How To Understand True Pump Horsepower - Up Rated vs Full Rated

Step 1

Instead of being treated to such classic yet equally vacuous answers “It’s always been that way,” or “Who knows,” a simple equation for determining total horsepower (THP), or true horsepower, will equip you to easily discern between the ratings and make substitutions without hesitation.

Step 2

A little background is probably beneficial at this point to help explain the “why” behind the use of the designations.  Historically, home water systems in the 1940’s were predominately found in more rural areas of the country where electrical service was not always consistent and low voltage situations were common.  For this reason, NEMA standards for service factor were set higher especially on lower horsepower motors for pump applications.  The higher service factor resulted in more start and run torque allowing the pumps to operate under low voltage conditions.

Step 3

The leisure water industry began to grow leading to pumps and motors being adapted to swimming pool and spa applications.  Original equipment manufacturers began to apply larger impellers using the full service factor horsepower – simply different terminology for total horsepower – in order to move more water (GPM) for each rating.  These developments led to high service factor (full rate) and low service factor (up-rated) models offered to the market.

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Step 4

Eventually regional preferences developed for either full rated or up-rated motors offered on new pump systems as well as replacements.  For illustrative purposes then, Florida remains a largely up-rated motor market today. Still, no region is exclusively full rated or up-rated – leading pool professionals to needlessly grapple with the confusing designations when presented with motors that are not equivalent even though the nameplate horsepower appears to indicate otherwise.

Step 5

For example, the full rate SQ1102 and the up-rated USQ1102 are both 1.0 nameplate horsepower square flange replacement motors.  As discussed in the March 2013 issue, the SQ1102 actually has a total horsepower (THP) of 1.65, compared to a 1.25 THP for the USQ1102.

Step 6

The solution – multiply the nameplate horsepower by the nameplate service factor to determine the total horsepower.  Make it a habit of determining the total horsepower before selecting a replacement motor, especially if going from one rating designation to another.  Choose a replacement with an equivalent total horsepower or slightly above that of the motor being replaced. 

Step 7

Horsepower (HP) x Service Factor (SF) = Total Horsepower (THP)

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