Motor Nameplate Reading: Full Guide to Understanding 3-Phase Motor Data

A technician sets the thermal overload relay to 21A on a motor wired for 320V. The motor runs fine at light load but trips repeatedly under normal conditions. The nameplate showed 42/21A: 42A is the FLA at 230V, and 21A is the correct value for 460V operation. The overload was set to half the right number.

Dual-voltage confusion is the most common nameplate misread, but insulation class in high-ambient locations, service factor in VFD applications, and the code letter when sizing breakers each have their own failure modes. Every field on the nameplate is covered below, with worked examples for 15 HP and 25 HP motors.

Table of Contents

1. Power Rating: HP vs. kW
2. Voltage and Dual-Voltage Motors
3. Full Load Amperes (FLA)
4. Speed, Frequency, and Slip
5. NEMA Frame Designation
6. Insulation Class
7. Service Factor
8. Design Letter, Enclosure, and Code Letter
9. 15 HP Motor Nameplate: Worked Example
10. 25 HP Motor Nameplate: Worked Example
11. Lenze Motor Nameplate: IEC Format
12. Common Mistakes

Power Rating: HP vs. kW

The nameplate HP is the motor's rated mechanical output at the shaft, not input power. Because no motor is 100% efficient, electrical input is always higher.

For a 15 HP motor at 91% efficiency:

$$P_{input} = \frac{15 \times 0.746}{0.91} = \frac{11.19}{0.91} = 12.3 \text{ kW}$$

NEMA motors are rated in horsepower; IEC motors (including Lenze) use kilowatts. The conversion is 1 HP = 0.746 kW, so 11 kW is approximately 15 HP.

Standard NEMA sizes follow a fixed sequence: 1, 1.5, 2, 3, 5, 7.5, 10, 15, 20, 25, 30, 40, 50 HP. If your calculated load falls between sizes, select the next one up.

Voltage and Dual-Voltage Motors

A rating of 230/460V does not mean the motor accepts any voltage in that range. It means two distinct winding configurations:

• 460V wye connection: 21A FLA for a 15 HP motor
• 230V delta connection: 42A FLA for the same motor

Both configurations deliver the same 15 HP at the shaft. The connection is made in the terminal box using T-lead jumpers.

• 460 V (high-voltage connection): T4-T7, T5-T8, and T6-T9 are tied together in pairs; line conductors connect to T1, T2, and T3.
• 230 V (low-voltage connection): T4, T5, and T6 are tied together; line conductors connect to T1-T7, T2-T8, and T3-T9.

Full Load Amperes (FLA)

FLA is the current drawn at rated voltage when producing rated HP. Three decisions hinge on it:

• Overload relay setting: 100-115% of FLA (up to 125% if SF (Service Factor) is 1.15 or higher, per NEC 430.32)
• Branch circuit conductor: minimum 125% of FLA (NEC 430.22)
• VFD output current rating: must meet or exceed motor FLA

FLA can be calculated from nameplate data:

$$I = \frac{HP \times 746}{\sqrt{3} \times V \times \eta \times PF}$$

For a 15 HP, 460V motor with efficiency of 91% and power factor of 0.88:

$$I = \frac{15 \times 746}{1.732 \times 460 \times 0.91 \times 0.88} = \frac{11,190}{643.5} = 17.4 \text{ A}$$

The nameplate shows 21A, slightly higher because NEC Table 430.250 values include a margin. Use nameplate FLA for protection settings, not the calculated value.

Speed, Frequency, and Slip

Nameplate RPM is the full-load speed, always less than synchronous speed. For a 4-pole, 60 Hz motor:

$$N_s = \frac{120 \times 60}{4} = 1800 \text{ RPM (synchronous)}$$

A nameplate speed of 1760 RPM gives a full-load slip of:

$$s = \frac{1800 - 1760}{1800} = 2.2\%$$

Slip increases with load. A heavily loaded induction motor runs measurably slower than nameplate speed. Induction motors are not constant-speed machines: if speed accuracy matters for the application, a VFD with closed-loop encoder feedback is needed.

