25 hp Pump Motor: Nameplate vs. Table, and a 'Temporary' 150 A Breaker
A 25 hp, 480 V, 3-phase TEFC pump motor is installed on a rooftop mechanical platform. Nameplate data: 32 A FLA, service factor 1.15, 40 degrees C ambient, code letter G. The branch circuit is 60 ft of EMT with three current-carrying THWN-2 copper conductors and a copper equipment grounding conductor; all terminations are rated 75 degrees C. The design shows 8 AWG copper branch-circuit conductors, a 90 A inverse-time circuit breaker, a 40 A overload relay, and a 60 A nonfused disconnect mounted on the platform railing. During commissioning the breaker tripped once on a locked-rotor start, and the crew wants to install a 150 A breaker 'temporarily' to get the pump running. They also ask whether the disconnect is required to be within sight of the motor.
1. Verify the branch-circuit conductors and the overload relay sizing. Which ampere value is used for each?
Read the worked answer
The trap is using the wrong ampere value for each calculation.
Conductors - 430.6(A)(1) requires the Table 430.250 value, not the nameplate.
- Table 430.250 for a 25 hp, 460 V, 3-phase motor = 34 A.
- 430.22 requires 125 percent: 34 A x 1.25 = 42.5 A.
- 8 AWG copper at 75 degrees C = 50 A, so 8 AWG is acceptable.
Overload relay - 430.32(A)(1) uses the nameplate FLA.
- Service factor 1.15 means the 125 percent factor applies: 32 A x 1.25 = 40 A.
- A 40 A overload relay is correctly sized (and 40 A is a standard rating, so no rounding question arises).
Summary rule to carry into the exam: Table 430.250 governs conductors and short-circuit/ground-fault protection; nameplate current governs overload protection. Mixing them is the classic motor-calculation error.
2. Evaluate the 90 A breaker and the crew's request to substitute a 150 A breaker.
Read the worked answer
The 90 A breaker is correct as designed; 150 A is not permitted.
- 430.52(C)(1) and Table 430.52 for an inverse-time breaker on a squirrel-cage motor: 250 percent of 34 A = 85 A.
- 85 A is not a standard rating (240.6), so the next higher standard rating, 90 A, is permitted.
- The exceptions in 430.52(C)(1) allow an increase where the motor will not start, but they cap the device. A 150 A device on 34 A FLA is roughly 441 percent of FLA, above the 400 percent ceiling that applies to inverse-time breakers under those exceptions. The maximum available would be about 136 A, and even that would require a documented starting problem.
Why the crew is wrong on the merits:
- The breaker tripping once on a locked-rotor start is a symptom. Check supply voltage and voltage drop on the 60 ft run, phase imbalance, mechanical binding, and the code-letter G locked-rotor kVA (which is high enough to explain the trip).
- Oversizing the breaker removes short-circuit protection from the 8 AWG conductors without solving the starting problem.
- The overload relay, not the breaker, is what protects the motor against overload.
Answer to the crew: do not install the 150 A device; troubleshoot the starting condition first.
3. Size the disconnecting means and state whether it must be within sight of the motor.
Read the worked answer
Rating:
- 430.110(A) requires the disconnecting means to have an ampere rating of at least 115 percent of the motor FLA: 34 A x 1.15 = 39.1 A.
- The 60 A nonfused disconnect satisfies the ampere requirement. Confirm it carries the correct horsepower rating for 480 V, 25 hp (430.109 and the disconnect's marking) - ampere rating alone is not sufficient for a motor disconnect.
Location:
- 430.102(B) requires a disconnecting means within sight of the motor and the driven machinery, and it must be readily accessible. 'Within sight' means visible and not more than 50 ft away.
- Exception: a lockable disconnecting means that is out of sight is permitted where it can be locked open, and the arrangement is acceptable to the AHJ.
Conclusion: the platform-railing location is fine if it is within sight and readily accessible; if not, either relocate it or use a lockable disconnect meeting the exception.