P033 [Motor OL Current] on a PowerFlex 525 holds exactly one number. Land two 3 HP motors on T1, T2 and T3 and that number is now watching the pair as though they were one 6 HP machine, which leaves neither motor protected.
That single parameter is the whole argument. Two standard induction motors will run from one drive, and Siemens puts it plainly for the G120: multi-motor operation is generally permissible for standard induction motors. What you give up is the drive’s motor protection, sensorless vector control and any usable autotune. What you take on is a cable problem that grows with every branch, and a set of sequencing rules for whatever you fit to replace the protection you lost. If the panel is being built to UL, the answer gets shorter still, and there is a sentence in the G120C manual further down that ends the conversation.
What the drive stops protecting
P033 sets the motor nameplate overload current, anywhere from 0.1 A to twice the drive’s rated current, and the drive faults F007 “Motor Overload” when the I²t counter built on that number runs out. Rockwell ties it to something specific rather than leaving it vague: class 10 motor overload protection per NEC article 430, motor over-temperature protection per NEC 430.126(A)(2), UL 508C File 29572. A493 [Motor OL Select] picks the derating curve — No Derate is the default, with Min and Max Derate pulling the allowed current down as output frequency falls — and A494 [Motor OL Ret] decides whether the counter survives a power cycle or starts again from cold. It is a real overload relay, in firmware, fed by the current the drive measures at its own output terminals. The G120 does the same job with a thermal model instead of a single trip curve. Thermal motor model 2 for induction motors is a three-mass model of stator core, stator winding and rotor, and p0610 [Motor overtemperature response] decides what happens when the model reaches the alarm threshold. Its factory setting is 12, which messages and stores the temperature without reducing I_max. The stored model temperature is what lets Siemens meet UL 508C with no sensor in the winding at all.
Both of those measure one thing. Total output current, at the drive.
Neither has any way to split it between two machines.
Picture the pair, then. Two identical 3 HP motors, drive rated 12 A, P033 set to 8.4 A because somebody added the nameplates together. Motor B’s conveyor jams and its rotor locks. Motor B is now pulling something close to its locked-rotor current while motor A idles at half load, and the sum at the drive terminals can sit under 8.4 A for a very long time. The counter never moves. Set P033 to one motor’s 4.2 A instead and you get the opposite failure: the drive trips on normal combined load, somebody decides the parameter must be wrong, and it quietly goes back up to 8.4 A by the end of the week. That is the loop this ends in, and it is the first thing everybody tries. Halving the number, doubling it, splitting the difference — there is no value of P033 that protects two motors, because the quantity it would need does not exist anywhere in the drive.
Motor B cooks.
A484 [Current Limit 1] has the same shape of problem, and it is worth saying out loud because people reach for it as a substitute for the overload. It defaults to drive rated amps × 1.1 on normal duty and 1.5 on heavy duty, and it is one limit applied to the whole output, so it clamps the pair rather than the machine that is in trouble. The same goes for A486 and A488 [Shear Pin1 and 2 Level], which fault on a current you set and a time you set, both of them measured on the sum.
It clamps the pair. It never clamps the one that is stalled.
The overload you now have to fit
Each motor gets its own.
That is what the motor manufacturers say when somebody rings and asks, and it is the answer here too. The question that follows it is always the same one: fit what, exactly, and where in the circuit.
A thermal overload relay with its own contactor per motor is the ordinary arrangement, and a motor protection circuit breaker per branch is the other. Both work. What kills them is the way they get used, and this is the failure that shows up on the forums more than any other: somebody fits an MPCB per motor downstream of the drive, the MPCB opens while the drive is running at 45 Hz into a loaded motor, and the contacts weld or erode. One thread on r/PLC describes exactly that, two motor circuit protectors on a drive output, tripping and then burning up. Interrupting a PWM output under load is not what those contacts were built to do, and the drive does not enjoy it either. The fix is sequencing rather than hardware. Wire the auxiliary contact of each device back to the PLC, drop the drive’s start bit on the way out, and let the drive stop the output before the branch device opens. A stop from the drive takes as long as your decel ramp, and the branch device then opens into nothing.
Motor thermistors are the other half of it, and they cost nothing when the motors already have them fitted. On the G120, p0601 selects the sensor type: 1 for a PTC, 4 for a bimetallic NC contact. One Control Unit has one temperature input, so a pair of motors means the PTCs go in series and you lose which one tripped. That is annoying and still better than nothing. If the machine matters, the thermistor chain goes to its own relay with a contact per motor and the PLC gets both.
Why sensorless vector goes away
P039 [Torque Perf Mode] on the PowerFlex 525 defaults to 1 “SVC”. Option 0 is “V/Hz”.
That is where two motors belong.
