Air-conditioning equipment with a hermetic refrigerant motor-compressor is marked with both a rated-load current and a branch-circuit selection current. Which of those values governs the sizing of the disconnecting means, the conductors, the controller, and the protective devices?
- AThe rated-load current, because it reflects the compressor running amperes under design conditions
Rated-load current is the correct basis only when no branch-circuit selection current is marked. Where both appear, using the lower one undersizes every part of the circuit.
- The branch-circuit selection current, because the Code directs you to use that marked value wherever it appears
- CThe average of the two marked values, rounded up to the next whole ampere
Averaging marked values is not a Code method anywhere. The Code always tells you which single marked value to use.
- DThe locked-rotor current, because it is the largest current the compressor will ever draw
Locked-rotor current is a momentary starting figure used for selecting certain disconnect types and for short-circuit studies, never for sizing conductors or overload protection.
Why B is correct
A branch-circuit selection current appears on the nameplate when the manufacturer wants the circuit sized above the compressor rated-load current, typically because a protective device inside the unit is set to allow a higher continuous current. When that value is marked, the Code substitutes it for the rated-load current in every sizing step for the circuit and its equipment.
What this question is testing
This tests careful nameplate reading, which is what Article 440 is really about. The exam wants to see whether you know that a branch-circuit selection current, when present, displaces the rated-load current everywhere.
On the job
Rooftop units and heat pumps are usually installed against a nameplate that lists a minimum circuit ampacity and a maximum overcurrent device, which the manufacturer already derived from these rules. Trouble starts when a unit is field assembled or when a compressor is replaced and the installer works from the compressor label alone. Reading the wrong current off a nameplate that carries three of them yields a conductor one or two sizes small and an overcurrent device that nuisance trips on hot afternoons, which is precisely when the equipment is needed.
Memory technique
If a selection current is marked, it wins.
Exam tip
On air-conditioning questions, scan the nameplate data in the stem first and ask which currents are marked. That decides which rule applies before any arithmetic starts.
Where to look it up
Article 440 Part I, the sections covering marking on hermetic motor-compressors and the ampacity and rating provisions that follow.