A portable generator can keep your home running during a power outage or emergency. The method discussed in this article is a standard, NEC-compliant method used by electricians throughout the United States: connect a generator to a manual transfer switch through an external input box and feed select circuits only.
This is the safest way to connect the generator to the main panel because the manual transfer switch provides complete mechanical isolation from utility power. Unlike manual switch kits or switches, it is compatible with every panel design and fully compatible with 120/240V split phase US wiring. It’s also easier for homeowners to work during a power outage, since you don’t have to handle the breaker at all. Official inspections approve this method of connecting a power generator to your home.
This guide expands the diagram into a clear step-by-step explanation, including the exact cable types you need to use.
System overview (based on information graph)
Before beginning the wiring process, it is important to understand the components:
Main board – Receives utility power to hot 1 (L1), hot 2 (L2), neutral, and ground.
Manual transfer switch It is a small, customized electrical panel that is installed next to the main breaker panel in your home. It has a set of individual switches, one for each circuit you want to operate during a power outage. Each adapter has two locations: LINE and GEN. Allows you to convert selected circuits from line (benefit) for Gen (Generator).
Alternator inlet (L14-20 or L14-30) – External connection point for the generator wire.
Portable generator – Provides 120/240V split power through a 4-wire outlet.
This setup isolates the generator from the utility grid, giving you precise control over which circuits receive backup power. You can choose what you want to run with your generator at any time.
Types of cables used in the transmission switching method
This method requires three main categories of cable:
SOOW 4-wire cable Between the entrance box and the transfer switch
10/4 SOOW for 30A input L14-30
12/4 SOOW for 20A input L14-20
Load: Hot 1 (black), Hot 2 (red), Neutral (white), Ground (green)
Heavy duty generator wire (L14-20 or L14-30) between the alternator and the input box
The presence of branch circuit wires In panel (14 AWG, 12 AWG, 10 AWG NM-B)
The exact usage of these cables is explained in the list below.
Step-by-step installation with expanded explanation
Step 1: Install the transfer switch next to the main board
Install the manual transfer switch directly next to the breaker panel.
This keeps the wires short and allows you to use the pre-attached wire bundle called a whip that comes with the transfer switch.
The whip has:
You do no Purchase these separately. They come pre-wired with the switch.
Step 2: Move each selected circuit to the transfer switch
To operate a utility or generator circuit, the load wire must pass through the transfer switch.
Here is how this happens:
The branch circuit load wire is removed from its breaker.
This wire is inserted into the corresponding labeled wire from the transfer switch (A, B, C, etc.).
The wire corresponding to the switch (for example, “Black A” or “Red A”) is placed back on the breaker.
This “switching” allows the switch to choose whether the load receives utility power or generator power.
The transfer switch only interacts with the wires inside the panel:
14 AWG NM-B wire for 15 amp circuits
12 AWG NM-B wire for 20 amp circuits
10 AWG NM-B wires for 30A 240V loads
No new cable was run to the branch circuits. The key intercepts what already exists.
Step 3: Run a 4-wire SOOW cable from the input box to the transfer switch
To transmit generator power to the home, electricians use a SOOW 4-wire cableRated for outdoor and flexible use:
10/4 SOOW for 30A L14-30 input (most common, supports ~7,500W)
12/4 SOOW for 20A L14-20 input (supports ~5000W)
8/4 SOOW for larger 50 amp systems (rare for portable devices)
This cable includes:
Black – Hot 1 (X)
Red – Hot 2 (R)
White – Neutral (W)
Green – Ground (G)
These correspond to the stations shown in the diagram.
The cable enters the transfer switch via a strain relief connector and terminates at the dedicated alternator input lugs.
Step 4: Install the external generator inlet box
The inlet box connects the generator with SOOW cable.
You will need:
Inside this input, the same four-wire SOOW cable is located on the X, Y, W, and G terminals.
This creates a complete, safe path for generator power to enter the home without having to open the panel.
Step 5: Use the correct alternator wire (SOOW, current matching)
From generator to inlet, use heavy lockout wire:
This rope must match:
Step 6: Grounding and Bonding
The generator’s ground conductor (green on the SOOW cable) connects to the home’s grounding system at the transfer switch or panel.
If requested by the local authority, a 6 AWG bare copper grounding electrode conductor It can be operated on an external ground rod; However, this is rarely necessary for portable units.
4. Operation during a service outage
When the tool is running:
When the tool is off:
Connect the generator wire to the inlet.
Start the generator.
Flip the selected circles from Line → gen.
Only these circuits receive generator power.
This allows:
The SOOW feeder cable carries the generator power through the switch exactly as shown in the diagram.
Cables available from National Nassau Cable
If you need materials for this installation, NNC has all types of cables used in the switching method, including 10/4 SOOW and 12/4 SOOW generator cables, as well as standard NM-B branch circuit wires such as 14/2, 12/2, 12/3, and 10/3 NM-B. These options cover both 20 amp and 30 amp generator setups and provide everything needed to connect the input box, transfer switch and circuits.
