Okay, let's break down how to size a service for variable speed pool pump motors, considering their continuous operation and the differences between nameplate and operational power levels.

Key Considerations



NEC (National Electrical Code) is the Governing Standard

Your calculations must comply with the NEC.

Continuous Load

Operating 10-24 hours per day classifies these motors as a continuous load. The NEC requires specific derating or oversizing factors for continuous loads.

Variable Speed Motor Characteristics

Variable speed motors can significantly reduce energy consumption by operating at lower speeds when full flow isn't required. This is where the discrepancy between nameplate and actual operating power comes in.

The NEC Approach: Starting Point - Nameplate

The NEC generally prioritizes nameplate ratings as a starting point for load calculations. This is because the nameplate represents the maximum potential power draw of the equipment. Ignoring the nameplate could lead to an undersized service that cannot handle the equipment's maximum demand, even if it rarely operates at that maximum.

NEC Articles Relevant to Variable Speed Motors (particularly pool pumps)



Article 430 (Motors, Motor Circuits, and Controllers)

This is the core article for motor-related calculations.

Article 680 (Swimming Pools, Fountains, and Similar Installations)

This article has specific requirements for pool pumps and their wiring.

Article 220 (Branch-Circuit, Feeder, and Service Calculations)

Deals with overall load calculations for services and feeders.

Steps for Load Calculation (Simplified Overview)


Determine Nameplate Data

For each motor, record the following from the nameplate:

Rated Voltage

(e.g., 120V, 240V)

Full Load Amps (FLA)

This is crucial. It's the current the motor draws at its rated horsepower and voltage under full load.

Horsepower (HP)



Service Factor

Some motors have a service factor greater than 1.0. This indicates the motor can handle occasional overloads.
2.

Determine Actual Operational Load

This is where it gets tricky, and accurate information is critical:

Typical Operating Speed/Flow Rate

What percentage of full speed will the pumps typically operate at for the majority of the time?

Power Consumption at Typical Speed

You need to determine the power consumption (Watts or Amps) at this typical speed. This can be done in a few ways:

Manufacturer's Data

The best option is to get a power consumption curve or table from the pump manufacturer that shows the power draw at various speeds/flow rates. Many manufacturers of variable-speed pool pumps provide this data because energy efficiency is a selling point.

Measurements

If manufacturer data is unavailable, you might be able to measure the actual current draw of the motor in operation using a clamp-on ammeter, along with voltage. However, be extremely careful when measuring electrical parameters, and only do so if you are qualified and experienced. It's recommended to have an electrician do this.



Estimations (Least Accurate)

As a last resort, you might be able to estimate power reduction based on the cube law for pumps (power is proportional to the cube of the speed). However, this is just an estimation and may not be accurate, especially with variations in pump efficiency and system head (resistance to flow). Do not rely on estimates if you can avoid it.3.

Continuous Load Adjustment

Because these motors are running continuously, you must apply the NEC continuous load factor. Typically, this requires sizing the conductors and overcurrent protection to at least 125% of the calculated continuous load. (NEC 210.19(A)(1), 215.2(A)(1), 230.42(A)(1)).
4.

Largest Motor Consideration

In many motor circuit calculations, the NEC requires you to size the conductors to at least 125% of the FLA of the largest motor in the group. (See NEC 430.24) Then add the sum of the other motors full load currents.
5.

Service Calculation

After calculating the ampacity of each motor circuit, you need to combine them (with appropriate demand factors, if applicable) to determine the overall service load.6.

Overcurrent Protection

Size the overcurrent protection (circuit breakers or fuses) according to NEC 430.52, which often allows for instantaneous trip circuit breakers, inverse time breakers, and dual element fuses sized per the NEC table 430.
52.

Example Scenario (Simplified)

Let's say you have three variable-speed pool pumps:

Pump 1

Nameplate FLA = 10A, typical operating power = 40% of nameplate FLA. 10 x 0.4= 4A

Pump 2

Nameplate FLA = 8A, typical operating power = 50% of nameplate FLA. 8 x 0.5 = 4A

Pump 3

Nameplate FLA = 6A, typical operating power = 60% of nameplate FLA. 6 x 0.6 = 3.6A1.

Continuous Load

Pump 1: 4A 1.25 = 5A Pump 2: 4A 1.25 = 5A Pump 3: 3.6A 1.25 = 4.5A2.

Largest Motor Consideration

Pump 1 has the largest FLA Pump 1 at 125% for continuous load is 5A Pump 2 and 3 are Added for a total ampacity of 5A + 4.5A = 9.5A Service Calculation would require 14.5A

Important Considerations and Caveats



Manufacturer's Instructions

Always follow the manufacturer's installation instructions for the pumps and their associated equipment.

Local Codes

Local electrical codes can be more stringent than the NEC. Check with your local electrical inspector for any specific requirements.

Future Expansion

Consider any potential future expansion or changes to the system when sizing the service. It's often prudent to add some capacity for future needs.

Voltage Drop

Always account for voltage drop in the conductors. Excessive voltage drop can cause motors to overheat and perform poorly. Size the conductors appropriately to minimize voltage drop.

Harmonics

Variable frequency drives (VFDs) can generate harmonics, which can affect power quality. Consider using line reactors or filters if harmonic distortion is a concern.

Grounding

Proper grounding is essential for safety and to prevent electrical noise. Follow NEC Article 250 for grounding requirements.

Pool Bonding

NEC Article 680 has stringent bonding requirements for pool equipment to ensure equipotential bonding and minimize the risk of electric shock.

In summary


Start with the nameplate FLA. This is the foundation of your calculation.2.

Obtain the most accurate operational power data possible. Manufacturer's data is preferred. Measurements are second best. Avoid relying on estimations unless absolutely necessary.3.

Apply the NEC continuous load factors.4.

Apply the NEC 125% to the largest motor.5.

Consult with a qualified electrician or electrical engineer* for a thorough and code-compliant service sizing calculation. They can help you navigate the complexities of the NEC and ensure a safe and reliable installation.Failure to properly size the service can result in nuisance tripping of breakers, overheating of conductors, motor damage, and potential safety hazards. Flag for review