Okay, let's break down the wiring for your makeup air system with the 20kW duct heater and the subpanel expansion. This involves some electrical code considerations, so remember that I'm providing general guidance, and you should always consult with a licensed electrician for a safe and code-compliant installation. They can assess your specific situation and local codes.

Duct Heater Wiring (Single Circuit vs. Two Circuits) Continuous Load

You're correct that a duct heater, especially one used for makeup air, is generally considered a continuous load. This means it's expected to operate for 3 hours or more at a time. The NEC (National Electrical Code) requires that you size the wiring for continuous loads at 125% of the rated current.

Calculating Ampacity

83 amps 1.25 = 103.75 amps. This is the minimum ampacity your conductors need to handle.

Temperature Rating

Since you don't have a temperature rating for the heater, you must assume the lowest permissible rating. As such, you should follow the 60°C column in the Ampacity chart.

Single Circuit

Based on the 60°C column of the Ampacity chart, you will need

#1 AWG Copper Wire to handle 110 Amps. Using a single circuit will require less overall parts, but you need to handle and pull larger wires, which can be difficult depending on your setup. Two Circuits (Splitting the Load)

This is often a practical solution. Divide the heater load: 83 amps / 2 = 41.5 amps per circuit. Apply the continuous load factor: 41.5 amps 1.25 = 51.875 amps per circuit. Based on the 60°C column of the Ampacity chart, you will need

#6 AWG Copper Wire to handle 55 Amps. This makes the wire easier to manage. You'll need two breakers in your panel, and you'll need to ensure the heater is designed to be wired in this configuration (usually, there are separate terminals for each heating element).

Ground Wire Sizing

This is based on the overcurrent protection device (the breaker) protecting the circuit, not the load current itself. Refer to NEC Table 250.122. Single Circuit (110A wire)

You'll likely need a 110A breaker. According to NEC Table 250.122, for a 100A overcurrent device, you'll need a

#6 AWG Copper Wire. Two Circuits (55A wire)

You'll likely need a 60A breaker. According to NEC Table 250.122, for a 60A overcurrent device, you'll need a

#10 AWG Copper Wire.New Subpanel Wiring Subpanel Feed

You're planning to feed the new subpanel from the existing 150A subpanel.

Wire Sizing

A 150A feeder requires at least

#1/0 AWG Copper Wire or #3/0 AWG Aluminum Wire. This assumes the terminals in your breakers and subpanels are rated for 75°C. Double-check the temperature ratings of the equipment you're using. If you're using aluminum, you MUST use connectors and equipment rated for aluminum conductors (AL-CU rated). Ground Wire (Equipment Grounding Conductor - EGC)

Again, refer to NEC Table 250.122, based on the 150A overcurrent device protecting the feeder to the subpanel. For a 150A breaker, you need a

#6 AWG Copper Wire. This ground wire must be run with the feeder conductors. Neutral

Run a neutral conductor of the same size as the ungrounded conductors (#1/0 AWG Copper or #3/0 AWG Aluminum). While the NEC permits reducing the neutral in some cases, it's generally best practice to run a full-sized neutral, especially for a subpanel that will be serving a variety of loads.

Grounding



Separated Neutral and Ground

Absolutely critical: In a subpanel, the neutral and ground must be separated. The neutral bus in the subpanel should be isolated from the enclosure. The ground bus should be bonded to the enclosure.

Bonding

The subpanel enclosure MUST be properly grounded.

Important Considerations and Recommendations



Local Codes

Always check and comply with your local electrical codes and regulations. They may have stricter requirements than the NEC.

Electrician

This is not a simple DIY project.

Hire a qualified, licensed electrician to do the work. They will ensure the installation is safe, code-compliant, and properly grounded. They can also perform load calculations to ensure your service and panels can handle the increased load. Breaker Size

The breaker size should be appropriate for the wire size and the equipment being served. Do not exceed the ampacity of the wire.

Conduit

Consider using conduit for running the wires, especially for the subpanel feed. This provides physical protection for the conductors.

Torque

Use a torque screwdriver or wrench to tighten all connections to the manufacturer's specifications. Loose connections are a common cause of electrical problems and fires.

Labeling

Clearly label all circuits in the panel for easy identification.

In summary



Duct Heater

You need to decide between a single circuit of #1 AWG or two circuits of #6 AWG (Copper) based on the 60C column. Grounding requires #6 AWG or #10 AWG Copper, respectively.

Subpanel Feed

#1/0 AWG Copper (or #3/0 AWG Aluminum) for the feeder conductors, #6 AWG Copper for the equipment grounding conductor, and a full-sized neutral.

Disclaimer: This information is for general guidance only and does not constitute professional electrical advice. Consult with a licensed electrician for a safe and code-compliant installation. Electricity is dangerous, and improper wiring can lead to fire, shock, and death. Flag for review