What Size VFD for a 5HP Motor? I Got It Wrong and It Cost Us $3,300

The Line Was Down, and It Was My Fault

In August 2023, we installed a new packaging line in our main assembly area. The conveyor ran on a 5HP, 460V, three-phase motor—1750 RPM, standard stuff. We'd paid a regional integrator $14,000 for the whole conveyor system. The VFD came bundled with it—some off-brand unit that I'd never heard of, listed at $420 on the quote.

Six weeks in, the drive failed. Not gradually. One Tuesday afternoon, it just stopped. The integrator said the voltage looked "off." But our supply was dead-on spec. They never figured it out.

I spent two weeks getting that line back up. And I had to buy a replacement VFD out of my budget. That's the thing about bundled equipment—when it breaks, you're on your own. I was staring at a $14,000 system sitting idle over a part that should've cost $780.

"You Size VFDs by Amps, Not Horsepower"

My first move was the same mistake I'd been making for years: find a 5HP VFD. That was my entire thought process. 5HP motor, 5HP drive. Simple.

Then our senior controls tech—a guy who's been doing motor work since before I had a driver's license—corrected me. "You size drives by amps, not horsepower. Look up the motor's FLA."

I pulled up the NEC 430.250 tables. A 5HP, 460V three-phase motor draws about 7.6 amps at full load. But here's where it gets tricky: for constant-torque loads (conveyors, extruders, anything that needs full torque at low speeds), the rule of thumb is that the drive's continuous current rating should be at least 110% of the motor's FLA. So I needed a drive rated for at least 8.4 amps.

Then I started digging into Teco Electric's selection guides. I remembered they'd been around since 1956—teco electric & machinery founded 1956—so their documentation should be solid. I downloaded their VFD catalog and worked through the sizing tables.

But comparing brands got confusing fast. Some drives rated for "5HP" had continuous current ratings of only 7.0A. Others had 9.5A. Same nominal horsepower, different actual capability. If you're just matching horsepower numbers, you could easily end up with a drive that trips on overload every time the conveyor starts under load.

I used Teco's online selection tool after logging into their support portal. I had to create an account first—teco electric login was straightforward enough, though the portal feels a bit dated. But the selection data was worth it. Their guide actually broke down drive sizing by load type, which I hadn't seen from other manufacturers at that point.

It reminded me of when we prototyped a small pick-and-place arm two years ago. The servo motor pinout was clearly documented—every wire, every signal. The Arduino stepper motor setup we did before that was even simpler. Industrial VFDs? It's a different world. More terminals, more parameters, more ways to get it wrong.

I narrowed it down to two options:

• Option A: A budget drive rated 5HP, 7.0A continuous. $420.
• Option B: A Teco E510 series, rated 7.5HP, 11A continuous. $780.

The difference was $360. On a $14,000 system, that's noise. But I was being pressured to keep costs down, and Option A was right there. "It's a 5HP motor," my manager said. "Why do you need a 7.5HP drive?"

I didn't have a good answer. I bought Option A.

The Overload Trips Started on Day 19

The first few weeks were fine. Then the tripping started. Not every run—just when the conveyor was fully loaded at startup. The drive would fault on overcurrent (OC), and we'd have to reset it.

Each trip cost us maybe 3-5 minutes of production. Sounds small, right? But when it happens 8-10 times a shift across two shifts, you're losing 45 minutes a day. At our throughput and margin, that's roughly $350 in lost production per day.

This went on for nine working days before I gave up on the drive. That's $2,100 in lost output.

The vendor wouldn't warranty it, either. "Improper sizing" isn't a defect—it's on the buyer. So I ate the $420 and ordered the Teco drive.

What the NEC Table Actually Tells You

The Teco E510 arrived in four days. I installed it, set the parameters, and we've run it for six months without a single trip. Not one.

Total cost of my mistake: $420 for the cheap drive + $2,100 in lost production + my own time. Call it $3,300 tuition.

Here's what I wish I'd known from the start—the actual sizing logic:

1. Look up the motor's full-load amps (FLA) from NEC 430.250. For a 5HP, 460V motor, that's 7.6A.
2. Determine your load type: Constant torque (conveyors, positive displacement pumps, extruders) requires a drive rated at 110% of FLA, so 8.4A minimum. Variable torque (centrifugal fans, centrifugal pumps) can use a drive rated at 100% of FLA, so 7.6A is acceptable.
3. Check the drive's continuous current rating, not its horsepower label. A "5HP" drive might be rated 7.0A or 9.5A depending on the manufacturer and series.
4. Account for carrier frequency derating. If you run the drive at a high switching frequency, the effective current rating drops. Most drives show this in a derating table.

For our conveyor, 7.6A × 1.1 = 8.4A. The Teco E510's 11A rating gave us about 30% margin. That's not overkill—that's breathing room for startup surges.

But Here's the Honest Part

I'm not going to tell you that everyone needs a drive sized 50% above their motor's FLA. That would be wrong.

If you're running a variable-torque load—like a centrifugal fan or a centrifugal pump—you probably don't need that margin. I've seen plenty of installations where a drive rated at exactly the motor's FLA ran for years without issues. Those loads just don't demand the same startup torque.

And if your motor is oversized for the application already—which happens more often than people admit—you might get away with a smaller drive.

The point is: match the drive to the load, not just the motor nameplate. That's the lesson I paid $3,300 to learn.

Our Teco drive has been running since October 2023. No issues. I check the fault log every few weeks out of habit now. Still clean.

Next time someone asks me "what size VFD for a 5HP motor," I'm not going to give them a number. I'm going to ask them what the motor is driving.