Load Dependent Fault Light vs Heavy vs Startup Decision Tree

Why this matters

A fault that only appears under specific load conditions is a diagnostic gift, because the load condition itself reveals the failure class. Startup-only faults point at start components, inrush-current handling, and motor start torque. Heavy-load-only faults point at high-resistance connections, undersized supply, and thermal margin failures. Light-load-only faults are rare but real: they point at sensor thresholds, hysteresis in protective devices, and intermittent contacts that only conduct under specific contact pressure. The tree below maps the load-dependence pattern to the right failure class so the tech does not waste a visit testing under the wrong load.

Symptom presentation

The customer reports a problem that only happens when something specific is happening to the system. The dishwasher only fails on heavy loads, the AC only fails on hot afternoons, the breaker only trips when the dryer is running with the washer, the well pump only stalls during morning peak use, the generator only faults at startup.

Confirm the load condition. Ask: what was the load when it last happened, how often does the load condition occur, has the load condition changed recently. The load-pattern answer pins the branch.

Quick checks by load class

For startup-only faults (the fault occurs at the moment of starting a motor, energizing a transformer, or initiating a heat cycle):

• Check start capacitors on capacitor-start motors. A start cap that has dried out or shorted internally produces start torque failures that manifest as humming, no rotation, or repeated overload trips at startup.
• Check inrush-current handling. Many circuits and small UPS systems handle steady-state current well but trip on inrush; HVAC compressor start, well-pump start, and resistive heating elements can produce 6 to 10x the run current for 0.5 to 2 seconds.
• Check supply voltage during start. A supply that sags below the motor's minimum start voltage during inrush stalls the motor and trips the overload.
• Check control sequencing. Soft-start drives, VFDs, and some HVAC controls sequence startup in steps; a failed step in the sequence produces a startup-only fault.

For heavy-load-only faults (the fault occurs only when the system is at or near rated capacity):

• Check for high-resistance connections. A connection with 1 to 3 milliohms of extra resistance does not affect a light load but produces voltage drop and heat at heavy load. Use a thermal-imaging camera under heavy load to find hotspots.
• Check undersized supply. A branch circuit with a voltage drop above 3 percent at heavy load (per NEC 210.19 informational note) will sag heavy-load voltage to the failure threshold.
• Check thermal margin. A component that runs at 80 percent of its thermal rating at light load may exceed 100 percent at heavy load.
• Check supply pressure on water and gas systems. A water-supply pressure that satisfies a single fixture may collapse with two fixtures open; a gas supply pressure that maintains 7 inches water column at single-burner draw may sag below 5 inches with multiple appliances drawing.

For light-load-only faults (the fault occurs only at minimum operating conditions):

• Check sensor thresholds. A pressure sensor or flow sensor with a worn deadband may chatter at the minimum condition.
• Check protective device hysteresis. An overload or temperature switch with worn contacts may release at light load when the contact pressure is at the low edge.
• Check intermittent contacts under low contact pressure. A relay or switch with worn contacts may conduct at heavy load (the contacts heat and weld momentarily) but fail at light load.

Isolation tree

Branch A (startup-only, motor system): test start capacitor and start relay. Replace per the platform spec.

Branch B (startup-only, inrush): measure inrush with a clamp-on RMS meter with peak hold. Confirm inrush is within the breaker / fuse / supply rating. Upsize the supply if inrush exceeds spec.

Branch C (startup-only, supply voltage sag): measure supply voltage at the load terminals during start. If sag exceeds 10 percent, the supply circuit is undersized or has high resistance somewhere in the run. Walk the supply.

Branch D (startup-only, sequencing): read the controller event log, verify each step of the start sequence completes per the platform service manual.

Branch E (heavy-load-only, high resistance): thermal-imaging survey under load. Tighten or replace heated connections, replace heated terminals with new ones per the manufacturer's torque spec.

Branch F (heavy-load-only, undersized supply): measure voltage drop at the load under heavy load. If drop exceeds 5 percent (NEC 215.2 informational note for feeders, 210.19 for branch circuits), upsize the conductor or shorten the run.

Branch G (heavy-load-only, thermal margin): correct ventilation, install per ambient temperature rating, document the thermal envelope.

Branch H (heavy-load-only, supply pressure): walk the supply system. For water, measure pressure at the source and at the point of use under load; pressure drop above the system spec indicates supply restriction. For gas, measure manifold pressure under multi-appliance draw per NFPA 54.

Branch I (light-load-only, sensor): replace the sensor per the platform spec, verify the platform-specified resistance / voltage range at the minimum condition.

Branch J (light-load-only, protective device): replace the device.

Branch K (light-load-only, contact): replace the contactor or switch.

Confirming the diagnosis

Re-test under the failure load. Light-load tests are easy; heavy-load tests may require a load bank or a controlled customer-load setup. Startup tests are easy to repeat but the inrush measurement requires the right instrumentation.

For Branch E (high resistance), confirm with a follow-up thermal image after the repair; the prior hotspot must be gone under the same load condition.

For Branch F (undersized supply), measure voltage drop after the upsize and confirm within the NEC informational note threshold.

For Branch H (supply pressure), confirm pressure stays within the manufacturer's spec under the multi-fixture or multi-appliance load that previously failed.

Remediation paths

Startup: start components, soft-start kits, sequencer replacement.

Heavy load: connection repairs, supply upsize, ventilation improvements, supply-system correction.

Light load: sensor replacement, protective device replacement, contactor replacement.

When to escalate or refer

References

• NFPA 70 (NEC) Article 210 (Branch Circuits), Article 215 (Feeders), Article 220 (Load Calculations).
• NFPA 54 (National Fuel Gas Code; gas supply pressure requirements).
• IEEE Std 43 (Recommended Practice for Testing Insulation Resistance of Electric Machinery).
• ASHRAE Handbook (HVAC Applications, 2023, Chapter on commissioning and load measurement).
• OSHA 29 CFR 1910.147 (Lockout-Tagout for energized testing safety).