Overload Relays for Fan and HVAC Motors
How do you size and set an overload relay for fan and HVAC motors? Fan and HVAC motors carry a torque-speed curve set by blade and wheel inertia, and under IEC 60947-4-1 that inertia decides the trip class — a bimetallic relay sized off a generic motor table nuisance-trips on the fan even though the FLC matches. Undersize the class and the relay opens before a large fan reaches running speed; leave too much margin and a jammed damper or a seized bearing cooks the winding before the bimetal reacts. This article covers run-up time for belt-driven versus direct-drive fans, trip class selection from 10A through 30, FLC and service-factor dial setting on star-delta fan starters, phase-loss and ambient exposure on rooftop and condenser units, and manual-versus-automatic reset for unattended mechanical rooms.
Why Fan Inertia Changes the Overload Sizing Problem
A motor accelerating a fan wheel draws close to locked-rotor current — typically 6-8x FLC — for the entire run-up, and run-up time on a fan is set almost entirely by the wheel's moment of inertia (WK²), not by the load torque at running speed. A small direct-drive condenser fan with a light aluminum blade reaches full speed in 3-6 seconds. A large centrifugal supply fan on a belt drive, or a cooling-tower fan with a wide-diameter blade, can take 12-25 seconds to spin up because the wheel itself is heavy relative to the motor's rated torque.
The affinity laws that govern fan performance (torque scales with speed squared, power with speed cubed) mean a fan needs very little torque at low speed, but that doesn't shorten the run-up, because the wheel still has to accelerate the same mass. A pump load and a fan load with the same FLC can have completely different run-up times, which is why sizing an overload relay by nameplate current alone, without checking run-up time against the relay's trip curve, produces nuisance trips on the fan side more often than on pumps.
Bimetallic or Electronic for HVAC Duty
Most packaged AHU fans, condenser fans, and small exhaust fans run fine on a bimetallic relay — TeSys LRD, ABB TA, or SIRIUS 3RU2 under the same frame's contactor — because their run-up sits inside a Class 10 or 10A curve and their setting range covers the FLC directly. Larger cooling-tower fans, chiller-plant supply fans, and any motor over roughly 100 A of FLC start pushing into electronic territory: LR9/TeSys T, ABB E-series, or SIRIUS 3RB3 give a wider 1:3-1:4 setting ratio, selectable Class 10-30 without swapping the relay body, and thermal memory that survives a power cycle mid-run-up. See thermal vs electronic overload relays for the full trade-off on cost and setting range.
What we see in the field: buildings retrofit with VFDs on the supply fan and forget the overload relay is still there doing double duty, sized for a DOL start that no longer happens day to day. If the VFD gets bypassed for maintenance, the relay is the only motor protection running. Check the class fits the DOL run-up, not just the VFD ramp rate.
Matching Trip Class to Fan Type
Belt drive changes the inertia the motor actually sees, because the sheave ratio and belt mass sit between the motor shaft and the wheel; a small-diameter motor sheave driving a large fan sheave reduces the reflected inertia and can shorten run-up compared to a direct-drive fan of the same wheel size. The table below gives typical starting figures. Always confirm against the fan curve, not the category.
| Fan Type | Typical Run-Up | Typical Trip Class |
|---|---|---|
| Direct-drive condenser or small AHU fan (under ~10 kW) | 3-6 s | 10A |
| Belt-driven centrifugal supply/return fan | 6-12 s | 10 |
| Large axial or cooling-tower fan, wide-diameter blade | 12-25 s | 20-30 |
Formula: Trip Class Verification for High-Inertia Fans — Source: IEC 60947-4-1, Clause 8.2.1.2
ttrip (at 7.2 × Iset, from cold) ≥ tstart
| Symbol | Description | Unit |
|---|---|---|
| ttrip | Relay trip time at 7.2x the dial setting, tested cold | s |
| Iset | Dial setting, equal to motor FLC | A |
| tstart | Fan run-up time to full speed, from the fan curve submittal | s |
Read the relay's published time-current curve at 7.2x setting and confirm it clears the fan's run-up time with margin. A Class 10 relay trips in 4-10 s at that multiple, which fits the belt-driven row above but runs tight for the cooling-tower row. Move to Class 20 or 30 and the same current setting tolerates the longer run-up without changing the dial. See overload relay trip classes for the full curve breakdown.
