No standard sets a maximum horsepower for a 480 V motor. The 200 hp figure that circulates as a limit is a plant-level policy, and the physical limits that do exist are line current, starting kVA on the supply transformer, and how many conductors fit in a termination.
Line current at 480 V as the deciding quantity
Full-load current for a three-phase motor is I = (746 x HP) / (sqrt(3) x V_LL x efficiency x power factor). Voltage sits in the denominator, so moving from 480 V to 4160 V cuts line current by the voltage ratio (about 8.7x). The table uses assumed values of 0.95 efficiency and 0.88 power factor (combined 0.836). These are illustrative assumptions. Replace them with the motor nameplate or datasheet values.
| Motor size | Full-load current at 480 V (A) | Full-load current at 4160 V (A) |
|---|---|---|
| 150 hp | about 161 | about 19 |
| 200 hp | about 215 | about 25 |
| 500 hp | about 537 | about 62 |
| 1000 hp | about 1073 | about 124 |
| 1500 hp | about 1610 | about 186 |
At 1500 hp on 480 V the feeder carries roughly 1600 A. That requires multiple parallel conductors per phase, and the lug space, bending radius, and termination labor become the limit. This matches the practical ceiling of about 1500 hp reported for 480 V systems, where cable becomes unwieldy and very difficult to terminate. Read the conductor count from the ampacity table your local electrical code applies, at the actual insulation temperature rating and installation method.
Reported cutoffs and what each one actually is
The numbers engineers quote fall into three different categories: plant policy, economics, and physical limits. Treating them as one rule is what makes 200 hp look like a standard.
| Figure | What it is | Category |
|---|---|---|
| 150 hp, 200 hp | Internal edicts at plants that already run 4160 V distribution (all motors at or above the cutoff go to 4160 V) | Plant policy |
| 500-600 hp | Usual point where going to medium voltage pays off in general practice | Economics |
| 500 hp at 480 V | Fairly common | Availability |
| 1000 to 1500 hp at 480 V | Possible at a price premium | Economics |
| About 1500 hp | Practical 480 V ceiling because of cable size and termination | Physical |
| 2500-3000 kVA | Usual upper size of the 480 V supply transformer | Physical / supply |
Small motors are the reason 150 to 200 hp edicts exist only in plants that already have 4160 V running. In most other places, medium voltage for motors that small is impractical, and the cutoff sits at 500-600 hp.
Transformer size and start voltage dip
The real bound on a 480 V motor is often the supply transformer, not the motor. A motor draws its locked-rotor current during acceleration, and that current flows through the transformer impedance. The resulting voltage sag hits the motor terminals and every other load on the bus. Two checks apply, both taken from field practice.
- Rule of thumb: transformer kVA greater than 3 x the largest motor hp. At the 2500-3000 kVA transformer sizes typical of 480 V systems, that rule caps the largest motor at about 833 hp (2500 kVA) to 1000 hp (3000 kVA). This is a derived screen, not a code limit.
- Dip check: with the transformer loaded to 80% or more, the voltage at the other running consumers must not fall below the permissible level during the start. A working figure is 10% under rated.
The bus voltage dip is approximately S_start / (S_start + S_sc) x 100, where S_sc is the short-circuit kVA at the motor bus. For a transformer-only source, S_sc is roughly transformer kVA divided by its per-unit impedance, so read %Z from the transformer nameplate. Use a motor-starting study for the final answer, especially where reduced-voltage starting or a soft starter is on the table.
Comparing 480 V and 4160 V on the criteria that decide the case
| Criterion | 480 V favored when | 4160 V favored when |
|---|---|---|
| Motor purchase price | One comparison of 1000 hp compressors found 480 V motors somewhat cheaper than 4160 V | Motor is above 1000-1500 hp or the 480 V premium is high |
| Starter and circuit cost | Unit substation can sit close to the motors, so circuits are short | Long runs make 480 V feeders large and costly |
| Existing plant voltage | Plant is all 480 V | Plant already runs 4160 V; staying with the established standard is usually better |
| Motor count above 200 hp | Few large motors | Large number of motors above 200 hp; higher voltage is generally more economical |
| Maintenance capability | Staff is not trained on medium voltage and training would be a significant cost | Staff already qualified on MV |
| Procurement lead time | Standard 480 V gear is available quickly, including for future replacements | MV lead times are acceptable |
| Supply transformer | Largest motor passes the kVA and dip checks | Largest motor fails the dip check on a 2500-3000 kVA class transformer |
Recommended decision path
Stay with the plant's established voltage unless a physical limit forces a change. Where the plant is greenfield or mixed, choose 480 V for motors up to the range where transformer kVA and cable termination still work (up to about 1500 hp is possible, with 500 hp common), and choose 4160 V when a large group of motors exceeds 200 hp, when the largest motor fails the start-dip check, or when the feeder would need an impractical number of parallel conductors. A 1000 hp compressor group with a nearby unit substation is a documented case where 480 V won on cost.
