How to Install a Transfer Switch for Your Perkins Generator: A 7-Step Checklist

I've coordinated 200+ emergency power installations over the past eight years, mostly for commercial and industrial clients who can't tolerate a building going dark. The pattern rarely changes. The generator arrives on schedule. The Perkins engine fires up on the first crank. Then somebody realizes the transfer switch is still in a crate, and the inspection is two days out.

Everything I'd read about generator projects said the engine was the hard part. In practice, the ATS is where projects stall. That's why this checklist exists. Seven steps, in the order they need to happen.

Who This Checklist Is For

Use this if you have a generator on site and you need it to connect to your building's electrical system automatically. That means an automatic transfer switch (ATS) is part of the project. It applies to an 80 kW Perkins commercial generator serving a retail storefront, a 350 kW Perkins standby generator in a data center yard, and a portable generator feeding a building through a 50A inlet.

And if you're here because you're comparing a solar generator or battery backup against a diesel unit for the same job, keep reading. The ATS wiring steps are the same. The sizing logic differs, and I'll get to that in Step 2.

Step 1: Decide Where the Transfer Switch Goes

The first decision is a simple question. Where does the ATS sit relative to your main breaker?

A service-entrance transfer switch is placed ahead of everything. It becomes the building's main disconnect, and the utility treats it as the boundary point. That version needs a fault-current rating matched to the utility's available short-circuit current. It's a bigger, more expensive piece of gear, and it takes longer to get utility approval.

A downstream transfer switch sits on the load side of the existing main breaker. It controls a subpanel or a specific set of circuits. This is the easier retrofit in most buildings, and it's what I usually spec unless the electrical service is being upgraded anyway.

Get this wrong, and you'll order the wrong switch. I've seen it happen more than once.

Step 2: Size the ATS for the Load, Not the Generator

Then comes sizing, and this is where I see the most confusion. People look at the generator nameplate, see 80 kW, and pick a transfer switch rated for the generator's output. That's backwards.

The ATS is not just in the generator circuit. It's in the utility circuit too. When the grid is up, the switch is passing utility current to the building. So it must be rated for the full connected load, not just what the generator can supply.

A few numbers to make this concrete. An 80 kW Perkins commercial generator at 480V three-phase puts out roughly 120 amps at 0.8 power factor. But if it backs up a subpanel with a 200A main, the ATS needs to be rated 200A, because it carries that full 200A from the utility during normal operation. The generator only carries it during an outage.

For a 350 kW Perkins standby generator, the picture is similar. You're often looking at a 600A or 800A ATS, depending on the service size. I've had clients try to argue that a 400A switch is enough. It might be enough for the generator, but it's not enough for the building load when the grid is live. The switch has to be rated for whatever current passes through it, period.

The same logic applies to solar generators and battery storage. If an inverter feeds a transfer switch, the switch rating still matches the electrical service and the loads, not the inverter's output. The real difference between solar-plus-battery and a diesel genset is runtime. A battery bank delivers clean power for a fixed window. A diesel generator keeps running as long as the tank has fuel. For a facility that needs 72 hours of backup after a hurricane, that math usually favors the generator. For short, predictable outages, battery can be the better call. Neither is a bad choice. I've quoted both.

Step 3: Pick the Transition Type

Next, you need to choose between an open transition and a closed transition switch.

Open transition is break-before-make. The switch disconnects from the utility, then connects to the generator. There's a gap of a few seconds. Lights flicker, computers ride through on UPS, HVAC restarts. Not ideal, but workable for most commercial buildings.

Closed transition is make-before-break. It parallels the utility and the generator for a moment, which means no break at all. That requires the generator to match the utility's frequency and phase, so you're adding paralleling gear and a significantly higher price tag.

I've seen salespeople push closed transition on clients who don't need it. Unless you're running a data center or a process line where a 100-millisecond blip ruins a batch, open transition is usually the right answer.

Step 4: Sort Out the Permit and the Code Classification

Before you buy anything, talk to the local building department. Every transfer switch installation I've been involved with required a permit, and the classification of the system under the National Electrical Code determines what gets inspected and how strict the rules are.

NEC 700 covers emergency systems for life safety, like hospitals and egress lighting. NEC 701 covers legally required standby systems, such as fire pumps and elevators. NEC 702 covers optional standby systems, which is where most commercial and industrial backup power falls.

