Okay Bobby, let's break down your EV charger wiring situation to make sure you're safe and compliant with electrical codes. It sounds like you're doing your homework, which is great!Here's a comprehensive analysis of your options

Understanding the Problem: Continuous Load and Wire Ampacity
Continuous Load

EV chargers are considered a continuous load. Electrical codes require that wiring and breakers be sized to handle

125% of the continuous load current. Therefore, if your charger will draw 24A continuously, you need wiring and a breaker rated for at least 24A 1.25 = 30A. For a 32A continuous draw, you need wiring and a breaker rated for at least 32A 1.25 = 40A. Wire Ampacity

Wire ampacity (how much current a wire can safely carry) depends on:

Wire Gauge (Size)

Larger gauge wires can carry more current.

Insulation Type

Different insulation materials have different temperature ratings. Common types are NM-B (Romex), THHN, THWN, etc. Higher temperature ratings allow for higher ampacity.

Installation Method

How the wire is installed affects its ability to dissipate heat. For example, wire bundled with other wires will have a lower ampacity than wire run in free air.

Ambient Temperature

High ambient temperatures reduce wire ampacity.

NEMA 14-50R

This is a common receptacle for EV chargers and other high-power devices. It's rated for 50 amps. It's perfectly fine to use this even if you are not drawing 50 amps.

Analyzing Your Options



Option 1: Sub-Panel (Your Question) Existing Setup

40A sub-panel with 8-3 NM-B cable.

Proposed

30A breaker, 24A charger setting, #10 THHN to 14-50R.

NM-B and Ampacity

8-3 NM-B typically has an ampacity of 40A, assuming it's installed correctly and meets code requirements (e.g., not bundled too tightly with other wires, properly protected). However, the 60°C rating is the weak point for 40 amps.

#10 THHN and Ampacity

#10 THHN can be rated for 30A, if its temperature rating is used.

The Problem

You are planning on using 10 gauge wire on a 40 amp rated panel. 10 gauge wire is only designed for 30 amps. I would not recommend this

Option 2: Main Panel Proposed

40A breaker, 32A charger setting, #8 THHN to 14-50R.

#8 THHN and Ampacity

#8 THHN is usually rated for 50A at 90°C, and for 40A at 75°C.

Analysis

A 40A breaker is correctly sized.

Key Considerations and Recommendations


NM-B Temperature Rating and Derating

NM-B cable is typically rated for 60°C at the terminals, according to the NEC. While the 8-3 might theoretically be able to handle 40 amps at its maximum temperature rating, it's generally safer to derate, and you will need to connect the wires to the breaker with wire nuts and a THHN pigtail to match the breaker requirements. It's best to err on the side of caution.2.

Breaker and Wire Compatibility

Breakers have temperature ratings for the wires they accept. Many breakers are rated for 75°C wire. If your breaker is only rated for 60°C wire, then you're limited to the 60°C ampacity rating of your wiring. Always check the breaker specifications.3.

Local Electrical Codes

Crucially, your local electrical codes always take precedence. Consult with a qualified electrician to ensure your wiring is compliant with all local regulations. Permits are often required for this type of work.4.

Proper Connections

Use appropriately sized wire connectors and torque them to the manufacturer's specifications. Loose connections cause heat and are a fire hazard.

Therefore, the preferred option is option 2

I recommend going with the 40A breaker from the main panel with #8 THHN wire to the 14-50R receptacle.

Why I Prefer Option 2



Better Ampacity Margin

#8 THHN has a higher ampacity, offering a greater safety margin.

90°C Requirement

It satisfies the manual's requirement of 90°C wiring.

Future-Proofing

Allows for a slightly higher charging rate in the future if needed.

Important Cautions



Do Not Exceed Breaker Rating

Never install a breaker larger than the wiring's ampacity.

Consult a Qualified Electrician

Seriously. Working with electrical systems is dangerous. If you're not completely comfortable and knowledgeable about electrical codes, hire a licensed electrician to do the work. They can inspect your existing wiring, ensure everything is up to code, and perform the installation safely. They can also verify that the #8-3 NM-B cable is in good condition and suitable for the load, or advise on alternatives.

Permits

In most jurisdictions, you'll need a permit for this type of electrical work. Getting a permit ensures that the work is inspected and meets safety standards.

Torque Specifications: All electrical connections (breaker terminals, receptacle terminals, etc.) must be tightened to the manufacturer's specified torque. Use a torque screwdriver or wrench to ensure proper tightening.I hope this detailed breakdown helps! Be safe, and good luck with your EV charger installation! Flag for review