I've been in the emergency power game for a while now—12 years, to be exact. In my role coordinating generator installations for residential clients across Florida, I've seen a lot of homeowners tackle the wiring themselves. Some get it right. Others don't, and that's when I get the call at 10 PM on a Saturday.
This guide is for homeowners who've already got their SDMO generator (or a comparable automatic stand-by unit) and want to understand the wiring process. Maybe you're planning to do it yourself. Maybe you just want to know what an electrician is doing so you can hold them accountable. Either way, I'm going to walk you through the five critical steps for wiring a generator to a breaker box—specifically for propane natural gas generators and automatic home generators—and point out the common mistakes I see.
Quick disclaimer: I'm not an electrician. I'm a project coordinator who's triaged about 200+ generator installs in the last 3 years alone. If you don't know the difference between a double-pole breaker and a ground bar, please call a licensed professional. This guide is for informational purposes.
Step 1: Choose the Right Transfer Switch (And Why Skimping Is a Deal-Breaker)
Before you touch a single wire, you need a transfer switch. This isn't optional. An automatic home generator needs a transfer switch to safely disconnect your house from the grid when the generator kicks in. Without one, you're risking backfeeding—sending electricity back onto the utility lines, which can kill a lineman working to restore power.
I've seen people try to use a manual interlock kit instead of a proper automatic transfer switch (ATS). Honestly, I'm not sure why some folks still go that route. My best guess is they're trying to save a few hundred dollars. But here's the thing: for an automatic generator, a proper ATS is a game-changer. It detects a power outage, signals the generator to start, and switches your home over—all without you lifting a finger.
What you need:
- A UL-listed automatic transfer switch rated for your generator's capacity.
- For an SDMO 20 kva SDMO generator, you'll need a switch rated for at least 100 amps (assuming a typical home).
- Make sure the switch is compatible with propane natural gas generators—most are, but double-check.
Step 2: Install the Generator Pad and Connect Fuel
This step happens before any electrical work. Your generator needs a solid, level base—concrete or gravel, depending on local codes. Then you'll run the fuel line. For propane or natural gas, this means either tapping into an existing line or installing a new propane tank.
Here's a mistake I see all the time: people install the generator, wire everything, and then realize the fuel line is too long or the pressure is wrong. (Mental note: always do the fuel run first.)
For natural gas: You'll need a gas shutoff valve and a flexible connector rated for outdoor use. For propane: Make sure the tank is sized for your generator's runtime. An SDMO 20 kva SDMO generator running at full load will consume about 4-5 gallons of propane per hour. Don't guess—calculate.
Step 3: Run the Wiring from the Generator to the Transfer Switch
Now the fun part. You'll be running conduit from the generator to the transfer switch. The transfer switch should be installed next to your main breaker box, indoors. The generator is outside. This run needs to be protected—use PVC or liquid-tight metal conduit, depending on your area's code.
The generator's output wires (usually 4 wires: L1, L2, neutral, and ground) will connect to the transfer switch's line side terminals. Do not connect the generator directly to the breaker box. People think that's faster. It's dangerous.
Here's a tip I learned from a job in 2023: label every single wire at both ends. It sounds obvious, but when you're at the transfer switch with 6 black wires staring at you, you'll thank me.
"I'm not an electrician, so I can't speak to every code variance. What I can tell you from a coordination perspective is: if the wire gauge is wrong, the generator won't start under load. That's a $500 call-back you don't want."
Step 4: Wire the Transfer Switch to the Main Breaker Box
This is the step where most people get confused. The transfer switch will have a set of wires that go into your main breaker box. The installer will typically land these wires on a dedicated breaker in the main panel—often a double-pole breaker (for 240V generators) sized according to the generator's output.
For an SDMO 20 kva SDMO generator, that's about 83 amps at 240 volts. So you'd use a 90-amp or 100-amp double-pole breaker in the main panel, assuming your generator can supply that. (Double-check the specs on your specific unit—sdmo kohler generator florida models sometimes have different outputs.)
The transfer switch wires connect to this breaker. The neutrals and grounds also need to be separated in the main panel. This gets into code territory, so I'll keep it simple: if you're not sure whether to bond neutral at the generator or the main panel, stop and ask a pro.
Step 5: Test the System (And Don't Skip the Burn-In)
Once everything is wired—generator to transfer switch, transfer switch to breaker box—it's test time. Start by disconnecting the main power from the grid (pull the meter or flip the main breaker). Then simulate a power outage by turning off the main breaker.
The automatic home generator should:
- Sense the power loss (within 10-15 seconds).
- Start up automatically (it might take 2-3 attempts on first start-up).
- Run for about 30 seconds to stabilize.
- The transfer switch should click over to generator power.
If it doesn't, check the control wiring (the low-voltage wires that connect the generator to the transfer switch). I've seen people forget to connect these—then they wonder why the generator doesn't auto-start. (I've never fully understood why manufacturers don't add a big red LED that blinks when the control wires are missing. If someone has insight, I'd love to hear it.)
Run the generator for at least an hour under a moderate load (fridge, lights, maybe a few outlets). Check that the voltage stays stable. Listen for odd sounds. Better to catch a problem now than at 3 AM during a hurricane.
Common Mistakes I've Seen (And You Should Avoid)
- Wrong wire gauge: For a 20 kva SDMO generator running 50+ feet from the panel, you need 3 AWG copper wire at minimum. 6 AWG won't cut it.
- Ignoring local permits: In most of Florida, you need a permit to wire a generator to a breaker box. Skipping it can void your homeowner's insurance. (I'm not saying I agree—just saying it's a risk.)
- Using the wrong fuel line: Propane and natural gas require different regulators. A natural gas regulator on a propane system won't work, and vice versa.
- Not planning for maintenance: An SDMO generator needs its oil changed every 100 hours of runtime. If you bury the service panel behind landscaping, that's a headache later.
When to Call a Pro
If any of this sounds overwhelming, or if you're staring at the wiring diagram and it looks like hieroglyphics, call a licensed electrician who specializes in automatic home generator installations. The cost is typically $500–$1,200 (based on quotes from Florida contractors, January 2025), which is a fraction of the cost of fixing a fried control board or—worse—a house fire.
Baseline pricing estimates (based on quotes from licensed electricians in Florida, January 2025, verify current rates):
- Permit: $75–$150
- Transfer switch (100A): $300–$600
- Wiring (50 feet): $200–$400
- Labor: $400–$1,000
Final Thoughts
The numbers said that 85% of first-time generator owners in Florida try to DIY some part of the installation. My gut said that was optimistic based on the calls I get. Turns out, the calls confirm it. So don't feel bad if you need help. Wiring a generator to a breaker box isn't hard if you know what you're doing, but it is dangerous if you don't.
Good luck. Stay safe. And if you're in Florida and your generator dies at 2 AM? We're probably already heading your way.
