What it is

Most residential equipment runs on single-phase 240V. Step up to light commercial and you're working with three-phase power — common at 208V or 480V. Three-phase isn't harder, it's just more: three hot legs instead of two, three sets of readings, and a couple of new failure modes (imbalance, phase loss, and rotation) that don't exist on single-phase. Getting comfortable with these is most of what you need to service a commercial RTU's electrical side. This is the orientation.

How it works

Single-phase power delivers one alternating voltage. Three-phase delivers three of them, each offset in timing by a third of a cycle. The practical payoff is that three-phase delivers power more smoothly and lets motors be simpler, smaller, and more efficient for a given output — which is why commercial compressors and blowers use it.

You'll see two common configurations on the building side, but for service the things that matter are:

  • Three hot legs (call them L1, L2, L3). You read line-to-line voltage between each pair (L1-L2, L2-L3, L1-L3) — those should all be close to the nameplate voltage and close to each other.
  • Phase rotation/sequence — the order the three phases peak (often called ABC vs. CBA). It determines which direction three-phase motors spin. Get it backwards and a scroll compressor runs backward (bad) or a blower turns the wrong way.
  • No neutral needed for the three-phase loads themselves — the three legs balance each other. (A separate neutral may exist for 120V control/accessories derived from the supply.)

In the field

What to actually check on a three-phase unit:

  1. Read all three line-to-line voltages. L1-L2, L2-L3, L1-L3. All three should be near nameplate and near each other. A leg reading low or zero is a problem.
  1. Check for voltage imbalance. Compare the three readings. If one leg is noticeably off from the others, you have imbalance — and three-phase motors hate it. A small percentage of voltage imbalance creates a much larger percentage of current imbalance and serious extra heat in the windings.
  1. Check for phase loss (single-phasing). If one leg drops out entirely — blown fuse, burnt connection, utility issue — a running three-phase motor tries to keep going on two legs, draws way too much current, and cooks. Phase-loss/phase monitors exist to shut the unit down before that happens; many commercial units have them.
  1. Verify rotation on new installs or after utility/wiring work. Use a phase-rotation meter (or confirm scroll compressor direction by suction/discharge behavior and sound — a scroll running backward won't pump, makes an awful noise, and the pressures won't separate). Swapping any two of the three legs reverses rotation.
  1. Clamp the current on each leg under load and compare to nameplate (RLA/FLA). Balanced legs should pull similar current; a big current spread points at imbalance or a winding problem.

Normal values & targets

  • Common commercial voltages: nominal 208V or 480V three-phase (you'll also hear 240V three-phase on some systems). Read against the nameplate.
  • Voltage imbalance: keep it small — a widely used rule of thumb is to stay under ~2% voltage imbalance between legs; beyond that, motor heating climbs fast and life drops. Treat a noticeably uneven set of readings as a red flag to investigate.
  • Current imbalance amplifies voltage imbalance — a couple percent voltage imbalance can produce many times that in current imbalance. Clamp all three legs.
  • Phase loss = emergency for a running motor. One leg gone and the motor draws toward locked-rotor levels on the remaining legs.

These are typical references; always compare to the unit's data plate and any posted utility data.

Common faults & what they mean

  • One leg low or dead (phase loss / single-phasing): blown fuse, loose/burnt lug, failed contactor pole, or utility issue. A running motor on two legs overheats fast — this destroys compressors and motors.
  • Voltage imbalance between legs: loose connections, undersized/long conductors, unbalanced building loads, or utility supply problems. Causes excess heat and premature motor failure.
  • Compressor won't pump / horrible noise / pressures won't separate after a wiring change: reversed rotation on a scroll — swap two legs. Don't let it run reversed.
  • Burnt one set of contactor contacts: that pole isn't passing current cleanly — a path to single-phasing the load.
  • Nuisance phase-monitor trips: the monitor is doing its job — find the imbalance or loss it's catching, don't bypass it.

Tech tips & gotchas

  • Always read all three legs, line-to-line. A problem hides in the comparison between legs, not in any single number. Reading just one pair can miss imbalance entirely.
  • Reversed rotation only matters on three-phase loads. A scroll compressor run backward won't build pressure and will be damaged if left running — kill it and swap two legs. Single-phase scrolls can't reverse this way; three-phase ones can.
  • Don't bypass a phase/voltage monitor to "get it running." It's protecting a very expensive compressor from single-phasing. If it's tripping, the supply has a real problem.
  • Tighten and inspect lugs at PM. Loose three-phase connections are a leading cause of imbalance and burnt legs — heat, vibration, and time loosen them.
  • Label rotation after you confirm it so the next leg-swap doesn't reverse a known-good unit.

Safety / code notes

  • Three-phase line voltage (especially 480V) is lethal and arc-flash capable — lock out/tag out, verify zero energy on all three legs, and use properly rated PPE and meters.
  • Confirm the disconnect is present and within sight of the unit per NEC §440, and that overcurrent protection matches the nameplate.
  • Phase-loss/imbalance protection is there to prevent motor destruction — keep it functional, never defeat it to force a unit to run.