A NEMA 284T frame motor operating at 460 volts (often cataloged under specific manufacturer part numbers like 284TSTDBD6001) is a standardized industrial 3-phase AC induction machine where the "284T" dictates the physical mounting footprint and shaft height, while the 460V amp rating governs your entire electrical supply design. If you are pulling this motor off a shelf to replace a failed unit on a shop floor, understanding the relationship between its physical frame and its electrical demands is the difference between a drop-in replacement and a week-long rewiring nightmare.
This specific configuration changes everything about your branch circuit: it dictates your wire gauge, your short-circuit breaker size, and your thermal overload relay settings. The most common mistake hobbyists and junior technicians make is confusing the motor's 460V utilization rating with the facility's 480V supply voltage, or assuming the physical NEMA frame size (284T) directly correlates to a single fixed horsepower. In reality, a 284T frame can house a 10 HP motor at 1800 RPM or a 15 HP motor at 1200 RPM, and the amp draw shifts accordingly.
The 284T Frame and 460V Supply: What the Nameplate Actually Means
Before you strip a single wire, you need to understand the two halves of this motor's identity. The 284T designation comes from the NEMA MG-1 standard for Motors and Generators. The "284" defines the physical dimensions: a shaft height of 7.0 inches (the first two digits, 28, divided by 4) and a specific bolt-hole spacing on the mounting feet. The "T" indicates it uses standard T-frame dimensions introduced in the 1960s, meaning it will physically bolt down to the same base as an older U-frame motor of the same size, though the shaft diameter may differ.
The 460 volts rating is the motor's designed utilization voltage. Utilities and transformers supply 480V 3-phase power, but the National Electrical Code (NEC) and motor manufacturers design equipment for 460V to account for a standard 5% voltage drop across the facility's feeders and branch circuits. If you measure 485V at the panel but 455V at the motor terminals under load, the motor is operating exactly as designed.
Never size your branch circuit conductors strictly by the nameplate Full-Load Amps (FLA). NEC Article 430 requires you to use the standardized values in Table 430.250 for wire sizing, regardless of what the physical nameplate says. The nameplate FLA is only used for sizing the thermal overload protection.
NEMA 284T Typical Specs at 460V (Spec Sheet Table)
Because a 284T frame can be wound for different horsepower and speed combinations, the amp draw varies. Below is the critical reference data for the most common 284T 460V 3-phase motors you will encounter in the field.
| Motor Rating | Speed (RPM) | NEC Table 430.250 FLA | Typical Nameplate FLA | Min. Wire Size (75°C THHN) | Max Inverse-Time Breaker |
|---|---|---|---|---|---|
| 7.5 HP | 1800 | 11 Amps | 9.8 - 10.5 Amps | 14 AWG | 30 Amp |
| 10 HP | 1800 | 14 Amps | 12.5 - 13.5 Amps | 12 AWG | 35 Amp |
| 10 HP | 1200 | 14 Amps | 13.0 - 14.0 Amps | 12 AWG | 35 Amp |
| 15 HP | 1200 | 21 Amps | 19.0 - 20.5 Amps | 10 AWG | 50 Amp |
Worked Example: Sizing Wire and Breakers for a 10HP 284T
Let's run the exact math for a 10 HP, 1800 RPM motor in a 284T frame operating at 460V. The nameplate on your specific 284TSTDBD6001 unit reads 13.2 Amps FLA. Here is how you legally and safely size the circuit according to NEC Article 430.
- Conductor Sizing (NEC 430.22): You must size wires for 125% of the motor's full-load current. However, per NEC 430.6(A), you use the table value, not the nameplate. Table 430.250 lists a 10HP 460V motor at 14 Amps.
Calculation: 14A × 1.25 = 17.5 Amps. Looking at the 75°C column of NEC Table 310.16, 14 AWG THHN is rated for 20A, which covers 17.5A. However, for mechanical durability in industrial environments, 12 AWG THHN (rated 25A) is the standard minimum pull. - Short-Circuit Breaker (NEC 430.52): The breaker protects the wire from short circuits, not the motor from overloads. For an inverse-time breaker, the code allows up to 250% of the table FLA.
Calculation: 14A × 2.50 = 35 Amps. A standard 35A 3-pole breaker is the exact fit. (If 35A isn't a standard size in your panel lineup, NEC 430.52(C)(1) Exception 1 allows you to round up to the next standard size, which is 40A). - Thermal Overloads (NEC 430.32): This is the only place you use the nameplate value. If your motor has a 1.15 service factor, you size the overload heaters or dial the electronic overload to 125% of the nameplate 13.2A.
Calculation: 13.2A × 1.15 = 15.18 Amps.
Think of motor starting current (LRA) like a heavy freight train getting up to speed—it requires a massive surge of energy to overcome static inertia, which is why your breaker must tolerate a brief spike without tripping, while the thermal overload watches the long-term heat buildup.
Where You Meet 284T 460V Motors in Practice
You won't find a 284T frame motor in a residential garage unless you are running a massive industrial dust collector or a heavy-duty rotary phase converter. These are the workhorses of commercial and light industrial spaces. You will typically encounter them in:
- HVAC Chillers and Large AHUs: Driving the centrifugal fans or screw compressors in commercial rooftop units.
- Material Handling: Powering the main drive rolls on conveyor belts in packaging facilities.
- Woodworking and Metalworking: Spinning the 12-inch planers, wide-belt sanders, or manual lathe headstocks in production shops.
- Agricultural Pumps: Driving deep-well vertical turbine pumps or large horizontal irrigation boosters.
When replacing a failed unit, the 284T frame ensures the new motor's shaft is exactly 7.0 inches off the mounting base. If you accidentally order a 286T frame (which has a longer body and different bolt spacing) or a 256T frame (smaller footprint), you will be drilling new holes in your steel mounting plate or dealing with a misaligned belt drive that will shred belts in a week.
FAQ: Common 460V Motor Wiring Mistakes
Can I run a 460V motor on a 208V or 240V 3-phase system?
No. A 460V motor requires a 480V/460V 3-phase supply. If you only have 240V 3-phase, you must either buy a dual-voltage motor (wired in the Low Voltage / Delta configuration) or use a step-up transformer. Running a 460V motor on 240V will cause it to draw double the current, overheat rapidly, and trip your overloads instantly.
Why does the nameplate say 460V but the transformer says 480V?
This is an intentional design margin. The utility supplies 480V at the transformer secondary. By the time that power travels through the main breaker, down the busbars, and out through 100 feet of wire, voltage drop reduces it to roughly 460V at the motor terminals. The motor nameplate reflects the voltage it expects to see at the terminals under load, not the source voltage.
What does the "ST" or "DBD" mean in a catalog number like 284TSTDBD6001?
These are manufacturer-specific suffixes. "ST" often denotes Severe Duty or a specific insulation class (like Class F or H inverter-duty wire). "DBD" might indicate a specific enclosure type (like TEFC - Totally Enclosed Fan Cooled) or a specific bearing configuration. Always cross-reference the exact catalog number with the manufacturer's spec sheet to verify the enclosure rating (e.g., IP55 vs NEMA 1) before installing it in a washdown or dusty environment.
Do I need to bond the motor frame to ground if it's bolted to a grounded steel table?
Yes. Never rely on the mounting bolts for your equipment grounding conductor (EGC) path. Rust, paint, and vibration can break that metallic bond. You must run a dedicated copper EGC (sized per NEC Table 250.122, which requires a 10 AWG copper ground for a 35A breaker) inside the conduit and terminate it to the motor's designated green grounding lug.