Procedure for qualifying one motor at 480 V
- Record the plant voltage standard and any internal hp cutoff (150 hp and 200 hp policies exist). A written edict overrides the economic comparison.
- Calculate full-load current from nameplate hp, voltage, efficiency, and power factor using the formula above, then size feeder conductors and count parallel sets from the applicable code ampacity table.
- Read locked-rotor current or kVA code from the nameplate and calculate start kVA.
- Screen the transformer: kVA greater than 3 x motor hp, then calculate the dip with the transformer at its expected running load (80% or more is the demanding case).
- Check the dip at the other loads' terminals against the permissible level (10% under rated as a working figure). If it fails, evaluate a reduced-voltage starter, a larger or lower-impedance transformer, or 4160 V.
- Confirm that the largest lug and conductor bundle physically terminate in the starter and motor terminal box.
- Price the motor, starter, circuit, and lead time at both voltages, and add maintenance training cost if MV is new to the site.
Telling thermal faults from voltage-sag faults after commissioning
A motor on an undersized source fails in two distinct ways. Heat faults come from a long acceleration at locked-rotor current. Logic faults come from bus voltage collapsing during the start. Measure motor current and terminal voltage during the start to separate them.
| Symptom | Quantity past a limit | Likely cause | Where to read it |
|---|---|---|---|
| Overload trip during acceleration | Start time x locked-rotor current (thermal) | Sagging terminal voltage lowers accelerating torque, so acceleration takes longer | Motor current and voltage trend during start |
| Contactors chatter or drop out; PLC or control power resets | Bus voltage below drop-out level | Start kVA too large for transformer; dip exceeds the permissible level | Voltage at other loads' terminals during start |
| Transformer runs hot, protection trips | Transformer loading above rating | Motor group plus starting load exceeds kVA; 80% or more base loading | Transformer nameplate kVA versus measured load |
| Hot lugs or damaged insulation at the terminal box | Termination current density | Too many parallel conductors forced into the lug space | Thermal scan of terminations under load |
Confirm the selection before releasing the design
- Record the terminal voltage of the motor and of a sensitive downstream load during a full start. Compare with the permissible level used in the calculation.
- Compare running current against the nameplate full-load current at rated voltage.
- Record the transformer loading during the start and at steady state.
- Scan terminations under load for hot spots.
- Record the acceleration time. If the time is far above the study value, the source impedance is higher than assumed.
Can I run a motor above 200 hp on 480 V?
Yes. No standard sets a maximum hp for 480 V, and 500 hp motors are fairly common, with 1000 to 1500 hp possible at a cost premium. Confirm the transformer and start-dip checks pass, and that the feeder terminations physically fit.
Does the 3 x hp transformer rule limit my largest 480 V motor?
It works as a screening rule: transformer kVA should exceed 3 x the largest motor hp, so a 3000 kVA transformer supports about 1000 hp and 2500 kVA about 833 hp. It replaces neither a motor-starting study nor the check that other loads stay above the permissible voltage during the start.
Can I keep 480 V for a 1000 hp motor if the plant is otherwise 4160 V?
Yes, if the plant policy allows it, a unit substation can sit close to the motor, and the dip check passes. A 1000 hp compressor comparison found 480 V somewhat cheaper than 4160 V, but a plant edict, staff training, and lead time can still decide against it.
Stop and escalate when the calculated start dip is near the permissible limit, when the terminal count exceeds the lug space, or when the source impedance is unknown. Request the locked-rotor data and starting curves from the motor manufacturer, and the %Z and short-circuit data from the transformer manufacturer or the utility. Use their official support channels for the final starting study.