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.

The decision flow at a glance:

  When does it fail?
  |
  +-- 1. Startup only? -----> start caps, inrush, start
  |                           sag, sequencing
  |
  +-- 2. Heavy load only? --> high-resistance joints,
  |                           undersized supply
  |
  +-- 3. Thermal margin? ---> 80% rated at light, over
  |                           at heavy
  |
  +-- 4. Supply pressure? --> water / gas collapse under
  |                           multi-draw
  |
  +-- 5. Light load only? --> sensor thresholds,
  |                           hysteresis, contacts
  |
  +-- 6. Confirm? ----------> re-test under the failing
  |                           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 transformer energization can produce 6 to 10x the run current for 0.5 to 2 seconds. Resistive heating elements are the exception: cold resistance sits only slightly below hot, so inrush is roughly 1.1 to 1.5x and is never the cause of a start-only trip.
• 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 branch-circuit drop exceeds 3 percent, or feeder plus branch drop to the farthest outlet exceeds 5 percent (informational notes at NEC 210.19 and 215.2), 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 figures.

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.

• Branch A: replace start capacitor and start relay as a set per the platform spec; a weak relay takes the new capacitor with it. Acceptance: motor comes up to speed on repeated starts without the overload dropping it.
• Branch B: the fix is ramp or capacity, not a bigger breaker on the same conductors. Fit the manufacturer's soft-start or start kit, or upsize the supply to the nameplate locked-rotor figure. Acceptance: measured inrush inside the protective device's let-through with the device holding.
• Branch C: walk the run and correct what is dropping it (loose lug, corroded splice, undersized conductor, long run). Acceptance: voltage at the load terminals during start stays inside the nameplate utilization range.
• Branch D: repair or replace the component at the failed step (sequencer, board, interlock, sensor) per the service manual. Acceptance: full start sequence completes with every step logging clean on three consecutive starts. A step that fails only intermittently gets logged and watched, not swapped blind.

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

• Branch E: remake the connection. Cut back damaged conductor, new lug or terminal, torque to the manufacturer's spec with a calibrated tool. Heat-damaged terminals and discolored conductor get replaced, not retightened. Acceptance: repeat thermal image at the same load with no hotspot.
• Branch F: upsize the conductor or shorten the run. Where that means a new feeder or a service upgrade it is a permitted, quoted project, not today's appointment. Acceptance: re-measured drop back inside the informational-note figures with the load running.
• Branch G: restore the airflow and clearance the equipment was rated for (clean coils, filters and vents, restore manufacturer clearances, correct the ambient). Where ambient genuinely exceeds the rating, the answer is a higher-rated component or a different location, not a derate on paper. Acceptance: component temperature under the failing load inside the manufacturer's rating.
• Branch H: correct the restriction the supply walk found. Water side that is a fouled screen, a failing pressure-reducing valve, or an undersized service line. Gas side it is undersized or obstructed piping, and resizing belongs to a licensed gas fitter with an NFPA 54 sizing calculation; turning up a regulator is not a fix for a sizing problem. Acceptance: pressure holds within the manufacturer's spec under the multi-fixture or multi-appliance draw that previously failed.

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

• Branch I: replace the sensor per the platform spec. Acceptance: platform-specified resistance or voltage reading at the minimum condition, no chatter across the deadband.
• Branch J: replace the protective device with the same rating and trip class the equipment specifies. A larger device to stop nuisance trips is not a repair, it removes the protection. Acceptance: device holds at the minimum condition and still trips on a deliberate overload test where the platform supports one.
• Branch K: replace the contactor or switch with the manufacturer's part, and find what burned the contacts (wrong coil voltage, control chatter, device undersized for the load) before you sign off, or the new one goes the same way. Acceptance: measured contact voltage drop at the minimum condition, no dropout across repeated light-load cycles.

When to escalate or refer

Stop and call the utility, same visit, when the problem is on the supply side of the meter. Concretely: voltage outside the equipment's rated utilization range measured at the service with the customer's load off, a significant imbalance between the two legs, voltage on the two legs moving in opposite directions as load changes (the classic loose-service-neutral signature), or a thermal hotspot at the meter base or service lugs. None of that is yours to repair, and a failing service neutral is a life-safety condition, not a scheduling item. Tell the customer to shed large 240V loads until the utility clears it.

Escalate to a licensed electrician when you are not one. An HVAC or appliance tech who has traced a heavy-load fault to a high-resistance connection in a panel, an undersized branch circuit, or a service that cannot carry the load has done their job correctly. Opening the panel to fix it is a different license and a different insurance position. Write up the finding with the measurements and hand it off.

Escalate on the gas side by where the pressure fails. If manifold pressure falls below the manufacturer's minimum with multiple appliances firing and the pressure at the meter outlet is also low, that is the gas utility. If meter pressure holds and the drop happens in the customer's piping, that is a pipe-sizing problem and belongs to a licensed gas fitter with an NFPA 54 sizing calculation, not to a regulator adjustment.

Two of the same part is the escalation trigger. If the same component has been replaced twice and the fault returns, stop replacing it. You are treating a symptom of something upstream - supply, control sequencing, thermal environment, or a design mismatch. Bring in a senior tech, the manufacturer's technical support line, or the equipment engineer with your logged conditions in hand.

Refer to an engineer when the answer is a study, not a repair. Commercial motor-starting analysis, coordination studies, harmonics, and any load calculation that will justify a service upgrade need a design professional and a stamped document. Same for water systems where the fix is a resized service main.

Refer to an authorized servicer when the equipment is under a manufacturer warranty whose remedy requires one. Performing the repair yourself can void what the customer is entitled to.

Refer any test you are not qualified or equipped to perform. Energized measurement inside equipment that requires arc-flash PPE and an NFPA 70E qualified person is a hard stop, not a judgment call. So is a confined-space entry or an energized test that has no safe measurement point.

Recognize when the fix stops being a service call. New feeder, service upgrade, repiping, added gas capacity: that is a permitted, quoted project with a design step. Say so plainly, quote it as a project, and do not attempt to squeeze it into today's appointment.

Document the hand-off. Load condition, measurements, what you ruled out, what you are recommending, and who it went to. The next party should not have to repeat your diagnostic to trust it.

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 1910.333(b)(2) (de-energizing and lockout for electrical work) and 1910.147 (control of hazardous energy where the stored energy is mechanical).