How to Size a VFD Without a Motor Nameplate: Five Methods

How to size a VFD without a motor nameplate comes down to two things: identify the frame size and pole count to bracket the rating, then confirm the bracket with a current measurement. Where measurement is impossible, NEC 430.6(C) lets you fall back to 150 percent of the Table 430.250 value.

Two warnings belong at the top. Do not size on a clamp reading taken at light load, and do not trust the widely repeated rule that full-load amps equal three times the no-load current. The first produces a drive that fails under load. The second is a corollary of a narrow case that field advice treats as universal.

An unmarked motor is common. Plates corrode, motors are rewound and never re-stamped, and equipment arrives from auction with its history missing. This article covers five documented methods and the arithmetic for each.

If the nameplate is present and legible, none of this applies. Start with how to size a VFD for a motor instead.

Key Takeaways

• No-load current is roughly one third of full-load current for two- and four-pole motors, but the ratio climbs with pole count and passes 60 percent at ten poles. The “FLA equals three times no-load” shortcut is a corollary of that narrow case, not a general method.
• A clamped reading taken at light load badly understates the rating. On a 40 hp motor EASA tested, current at 25 percent load was 17.8 A against a 46.1 A full-load figure.
• NEMA frame numbers encode shaft height, not horsepower. Shaft height in inches is the first two digits divided by four, so a 143T and a 145T are both 3.5 inches.
• NEC 430.6(C) gives the code fallback: use the nameplate maximum operating current, or 150 percent of the Table 430.250 value if the nameplate has none.
• An unidentifiable motor is not automatically one to scrap. A non-standard or obsolete frame often argues for repair, because nothing modern bolts into the same space.

How to Size a VFD Without a Motor Nameplate: What You Need First

A VFD is a current-rated device. Its continuous output current must cover the motor’s worst-case continuous demand, so every method below answers one question: what is this motor’s full-load current at its rated voltage?

Horsepower is a useful intermediate, because current tables are indexed by it. Current is what the selection turns on.

Two Different Problems

A motor nameplate missing entirely is not the same problem as one that is partly legible. If corrosion has taken only the current rating and left voltage, speed, frame, and enclosure, Method 1 usually settles it. The fully missing case is where the frame number and pole count earn their keep.

What Even a Good Nameplate Hides

NEMA MG 1-12.47 permits a motor’s actual full-load current to differ by up to 10 percent from the nameplate figure, so a motor reading 8 percent over its plate is not automatically faulty. And a nameplate describes the motor, not the installation, which is what makes a running-current reading so easy to misread.

Method 1: Read What Survived

Before measuring anything, look harder at what is still there. Frame, voltage, speed, and enclosure often survive when the current field has corroded.

Photograph the plate before cleaning it. A wire brush removes corrosion and often removes the stamping with it. If the plate is stamped rather than printed, raking light at a low angle reveals characters a direct view does not.

Method 2: Identify the Frame and the Pole Count

This is the method a motor shop reaches for first, and it requires energizing nothing.

What a Frame Number Encodes

For integral NEMA “T” frames, the first two digits divided by four give the shaft height in inches, measured from the base to the shaft centreline. That is the “D” dimension, and it is a standards-based identifier rather than a rule of thumb.

A 143T and a 145T are both 3.5 inches. The third digit raises the bolt-hole spacing and length, so the 145T is longer at the same shaft height.

The “T” suffix marks the 1965 NEMA re-dimensioning. Older “U” frame motors share many shaft heights but are not dimensionally interchangeable. Treating a U frame as a T frame is a reliable way to order a drive that will not fit.

IEC frames are simpler. Per IEC 60072, the frame number is the shaft centre height in millimeters: a 90 frame is 90 mm, a 132 frame is 132 mm. A published motor frame size chart converts both systems into mounting dimensions and typical ratings.

Getting Pole Count From Speed

Pole count comes from a free relationship. If you can read or measure the rated speed:

poles = 120 x frequency / synchronous speed

A motor at 1,450 rpm on 50 Hz is four-pole. At 2,900 rpm on 50 Hz it is two-pole. Round to the nearest even number because slip is small.

Why You Need Both

There is no fixed frame-to-horsepower relationship. The same frame serves different ratings at different speeds, because a slower motor needs more active material for the same power.

EASA’s no-nameplate procedure combines the two: frame gives a power range, pole count says where in that range the motor sits.

