interlocks in your jurisdiction.

Tools, Materials, and Sizing for a 30A System

Sizing the wire and breaker correctly is non-negotiable. A 30A system requires 10 AWG copper wire. Do not upsize the breaker to 40A or 50A without upsizing the inlet box and wire to match, or you risk melting the inlet plug under a sustained 7,500W load.

Materials List

  • Generator Inlet Box: 30A, 125/250V, NEMA L14-30R (e.g., Reliance Controls PB30 or Bryant 9430FR).
  • 2-Pole Breaker: 30A, matching your panel brand exactly (e.g., Eaton BR230 for BR panels, Siemens Q230 for Siemens, Square D HOM230 for Homeline). Never mix brands.
  • Interlock Kit: Specific to your panel model (e.g., Eaton CHINTLK, Siemens ECSBPK01, or Square D HOMCGCAFI).
  • Wire: 10/3 NM-B (Romex) with bare ground for indoor runs through studs. If running outside or through masonry, use four #10 AWG THHN wires (Black, Red, White, Green) inside 3/4-inch liquidtight flexible metallic conduit.
  • Hardware: 1/2-inch or 3/4-inch NM-B cable clamps, 10 AWG pin ferrules (if using stranded THHN), and a tube of Noalox anti-oxidant compound (if terminating on aluminum bus bars).

Tools List

  • Digital Multimeter (DMM) rated CAT III 600V or CAT IV
  • Non-Contact Voltage Tester (NCVT)
  • Insulated screwdrivers (flathead and #2 Phillips)
  • Wire strippers and lineman's pliers
  • Torque screwdriver (calibrated to inch-pounds)

Mains Safety Protocol: De-Energize and Verify

CRITICAL MAINS HAZARD: Working inside a main service panel exposes you to lethal utility voltage. Even with the main breaker turned OFF, the utility lugs at the top of the main breaker remain energized and can deliver fatal arc flashes.
  1. Turn OFF the main service breaker.
  2. Lock out the main breaker or place a warning tag on the panel cover.
  3. Test your NCVT on a known live source (like an exterior outlet) to verify the tester works.
  4. Test the branch circuit bus bars with the NCVT. It should read DEAD.
  5. Verify with your DMM set to AC Volts: measure from the main breaker load lugs to the neutral bar. It must read 0V. Never trust a breaker position indicator; always test for dead.

Note: NEC-style guidance requires a permit and inspection for this work. Your local Authority Having Jurisdiction (AHJ) has final authority and may require a licensed electrician to perform the final termination.

Step-by-Step Installation: Inlet Box to Panel Terminations

With the panel verified dead, proceed with the physical installation. This procedure assumes you are using 10/3 NM-B cable routed from an exterior wall directly behind or adjacent to the main panel.

  1. Mount the Inlet Box: Secure the NEMA L14-30R inlet box to the exterior siding at least 18 inches above grade. Route the 10/3 NM-B through the knockout into the back of the inlet box.
  2. Terminate at the Inlet Box: Inside the inlet box, land the Black wire (X) on the brass screw labeled X. Land the Red wire (Y) on the brass screw labeled Y. Land the White wire (W) on the silver screw labeled W. Land the Bare/Green wire (G) on the green ground screw. Torque all terminal screws to 35 in-lbs (or per the manufacturer's stamp).
  3. Route to the Panel: Run the 10/3 NM-B through the wall cavity into the main panel. Secure the cable with an NM-B clamp at the panel knockout, leaving at least 8 inches of slack inside the panel.
  4. Prepare the Wire: Strip 3/4-inch of insulation from the Black, Red, and White conductors. Strip 1 inch of insulation from the Bare ground wire.
  5. Land the Ground: Route the Bare wire to the panel's ground bar. Insert it into an empty hole and tighten the green screw. Give it a firm tug to ensure it is seated.
  6. Land the Neutral: Route the White wire to the panel's neutral bar (the silver bar where all the other white branch circuit wires are landed). In a main service panel, the neutral and ground bars are bonded together, but you must land the inlet neutral on the neutral bar to maintain proper current return paths. Tighten the silver screw securely.
  7. Terminate the Hot Legs: Connect the Black wire to the left brass lug of the new 30A 2-pole breaker. Connect the Red wire to the right brass lug of the breaker.
    Crucial: Use your torque screwdriver to tighten these lugs to the exact spec printed on the breaker label (typically 35 to 40 in-lbs for 10 AWG). Under-torquing causes arcing; over-torquing strips the threads.
  8. Seat the Breaker: Firmly press the 30A breaker onto the panel's bus stabs until it clicks into place.
  9. Install the Interlock Plate: Follow the manufacturer's template to drill the required holes in the panel dead-front cover. Mount the mechanical interlock plate so that it physically blocks the generator breaker from being turned ON unless the main utility breaker is OFF.
  10. Replace the Cover: Reinstall the panel dead-front cover, ensuring no wires are pinched.

