You're on the right track, but let's break down each scenario and clarify a few key points using the NEC (National Electrical Code) as a guide.

Scenario 1: MCA = 13.3A, Max Breaker = 20A Breaker Size

Your understanding is

correct. The MCA (13.3A) dictates the minimum ampacity the circuit must handle. Since standard breaker sizes jump from 15A to 20A, and the maximum breaker is 20A, a 15A breaker is the appropriate and compliant choice. NEC 430.52(C)(1) typically allows for rounding up to the next standard overcurrent device rating if the calculated MCA doesn't match a standard size, but you also have to stay within the maximum breaker rating provided by the equipment manufacturer. Conductor Size

This is where the confusion lies.

Ampacity

#12 AWG copper wire typically has an ampacity of 20A (based on the NEC 310.16 table for 75°C terminals, which is often the default assumption). Since your MCA is 13.3A and your breaker is 15A, #12 AWG would technically be sufficient from a purely ampacity perspective.

Terminal Compatibility

However, the datasheet stating "Terminal Block to Accept #10 AWG" is crucial.

You likely need to use #10 AWG, even though #12 AWG has sufficient ampacity. Why? This is a listing requirement. The equipment manufacturer has designed the terminal block to properly and safely accommodate only #10 AWG wire. Using #12 AWG could lead to: Loose Connection

The smaller gauge wire may not make a secure connection in the larger terminal. This can cause arcing, overheating, and potential fire hazard.

Improper Torque

The terminal block may not be able to be properly tightened down on the smaller wire, leading to the same issues as a loose connection.

Warranty Issues

Using a wire size other than what the equipment manufacturer specifies could void your warranty.

NEC Implication

NEC 110.3(B) mandates that listed or labeled equipment be installed and used in accordance with any instructions included in the listing or labeling. The "Terminal Block to Accept" specification is part of that listing.

How to Handle It

Use #10 AWG from the breaker to the equipment's terminal block.

Scenario 2: MCA = 17.7A, Breaker = 20A Breaker Size

Your understanding is

correct. An MCA of 17.7A requires a breaker sized at least that. A 20A breaker is appropriate. Again, check the equipment documentation for any maximum breaker size restrictions. Conductor Size



Ampacity

#12 AWG copper wire typically has an ampacity of 20A. So, it seems sufficient to handle the 17.7A MCA with a 20A breaker.

Terminal Block Compatibility



You must know what size wire the equipment's terminal block is designed to accept. This is the deciding factor. If the terminal block accepts #12 AWG

You can use #12 AWG for the entire run.

If the terminal block only accepts #10 AWG

You must use #10 AWG, as explained above.

Key Takeaways and Recommendations


Always prioritize the equipment manufacturer's instructions and specifications. The MCA and maximum breaker size are critical, but so is the specified terminal block wire size.2. Check the Equipment Documentation

Refer back to the equipment's datasheet, installation manual, and any other documentation provided by the manufacturer. Look specifically for wiring instructions, allowable wire sizes for the terminal block, and any notes about specific installation requirements.3.

Local Codes and Inspectors

While the NEC is a national standard, local codes and inspectors may have additional requirements. It's always best to check with your local authority having jurisdiction (AHJ) to ensure compliance.4.

Oversizing

There's generally nothing wrong with oversizing your conductors (using a larger gauge wire than required by ampacity), as long as it fits within the terminal block's specifications. Oversizing can sometimes improve voltage drop, especially over longer runs. However, it also adds to the cost.5.

Voltage Drop

While not directly related to your questions, it's good practice to consider voltage drop, especially for longer runs. If the equipment is far from the breaker panel, you may need to upsize the conductors to compensate for voltage drop.

In summary: Ampacity is important for wire size, but always adhere to the equipment manufacturer's terminal block wire size specifications. When in doubt, contact the equipment manufacturer for clarification. Flag for review