NEMA frame	Shaft height	Typical HP range
143T	3.5 in	1 to 5
145T	3.5 in	2 to 7.5
182T / 184T	4.5 in	7.5 to 20
213T / 215T	5.25 in	20 to 40

Read these as a bracket, then confirm with a measurement. The ranges overlap deliberately.

Halvor maintains the drive room at a paper mill in southern Norway and inherited a conveyor motor whose plate had been painted over twice. The frame read 284T, putting shaft height at 7 inches. Speed was 1,180 rpm on 50 Hz, so six poles. That bracketed the motor between 25 and 40 hp, and a running-current check at full conveyor load returned 44 A. The drive they ordered was right first time.

If the frame cannot be read or does not appear on a standard chart, the motor is non-standard or very old. That changes the decision at the end of this article, not the method here.

Method 3: Measure No-Load Current, Carefully

Most field advice leads with this method. It is also the one most often applied outside its valid range.

What Is Actually Documented

EASA states the ratio, not the multiplier: a motor running at rated voltage with no load typically draws about one third of its rated current. That is a documented rule of thumb for the common case.

The “FLA equals three times no-load current” shortcut is that ratio inverted. It is a corollary, not a published method, and it inherits every limit the ratio has.

The Limit That Gets Ignored

The one-third figure holds for two- and four-pole motors. It degrades as pole count rises, because more poles mean more air-gap crossings per revolution and therefore more magnetizing current.

Poles	Motor no-load current as a share of FLA
2	25 to 33 percent
4	33 to 40 percent
6	33 to 45 percent
8	33 to 65 percent
10 or more	60 percent and above

At ten poles and above, no-load current can equal or exceed full-load current. A lightly loaded ten-pole motor may draw more standing unloaded than its own nameplate figure, and multiplying that by three produces a number with no relationship to the motor.

The ratio also runs inversely with motor size. EASA notes it can be twice as large for a 10 hp motor as for the same manufacturer’s 300 hp motor.

Why This Method Needs a Second Opinion

No-load current is affected by steel grade, annealing, and small variations in air gap, and EASA warns a manufacturer’s published figure may vary by 25 percent or more. Multiplying by three amplifies any measurement error threefold. That is why a motor shop treats it as a check on an estimate from the frame, never as the primary method.

Doing It Safely

EASA’s documented sequence for an unknown motor:

1. Start at a significantly reduced voltage, just enough to accelerate the motor.
2. Measure no-load speed and derive pole count.
3. Raise voltage toward the assumed rated value while watching no-load current.
4. Stop immediately if current rises faster than voltage. That means you have passed rated voltage, or the motor is wound for a different frequency.

Run the motor uncoupled and guard the shaft, because a key can eject from an open keyway. If the motor is a permanent-magnet type, spinning it generates voltage, so the drive terminals can be live with the supply off.

Method 4: Measure Running Current at Real Load

This method is the most accurate of the five and the easiest to get badly wrong.

The Trap

A motor at light load draws current well below its rating, and the relationship is not linear enough to reason your way back from a small number.

EASA’s dynamometer test on a 40 hp four-pole motor measured:

Load	Current	Share of FLA
0 percent	14.3 A	31 percent
25 percent	17.8 A	39 percent
50 percent	25.6 A	56 percent
75 percent	35.2 A	76 percent
100 percent	46.1 A	100 percent

A technician who clamped 17.8 A and sized a replacement on it would buy roughly a 7.5 to 10 hp drive for a 40 hp machine. It would run, right up until it was asked to do real work.

How to Take the Reading

Measure at 50 percent of rated load or above, ideally between 75 and 100 percent. Below about 40 percent load, current-based estimates are not reliable enough to select from.

If the load varies through a cycle, record current across the whole cycle and take the peak, not the average. A conveyor lightly loaded for most of a shift still has a worst case, and that is the case the drive must survive. Measure all three phases and use the highest reading.

Use a clamp meter that computes true RMS. If the motor already runs on a drive, measure at the drive input rather than the output for a cleaner waveform.

The EASA Load Formula and What It Is For

There is a widely circulated formula attributed to EASA:

hp required = hp nameplate x [1 - (FLA - actual amps) / (FLA - no load amps)]

It is real, and it is from EASA. It is also not an identification method.

That formula estimates the actual load a motor is carrying to catch a motor oversized for its job. It requires a known nameplate FLA and horsepower, so when the nameplate is missing, it has no inputs. It has been misattributed in the field as a way to identify an unknown motor. It cannot do that.

