Okay, I understand. You're looking at a previous forum thread about sizing a subpanel for 5 HVAC units, and you want to confirm your calculations. Let's break down the issues and considerations for properly sizing the subpanel.

Understanding the Information from the Forum Post



Min. Circuit Ampacity (MCA)

40 Amps. This is the minimum amount of current the circuit must be able to handle to operate the unit safely. It takes into account the running current of the compressor and other components, as well as potential motor starting surges.

Max Overcurrent Protection (MOCP) or Max. Breaker Size

60 Amps. This is the largest breaker allowed to protect the equipment. This prevents damage in case of a short circuit or overcurrent.

Why a Simple Multiplication is Not Sufficient

While 60 amps 5 units = 300 amps might seem logical at first glance, it's not the correct way to size a subpanel for several reasons:1.

Diversity/Demand Factors

It's highly unlikely that all five HVAC units will be running at their maximum load simultaneously for extended periods. HVAC systems cycle on and off based on temperature and thermostat settings. Building codes and engineering practices allow for a "demand factor," which reduces the assumed load based on the number of units and their usage patterns.2.

Motor Starting Current (Inrush)

HVAC units, especially those with compressors, draw a significantly higher current when they initially start up compared to their running current. This inrush current is usually only momentary but needs to be considered. A breaker that's exactly sized for the running load would trip constantly during startup.3.

Future Expansion

Consider if more HVAC units (or other loads) might be added to the subpanel in the future.

Steps to Determine the Correct Subpanel Size

Here's a better approach to calculating the required subpanel size:1.

Gather Complete Information



Unit Information

You've got the MCA and MOCP, but also look for:

Locked Rotor Amps (LRA)

This is the current drawn when the motor is stalled (like during initial startup). It's very important for breaker sizing and calculating voltage drop.

Full Load Amps (FLA)

This is the current the unit draws under normal full-load operating conditions.

Other Loads

Are there any other loads connected to the HVAC units that are not accounted for in the MCA? (e.g., electric heaters, condensate pumps, etc.)

Local Codes

Check your local electrical codes (e.g., NEC in the US) for specific requirements for HVAC load calculations and demand factors. These codes are in place for safety and often dictate how you must calculate the load.

Utility Requirements

The power company may have specific requirements for service sizing that could affect your subpanel choice.2.

Calculate Total Connected Load

Add up the MCA for all five units. This gives you a baseline connected load. Consider other connected loads to the HVAC as mentioned above.3.

Apply Demand Factors (Refer to NEC or Local Codes)

This is where the biggest savings come in. Codes typically allow you to reduce the connected load based on the number of units.

Important

You must consult the NEC (or your local electrical code) for the allowed demand factors. There is no one-size-fits-all answer.4.

Calculate Motor Starting (Inrush) Requirements

The NEC and local codes have detailed rules about sizing breakers and conductors to handle motor starting currents. The most common approach is to size the breaker for at least 125% of the unit with the largest FLA, then add the full load amps of the other units.

Sequential Starting

If the HVAC units are designed to start sequentially (one after another), this can significantly reduce the inrush current seen by the subpanel. If they start simultaneously, you need to account for the combined inrush.

Voltage Drop

The inrush current can cause a voltage drop on the circuit. You need to ensure the voltage drop remains within acceptable limits (typically 3-5%) during startup to prevent equipment malfunction or damage. This might require increasing the wire size.5.

Add a Safety Margin

After applying demand factors and accounting for motor starting, add a reasonable safety margin (e.g., 15-20%) to account for potential future loads or unforeseen circumstances.6.

Select Subpanel Size and Overcurrent Protection

Choose a subpanel with an ampacity rating that is equal to or greater than your calculated load (including safety margin). The main breaker in the subpanel should be sized according to your calculations and comply with the NEC and local codes. It cannot exceed the ampacity rating of the subpanel.

Example (Illustrative - Consult Your Local Codes!)

Let's assume (for illustration only) that the NEC allows a demand factor of 80% for 5 identical HVAC units:1.

Total Connected Load (MCA)

5 units 40 amps/unit = 200 amps2.

Apply Demand Factor

200 amps 0.80 = 160 amps3.

Largest Unit FLA

Suppose the FLA of each unit is 30amps and the LRA is 120 amps. We need to accommodate the inrush current of the largest motor starting, add the current of all the other units running, and add this number to the previously calculated 160 amps.1.

Add a Safety Margin

160 amps 1.20 = 192 amps

In this highly simplified example, you might be able to use a 200-amp subpanel. But you must do a full calculation according to the NEC and local codes and consult with a qualified electrician.Important Considerations



Licensed Electrician



This work MUST be done by a licensed and qualified electrician. Electrical work is dangerous and requires a thorough understanding of codes and safety practices. This explanation is for informational purposes only and should not be used as a substitute for professional advice. Permits

Electrical work typically requires permits from your local building department. Make sure you obtain the necessary permits before starting any work.

Coordination

Work closely with the HVAC contractor to ensure that the electrical system is properly sized and installed to meet the needs of the HVAC equipment.*

Voltage Drop

Always calculate voltage drop for the wire size you are planning to use. This is especially important on long runs and with high inrush currents.

In summary: While the original post's simple multiplication might seem intuitive, it's not the correct way to size a subpanel. You need to consider demand factors, motor starting currents, voltage drop, future expansion, and, most importantly, the requirements of the NEC and your local electrical codes. Consult with a licensed electrician to perform the proper calculations and ensure a safe and compliant installation. Flag for review