Compressor pulls high amps at startup and sometimes trips.

Diagnose before you add a hard-start kit.

1. Measure startup current vs. rated load amps (RLA). - Use a clamp meter with a "inrush" or "peak hold" function. - If startup current is 3–6× RLA and drops quickly, the compressor is likely fine. - If it stays >6× RLA or doesn’t drop, suspect a mechanically seized or worn compressor.

1. Check supply voltage. - Measure voltage at the compressor terminals during startup. - If voltage sags below 85% of nominal (e.g., <204V on 240V), the issue is power supply, not the compressor. - Low voltage causes high amp draw and can mimic a bad compressor.

1. Inspect the start components. - Look for a burned start capacitor or failed start relay. - Test the capacitor with a megohmmeter or capacitance meter. - A weak capacitor (<80% of rated µF) is a common cause of high startup amps.

1. Compressor wear signs: - Excessive heat, oil contamination, or low refrigerant can increase load. - If the compressor is hot to the touch after short runs, it may be failing internally.

When is a hard-start kit appropriate?

• The compressor is mechanically sound (amps normalize after startup).
• Supply voltage is adequate.
• Start components are functional or replaced.
• The system has high inertia (long lines, large condenser) or frequent starts.

Code note: NEC 440.22 requires overcurrent protection to handle startup currents. A hard-start kit can reduce startup load, but if the compressor is failing, replacement is needed.

Bottom line: Replace capacitors/relays first. Only add a hard-start kit if the compressor tests good and voltage is stable. If amps remain high after startup, call a pro—likely a failing compressor.

Diagnosis Steps (in order):

1. Verify Power Supply: Measure voltage at the contactor during start attempt. Must stay ≥90% nameplate (e.g., ≥208V on 230V unit). Low voltage = utility/wiring issue, not compressor.
2. Check Capacitors: Test run & start caps with meter (power off, discharged). Must be within ±6% (run) / ±10% (start) of µF rating. ESR test preferred. Bad cap = high amps/stall.
3. Measure Locked Rotor Amps (LRA): Clamp the common wire only during start attempt (use max-hold or min/max). Compare to nameplate LRA. ≈ Nameplate LRA (briefly): Start circuit (cap/relay/wiring) likely OK. Compressor mechanical binding or windings shorted. << Nameplate LRA (e.g., 50-70%): Start circuit failure (bad start cap, potential relay, start winding open).
4. Winding Resistance (Power Off): Measure R-S, C-S, C-R. Sum of two smallest ≈ largest. Infinite = open winding. Low resistance to ground = shorted (megger at 500V+).
5. Mechanical Check: If LRA is nameplate-correct but trips instantly on thermal overload: hot pull-down, low voltage, or seized bearings. Try cooling shell with water mist; if it starts, it’s thermal.

Hard-Start Kit (Potential Relay + Start Cap) is Correct ONLY When:

• Start capacitor & relay test bad (NEC 440.6, 440.9).
• Voltage is confirmed good but compressor LRA is slightly high only on marginal power (long line sets, generator).
• Not a fix for: Shorted windings, mechanical seizure, low voltage, undersized breaker/wire (NEC 210.19, 440.22), or failed run capacitor.

Red Flag: If compressor draws >125% RLA after start (run amps), it’s overcharge, airflow, or bearing failure — kit won't help.

Bottom Line: Fix the root cause. Kits mask voltage/wiring issues and delay inevitable replacement. If windings/shorted/mechanical → replace compressor/condensing unit.