Setting the Dial: FLC, Service Factor, and Star-Delta Fans
The dial is set to the motor's nameplate FLC, same rule as any motor. HVAC motors commonly carry a 1.15 service factor, which gives a little headroom on the setting but doesn't change the base number: set to FLC, not to FLC times the service factor. On large fans started star-delta to limit inrush on a weak rooftop feeder, the overload relay usually sits in the delta leg and sees phase current at roughly 0.58x the line FLC, not the full line current. Get this wrong and the relay ends up set almost double what it should be, which defeats the overload protection entirely. Overload relay sizing for star-delta starters and star-delta starter wiring cover the wiring and the math side by side.
Phase Loss and Ambient Extremes on Rooftop and Condenser Units
Rooftop AHUs and condenser fan motors sit outdoors, which puts two things on the overload relay that an indoor pump panel rarely sees: wide ambient swings and long, exposed feeder runs prone to a lost phase. A bimetal relay's ambient compensation keeps the trip point roughly stable between about -5 and +55/60 C, but a relay mounted in an uninsulated rooftop control panel in direct sun can sit well above the compensated band on a summer afternoon. That is why some HVAC overload relays trip only in hot weather even though the motor current hasn't changed. Phase loss on outdoor runs — connector corrosion, rodent damage, a loose terminal at the disconnect — forces roughly 1.7x current onto the remaining two windings; a phase-loss-sensitive relay's differential trip bar catches this faster than the plain thermal element would. See phase-loss and single-phasing protection for the mechanism.
Reset Mode in Unattended Mechanical Rooms
Hand reset stays the default for supply, return, and exhaust fans: a trip usually means a blocked damper, a seized bearing, or a jammed wheel, and none of those clear themselves. Restarting automatically just repeats the fault until something burns. Auto reset shows up on some condenser fan circuits in unattended penthouse mechanical rooms, on the logic that a nuisance trip from a hot afternoon should clear itself rather than leave a chiller running without condenser airflow until someone notices. That trade-off depends on what else protects the chiller if the fan doesn't restart. A high-head safety switch downstream makes auto reset defensible; without one, hand reset is the safer default even if it means a service call.
Frequently Asked Questions
What trip class do rooftop AHU fans need?
Small direct-drive AHU fans usually run fine on Class 10A. Larger belt-driven supply fans with a 6-12 second run-up typically need Class 10, and only the largest, wide-diameter fans or cooling-tower units with a 12-25 second run-up need Class 20 or 30. Confirm against the actual fan curve rather than sizing by horsepower alone.
Can I use auto reset on HVAC overload relays?
Only where an unattended restart is safe, for example a condenser fan circuit backed by a separate high-pressure safety cutout. For supply, return, and exhaust fans, hand reset is the safer default because most trips come from a mechanical fault that won't clear on its own.
Do VFD-driven fans still need an overload relay?
If the fan can be run direct-on-line during a VFD bypass for maintenance, yes: the overload relay is the only motor protection active during that bypass period. Size it for the DOL run-up time, not the VFD's ramp rate.
What size overload relay fits a cooling-tower fan motor?
Set the dial to the motor's nameplate FLC and check the relay's trip-class curve against the fan's run-up time from the fan curve submittal. Large cooling-tower fans commonly need Class 20 or 30 and, above roughly 100 A FLC, an electronic relay for the wider setting range.
Why does my fan motor overload relay keep tripping only on hot days?
A rooftop panel in direct sun can push the relay's own ambient above its compensated range, shifting the effective trip point down even though motor current hasn't changed. Check panel ventilation and shading before assuming the motor itself is overloaded.
Conclusion
Fan and HVAC motor overload sizing comes down to run-up time, not horsepower. Pull the run-up figure from the fan curve, check it against the relay's trip-class curve at 7.2x setting, set the dial to nameplate FLC (or 0.58x line FLC on a star-delta delta leg), and default to hand reset unless a downstream safety covers an unattended restart. For the broader sizing and coordination rules that apply across motor types, see the thermal overload relay engineering guide, and browse thermal overload relays or contactors for stocked ranges from Schneider, ABB, and Siemens.