Adaeze runs maintenance at a cement plant in Ogun State and had a mill motor shot-blasted clean during a rebuild, taking the plate with it. The frame read 404T, putting shaft height at 10 inches, and speed came back at 990 rpm on 50 Hz, so six poles. That bracket plus a loaded current check identified it as 75 hp. The drive ran four months before tripping repeatedly on overload: the frame bracket had been treated as an answer rather than a range. The motor was a 100 hp unit all along.

Method 5: The Code Fallback

When measurement is impossible, or you need a defensible number for a permit, the code provides one.

NEC 430.6(C)

The text reads:

“For motors used in alternating-current, adjustable voltage, variable torque drive systems, the ampacity of conductors, or ampere rating of switches, branch circuit short-circuit and ground-fault protection, and so forth, shall be based on the maximum operating current marked on the motor or control nameplate, or both. If the maximum operating current does not appear on the nameplate, the ampacity determination shall be based on 150 percent of the values given in Table 430.249 and Table 430.250.”

So the code path is: use the nameplate maximum operating current if there is one. If there is not, use 150 percent of the Table 430.250 value.

Note the scope. That subsection covers variable torque drive systems, the fan and pump family. It has been carried from the 2011 through the 2023 editions.

Table 430.250 Values

HP	230 V	460 V
2	6.8 A	3.4 A
3	9.6 A	4.8 A
5	15.2 A	7.6 A
10	28 A	14 A

A 10 hp, 460 V motor with no nameplate current therefore has a code basis of 21 A.

What the Fallback Is and Is Not

The 150 percent figure is a floor for code compliance, not a target for drive selection. It is deliberately conservative because it assumes the worst motor in that class at that voltage.

Select the drive on it and you will often buy a frame larger than needed. That is defensible when you genuinely cannot measure on a critical application, and expensive when a twenty-minute measurement would have settled it.

NEC 430.6(A)(1) separately requires the table values for conductor ampacity and switch ratings, which is why a drive quote and a conductor schedule can legitimately show different currents for the same motor.

What to Do With a Rewound Motor

A rewound motor adds uncertainty, and the magnitude depends on rewind quality.

EASA and AEMT’s 2019 rewind study of 40 to 100 hp premium-efficiency motors found efficiency changes between plus 0.3 and minus 0.5 percentage points, an average of minus 0.1, which sits inside the test method’s own tolerance. A good rewind does not meaningfully change the motor.

A poor one can. Dropped turns or a reduced coil span raise flux density toward core saturation, and core damage during stripping or burnout raises losses further. Reconnecting a wye winding as delta raises flux density by roughly 1.73 times, and reconnecting a one-circuit winding as two-circuit doubles it.

Older industry data reports larger penalties than the EASA study found, including figures such as 18 percent higher total losses. The reconciliation is rewind quality.

Practically: if a rewound motor now draws more than its original nameplate, treat that as a signal, not proof of a bad rewind.

Five Mistakes That Cause the Wrong Drive

Applying the three-times rule without checking poles. Correct for a four-pole pump motor, badly wrong for a ten-pole one.

Sizing on a light-load clamp reading. The most expensive error in this article, and the one that produces a drive working perfectly until the machine is loaded.

Treating the EASA load formula as identification. It estimates load on a known motor. It cannot identify an unknown one.

Reading the frame as a horsepower. Frame gives shaft height, speed gives pole count, and neither gives horsepower alone.

Leaving a power factor correction capacitor in the circuit. ABB’s electrical planning guidance states it directly: “Do not connect power factor compensation capacitors or harmonic filters to the motor cables (between the drive and the motor). They are not designed for use with AC drives and can cause permanent damage to the drive or themselves.”

That last one deserves emphasis. Capacitive reactance falls as frequency rises, so a capacitor that looks harmless at 50 Hz looks close to a short circuit to a drive switching at several kilohertz. When replacing a direct-on-line starter with a VFD, check for capacitors already installed across the motor terminals. They are often hidden in a terminal box and commonly cause a drive to fail at commissioning.

Configuring the Drive Once You Have a Number

Identification is half the job. An unmarked motor needs its parameters entered honestly.

Set the drive’s motor data to the actual motor, not the drive’s own rating. Rated current, voltage, frequency, and no-load current all feed the electronic thermal overload, and if they are wrong the protection is wrong with it. A drive configured for its own nameplate current will happily cook a smaller motor.

