Nuisance Tripping on a Lift Station Pump Motor
An unattended wastewater lift station has a 10 hp, 208 V, three-phase, 3,500 rpm submersible pump motor started across the line by a NEMA combination starter with an adjustable thermal overload relay and a molded-case circuit breaker. The nameplate reads 26.0 A, service factor 1.15, 60 Hz, continuous duty, 40 °C ambient, code letter F. The supply is 208Y/120 V with 75 °C terminations throughout, and 8 AWG THHN copper is already installed from the MCC. The motor trips on overload about 25 minutes into each pumping cycle. The mechanical seal is new, the impeller turns freely by hand, and the replacement starter was selected 'to match the wire.'
1. Verify the branch-circuit conductors, overload setting, short-circuit protection, and disconnecting means.
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Motor FLA for sizing - Table 430.250, 10 hp at 208 V = 30.8 A. (Table value governs conductor and short-circuit sizing, not the nameplate.)
Branch-circuit conductors - 430.22: 125% × 30.8 = 38.5 A. 8 AWG THHN at 75 °C = 50 A. Compliant.
Overload - 430.32(A)(1): for a continuous-duty motor with a service factor of 1.15 or greater, the overload must not exceed 125% of the nameplate current: 26.0 × 1.25 = 32.5 A. The relay element selected must not be larger.
Short-circuit and ground-fault protection - 430.52(C)(1), inverse-time breaker at 250%: 30.8 × 2.50 = 77 A. 77 A is not a standard size; the next higher standard rating (240.6) is 80 A, permitted by Exception No. 1 to 430.52(C)(1). The 400% ceiling for an inverse-time breaker is not approached.
Disconnecting means - 430.110: not less than 115% of FLA = 35.4 A, so a 40 A rated disconnect. The combination starter's disconnect must meet this.
Note the two different reference currents: 30.8 A for conductors, breaker and disconnect; 26.0 A for the overload.
2. At the trip point the tech records line currents of 31.2 A, 30.9 A and 31.4 A, and line voltages of 208 V, 206 V and 209 V. The maintenance lead proposes increasing the overload setting and the breaker. Evaluate that proposal.
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Do not increase the overload - the relay is doing exactly its job.
- Service-factor amps = 26.0 × 1.15 = 29.9 A. The motor is running at about 119% of nameplate and above its service-factor limit on all three phases.
- The voltage unbalance is negligible: average = (208 + 206 + 209)/3 = 207.7 V; maximum deviation = 1.3 V; unbalance = 1.3/207.7 ≈ 0.6%. That is far below the 1% level at which NEMA derating becomes a practical problem, so voltage quality is not the cause.
- Balanced overcurrent on all three phases with a balanced supply points to a mechanical/hydraulic cause: pump running out on its curve (excess flow, worn or mis-trimmed impeller), a partially plugged or throttled discharge, debris in the suction, or a change in specific gravity. The electrical measurements are the symptom, not the fault.
The overload is already at the 125% maximum for the nameplate; raising it removes the motor's overload protection and converts a nuisance trip into a burned winding. Raising the 80 A breaker is worse - that breaker protects the circuit, not the motor, and 430.52 already allowed the next size up. The correct response is to correct the hydraulic condition, then re-verify that running current falls to or below the nameplate rating.
3. The combination starter is at the MCC, 60 ft from the motor and not within sight of it. Is a separate disconnecting means required at the motor?
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430.102(B) requires a disconnecting means within sight of the motor and driven machinery, unless the exception is satisfied. The exception permits the disconnect to be located elsewhere if it is capable of being locked in the open position and the arrangement is under the control of qualified persons.
Because the station is unattended and the motor is out of sight of the MCC, the practical compliance requirements are:
- A lockable disconnect at the MCC (a lockable combination starter disconnect qualifies), with a documented lockout/tagout procedure.
- Confirmation that the lockout point is accessible only to qualified persons and that the procedure addresses the submersible's location.
If those conditions cannot be met, a local disconnecting means within sight of the motor is required. This is also the point where 430.102(A) - the controller location - must not be confused with the disconnect location; they are separate rules.