The Hidden Voltage Drop: How Small Electrical Imbalances Wreck HVAC Performance

You know that “everything looks fine” call that still cooks a motor a week later? Tiny voltage issues are often the ghost in the machine. This is a deep-dive for pros who want to catch those ghosts before they become callbacks.

What voltage drop and imbalance really are

Voltage Drop

• Voltage drop (VD)
  The difference between source voltage and what the equipment actually sees under load. Excessive drop usually points to resistance somewhere in the path (conductor length/size, loose/corroded terminations, tired disconnects, pitted contactors).

• Single‑phase imbalance
  Unequal voltage on L1–N versus L2–N in split‑phase systems. Often tied to panel load distribution, neutral issues, or shared neutrals with poor terminations.

• Three‑phase imbalance
  Unequal line‑to‑line voltages across phases. Even small imbalances create disproportionate current imbalance and extra heat in windings and conductors.

  Targets to live by

  • Single‑phase drop: keep total feeder+branch under about 5%; investigate above ~3% at the load.

  • Three‑phase imbalance: aim for ≤1% ideal; start corrective action above ~2%.

3 Phase Imbalance

Why tiny differences hit so hard

• Induction motors run hotter fast
  A slight voltage dip means higher current for the same torque, driving copper losses up and raising winding temperature. Elevated heat cooks insulation, dries oil, and shortens bearing life.

• Torque ripple from phase imbalance
  Uneven phase voltage distorts the rotating field, increasing vibration and noise and pushing compressors toward thermal trips.

• Capacitor‑run motors get finicky
  Low or uneven voltage changes phase angle and effective capacitance, causing over‑amp conditions and nuisance thermal cutouts. A “bad cap” reading may be a symptom of a supply problem.

• ECMs and VFDs aren’t invincible
  Drives will compensate—until they run out of headroom. Expect higher DC bus ripple, derating, overcurrent trips, and random fault codes that look like “software glitches.”

• Controls brownout before power fails
  24V circuits sagging to 18–20V under load cause sticky relays, chattering contactors, erratic board resets, and intermittent economizer faults that masquerade as mechanical issues.

Field procedure: find it, prove it, fix it

Instruments that make the difference

• True‑RMS DMM and clamp meter: Accurate under non‑sinusoidal waveforms.

• Power quality/data logger: Catch intermittent sags and flicker tied to large loads or utility events.

• IR camera: Rapidly spots hot lugs, fuses, and splices.

• Insulation tester: Rules out winding breakdown so you don’t chase ghosts.

• Torque screwdriver/wrench: To finish the job right after you find a loose termination.

Test under actual load

• Baseline at the panel

  • Measure L–L (or L–N) and record.

  • Load profile: Note other large loads on the same service; repeat checks when they’re ON.

• Measure at the equipment line side (disconnect)

  • Compare panel vs. disconnect readings to isolate feeder vs. branch issues.

  • IR scan the disconnect and lugs; heat = resistance.

• Measure across the contactor (line vs. load)

  • Millivolt drop across closed contacts reveals pitting you won’t “see.”

  • Coil voltage during pull‑in and hold; log the sag as fans/compressors start.

• Measure at the load

  • Compressor terminals and indoor blower voltages under steady state.

  • Record currents on each phase/leg and note imbalance.

• Calculate and decide

  • Percent voltage drop at each segment (panel→disconnect, disconnect→contactor, contactor→load).

  • Percent phase imbalance using the formula above; correlate to current imbalance.

Root causes you’ll find (and how to fix them)

• Loose/oxidized terminations

  • Tell: Warm lugs, mV drop across joints, IR hotspots.

  • Fix: Clean, de‑oxidize (especially Al), re‑terminate, torque to spec, recheck under load.

• Undersized or long conductors

  • Tell: Acceptable voltage at panel; high drop at load with heavy start currents.

  • Fix: Upsize conductors, shorten runs, consider soft‑start or drive on large compressors.

• Tired disconnects and pitted contactors

  • Tell: mV drop across closed contacts; heat discoloration.

  • Fix: Replace hardware; verify coil is getting stable voltage during pull‑in.

• Panel load imbalance (split‑phase)

  • Tell: L1–N vs. L2–N differ several volts at the same moment.

  • Fix: Rebalance breakers across legs; move large intermittent loads to the heavier leg’s opposite.

• Utility side phase imbalance (3‑phase)

  • Tell: Imbalance present at the service ahead of your feeders.

  • Fix: Document, log, and coordinate with utility; you can only mitigate locally (derate, rebalance loads).

• Shared or compromised neutrals

  • Tell: Neutral‑ground voltage swings under load; heating on neutral conductors.

  • Fix: Correct shared neutral topology, tighten/replace terminations, ensure proper MWBC handle ties/opposite legs.

• Weak run/start components masking as supply issues

  • Tell: Voltage okay, current high; cap values out of spec; hard starts “fix” the symptom.

  • Fix: Replace out‑of‑spec components, then re‑measure drop to confirm the supply path is healthy.

• Grounding/bonding defects

  • Tell: Chatter, nuisance trips, touch voltage, control anomalies.

  • Fix: Correct bonds, replace corroded straps, verify equipment grounding path integrity.

Case study + a fast field checklist

Rooftop 3‑phase, intermittent trips

• Symptoms: Random compressor OL trips on hot afternoons; nameplate within amps when checked in the morning.

• Findings: Afternoon L‑L voltages 479/468/471 V (avg 472.7 V; max deviation 4.7 V ≈ 1.0% imbalance). Current imbalance was noticeably higher; IR showed a warm line lug in the fused disconnect.

• Fix: Cleaned/re‑terminated line and load lugs, replaced worn fuses and contactor, torqued to spec, rebalanced panel loads in building. Afternoon retest showed 477/475/476 V (≈0.2%); nuisance trips stopped.

What to rule out first (every time)

• Under‑load readings: Don’t trust idle voltages; measure when the compressor/blower is running.

• Segment the path: Panel → disconnect → contactor → load. Find where the drop occurs.

• Coil stability: Verify 24V (or coil spec) stays solid during inrush.

• Millivolt checks: Across fuses, contacts, and lugs; heat means resistance.

• Re‑torque and re‑test: Correct, then prove the improvement with numbers.

Takeaways you can use on your next call

• Start with power quality before refrigerant: Airflow and electrical health are foundational.

• Quantify, don’t guess: Compute percent drop and imbalance; set your own red/yellow/green thresholds.

• Fix the cause, not the casualty: Burned components are often symptoms of small, chronic electrical issues.

• Log the tough ones: A 24‑hour logger beats chasing intermittent faults by luck.