You're asking a great question and highlighting a common observation in residential pool wiring. While your calculation seems straightforward, there are several reasons why 20A breakers are almost universally used for pool pump motors, even those with lower running amp ratings. Here's a breakdown

Why 20A Breakers are Used, Even When it Seems Oversized:
Motor Starting Current (Inrush Current)

You're on the right track with the 150% calculation. However, pool pump motors (especially single-phase induction motors) can have very high inrush currents, significantly exceeding even the 150% rule of thumb. This inrush current is brief but can easily trip a breaker that's sized too close to the running amps. While 13.5A seems correct, the actual inrush current might be much higher. This is also affected by the type of motor used.

Motor Service Factor

Motors often have a "service factor" rating. This indicates the motor can safely handle a load higher than its nameplate rating for short periods. However, operating continuously above the nameplate rating will shorten the motor's lifespan. The breaker needs to allow for this temporary overload capacity without nuisance tripping.

Conductor Ampacity

While the motor itself might be rated for 9 amps, the wiring feeding the motor needs to be adequately sized for the breaker protecting it. Using a 20A breaker generally requires 12 AWG wire, which provides a safety margin for potential overloads and voltage drop. If you downsize the breaker to 15A, you might be able to use 14 AWG wire (depending on the installation method and local codes), but this is generally avoided in pool applications due to the increased risk of voltage drop.

Voltage Drop

Pool pumps are often located some distance from the electrical panel. Voltage drop is a concern, especially with a motor that might be under significant load. A larger wire size (necessitated by the 20A breaker) helps minimize voltage drop, improving motor performance and lifespan.

Future-Proofing

Pool equipment often gets upgraded. A homeowner might replace a single-speed pump with a variable-speed pump (which could draw more amperage, even though it often doesn't). Using a 20A breaker initially provides some headroom for future upgrades without requiring a complete re-wiring of the circuit.

Manufacturer Recommendations

Many pool pump manufacturers explicitly recommend a 20A breaker for their pumps, regardless of the nameplate amperage. This is to account for all the factors above and ensure reliable operation. Always follow the manufacturer's recommendations!

Local Codes and Practices

Local electrical codes can vary. Some jurisdictions might have specific requirements for pool equipment wiring that mandate a minimum breaker size. It's crucial to check your local codes before making any changes.

Nuisance Tripping

A slightly undersized breaker (like a 15A in your scenario) is more prone to "nuisance tripping" – tripping unnecessarily due to brief spikes in current, even if the circuit isn't truly overloaded. This is frustrating for the homeowner and can damage the motor over time due to frequent starts and stops.

Why Your Calculation, While Correct in Principle, Doesn't Tell the Whole Story

Your calculation of 9 amps x 150% = 13.5 amps is a good starting point, but it's overly simplistic for motor circuits. The inrush current for a motor can be significantly higher and lasts for a very short duration. Standard circuit breakers are designed to tolerate brief overloads to allow for motor starting.

What You Should Do


Consult the Motor's Nameplate

The nameplate should provide the Full Load Amps (FLA) and any recommendations for breaker size or overcurrent protection.2.

Check the Manufacturer's Installation Instructions

Always refer to the manufacturer's installation instructions for the specific pump model. They will often specify the recommended breaker size.3.

Consult Local Codes

Local electrical codes always override general rules of thumb. Check with your local permitting office or a qualified electrician to ensure your wiring complies with all applicable codes.4.

Err on the Side of Caution

In general, it's better to slightly oversize the breaker (within reasonable limits) than to undersize it. A 20A breaker, with 12 AWG wire, is the common and generally accepted practice for most residential pool pump motors.

In Summary

While your calculation is technically correct in a very limited sense, the 20A breaker is almost always the better choice for a pool pump motor. It provides a margin of safety for inrush current, service factor, voltage drop, future upgrades, and prevents nuisance tripping. Always prioritize the manufacturer's recommendations and local codes. Undersizing a breaker can lead to frequent tripping, motor damage, and potentially dangerous situations.

Important Disclaimer: Electrical work can be dangerous. If you are not a qualified electrician, do not attempt to perform any electrical work yourself. Always consult with a licensed and experienced electrician for any electrical installations or repairs. Flag for review