Okay, Dan, I've reviewed your plan with the (3) 3/4HP 480/3 motors on a single 20A 3-pole circuit, fed through PVC underground, a wireway/junction box near the panel, and disconnects at the motors. Here's a breakdown of my assessment and some key considerations to ensure it meets NEC (National Electrical Code) requirements

Overall Assessment: Feasible, but Needs Careful Consideration of Several Factors

The basic concept is sound, but the devil is in the details. You can run multiple motors on a single branch circuit, but you need to meticulously follow NEC rules regarding motor circuit protection, conductor sizing, and disconnects. Here's a detailed breakdown

Key Considerations and NEC References:

1. Motor Full Load Amps (FLA) and Overload Protection



Find the FLA

You must use the NEC tables (typically Table 430.250) to determine the Full Load Amps (FLA) for a 3/4 HP, 480V, 3-phase motor. Do not use the nameplate amps unless they are higher than the NEC table value. For planning purposes, a 3/4 HP, 480V 3-phase motor typically has an FLA around 1.1 - 1.2 amps. (Check NEC Table 430.250 for the precise value).

Overload Protection (Motors)

Each motor must have individual overload protection to protect against running overload. This is usually provided by overload relays (OLRs) in the motor starters/contactors.

NEC 430.32 outlines the requirements. These OLRs must trip at no more than 125% of the motor's FLA (for motors with a service factor of 1.15 or greater, or a temperature rise of 40°C or less). Important

Since you're using disconnects at each motor, consider using combination motor starters/disconnects that integrate the overload protection. This simplifies wiring and ensures compliance. If you use separate disconnects and starters, make sure the overload protection is integral to the starter.

Calculate Total Motor Overload Amperage

Total the OLRs that will be used. You will use this number to size your panel breaker.2.

Branch Circuit Conductor Sizing



NEC 430.24

The branch circuit conductors supplying two or more motors must have an ampacity not less than 125% of the full-load current rating of the highest rated motor in the group, plus the sum of the full-load current ratings of all the other motors in the group.

Calculation

Let's assume the NEC Table FLA for your 3/4 HP motor is 1.2 amps. Conductor Ampacity Required = (1.25 1.2 amps) + (1.2 amps + 1.2 amps) = 1.5 + 2.4 = 3.9 amps. Since the 3/4HP motors are the same, the same will apply to any of the motors.

Considerations



Derating

You must consider derating for conduit fill (NEC 310.15(C)(1) and the notes to Table 310.15(B)(3)(a)) and ambient temperature (NEC 310.15(B)). Underground PVC conduits can experience high temperatures, especially in direct sunlight. Derate your wire accordingly. This is VERY important.

Voltage Drop

Calculate voltage drop (210.19 Informational Note #4 and 215.2(A)(4)) . Long runs can cause voltage drop, which can damage motors. Use a voltage drop calculator and adjust wire size if necessary.

Minimum Wire Size

Even with the low calculated ampacity, you must still use a wire size that is suitable for a 20A breaker. Normally this will be 12ga wire.

Grounding

Proper grounding is essential. Use a properly sized Equipment Grounding Conductor (EGC) run with the circuit conductors. Consult NEC Table 250.122 for sizing.

Neutral

Since this is a 3-phase system, you generally don't need a neutral for the motor branch circuit.3.

Short-Circuit and Ground-Fault Protection (Branch Circuit Protection)



NEC 430.52

The branch-circuit short-circuit and ground-fault protective device (your 20A breaker) is intended to protect the branch-circuit conductors, the motor control apparatus, and the motor itself against short circuits or ground faults.

Maximum Breaker Size

The maximum size of the breaker depends on the motor's code letter (found on the motor nameplate) and the type of breaker (inverse time, instantaneous trip, etc.). If the motor does not have a code letter, you must use Table 430.52. A 20A breaker may or may not be acceptable, depending on the motor characteristics.

Typically

For motors without a code letter and using standard inverse time breakers, you are typically allowed up to 250% of the motor's FLA. In this case, that would be 1.2amps 250% = 3amps.

This is the most limiting factor. IMPORTANT

The breaker primarily protects the branch circuit wiring from short circuits, not the motors from overloads (that's the job of the overload relays). The overloads must be sized no more than 125% of the motor FLA.

Coordination

The breaker and overload relays must be properly coordinated. The breaker should only trip on a short circuit or ground fault, not on a motor overload (that's what the OLRs are for).4.

Disconnecting Means



NEC 430.102

Each motor must have a disconnecting means within sight of the motor and the controller location (if the controller is not within sight of the motor). This disconnect must be rated in horsepower and suitable for interrupting the motor's locked-rotor current (LRC).

Horsepower Rating

The disconnect must have a horsepower rating equal to or greater than the motor's horsepower.

Location

Your plan to have disconnects at the motors is good.

Lockout/Tagout

The disconnects must be capable of being locked in the open (off) position for safety during maintenance (lockout/tagout procedures).5.

Wireway/Junction Box



NEC 376 (Wireways) or 314 (Junction Boxes)

Make sure the wireway or junction box is sized correctly based on the number of conductors entering and leaving and the size of the conductors (fill calculations). Use appropriate fittings for connecting the PVC conduit to the wireway/junction box. The wireway/junction box must be accessible (NEC 314.29).
6.

PVC Conduit



NEC Chapter 3

Follow all NEC requirements for PVC conduit installation, including burial depth (Table 300.5), proper supports, and expansion fittings (if needed for temperature changes).

Conduit Fill

Do not exceed the maximum allowable conduit fill (NEC Chapter 9, Table 1 and notes).

Sealing

Consider sealing the ends of the PVC conduits to prevent moisture and debris from entering.

Recommendations and Cautions



Consult a Qualified Electrician

This is crucial. Working with motor circuits and underground wiring requires expertise and a thorough understanding of the NEC. Have a qualified electrician review your plan and perform the installation.

Local Codes

Always check and comply with local electrical codes and regulations, which may be more stringent than the NEC.

Nameplate Data

Always use the motor nameplate information (especially the service factor and code letter) when making calculations.

Motor Starting Characteristics

If these fans start frequently or under heavy load, the inrush current can be significant. Consider using soft starters or variable frequency drives (VFDs) to reduce inrush current. This would affect the breaker sizing calculations.

Consider Individual Circuits

While you can run multiple motors on a single branch circuit, it is often preferable to run each motor on its own dedicated circuit. This provides better protection and isolation and simplifies troubleshooting. However, depending on the distance and voltage drop, that may not be an option.

In Summary

Your plan is potentially feasible, but you must perform all the required calculations (FLA, conductor ampacity, breaker size, voltage drop, derating) and ensure compliance with all applicable NEC requirements. Do not proceed without consulting a qualified electrician. The branch-circuit breaker size will be the limiting factor. You may need to run a dedicated circuit to each motor.

Disclaimer: This information is for general guidance only. Always consult with a licensed and qualified electrician for your specific project. Flag for review