NEMA Frame Designation

The frame number encodes the motor's physical dimensions. For frame 215T:

• Shaft height (D): 215 / 16 = 13.44 inches from base to shaft centerline
• "T" suffix: NEMA T-frame standard (post-1964), which standardized shaft diameter, keyway, and bolt pattern across manufacturers

A 215T from any NEMA-compliant manufacturer is mechanically interchangeable with a 215T from any other.

HP	Typical Frame	Shaft Height
15	215T	13.44"
20	256T	16.00"
25	284T	17.75"
30	286T	17.75"

Insulation Class

Insulation class sets the maximum permissible winding temperature:

Class	Max Winding Temp	Rise at 40°C Ambient
B	130°C	80°C
F	155°C	105°C
H	180°C	125°C

Class F is standard on most new motors. Running a Class F motor at Class B temperature rise (80°C instead of 105°C) roughly doubles insulation life. Some manufacturers label this "F/B" on the nameplate and market it as an extended-life feature.

Above 40°C ambient, the permissible rise decreases. A Class F motor in a 50°C enclosure has 95°C of rise budget instead of 105°C, which reduces its continuous load capacity. At 55°C ambient, that margin narrows to 90°C of rise before the winding hits its thermal limit.

Service Factor

SF is the permitted overload multiplier under nameplate conditions: rated voltage, rated frequency, and 40°C ambient. SF 1.15 on a 15 HP motor means 17.25 HP is permissible continuously under those conditions.

Running in the service factor zone increases winding temperature significantly. For every 10°C rise above Class B levels, insulation life roughly halves. A motor running at its SF ceiling continuously will have a shorter service life than one operating at 100% rated load.

For VFD applications: treat SF as 1.0. Harmonic currents and voltage spikes from the drive add winding stress beyond what the nameplate SF accounts for.

Design Letter, Enclosure, and Code Letter

NEMA Design Letter

Design	Starting Torque	Locked Rotor Current	Typical Application
B	Normal	6-7x FLA	Pumps, fans, general purpose
C	High	6-7x FLA	Conveyors, loaded compressors
D	Very high	6-7x FLA	Punch presses, cranes, hoists

Design B covers most industrial applications. If no design letter appears on the nameplate, it is Design B.

Enclosure Types

Code	Name	Use Case
ODP	Open Drip-Proof	Indoor, clean, horizontal mounting
TEFC	Totally Enclosed Fan Cooled	Outdoor, dusty, wet environments
TENV	Totally Enclosed Non-Ventilated	Small motors, clean environments
XP	Explosion-Proof	Class I, II, III hazardous locations

Code Letter

The code letter defines locked rotor kVA per horsepower. For Code G (5.6 to 6.3 kVA/HP), a 15 HP motor:

$$\text{LR (Locked Rotor) kVA} = 15 \times 6.0 = 90 \text{ kVA}$$ $$I_{LRC} = \frac{90{,}000}{\sqrt{3} \times 460} = 113 \text{ A} \approx 5.4 \times \text{FLA}$$

This locked rotor current feeds into breaker sizing per NEC Table 430.52 and determines the transformer inrush load when the motor starts.

15 HP Motor Nameplate: Worked Example

Figure: Typical nameplate for a 15 HP, 460V, TEFC induction motor.

Parameter	Value	What It Means
HP	15	Rated shaft output (11.2 kW)
Volts	230/460	Delta at 230V, wye at 460V
Amps	42/21	FLA: 42A at 230V, 21A at 460V
RPM	1760	Full-load speed, 4-pole
Hz	60	Supply frequency
Phase	3	Three-phase
Frame	215T	13.44" shaft height
Ins. Class	F	Max 155°C winding temp
S.F.	1.15	17.25 HP permissible under nameplate conditions
Duty	Cont.	Continuous
Design	B	Standard torque and starting current
Encl.	TEFC	Outdoor-rated enclosure
Amb.	40°C	Standard ambient
Code	G	LRC: 5.6-6.3 kVA/HP
PF	0.88	Full-load power factor
Eff.	91.0%	NEMA Premium efficiency

Field engineering decisions from this nameplate:

