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To determine the ultimate trip overload device rating for a thermally protected 1 ½ horsepower 120-volt motor, it's essential to first convert the horsepower rating into an amperage rating. Generally, a standard calculation can be applied for single-phase motors, which often takes into account a typical efficiency and power factor.

For a 1 ½ horsepower motor running at 120 volts, the formula to find the full-load current (amps) is:

\[ \text{Amps} = \frac{\text{Horsepower} \times 746}{\text{Voltage} \times \text{Efficiency} \times \text{Power Factor}} \]

Assuming an efficiency of around 85% and a power factor of about 0.9 (common values), the calculation would yield a full-load current of approximately 14.6 amps for a 1 ½ horsepower motor operating at 120 volts.

However, for overload protection, it's common practice to select a trip setting that is higher than the full-load current, typically around 125% to 150% of that value. Using 125% of the full-load current calculated gives a value close to 18.25 amps, which indicates that the overload device needs