Well Pump Motor Branch Circuit and Overload Protection
A 2 hp, 230 V single-phase submersible well pump serves a home near Palmer. The nameplate reads 230 V, 12.0 A, SF 1.15. The pump is 180 ft from the panel, wired with 10 AWG copper in PVC conduit buried 24 in. The installer used a 30 A standard thermal-magnetic breaker and a 30 A fusible disconnect at the wellhead. The pump trips the breaker after 20–30 minutes of runtime on hot days. The homeowner reports low pressure and the pump cycling. The inspector questions the breaker size and the conductor ampacity at the 75°C terminals.
1. Determine the correct branch-circuit conductor size, short-circuit/ground-fault protection, and overload protection for this motor, and explain why the 30 A breaker trips.
Read the worked answer
Motor branch-circuit conductors must be at least 125% of the motor full-load current from the appropriate table (for a 2 hp 230 V single-phase motor, FLC ≈ 12 A; use table value, not nameplate, for sizing). 12 × 1.25 = 15 A minimum → 14 AWG copper would satisfy ampacity, but voltage drop over 180 ft favors 10 AWG. 10 AWG at 75°C = 35 A - fine.
Short-circuit/ground-fault protection (breaker) may be sized up to the percentages in the code table for inverse-time breakers - for a standard motor, up to 250% of FLC is permitted if the motor won't start otherwise. 12 × 2.5 = 30 A maximum - so the 30 A breaker is actually within the allowed range. However, the trip after 20–30 min suggests overload, not short-circuit: the breaker is acting as overload protection, which it should not.
Overload protection must be sized per the nameplate service factor: 12.0 A × 1.15 = 13.8 A; overload device set at ≤ 13.8 A (typically a heater or electronic overload in the controller). If the pump has no separate overload, the breaker is doing double duty and will nuisance-trip.
Diagnosis: The 30 A breaker is likely tripping due to running overload - low pressure/cycling suggests a failing pump, clogged intake, or worn impeller drawing above FLC, or a voltage-drop-induced current rise. Check actual running amps with a clamp meter; if > 13.8 A, the pump is the problem, not the breaker. Also verify the disconnect is rated and that the 10 AWG is not undersized for the actual run.
2. The installer proposes simply increasing the breaker to 40 A to stop tripping. Evaluate this proposal.
Read the worked answer
Reject it. The breaker is already at the maximum allowed for short-circuit protection (250% of 12 A = 30 A). Increasing to 40 A exceeds the code maximum and, more importantly, removes any overload protection the breaker was incidentally providing. The motor's overload protection must come from a separate overload device set at ≤ 13.8 A (nameplate FLC × SF).
A 40 A breaker on 10 AWG (35 A at 75°C) would also overprotect the conductor - the conductor's ampacity would be exceeded before the breaker trips, creating a fire hazard. 10 AWG is not permitted on a 40 A breaker for this application.
Correct approach:
- Measure running current; if above 13.8 A, repair/replace the pump.
- Install a proper motor controller with overload heaters or an electronic overload set at 13.8 A.
- Keep the 30 A breaker as short-circuit protection (or reduce if starting allows).
- Verify voltage at the pump under load; 180 ft of 10 AWG at 12 A gives VD ≈ 2 × 180 × 12 × 1.24 ÷ 1000 ≈ 5.4 V (2.3%) - acceptable, but check connections.
- Confirm the disconnect is a motor-rated switch, not a general-use switch, and that the wellhead is properly grounded and bonded.
The inspector should reject the 40 A proposal and require the overload device.