The Use Case

A church’s rooftop air‑conditioning system consists of a 5‑ton (≈7.5 HP) centrifugal compressor rated at 230 V three‑phase. The building has only 120/240 V single‑phase service, so a phase converter must provide the needed three‑phase voltage while handling the compressor’s high inrush currents.

What Usually Matters for Sizing

  1. Compressor horsepower – Directly translates to required three‑phase current (≈24 A at 460 V).
  2. Locked‑Rotor Amps (LRA) – HVAC compressors can draw 3–4× FLA on start; the converter must sustain this without voltage collapse.
  3. Compressor‑duty vs. standard NL – AutoLink or “compressor‑duty” models have larger idler motors and oversized capacitors to survive LRA spikes.
  4. Voltage level – The unit expects 230 V three‑phase; if the converter outputs 460 V, a step‑down transformer is required (or select a converter that can be set to 230 V).
  5. Continuous operation – HVAC systems run for many hours per day; thermal overload protection on the converter must be adequate.

Common Mistakes to Avoid

  • Selecting a bare NL unit – Standard NL converters often fail under compressor LRA, leading to tripping or motor damage.
  • Overlooking transformer needs – Trying to feed a 230 V compressor from a 460 V output without proper step‑down can destroy the compressor.
  • Undersizing for continuous duty – A converter sized only for start‑up but not for sustained load will overheat.
  • Ignoring nameplate LRA – Some compressors list only HP; the LRA must be estimated (≈3× FLA) or obtained from the manufacturer.

Practical Phoenix Recommendation

The call pattern that matched a 5‑ton compressor with model GP20NLA suggests the following:

  1. Choose an AutoLink/compressor‑duty converter – GP20NLA (20 HP rated) – The extra capacity handles LRA comfortably and leaves headroom for future HVAC additions.
  2. Configure output to 230 V three‑phase – Many Phoenix converters have selectable taps; verify the 230 V setting is available, or use a small step‑down transformer if only 460 V is offered.
  3. Install a dedicated 40 A double‑pole breaker on the single‑phase side to accommodate start‑up current.
  4. Add a thermal overload relay sized for continuous operation at the compressor’s FLA.

If site constraints limit space, a compact AutoLink unit with integrated transformer can be specified.

What to Have Ready Before Calling

  • Compressor nameplate: HP, voltage, LRA/FLA (if listed).
  • Existing single‑phase service rating (amps at 240 V).
  • Desired three‑phase output voltage (230 V preferred).
  • Information on how many hours per day the unit will run.
  • Any plans to add additional HVAC units later.

Final note: Confirm final sizing against the compressor’s nameplate, motor HP, voltage, LRA/FLA, and continuous duty before purchasing. An AutoLink/compressor‑duty converter like GP20NLA ensures reliable start‑up and steady operation for a 5‑ton AC unit on single‑phase power.