Okay, let's break down the confusing world of wire ampacity, breakers, and the NEC. You're right, there's a lot of conflicting information out there.

Understanding the Basics Ampacity

This is the current-carrying capacity of a conductor (wire) in amps. It's influenced by factors like wire gauge, insulation type (THHN in your case), ambient temperature, and how the wires are bundled together.

Overcurrent Protection

This is the job of a breaker or fuse. Its purpose is to protect the wire from overheating and potentially causing a fire.

NEC (National Electrical Code)

This is the standard electrical code in the US. Local jurisdictions often adopt and sometimes amend the NEC. It's not a set of instructions, but a set of rules that prioritizes safety.

Your Specific Situation: 12 AWG THHN in Conduit

Here's where the confusion arises.1. NEC Table 310.16 (Ampacity of Conductors)

This is the starting point. For 12 AWG THHN, the table generally lists an ampacity of 30 amps (at an ambient temperature of 30°C / 86°F). This is often where the 30A rating comes from.2.

Derating (Adjustment Factors)

Yes, it's true that if you have more than three current-carrying conductors in a conduit or raceway, you must reduce the ampacity from Table 310.16. Since you only have two current-carrying conductors (hot and neutral, ground is NOT considered a current-carrying conductor), derating for conductor count is

not required in your case.3. 12 AWG and 20 Amp Breakers: The Key Rule (NEC 240.4(D))

This is the critical section. NEC 240.4(D) specifically addresses the overcurrent protection of conductors and states: "Small Conductors. Unless specifically permitted in 240.4(E) or (G), the overcurrent protection shall not exceed 15 amperes for 14 AWG copper, 20 amperes for 12 AWG copper, and 30 amperes for 10 AWG copper after any correction factors for ambient temperature and number of conductors have been applied."Translation

Unless there's a specific exception allowed in 240.4(E) or (G), 12 AWG copper wire can only be protected by a

20-amp breaker.4. Exceptions to the 20 Amp Rule (240.4(E) and (G))

These exceptions are very specific and are unlikely to apply to your situation unless you are supplying motor-running overcurrent protection or a few other very specialized circumstances (taps, certain transformer secondaries, etc.). For a general-purpose circuit, these exceptions do NOT apply.

Therefore, in nearly ALL residential wiring situations, you should not use a 30 amp breaker with 12 gauge wire.Why the Confusion? Misinterpretation of Table 310.16

People see "30 amps" in the table and stop there, without reading the rest of the Code.

Lack of Understanding of 240.4(D)

This is the rule that restricts the breaker size for smaller conductors.

Older Installations/Grandfathering (Maybe)

If the circuit was installed many years ago, before the current NEC version was adopted in your area, there might have been different rules in effect. However, it is generally recommended to upgrade to current code when making changes. Check with a local electrician if you think this applies.

What to Do Now1. Replace the 30A Breaker

Replace the 30-amp breaker with a

20-amp breaker immediately*. This is a safety issue.2. Double-Check the Wiring

Ensure that all connections are tight and in good condition.3.

Assess the Load

Make sure the devices or appliances you're plugging into this circuit are rated to operate on a 20-amp circuit. Adding up the wattage of everything plugged in and dividing by the voltage (120V) will give you the current in amps. Don't exceed 16 amps (80% of the breaker rating) for continuous loads.4.

Consider Why the 30A Breaker Was Installed

If you're now tripping the 20A breaker frequently, it indicates you may need to consider adding a new circuit with larger-gauge wire if there is no misapplication of the circuit.5.

Consult a Qualified Electrician

If you are not comfortable working with electrical wiring, or if you have any doubts about your understanding of the NEC, hire a licensed electrician to inspect the circuit and make the necessary corrections. Electrical work can be dangerous, and mistakes can have serious consequences.

In Summary

While 12 AWG THHN has a listed ampacity of 30 amps in NEC Table 310.16, NEC 240.4(D) mandates that you protect it with a 20-amp breaker in most common applications. Using a 30-amp breaker on 12 AWG wire is a fire hazard and a violation of the NEC. Replace the breaker with a 20-amp breaker to ensure safety. Flag for review