Motor Overload Calculator

A motor overload device must be sized off the motor nameplate FLA, not the branch-circuit conductor ampacity or the NEC Table 430.248/430.250 full-load current. This calculator takes the nameplate Motor nameplate FLA and an Overload setting factor (percent) and produces the arithmetic result that a running overload relay, thermal element, or electronic overload should be checked against. It also compares that calculated value to a field-recorded or specified Entered overload setting, and reports Motor load in kVA based on System voltage and Phase.

The output answers one practical question: does the overload setting on the equipment (dial, programmed value, or relay tap) match the arithmetic result derived from the nameplate FLA and the applicable setting percentage, and is the resulting motor load consistent with the circuit voltage.

Inputs and Their Role

• Motor nameplate FLA — the full-load ampere rating stamped on the motor nameplate, used as the base value for overload sizing per NEC 430.32(A)(1). This is not the NEC table FLC used for conductor and branch-circuit protection sizing.
• Overload setting factor — the percentage applied to nameplate FLA to generate the calculated setting (in this example, 115%, corresponding to the 115% cap for motors that do not carry a marked service factor of 1.15 or greater, or a marked temperature rise of 40°C or less, per 430.32(A)(1)).
• Entered overload setting — a separate, independently entered amperage value representing the actual relay dial, programmed trip point, or spec value in the field. This field is for comparison only; it does not feed into the calculated setting.
• System voltage and Phase (single-phase or balanced three-phase) — used solely to compute motor load in kVA for load-review context, not for overload sizing.

Calculation Method

Calculated overload setting is a direct percentage multiplication:

Calculated overload setting = Motor nameplate FLA × (Overload setting factor ÷ 100)

Entered setting margin compares the field/spec value against the calculated arithmetic result:

Entered setting margin = Entered overload setting − Calculated overload setting

Motor load (kVA) uses standard three-phase apparent power math on the nameplate FLA and system voltage:

Motor load (kVA) = (V × I × √3) ÷ 1000 for balanced three-phase, or (V × I) ÷ 1000 for single-phase

Worked Result

With Motor nameplate FLA = 32 A, Overload setting factor = 115%, Entered overload setting = 36 A, System voltage = 480 V, Phase = balanced three-phase:

Calculated overload setting = 32 × 1.15 = 36.8 A

Entered setting margin = 36 − 36.8 = −0.8 A

Motor load = (480 × 32 × 1.732) ÷ 1000 = 26.6043 kVA

Setting status = review_recommended

The negative margin means the entered/field overload setting (36 A) sits below the arithmetic result generated by the setting factor (36.8 A). A negative margin does not by itself indicate a code violation — trip settings below the calculated ceiling are generally permitted since 430.32 states a maximum percentage, not a fixed value — but it does flag that the entered value does not match the percentage this screen was run against, which is why the tool returns a review status rather than a pass/fail result.

Where This Number Is Used

The calculated overload setting is a check figure for motor running overload protection under NEC 430.32, applied after branch-circuit conductors, short-circuit protection, and controller sizing have already been established under 430.22, 430.52, and related sections. It is not used for conductor ampacity, raceway fill, or voltage-drop calculations. The Motor load (kVA) figure is a separate cross-check for confirming that the nameplate current and system voltage are consistent with the expected load on that feeder or branch circuit, useful when reconciling nameplate data against panel schedules or one-line diagrams.

Field and Code Limitations

This calculator performs the arithmetic step only. It does not determine which percentage applies to a given motor — that depends on the motor’s marked service factor (1.15 or greater) and marked temperature rise (40°C or less), which set the 115%/125% ceiling under 430.32(A)(1), with higher allowances (130%/140%) available under 430.32(C) where a motor cannot start with the initial overload device. Selection of the correct percentage, the overload device’s trip class, and compliance with the applicable code edition adopted by the AHJ must be verified independently of this tool. The Entered overload setting field exists specifically to hold that field-verified or manufacturer-specified value so it can be checked against the arithmetic ceiling rather than assumed correct.