How to Hook Up a Generator to Your House with a Transfer Switch

In March 2024, I helped a customer who had a 175 kva kohler sdmo generator delivered 36 hours before a planned outage. The machine was fine. The problem was that nobody had thought through how it would connect to the building. They had no automatic transfer switch on site, no disconnect plan, and no idea what the electrician was supposed to wire first.

That job ended well because we found a way to rush the right gear. But it was closer than it should have been. Ever since then, I have used the same practical checklist for every backup power install I touch. This article is not a codebook and it is not a substitute for a licensed electrician. It is the field checklist that helps you ask the right questions before the generator shows up.

1. Work out the actual load before you buy anything

The first question is not which generator brand to buy. The first question is what has to keep running when the grid goes away. If the answer for a home is a well pump, furnace blower, refrigerator, lights, and internet, a 25 kw kohler sdmo generator is not a compromise. It is a properly sized machine.

If the building depends on three-phase equipment, motor loads, or a production line, that is when a 175 kva kohler sdmo generator starts to make sense. I have seen both installations fail when somebody sized the generator emotionally instead of electrically. A house does not need 175 kVA. A workshop with large motors may need more than the nameplate number suggests, because motors draw extra current when they start.

I should add that small projects deserve serious engineering, too. In our shop, a 25 kW request gets the same treatment as a 175 kVA job. If a supplier makes you feel like your project is too small to think through, that is not a good sign. A small generator failure costs the same sleepless night as a big one.

2. Make the automatic transfer switch the center of the design

The phrase everybody searches is how to hook up generator to house with transfer switch. The dangerous part is not the generator wiring. The dangerous part is letting the generator and the utility feed the same panel at the same time.

Never backfeed a building by plugging a generator into a dryer outlet, welder outlet, or any other wall receptacle. If the utility line is being worked on, backfeeding can electrocute a lineman. It can also start a fire when the neutral and ground paths are not set up correctly.

An automatic transfer switch fixes that by making it impossible for both sources to power the same load bus at the same time. When utility power fails, the switch sends a start signal to the generator. Only after the generator is up to speed and producing stable power does the switch move the load over. When utility power returns, the switch moves back and lets the generator cool down.

For many 200 amp residential and light commercial services, an ASCO 200 amp automatic transfer switch is a workhorse. It is rated to handle a 200 amp service entrance. That does not mean the connected generator produces 200 amps. A 25 kW generator at 240 volts single-phase produces roughly 104 amps continuously. The 200 amp rating on the transfer switch is about carrying the service capacity, not about the generator output.

If you are buying a large unit like a 175 kva kohler sdmo generator, do not assume the same 200 amp switch is correct. At 480 volts three-phase, a 175 kVA generator can draw more than 200 amps at full load. The switch has to be sized from the generator full-load current and the overcurrent protection, not from the kVA number on the brochure.

3. Get the permits and disconnect sequence straight

Before the electrician starts pulling wire, I check three things:

First, whether the local authority having jurisdiction requires a permit for the generator and the transfer switch. Most do. Second, whether the service needs a visible disconnecting means between the generator and the transfer switch. Many installations include a generator breaker outside the building so maintenance personnel can lock it out without going inside. Third, whether the ATS is rated for the fault current available on the utility side. This is not a detail you guess. It is on the switch nameplate and it has to match the engineered service capacity.

This is also the moment to decide whether you are backing up the whole panel or only an essential loads subpanel. If a 25 kW generator is going into a house with a 200 amp main service, I usually prefer a subpanel with only the critical circuits. Trying to power every outlet and every appliance in the house will overload the generator whenever the AC and oven cycle on together. The ATS can be rated at 200 amps while feeding that subpanel, but the subpanel protects the generator from being expected to carry loads that were never part of the plan.

4. Terminate the ATS in the right order

For the electrician doing the final hookup, the physical order matters. I have walked onto jobs where the utility feed was already terminated but nobody had checked the generator side. That creates an unsafe condition if the generator accidentally starts during wiring.

• Lock out and tag out the utility service before opening the panel.
• Terminate the normal utility feed first, following the ATS manufacturer markings.
• Terminate the generator feed next. Make sure the generator output breaker is off and locked before you land those wires.
• Terminate the load circuit last.
• Check that the neutral and ground are bonded only in the correct location. If the ATS switches the neutral, the generator neutral should not be bonded to the generator frame unless the installed configuration requires it.
• Torque every lug to the value printed on the switch. Loose lugs are a common cause of heat failures.

After termination, I always verify the voltage and phase rotation before loading anything. A 175 kva kohler sdmo generator can be connected to a building that is still labeled or wired incorrectly from an old repair. Label every conductor and compare it to the one-line drawing before closing the cover.

5. Treat the fuel system as part of the install

A generator that starts on paper but not on diesel is just expensive decoration. The most frustrating part of my job is showing up for an outage start and finding a clogged fuel filter or water in the tank.

If you are also maintaining a pickup truck, you already know the pattern. A Ram 2500 diesel fuel filter gets changed on a schedule because that truck has to start every day. The generator does not get used every day, so its fuel system gets ignored for months. That is the wrong way to think about it. Diesel can sit for a long time, but it can also grow algae, absorb water, and leave varnish in the injection system.

For any diesel SDMO or Kohler-SDMO generator, I check the fuel filter and water separator before I check the battery. If the fuel is being drawn from a bulk storage tank or from the same tank you use for a Ram 2500, do not just dip a hose into the bottom. Bottom-of-tank water and debris are exactly what will clog a generator filter. Use a transfer pump with filtration, or use a dedicated, treated fuel supply for the standby unit.

6. Performance-test before the storm

The final step is not flipping the switch to Auto and walking away. I run an actual loss-of-utility test whenever possible.

Start the generator manually and let it warm up. Check the output voltage and frequency. Then open the utility supply to the ATS and let the switch do its job. The ATS should signal the generator to start, wait for stable power, and then transfer the load. Let it carry the real building load for at least ten minutes. Watch the frequency meter and listen for heavy sag when motors start. If the generator struggles, the load management plan needs adjustment.

Once utility power is restored, the switch should return to the normal position. Let the generator run unloaded for several minutes to cool the turbocharger and engine, then shut it down. I know it is tempting to finish quickly, but a generator that has never been tested under load is not standby power. It is a hope.

Field notes: what I check every time

Most startup failures I see are small, repeatable mistakes. The load calculation was done after the generator was ordered. The ATS was sized for the generator instead of the load and service. The fuel was not filtered. The ground bond was wrong. The load wires were landed before the control wires, so the generator started with no way to signal the ATS.

If I remember correctly, the last one happened on a 175 kva install in 2023. Everything was polished, but the remote start wires were still coiled in the bottom of the switch enclosure. The generator started manually, the ATS did exactly what it was told, and the customer was left with an expensive way to run a generator that had to be flipped by hand. A 20 minute check would have caught it.

So here is my bottom line. Use the load calculation to choose the generator and the transfer switch. Make sure the ATS rating matches the service and the full-load current. Get the wiring order right, filter the fuel, and test the whole system under load. If you do those things, a 25 kw kohler sdmo generator will cover your essentials with room to spare, and a 175 kva kohler sdmo generator will carry a building without becoming a second outage. That is what the phrase backup power is supposed to mean.