Motor Branch Circuit vs. Nameplate: A Compressor That Trips
A 5 hp, 208 V, three-phase, 1,750 rpm open drip-proof motor drives a reciprocating air compressor in a small machine shop. The nameplate reads 14.6 A, 1.15 service factor, 40 °C rise, with 75 °C terminations. The branch circuit is 60 ft of 10 AWG Cu THWN-2 in EMT from a panel where 45 A inverse-time breakers are already on hand, and the combination starter has an adjustable thermal overload. Because the nameplate FLA was read as the governing number, the installer set the overload to 14.6 A, and the motor nuisance-tripped on hot afternoons under normal load. On the same panel, a rooftop package unit is being connected; its nameplate shows MCA 32.4 A and MOP 50 A.
1. Determine the minimum branch-circuit conductor size and the inverse-time breaker rating for the compressor motor, and explain why the breaker may be larger than the conductor ampacity.
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Motor FLA comes from the table, not the nameplate (430.6(A)(1)): Table 430.250 for a 5 hp, 208 V, three-phase motor gives 16.7 A.
Conductors (430.22): 125% × 16.7 = 20.9 A minimum ampacity. 10 AWG Cu THWN-2 at the 75 °C column (35 A) is more than adequate; 12 AWG (25 A) would technically satisfy ampacity but 10 AWG is the sensible choice for a 60 ft run and voltage drop.
Short-circuit/ground-fault protection (430.52, Table 430.52): Inverse-time breaker at 250% of FLA = 41.75 A. Since this is not a standard rating, 430.52(C)(1) Exception No. 1 permits the next higher standard device per 240.6 → 45 A.
Why the 45 A breaker is legal on 35 A conductors: Motor branch circuits are one of the specific exceptions to 240.4's general rule - 240.4(G) exempts 430 Part IV circuits. The breaker is providing only short-circuit and ground-fault protection; the overload device protects the motor and conductors against overload.
EGC: Table 250.122 for a 45 A device → 10 AWG Cu, which matches the circuit conductors.
2. Was the 14.6 A overload setting correct? What setting should be used, and what does the code allow if the motor still trips under normal starting and running?
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The setting was wrong. Overload protection is based on the nameplate full-load current (430.32), not the table value - but 14.6 A is the nameplate itself, not a percentage of it. For a motor with a 1.15 service factor or a 40 °C rise, 430.32(A)(1) allows the overload to be set at up to 125% of nameplate FLA: 14.6 × 1.25 = 18.25 A. The installer set it at 100%, so it trips on normal running.
Set the thermal unit/heater to a value not exceeding 18.25 A.
If it still trips: 430.32(C) permits increasing the overload setting where the device selected is not sufficient to start the motor or carry the load, up to a maximum of 140% of FLA for a motor with a marked service factor of 1.15 or a 40 °C rise: 14.6 × 1.40 = 20.4 A. Document the increase and the reason.
What not to do: Never solve a nuisance trip by removing or jumpering the overload, or by going past the 140% ceiling. If 140% will not hold, the cause is wrong sizing, refrigerant/compressor loading, ambient, or a failing motor - investigate rather than defeat the protection. Overload and short-circuit protection serve different purposes and cannot substitute for each other.
3. For the rooftop package unit, what conductor size and maximum overcurrent device apply, and why is the method different from the compressor in Tasks 1–2?
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Different article, different source of numbers. Hermetic refrigerant motor-compressors are covered by Article 440. The unit nameplate has already applied 440.32/440.33 (125% of the largest compressor load plus 100% of the other loads) to arrive at the minimum circuit ampacity (MCA). You use the nameplate MCA directly for conductor sizing - you do not re-apply 125% and you do not go to Table 430.250.
Conductors: MCA 32.4 A → 8 AWG Cu THWN-2 gives 50 A at 75 °C. 10 AWG (35 A) would also carry 32.4 A, but the unit's terminations and the 50 A device make 8 AWG the clean choice.
Overcurrent device: 440.22(B) allows the marked maximum overcurrent protective device to be used. MOP 50 A means a 50 A device is the ceiling - the 45 A breakers on hand are acceptable only if the unit starts and runs without tripping; you may never exceed 50 A.
Related requirements to check at the unit: a disconnecting means within sight of and readily accessible from the equipment (440.14), and a 125 V, 15 or 20 A receptacle outlet within 25 ft of the equipment for servicing (210.63).