Okay, let's break down this scenario and see what your options are, keeping safety and code compliance at the forefront.

Understanding the Situation Existing AHU

150A circuit for both cooling/air handling and electric heat. This is being removed.

New AHUs

Two units, part of a VRV system. Each unit has a 15A MOP (Minimum Overcurrent Protection) rating for the AHU itself (likely the fan motor and controls). Each unit has a 30A electric heat pack.

Feeder

Currently a 150A feeder feeding the existing AHU.

Location

Attic with high temperatures (potentially exceeding 120°F).

Goal

Safely and compliantly power the two new AHUs.

Feasibility and Code Considerations1. Load Calculation is Crucial

You must perform a proper load calculation to determine if the existing feeder is adequate to serve the two new AHUs. This is not optional. NEC 220 covers branch-circuit, feeder, and service calculations. Consider the following:

Total Connected Load

This is the sum of all loads. For each new AHU, it would be 15A (AHU MOP) + 30A (Electric Heat) = 45A Two Units: 45A x 2 = 90A

Demand Factors

Demand factors might allow you to reduce the calculated load if you can reasonably demonstrate that not all loads will operate simultaneously at their maximum. However, for HVAC systems, it's often safer to assume near-full-load operation, especially for heating. It's unlikely you'll get significant demand factor credit for electric heat in an attic space, especially in a commercial application.

Diversity Factor

You might be able to apply a diversity factor if the AHUs serve different zones and it's unlikely they'll both be running at peak heating simultaneously. Consult with a qualified HVAC engineer for this.

Existing Load

If anything else is on this feeder you need to account for that.

80% Rule

NEC article 210.19(A)(1) states a branch circuit must not exceed 80% of its capacity when supplying a continuous load. Electric heating is a continuous load so 150A x .80 = 120A.

Load Calculation Conclusion

If your load calculation shows that the new AHUs, with a safety margin, fit within the existing 150A feeder's capacity, and the wire size and conduit are adequate, then maybe you can proceed. If the calculation shows the load exceeds the feeder's capacity, you MUST upgrade the feeder.2.

Tapping the Feeder

Tapping the existing feeder is possible, but it requires careful planning and compliant installation. Consider these points:

Tap Rules

NEC 240.21 outlines the rules for tap conductors. A critical factor is the length of the tap conductors relative to the feeder conductor size. Because you want to be within 5ft of the feeder this makes this more feasible.

Tap Conductor Size

The tap conductors must have an ampacity of not less than the rating of the device or overcurrent protective device supplied by the tap conductors. In your case the 30 A breakers require at least a 10 AWG conductor. The 15 A MOP requires at least a 14 AWG.

Overcurrent Protection

You must install appropriately sized overcurrent protection (circuit breakers or fuses) at the point where the tap conductors connect to the feeder conductors (150A circuit). This is what your two new disconnects that will house the two new circuit breakers that will need to be installed.3.

Disconnects

You must install a disconnecting means (disconnect switch or circuit breaker) for each AHU, located within sight of the unit and readily accessible. This is code requirement.4.

Electric Heat Pack Installation

The contractor's installation of the electric heat packs is critical. Ensure they follow all manufacturer's instructions and relevant NEC requirements. Verify the heater's voltage, wattage, and ampacity.5.

Environmental Considerations (Attic Temperature)

The high attic temperature is a major concern.

Derating Conductors

High temperatures significantly reduce the ampacity of conductors. You must derate the conductors based on the temperature rating of the wire insulation and the ambient temperature. Consult NEC Table 310.15(B)(1) and the associated notes.

Equipment Temperature Ratings

Ensure that all equipment (disconnects, circuit breakers, load centers - if used) are rated for the expected ambient temperature. Standard residential-grade equipment may not be suitable for a hot attic. Look for equipment with higher temperature ratings (e.g., rated for 75°C or 90°C).

Avoid a Load Center if Possible

Given the high heat, avoiding a load center in the attic is a good idea. If you absolutely need one, use a NEMA 4X enclosure to help protect it, but still check temperature ratings.

Possible Solutions1. Taps and Disconnects (If Load Calculation Allows)

This is likely the most straightforward option if the load calculation works. Install a suitable junction box close to the existing 150A feeder. Install appropriately sized tap conductors from the 150A feeder to the two disconnects. Run wiring from the disconnects to the AHUs and their electric heat packs, ensuring correct wire size, conduit, and derating.2.

Subpanel (Only if Suitable Equipment Can Be Found)

If you must use a subpanel, look for one specifically designed for high-temperature environments. This might be difficult to find and expensive. Ensure the subpanel is properly sized for the load and has adequate overcurrent protection. Consider using a NEMA 4X enclosure to protect the subpanel from dust and moisture.3.

Upgrade the Feeder

If the load calculation shows the existing feeder is inadequate, upgrading it to a larger size is the safest and most code-compliant solution. This might involve running a new circuit from the main panel.

Important Considerations and Recommendations Consult a Qualified Electrician

Do not attempt this work yourself unless you are a licensed and experienced electrician. This is not a DIY project. Incorrect wiring can lead to fire, shock, and code violations.

Permits and Inspections

Obtain all necessary permits and inspections from your local electrical authority.

Manufacturer's Instructions

Follow all manufacturer's instructions for the VRV system, AHUs, electric heat packs, and any electrical equipment.

Documentation

Keep accurate records of all work performed, including load calculations, wire sizes, conduit types, and equipment specifications.

In summary, this project is feasible only if a load calculation shows the existing feeder has sufficient capacity, and if you take into account the high attic temperatures. You'll need to carefully plan the taps, disconnects, and wiring, and most importantly, involve a qualified electrician. Flag for review