Oversizing by one frame is generally acceptable for light and moderate starting loads, provided the motor parameters and thermal overload are set for the real motor. It buys margin without pretending to be precise.

For the parameter details, VFD auto-tuning covers what the drive measures about the motor, and motor nameplate parameters cover the fields you are reconstructing.

When Sizing a VFD Without a Motor Nameplate Is Not Enough

No rule says an unidentifiable motor should be scrapped. Standard NEMA and IEC motors can almost always be identified from frame dimensions, pole count, and winding data, and a motor shop can assign a new nameplate afterwards. That beats a guess, and for a large or unusual motor it is usually cheaper too.

The decision is a repair-versus-replace calculation, not an identification-difficulty one.

Factor	Points toward replacement
Size	Under roughly 50 hp
Age	Over roughly 15 years
History	Previously rewound
Repair cost	50 to 60 percent of a new motor or more
Efficiency	Standard efficiency, continuous duty

Note the entry missing from that table. A custom, obsolete, or non-standard frame argues for repair, not replacement, because no off-the-shelf motor bolts into the same space. Non-standard frames are frequently rewound precisely because nothing modern fits.

The cost threshold varies by source: 50 percent in plant engineering guidance, 60 percent in US Department of Energy motor management material. Treat it as a band, and note that rewinding a large motor is proportionally cheaper than replacing it.

Piotr runs maintenance at a food processing plant near Poznan with a 30-year-old mixer motor on a non-standard frame. The nameplate was long gone and the replacement quote came back at eleven weeks because nothing current matched the mounting. A motor shop identified it from frame dimensions and winding data in three days, rewound it, and issued a new nameplate at under a fifth of the replacement cost. The plant now has a documented motor where it previously had an unknown one.

Frequently Asked Questions

How do I find the horsepower of a motor with no nameplate?

Start with the frame number, which gives shaft height, and the speed, which gives pole count. Together they bracket the horsepower range. Confirm with a current measurement at 50 percent load or above, or send the motor to a shop.

Can I identify motor full-load amps from the no-load current?

Only as a check on an estimate you already have. Motor no-load current is roughly one third of full-load current for two- and four-pole motors, but the ratio rises with pole count and passes 60 percent at ten poles.

Why does my motor draw less than its nameplate says?

Because it is not fully loaded. Current falls steeply as load drops, and a motor at 25 percent load may draw under 40 percent of rated current. With motor FLA unknown, a light-load reading tells you about the load, not the motor.

What is the 150 percent rule in NEC 430.6(C)?

Where a motor’s maximum operating current is not marked on the nameplate, the code bases conductor ampacity and protection on 150 percent of the Table 430.250 value. It is a fallback for code compliance, not a target for drive selection.

Can I run a VFD on a motor I have not identified?

Yes, if you configure the drive for the motor rather than for itself, and if the insulation suits the drive output. Unidentified motors are usually old motors, and old motors may not be inverter-rated. Motor compatibility with VFD covers that separately.

Is it safe to run an unmarked motor to test it?

Not at full voltage on an unknown winding. EASA’s approach starts at significantly reduced voltage just to accelerate the motor, then raises voltage while watching no-load current, stopping if current rises faster than voltage.

What if the motor turns out to be non-standard?

That usually argues for repair. Non-standard and obsolete frames are often rewound because no current motor matches the mounting, and a shop that identifies the winding can issue a new nameplate.

Conclusion

Sizing a drive without plate data is a five-step problem: read what survived, identify frame and pole count, measure no-load current as a check, confirm with a loaded current reading, and fall back to NEC 430.6(C) where measurement is impossible.

The two errors that cost most are applying the three-times no-load rule outside the two- and four-pole case it came from, and sizing on a clamp reading taken at light load. Both produce a drive that works until the machine is asked to do real work.

Before ordering, have four things ready: the frame number, the pole count, a current reading at real load, and a decision about whether the motor is worth identifying. If the last is uncertain, a motor shop can answer the first three and hand back a motor with a new nameplate.

Sizing a Drive for an Unmarked Motor
Send us the frame number, the shaft height if you can measure it, the speed, and a current reading taken under normal working load. Our engineers will bracket the rating, confirm the duty class the application needs, and come back with a drive and the motor parameters to enter. Talk to our engineers before you order, and we will tell you if the answer needs a motor shop rather than a drive.