Verify, Test, and Expected Meter Readings

Do not plug in the generator until you have verified the wiring. Turn the main utility breaker back ON. The interlock should physically prevent you from turning the new 30A generator breaker ON while the main is ON. This confirms mechanical isolation.

Next, turn the main breaker OFF, slide the interlock plate down, and turn the 30A generator breaker ON. Plug your generator into the inlet box, start the generator, and let it stabilize for 30 seconds. Set your DMM to AC Volts and take the following measurements at the breaker terminals:

  • Black (L1) to White (Neutral): Expect 120V (Acceptable range: 114V - 126V).
  • Red (L2) to White (Neutral): Expect 120V (Acceptable range: 114V - 126V).
  • Black (L1) to Red (L2): Expect 240V (Acceptable range: 228V - 252V). If this reads 0V or 120V, your generator is not outputting split-phase power correctly.
  • Ground Bar to Neutral Bar: Expect < 2V (Ideally 0.0V to 0.5V). If this reads higher, you have a loose neutral connection or a bad ground rod bond.

If all readings are within spec, turn on a few 120V branch circuits (lights, fridge) and one 240V circuit (like an electric dryer or well pump) to load-test the system. Listen for any buzzing from the panel.

The Most Common Botch: Stranded Wire Splay and Lug Failure

The most frequent and dangerous installation botch occurs when DIYers use stranded THHN wire pulled through conduit, but fail to use pin ferrules before terminating the wire under the breaker's set-screw lugs.

The Symptom: When you tighten the set-screw directly onto stranded 10 AWG wire, the copper strands splay out from under the screw head. Initially, it makes contact. However, as the breaker heats up under a 25A load and cools down (thermal cycling), the splayed strands loosen. Within a few months, you will hear a distinct, angry buzzing or crackling sound from the panel. Eventually, the high-resistance connection will arc, melt the breaker lug, and potentially scorch the panel bus bar, requiring a costly panel repair.

The Fix: If you are using stranded THHN wire, you must crimp a 10 AWG copper pin ferrule onto the end of the wire using a proper ferrule crimping tool before inserting it into the breaker lug. The ferrule keeps the strands perfectly bundled, allowing the set-screw to clamp down on a solid, uniform surface. Alternatively, just use solid-core 10/3 NM-B cable, which does not require ferrules and is specifically designed for residential breaker terminations.

A Note on Generator Neutral Bonding: If your GFCI outlets inside the house trip randomly when running on generator power, your generator likely has a 'bonded neutral' (neutral and ground tied together at the generator), and your house main panel also has a bonded neutral. This creates a parallel path for neutral current to flow on the ground wire. Check your generator manual; for this interlock setup, you typically need a 'floating neutral' generator, or you must physically remove the neutral-to-ground bonding jumper inside the generator's alternator housing.

By following this exact 30A interlock procedure, using 10/3 NM-B, and terminating to the correct brass, silver, and green screws, you will have a robust, code-compliant backup power system that keeps your home running safely when the grid goes down.