Rockwell does not print a line forbidding SVC with two motors on the output. What it prints is what autotune actually does, which is more useful. P040 [Autotune] runs a static or rotating measurement and writes A496 [IR Voltage Drop] and A497 [Flux Current Ref] from what it finds. Run it with two motors wired up and what it measures is the parallel combination of two stators — a machine that does not exist, with roughly half the resistance of either real motor. Vector control then regulates a model of that imaginary machine, and P036 [Motor NP RPM], which the drive uses to calculate rated slip, holds one figure serving both. A rotating tune wants the motor uncoupled from the load in any case, which nobody is going to do to two coupled conveyor drives, and if it goes wrong you get F080 Autotune Failure and an hour gone. Set P039 = 0 and use A530 [Boost Select] to get the starting torque back. That is the trade: open loop, a boost setting you tune by watching current on the keypad, and no slip compensation worth the name. Siemens draws the same line from the other direction. p1300 [Open-loop/closed-loop control operating mode] runs 0 to 22, where 0 is U/f with a linear characteristic and 20 is encoderless speed control. The rotating motor data identification, p1900 = 1 or 3, is documented as not available for p1300 < 20. Choose U/f and the rotating measurement is simply not on the menu.
One more is easy to miss.
The G120 operating instructions state that the flying restart function must not be enabled when the inverter is driving several motors, with a single exception: a mechanical coupling that guarantees all the motors always run at the same speed. If you were relying on p1200 to pick up a coasting fan after a brownout, it comes out of the design, and you find out on the first power dip rather than at commissioning.
The cable is now the sum of both runs
Two motors means two cable runs.
The drive sees them added together.
Figure 11 in DRIVES-IN001 is worth looking at once, because it puts four arrangements side by side and every one of them is 182.9 m (600 ft) of motor cable. One motor on 600 ft. Two motors at 300 ft each. One motor at 50 ft and one at 550 ft. A 500 ft trunk to a junction box with 50 ft out to each motor. Same total, same load on the drive output, and the note on that page is direct about what happens when the total gets away from you: high-peak cable charging currents can cause drive overcurrents or ground faults, and Rockwell asks you to talk to a drive specialist before going past two motors at all.
Hold on to that when a F013 Ground Fault turns up on a drive that has run for three years without one, a week after a second motor was added.
The insulation is usually fine. The charging current is not.
The exact length you are allowed depends on the drive frame, the horsepower, the carrier frequency and the motor’s insulation rating, and Appendix A of DRIVES-IN001 carries the table for the 520-series. A440 [PWM Frequency] defaults to 4.0 kHz and ranges 2.0 to 16.0 kHz on the 525, and the lead-length tables are indexed by that setting, so dropping the carrier buys you cable at the price of audible noise out of the motors. Two other things from 520-UM001 matter more with two runs than with one. Do not route more than three sets of motor leads in a single conduit, and where more than three drive-to-motor connections share a conduit, shielded cable is required rather than recommended. And watch which shielded cable: the constructions that twist four THHN conductors and wrap the bundle tightly in foil raise the charging current enough that the published distances no longer apply to them.
When it is genuinely fine
Identical motors.
Same frame, same nameplate, same manufacturer where you can manage it.
Permanently coupled, or driving one load in a way that ties their speeds to each other — the two motors on a vibrating screen, a pair of fans in one duct, twin drives at either end of a long conveyor. Siemens names exactly this case as the exception to the flying-restart rule, which tells you what shape of application the vendor considers safe.
Same duty, both of them. Both loaded, both unloaded, both varying together. A pair where one runs hard and the other idles is the case that eats motor B, and no drive parameter will catch it.
Starting and stopping together, always.
If there is any requirement to run one without the other, you are switching contactors on a live drive output, and you are back in the paragraph about welded contacts.
And one that gets forgotten until the machine is built: both motors have to turn the same way. A PLCtalk thread from October 2024 records an engineered system that arrived on site before anyone noticed two of the motors needed to reverse. One drive gives you one direction command. Swapping two phases at the motor terminal box fixes it, if somebody catches it while the drawing is still on a screen.
The sentence that ends it, if you are building to UL
Under “Installation in the United States and Canada (UL or CSA)”, the SINAMICS G120C operating instructions list the measures for a compliant installation, and one of them reads: a multi-motor drive is not permissible, meaning several motors connected to one converter at the same time.
That is the G120C manual, not every drive and not every standard, so read the manual for the converter in front of you before you argue with an inspector. But it tells you what position a vendor takes when the listing is on the line, and it is the reason a lot of these questions end with two drives in the panel instead of one.
What to do next
Read P033 on the drive you have. If two motors are already on it, work out what number is in there and what it would have to be to protect either one of them on its own, and you will know whether this installation was designed or inherited.
If you are still specifying, price the second drive before you price the two overload relays, the two contactors, the two auxiliary contacts and the extra wiring. A PowerFlex 525 in a small frame often loses that comparison, and you keep the drive’s own motor protection, the autotune and the vector mode.
If it is going ahead anyway: P039 = 0, an overload per motor with its aux contact into the PLC, branch devices sequenced behind the drive’s stop, and the two cable runs added together before you check them against Appendix A. Then, when you come to drive it from the controller, remember that one start bit now starts two machines.
Sources used: Rockwell Automation 520-UM001 (PowerFlex 520-Series user manual, revision O, September 2025) and DRIVES-IN001 (wiring and grounding for PWM AC drives, revision Q, June 2019); Siemens SINAMICS G120 CU240B-2/CU240E-2 operating instructions (04/2018, A5E34259001B AF), the matching list manual (02/2023, A5E33839529) and the SINAMICS G120C operating instructions (10/2020, A5E34263257B AJ). Motor basics are covered separately in Electric Motors.