Three independent protection settings for the same motor — overload trip class chosen from its starting time, contactor size from its duty, and ground-fault pickup from the system's earthing arrangement.
| Motor full-load current | 34 A |
| Service factor / temp rise marking | Standard (not ≥1.15 SF or ≤40°C rise) |
| Starting time | 8 seconds |
| Duty class | S1 — continuous running |
| Starts per hour | 4 |
| Contactor duty | AC-3 (normal start/stop) |
| System grounding | Solidly grounded |
Without a high-service-factor or low-temperature-rise nameplate marking, NEC 430.32 defaults to the more conservative 115% setting.
IEC 60947-4-1 trip classes are named for their maximum trip time (seconds) at 7.2x rated current — a longer starting time needs a trip class that tolerates that duration without nuisance tripping during a normal start.
AC-3 duty (normal motor starting/stopping) uses the motor's FLA directly, with no additional multiplier.
| Check | Requirement | Actual | Status |
|---|---|---|---|
| Overload relay setting | n/a (this is the sizing result) | 39.1 A, Trip Class 10 | ✓ PASS |
| Contactor size | ≥ 34 A (AC-3) | 40 A standard frame | ✓ PASS |
| Ground-fault pickup | n/a (this is the sizing result) | 6.8 A, 0.1 s delay | ✓ PASS |
Key insight: Overload trip class and contactor duty rating solve two completely different problems — trip class is about tolerating a normal motor start's inrush without nuisance tripping, while contactor rating is about the switching duty of making and breaking the circuit itself. A motor with a long starting time and light switching duty (or vice versa) can need very different combinations of these two settings, which is why they're sized independently rather than from one single number.
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Open Motor Protection Sizer calculator →AC-4 duty applies a 2.0x conservative multiplier to the required equivalent current (a rule-of-thumb, not a single universal standard ratio, since it's genuinely manufacturer/model specific) — for this 34 A motor, that would push the requirement to 68 A, jumping to an 80 A standard contactor frame instead of 40 A, reflecting the much more severe electrical wear of frequent jogging or plugging duty compared to normal start/stop.
On a solidly grounded system, ground faults draw high current that a simple percentage-of-FLA pickup can reliably detect quickly. HRG systems are deliberately designed to limit ground-fault current to a small, controlled charging current — so ground-fault pickup instead has to be set relative to that system's own charging current (roughly 2x it) with a longer alarm/delayed-trip time, since the goal on an HRG system is often to alarm and allow orderly shutdown rather than instantly trip.