1. Overload relay: 21A x 1.15 = 24.2A (NEC 430.32 permits 125% of FLA when SF is 1.15 or higher)
2. Branch circuit conductor: 21A x 1.25 = 26.3A minimum, select #10 AWG at 75°C (30A rated)
3. Starting current: 113A locked rotor, verify transformer capacity can handle the inrush without pulling voltage below 85% on adjacent loads

25 HP Motor Nameplate: Worked Example

Parameter	Value	What It Means
HP	25	Rated shaft output (18.6 kW)
Volts	230/460	Delta at 230V, wye at 460V
Amps	68/34	FLA: 68A at 230V, 34A at 460V
RPM	1760	Full-load speed, 4-pole
Frame	284T	17.75" shaft height
Ins. Class	F	Max 155°C winding temp
S.F.	1.15	28.75 HP permissible
Code	G	LRC: 5.6-6.3 kVA/HP
PF	0.87	Full-load power factor
Eff.	93.0%	NEMA Premium efficiency

At 460V, the 25 HP motor draws 34A: 62% more current for 67% more output power. The close correspondence between those two percentages reflects the efficiency gain from 91% to 93% at the larger frame size. Larger motors are generally more efficient, and the 25 HP tier benefits from lower I²R losses relative to output.

Starting current (Code G):

$$\text{LR kVA} = 25 \times 6.0 = 150 \text{ kVA}$$ $$I_{LRC} = \frac{150{,}000}{1.732 \times 460} = 188 \text{ A} \approx 5.5 \times \text{FLA}$$

Branch circuit conductor: 34A x 1.25 = 42.5A, select #8 AWG at 75°C (50A rated).

For reference, overload relay setting: 34A x 1.15 = 39.1A (SF 1.15, same NEC 430.32 logic as the 15 HP).

Lenze Motor Nameplate: IEC Format

Lenze motors turn up frequently on imported conveyors, packaging lines, and servo systems. The IEC nameplate differs from NEMA in several fields that matter for replacement and protection.

Parameter	NEMA (North America)	IEC/Lenze (Europe)
Power	Horsepower (HP)	Kilowatts (kW)
Frame size	215T, 284T (inches)	IEC 160M, 160L (mm)
Voltage (60 Hz)	460V	460 delta
Voltage (50 Hz)	—	400 delta / 690 wye
Efficiency class	NEMA Premium	IE2, IE3, IE4
Power factor label	PF	cos phi
Duty cycle	Cont.	S1
Insulation class	F, H	Same letters, same limits

An 11 kW Lenze motor is the IEC equivalent of a 15 HP NEMA motor (11 / 0.746 = 14.75 HP). The dual voltage rating 400 delta/690 wye means delta connection at 400V for standard European distribution, or wye at 690V for high-voltage industrial supplies.

When replacing a Lenze motor with a NEMA motor, shaft output and speed can be matched, but mounting dimensions cannot. IEC 160M and NEMA 215T are not bolt-compatible: an adapter plate is required.

Common Mistakes When Reading Motor Nameplates

Using the wrong FLA for a dual-voltage motor

On a 460V system, the overload relay gets set to 21A, not 42A. Set it to 42A and the motor can run 100% overloaded before protection activates. Both values appear on the nameplate; the smaller one applies at the higher voltage.

Ignoring insulation class in high-ambient locations

A Class F motor in a 55°C enclosure has 100°C of temperature rise budget, not 105°C. That 5°C reduction sounds minor but it is the difference between a motor with adequate thermal margin and one that runs right at its limit under full load. Class H is the correct choice for sustained elevated ambient conditions.

Overlooking the code letter

The code letter determines locked rotor kVA, which feeds directly into NEC Table 430.52 for breaker selection and into transformer inrush calculations at startup. Skipping it leads to nuisance tripping on motor start or undersized protection that passes inspection but fails in service.

Assuming IEC and NEMA frames are physically interchangeable

Same kilowatt rating, same shaft speed, different bolt pattern and dimensions. An IEC 160M motor does not drop into a NEMA 215T mounting without an adapter plate. Check frame compatibility before ordering any replacement, especially on older European-origin machinery.

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Credits

• Photo by MChe Lee on Unsplash