Roles handled: Industrial generator procurement and installation coordination for 6 years. I've personally made (and documented) 23 significant mistakes during generator installations, totaling roughly $45,000 in wasted budget and rework. Now I manage our internal pre-install checklist to prevent others from repeating my errors.
This article is for anyone involved in installing a transfer switch for a commercial-grade diesel generator—specifically in the 50-1000 kVA range, like the SDMO or Kohler-SDMO units we deal with. If you're wiring up a portable 5kW unit, this isn't the guide you need. If you're connecting a permanent standby system, keep reading.
Below are the seven steps I check, re-check, and have on a laminated card in my toolbox. Each step represents a lesson that cost me something—time, money, or pride.
Step 1: Verify Voltage and Phase Configuration Before You Touch a Wire
The mistake I made (circa 2018): I assumed the generator and the facility were both 480V/3-phase. They were. But the generator was wired in a delta configuration, and the facility's main panel was wye. The transfer switch I'd ordered? Wye only. That mismatch cost $1,200 in replacement parts and a 3-week delay.
What to do:
- Confirm the generator's output voltage and phase configuration (check the nameplate—don't trust the spec sheet alone).
- Confirm this matches the facility's utility feed. Get this from the utility company if necessary—or rather, get it in writing from their engineering department.
- Order the transfer switch (or verify the one on site) matches both.
Checkpoint: Don't proceed until you have three data points: generator voltage/phase, utility voltage/phase, and the transfer switch rating. All three must agree.
Step 2: Size the Transfer Switch to the Generator, Not the Building Load
This one's common sense once you hear it, but it's easy to overlook if someone hands you a building load calculation. The transfer switch must handle the generator's maximum output, not the building's current load.
Example: We installed a 400 kW Kohler-SDMO generator at a data center. The building's running load was 280 kW. Someone on the team spec'd a 400A switch. The generator can output 520A. When we load-tested at full capacity—which we do for commissioning—the switch would have been undersized. We caught it during review. But I want to say I knew someone who didn't, and the switch failed within two years. Don't quote me on the exact timeframe.
Checkpoint: Transfer switch ampacity must be ≥ generator maximum ampacity. Period.
Step 3: Check the Available Fault Current
This gets into electrical engineering territory, which isn't my expertise. What I can tell you from a procurement and installation perspective is that failing to check available fault current will bite you during the electrical inspection.
The utility company will provide the available fault current at the service entrance. Your transfer switch must have an interrupting rating equal to or greater than that number. If it doesn't, the inspector will reject the installation. I've seen it happen on three separate projects.
Checkpoint: Get the utility's fault current data. Compare it to the transfer switch's interrupting rating. Document both.
Step 4: Plan for Neutral and Grounding Configuration
This is the step I see people get wrong most often. The generator manufacturer—whether it's SDMO or a Kohler-SDMO unit—will specify whether the neutral is bonded to the frame or floating. This must match the transfer switch's configuration and the facility's grounding scheme.
Real example: In September 2022, I approved an installation where the electrician bonded the neutral in the transfer switch, but the generator already had a bonded neutral. We had parallel neutral-to-ground paths. It didn't trip immediately, but it created a ground loop that caused intermittent equipment issues for months. Diagnosis alone cost $1,500.
Checkpoint: Confirm: generator neutral bond (bonded or floating) → transfer switch configuration → facility grounding. These must be consistent. If in doubt, consult the generator manual (note to self: actually do this, don't just pretend you will).
Step 5: Verify the Transfer Switch's Control Voltage Matches the Generator's Output
This one feels obvious, but I'll share a smaller mistake I made. Many automatic transfer switches require a control voltage to power their logic boards and motor operators. This often comes from a tap on the generator, or from a separate control transformer.
I once ordered a transfer switch that required 120VAC for its control board. The generator's control output was 24VDC. The mismatch meant we needed an additional control transformer and a separate circuit breaker, which we hadn't planned for. It wasn't a huge cost—maybe $400—but it caused a day of head-scratching and a rushed order for parts.
Checkpoint: Confirm the transfer switch control voltage requirements match what the generator provides. Look for this in the generator's control wiring diagram. (I really should keep a copy of that in my installation binder.)
Step 6: Test the Transfer Switch Functionality Before Connecting the Load
Even after choosing the right switch, I kept second-guessing. What if the timing was off? What if the voltage sensing was wrong? The hours until we ran the first test were stressful. Approved the test procedure and immediately thought, 'did I cover everything?'
The procedure I use now:
- Verify the generator starts and reaches proper voltage and frequency under no load.
- Using the generator's breaker, apply power to the transfer switch's generator input (with the utility side disconnected or locked out).
- Verify the switch transfers to the generator position and delivers power to the load side.
- Simulate a utility failure by opening the utility breaker (if safe) or using the switch's test function, then observe the automatic transfer sequence.
Approved the switch and immediately thought—wait, have I tested the retransfer? If I could redo my early installations, I'd always test the full cycle: utility fail → generator start → transfer → utility restore → retransfer → generator cooldown. But given what I knew then—nothing about the nuances of retransfer timers—my neglect was reasonable up to a point.
Checkpoint: Run at least three full transfer cycles during commissioning. Document any anomalies.
Step 7: Label Everything—And Then Label It Again
This isn't technical, but it's practical. In a crisis (power outage at 2 AM, facilities manager on-site who wasn't involved in the install), clear labeling can prevent mistakes.
What I now include:
- Main labels: "Utility Source," "Generator Source," "Load," all with voltage and phase markings.
- A laminated single-page instruction card near the switch: basic operation, what to do if it fails to transfer.
- Contact info for the installer and generator service company (with expiration dates on the labels—I really should update those).
The wrong label on a single item isn't a catastrophic failure, but missing a critical label when someone needs it is a $50 fix that saves a $5,000 emergency service call.
Common Mistakes to Avoid
1. Assuming 'universal' compatibility. Transfer switches from different manufacturers aren't necessarily interchangeable. A switch designed for one generator brand's control scheme may not work correctly with a Kohler-SDMO unit without additional interface modules. Check compatibility specs explicitly.
2. Ignoring the load shed requirements. If your generator can't supply the full building load, your transfer switch might need a load shed module. This is common in installations where the generator is sized for critical loads only. Failing to plan for this means either an overloaded generator or manually turning off non-critical loads.
3. Forgetting future expansion. If there's a chance you'll add more equipment to the backup circuit, consider sizing the transfer switch larger than current needs. Replacing a transfer switch isn't just the cost of the new unit—it's the labor, re-wiring, downtime, and re-inspection. As of early 2025, at least, the cost of upsizing one step is usually less than 15% of the total install labor for a replacement.
Final Thoughts (Yes, I'm Breaking My Own Rule)
There's no single "best" transfer switch for every generator. A switch that works perfectly for a 280 kVA SDMO generator in a manufacturing plant might be completely wrong for a 400 kW Kohler-SDMO unit in a data center. The transfer switch's job is to reliably connect the right source to the right load at the right time. Getting that right is a matter of specification, verification, and testing—not luck.
If this checklist helps you avoid even one of the mistakes I've made, it's worth the time I spent writing it. And if you catch something I missed, drop me a note. I'm still learning.
