The full load amps (FLA) of a US Motors D20P1G 575V motor is the exact continuous current the stator windings draw when the shaft is delivering its rated 20 horsepower mechanical output at 575 volts. If you are wiring this 20 HP, 3-phase, 1800 RPM TEFC (Totally Enclosed Fan Cooled) workhorse—now manufactured by Nidec under the US Motors brand—that single number on the nameplate dictates your entire electrical infrastructure. It is the anchor point for every protective device and conductor you install between the panel and the motor junction box.
How 575V Full Load Amps Dictate Your Installation
In a real circuit, the FLA changes everything about your material list and safety margins. It is not the maximum current the motor can ever draw (that would be locked rotor current), but rather the thermal baseline used to size conductors and overload heaters. According to NFPA 70 (NEC) Article 430, motor circuits are sized differently than standard resistive loads because of the massive inrush currents required to spin a rotor up to synchronous speed.
Here is a worked numeric example for installing a standard US Motors D20P1G with a nameplate FLA of 21.0A and a Service Factor (SF) of 1.15:
1. Conductor Sizing (NEC 430.22)
Branch circuit conductors must have an ampacity of at least 125% of the motor’s FLA.
Calculation: 21.0A × 1.25 = 26.25 Amps.
Action: You must select a wire with an allowable ampacity of at least 26.25A. Looking at the 75°C column of NEC Table 310.16 (which governs most motor terminal lugs), 10 AWG THHN copper is rated for 35A, making it the correct, code-compliant choice. Do not use 12 AWG, even though its 25A rating seems close; it falls short of the 26.25A minimum.
2. Short-Circuit and Ground-Fault Breaker (NEC 430.52)
Standard breakers would trip instantly on motor startup due to inrush current. Therefore, the NEC allows significantly larger inverse-time circuit breakers. The maximum rating for an inverse-time breaker is 250% of the FLA.
Calculation: 21.0A × 2.50 = 52.5 Amps.
Action: Since 52.5A is not a standard breaker size, NEC 430.52 permits you to round up to the next standard size, which is 60 Amps. However, if the motor starts successfully without tripping, many engineers will specify a 50 Amp breaker to provide tighter short-circuit protection while still accommodating the starting surge.
3. Overload Protection (NEC 430.32)
Overloads protect the motor from burning out under sustained mechanical strain. For a motor with a 1.15 Service Factor, the overload is sized at 125% of FLA.
Calculation: 21.0A × 1.25 = 26.25 Amps.
Action: Set your motor starter’s electronic overload relay or select your bi-metallic heater elements to trip at 26.2A. If the motor draws 27A for more than a few minutes, the overload will drop the contactor to save the windings.
Where You Meet the D20P1G in Practice
The US Motors D20P1G is a heavy-duty, general-purpose NEMA frame motor (typically a 254T or 256T frame for 20HP at 1800 RPM). You will almost exclusively encounter the 575V variant in Canadian industrial facilities, mining operations, and large-scale municipal water treatment plants.
Why 575V? In North America, utility transformers often output 600V, but utilization equipment like motors is nameplated at 575V to account for a standard 4% voltage drop across the facility’s distribution wiring. Running a 20HP motor at 575V instead of 230V cuts the current draw by more than half. This drastically reduces $I^2R$ heating losses in the cables, allows for much longer feeder runs without unacceptable voltage drop, and shrinks the physical size of the conduit and switchgear required.
FLA vs. LRA vs. SFA: Clearing Up Nameplate Confusion
The most common mistake junior electricians and maintenance techs make is confusing Full Load Amps with other current metrics printed on the motor nameplate. Sizing a breaker based on the wrong metric will either result in nuisance tripping or a burned-out stator.
| Metric | Approx. Value (20HP 575V) | What It Actually Means | What It Is Used For |
|---|---|---|---|
| FLA (Full Load Amps) | 21.0 A | Current at rated 20HP mechanical output. | Sizing wires, overloads, and baseline thermal limits. |
| LRA (Locked Rotor Amps) | ~126.0 A | Current drawn if the shaft is physically prevented from turning at startup. | Sizing short-circuit breakers and verifying voltage drop during starting. |
| SFA (Service Factor Amps) | ~24.1 A | Current drawn when the motor is pushed to its 1.15 SF limit (23 HP). | Setting the absolute maximum trip threshold for overload relays. |
| NLA (No-Load Amps) | ~6.5 A | Current drawn when spinning freely with no mechanical load attached. | Troubleshooting; if unloaded current is high, the rotor may be rubbing or windings are shorted. |
Frequently Asked Questions
What size breaker do I need for a US Motors D20P1G 575V motor?
For a 21.0A FLA motor, the NEC allows an inverse-time circuit breaker up to 250% of the FLA (52.5A). Because 52.5A is not a standard size, you can legally install a 60A breaker. However, if your facility experiences minimal voltage sag during motor starts, a 50A breaker is highly recommended as it provides tighter protection against short circuits while still clearing the starting inrush.
Can I run the D20P1G 575V motor on a 480V supply?
No, not directly. A 575V motor connected to a 480V supply will experience a 16% voltage deficit. To maintain its 20HP mechanical output, the motor will draw significantly more current (likely exceeding 25A continuously), causing the windings to overheat and the overloads to trip. If you must run this motor on a 480V system, you need to install a step-up transformer or use a Variable Frequency Drive (VFD) configured to output 575V.
Why is my 20HP 575V motor drawing more than 21 amps?
If your clamp meter reads 24A or higher while the motor is under its normal mechanical load, investigate these three culprits in order:
1. Low Supply Voltage: If your 575V system is sagging to 540V under load, the motor will draw excess current to compensate. Check the voltage at the motor junction box while it is running.
2. Mechanical Binding: Misaligned couplings, failing pump bearings, or clogged HVAC dampers force the motor to work harder than its rated 20HP, pushing it into the Service Factor range.
3. Single-Phasing: If one of the three phases is lost due to a blown fuse or broken contactor pole, the remaining two phases will spike to dangerous current levels to keep the rotor spinning. Your overload relay should catch this, but it is a leading cause of motor death.
