Okay, let's break down the wiring requirements for your Mitsubishi mini-split systems, considering the NEC (National Electrical Code) and your specific situation.

Understanding the Terminology



MCA (Minimum Circuit Ampacity)

This is the minimum ampacity your circuit conductors must have. It's calculated by the manufacturer to account for the continuous load of the unit, plus an additional safety factor.

MOCP (Maximum Overcurrent Protection)

This is the maximum size breaker or fuse allowed to protect the circuit. This is generally sized to protect the equipment and not necessarily the conductors.

Code Considerations - Key Concepts NEC 310.15 (Ampacity of Conductors)

This section dictates the ampacity (current-carrying capacity) of wires based on insulation type, ambient temperature, and the number of conductors in a raceway (conduit).

NEC 440 (Air Conditioning and Refrigerating Equipment)

This entire article covers the specific requirements for wiring and protecting HVAC equipment.

Analyzing Your Situation



Unit #1 (3 Zones)

MCA = 31.5A, MOCP = 40A

Unit #2 (2 Zones)

MCA = 26.9A, MOCP = 40A

Distance

~20ft and ~24ft runs

Conduit

3/4" gray conduit

1. Is 10/2 Suitable?

Let's first consider the ampacity of 10 AWG copper wire. The exact ampacity depends on the insulation type (THHN, THWN, etc.) and the temperature rating, but generally, 10 AWG copper is rated for 30 amps. Unit #1

MCA is 31.5A. A standard 10 AWG wire rated for 30A is insufficient based on MCA. A 10 AWG would be overloaded and unsafe.

Unit #2

MCA is 26.9A. This seems to be enough for standard 10 AWG rated for 30A.

2. Derating for Multiple Conductors in Conduit

You must derate the ampacity of the conductors when you have more than three current-carrying conductors in a conduit. In your case, you'll have six current-carrying conductors (two hot, two neutral from the 10/2 cables) in the conduit. NEC Table 310.15(C)(1) states that for 4-6 current-carrying conductors in a raceway, you must apply a derating factor of 80%. Therefore, the adjusted ampacity of your 10 AWG wire is 30A 0.8 = 24A. (Assuming 75C or 90C rated insulation, verify your wire's rating).

With the 80% derating, 10 AWG is now UNDERSIZED for BOTH units.3. Is 8 AWG Required for Unit #1?

Yes, because you are using two units with different MCA's Unit #1

8 AWG copper wire is rated for 40A. After derating by 80%, the ampacity of 8 AWG is 40A 0.8 = 32A. 32A is sufficient to feed the MCA of 31.5A

Recommended Wire Sizes & Other Considerations Unit #1 (3 Zones): 8/2 AWG copper with ground. Unit #2 (2 Zones): 10/2 AWG copper with groundImportant Considerations and Recommendations



Wire Type

Use THHN or THWN-2 insulated wire. These are commonly used for conduit installations and are rated for higher temperatures.

Grounding

Run a separate grounding conductor (green or bare copper) along with your hot and neutral wires. Make sure the ground wire is appropriately sized according to NEC 250.122, based on the size of the overcurrent protection device (40A in your case).*

Local Codes

Always check with your local electrical inspector or permitting office. They may have specific requirements that supersede the NEC.

Disclaimer: This is for informational purposes only and should not be considered a substitute for advice from a licensed electrician. Electrical work can be dangerous, and you should always consult with a qualified professional before performing any electrical work. Flag for review