25 hp Irrigation Pump: Which Current Do You Use, and Where
A 25 hp, 480 V, 3-phase centrifugal irrigation pump is being installed at a farm in upcountry Maui. The motor nameplate reads 32 A at 460 V, service factor 1.15, 75°C rise, code letter F. The supply is 480 V, summer ambient in the pump shed reaches 40°C, terminations are rated 75°C, and the run is 150 ft of THWN-2 in EMT. The controller is a combination starter with a melting-alloy overload relay and a thermal-magnetic inverse-time breaker for short-circuit and ground-fault protection. The installing electrician sized the branch-circuit conductors at 125% of the nameplate 32 A (40 A), selected 8 AWG copper, and set the breaker at 250% of 32 A, rounded to 80 A.
1. Check the conductor sizing. Which current value governs, and is 8 AWG copper adequate at 40°C ambient?
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Which current: for branch-circuit conductors, 430.6(A)(1) sends you to Table 430.250, not the nameplate. For a 25 hp, 460 V motor the table value is 34 A. Nameplate current is used for the overload relay, not for conductor ampacity.
Required ampacity: 430.22 requires 125% of the table FLC for a continuous-duty motor: 34 x 1.25 = 42.5 A.
Ambient correction: 8 AWG Cu at 75°C = 50 A (Table 310.16). At 40°C ambient, the 75°C correction factor from Table 310.15(B)(1) is 0.88: 50 x 0.88 = 44 A, which is still at or above 42.5 A. 8 AWG is acceptable - but only barely, and only with no conduit-fill derating. If four to six current-carrying conductors share the EMT, Table 310.15(C)(1) applies a 0.80 factor: 44 x 0.80 = 35.2 A, and the conductor would have to go to 6 AWG.
The EGC is sized from Table 250.122 on the branch-circuit overcurrent device, not on the motor FLC.
2. Is the 80 A breaker correct? What is the maximum permitted rating?
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430.52(C)(1), Table 430.52 permits an inverse-time breaker at 250% of the motor FLC for a standard motor: 34 A x 2.50 = 85 A. Section 240.6 permits rounding up to the next standard size, which is 90 A.
The 80 A breaker is not a violation - the rule sets a maximum, and 80 A is below it - but it was chosen by rounding 250% of the nameplate current, which is the wrong current and leaves capacity on the table. A breaker that is too close to the motor's starting inrush will nuisance-trip on every start, especially on a centrifugal pump with a long deceleration/restart cycle.
Correct answer: 90 A is the maximum standard rating permitted without going to an exception. If starting current still trips a 90 A breaker, 430.52(C)(1) Exception No. 2 allows the next higher setting, but not more than 400% of FLC (136 A), and the increase must be documented and acceptable to the AHJ - you cannot simply keep stepping up.
Also remember that the overload relay, not the breaker, is the motor's running protection; the breaker protects the conductors and clears short circuits.
3. Set the overload relay and explain why the number differs from the conductor and breaker calculations.
Read the worked answer
Overload protection is set on nameplate full-load current, not the table value. Under 430.32(A)(1), a motor with a service factor of 1.15 or greater may be protected at 125% of nameplate FLA:
32 A x 1.25 = 40 A
Select the heater or electronic setting that does not exceed 40 A. If the 40 A setting will not carry the actual load (trips at normal running current with correct voltage and balance), 430.32(C) permits increasing the setting, but not above 140% of nameplate FLA for this motor: 32 x 1.40 = 44.8 A. Any increase beyond 125% is a red flag that should be investigated - low voltage, single-phasing, a misapplied pump curve, or a wrong impeller - not simply dialed out.
The contrast is the exam point: 34 A (table) drives conductors and short-circuit protection; 32 A (nameplate) drives overload protection. Mixing them either under-protects the motor or causes nuisance trips, and it is one of the most common field errors on agricultural pump installations.