Get the classification confirmed in writing. It affects transfer time, wiring methods, testing requirements, and signage. I coordinated a project where the owner assumed the system was optional standby, and the building department classified it as legally required because an elevator was connected. The switch and control wiring had to be upgraded mid-project. That's the kind of surprise that blows a budget and a schedule.

If you're installing an ATS for a portable generator, the same classification rules apply. The size of the generator doesn't change the code requirements for the transfer equipment.

Step 5: Mount the ATS with Proper Working Clearance

The physical mounting is straightforward, but clearance rules catch people off guard. According to NEC 110.26, you need at least 36 inches of working space in front of the enclosure and 30 inches of width. I've walked into electrical rooms where someone planned a new panel directly in front of the proposed ATS location. That doesn't work.

Make sure the location is dry and accessible. For outdoor installations, order a NEMA 3R enclosure. For a 350 kW standby unit, the ATS is typically a floor-mounted cabinet that needs dedicated floor space. Plan that early, because it's hard to argue with a concrete pad that's already poured in the wrong place.

Step 6: Wire It, and Watch the Neutral-Ground Bond

Wiring is where things get serious. The most common inspection failure I see is a neutral-ground bond issue.

The bond between the neutral and the ground conductor has to exist in exactly one place. In a 3-pole ATS, the neutral stays solidly connected, and the main bonding jumper stays at the service entrance. The generator must then be configured with a floating neutral. If you're using a portable generator that came with a bonded neutral from the factory, you have to change it to a floating neutral before connecting it to a permanently installed ATS.

A 4-pole ATS switches the neutral as well as the phases. It's often specified for systems where you need to isolate the generator from the utility neutral, like when the generator becomes a separately derived system. Your electrician needs to know the difference, and the generator's wiring diagram needs to be followed exactly.

This is where I'll admit to my own mistake. In May 2024, I was called in to troubleshoot a 350 kW Perkins standby system that had passed the wiring inspection but failed the load test. The engine ran perfectly. The ATS transferred. And then ground-fault alarms went off immediately. The cause was a neutral jumper that should have been removed but wasn't. The fix took ten minutes. Ten minutes. The system had sat dark for a week while four contractors argued about whose fault it was.

Do not assume the electrician's crew has installed an ATS before. I made that assumption once and learned during a first automated transfer test that they'd only ever wired standalone generators. Verify experience before you sign off.

Step 7: Test the System Before You Need It

Finally, test the system before the utility outage does it for you. I've lost count of how many generators failed their first real test because nobody ever actually transferred a load.

The sequence is simple. Start with a manual test. Operate the ATS by hand, confirm the generator starts, and let it carry the connected load. Then run an automatic test. Simulate a utility failure and time the full sequence.

For emergency systems, NFPA 110 requires the generator to come up to speed and accept the load within 10 seconds. For optional standby, the timing is less prescriptive, but I still want to see the transfer complete in 10 to 15 seconds. If it's slower than that, something is wrong.

Then do a load bank test. An 80 kW commercial generator should be run at 75 to 80 percent load for at least two hours. A 350 kW standby unit deserves a full load bank test before it's trusted with a whole facility. A generator that has never carried real load is an unproven generator. I do not care how clean the wiring looks.

Common Mistakes to Avoid

• Assuming the ATS is just a switch. It's a code-governed safety device with specific wiring requirements. The contractor who installs it needs to have done this before. My assumption about someone else's experience cost a week of downtime.
• Sizing the switch to the generator nameplate. The ATS carries utility current in normal operation. Size it to the connected load and the service.
• Ignoring the neutral-ground bond. It's the most common cause of inspection failures and ground-fault alarms, and it's simple to verify.
• Skipping the load bank test. A no-load transfer test proves only that the contacts move. It doesn't prove the generator and switch can carry real current for hours.

The Budget Conversation Nobody Wants to Have

I'm a believer in transparent pricing. The vendor who lists the ATS, the conduit, the installation, the permit, and the load bank test as separate line items might show a higher total than the one who quotes a bare generator. That's the quote you want.

I've watched clients pick a low bid and then face a $5,000 change order for a transfer switch installation they thought was included. I've learned to ask what's NOT included before I ask the price.

And one last thing. Don't assume that because the prime mover is a Perkins engine, the transfer switch will maintain itself. Contactors wear, timers drift, and connections loosen. Annual inspections and regular transfer testing are part of owning the system. That's not a sales pitch. It's the difference between a generator that works when you need it and a very expensive ornament.