Lesson 22 · Field Skills

Reading a Motor Nameplate

The numbers stamped on a motor tell you how to wire it, protect it, and replace it. Here’s how to read every one that matters.

The motor’s ID card

Every motor has a metal nameplate stamped with the numbers you need to wire, protect, and replace it. Learning to read it is a core field skill — these numbers drive your overload sizing, conductor sizing, and whether the motor even fits the job.

The key fields

Voltage: what the motor is built for (e.g. 230/460V — a dual-voltage motor you connect one way for 230, another for 460). Match it to your supply.
FLA / FLC (Full-Load Amps/Current): the current it draws at full rated load. This is the number you size overloads and conductors from. Arguably the most important figure on the plate.
HP (horsepower): mechanical output rating.
RPM: full-load speed — tells you the pole count and roughly how fast the shaft turns (e.g. ~1750 = 4-pole, ~3500 = 2-pole at 60Hz).
Phase & Hz: single or three phase, and frequency (60 Hz in North America).
Service Factor (SF): how much over its rating it can run continuously (1.15 SF = 15% overload capacity). Affects overload sizing.
Frame: the standardized physical size/mounting (e.g. 145T) — so a replacement bolts up the same.
Code letter / Design letter: indicate starting inrush and torque characteristics.
Insulation class & temp rise: how much heat the windings tolerate.
Duty: continuous or intermittent.
Efficiency / Power factor: for energy and sizing calcs.

The big three for wiring: voltage (connect it right), FLA (size overloads and wire), and service factor (fine-tune the overload). Get those and you can power and protect the motor correctly.

Dual-voltage connections

Many motors run on two voltages depending on how their leads are connected (often shown by a connection table or diagram on the plate or in the peckerhead). Wire the leads for 230 or 460 per that diagram — get it wrong and you either get no torque or a burned motor. Always follow the plate’s connection table.

What to take away

The nameplate tells you everything to wire and protect a motor: voltage (and how to connect it), full-load amps (for overloads and wire), HP, RPM, service factor, frame, and more. Reading it confidently is foundational — the FLA and voltage in particular drive your whole installation.

Concept lesson: this one is about understanding equipment and ideas rather than wiring a circuit, so there’s no Sandbox build for it. When you set a Motor{A}s voltage in the Sandbox, you{A}re doing what the nameplate voltage tells you to match.