You are not beating this to death! It's important to understand these fundamental concepts. You're asking great questions, and it's better to be thorough than to make assumptions that can lead to dangerous situations.Let's break down those two statements with a little more explanation

1. Wire size can be larger than OCPD (Overcurrent Protective Device)? YES
Absolutely! This is a common and often preferred practice. Think of it like this: you can have a pipe that can handle a large volume of water, but only run a smaller volume through it. Why do we do this? Future Proofing

You might want to increase the load on the circuit in the future. Installing larger wires initially saves you the hassle and expense of rewiring later.

Voltage Drop

Longer wire runs can experience voltage drop. Using a larger wire reduces the resistance, thus minimizing voltage drop and ensuring adequate voltage at the load.

Derating

Certain conditions require you to increase the size of your wire. Ambient temperature, number of current carrying conductors, and conduit fill can all require larger wires than the OCPD.

Specific Equipment Requirements

Some equipment might specify a minimum wire size regardless of the OCPD.

2. OCPD (Overcurrent Protective Device) can't be larger than wire current carrying capacity? YES Absolutely! This is a fundamental safety principle. The purpose of the OCPD (fuse or circuit breaker) is to protect the WIRE from overheating and potentially causing a fire. Think of it like this

The OCPD is the "guardian" of the wire. If the current in the wire exceeds the wire's safe current carrying capacity (ampacity), the OCPD trips, interrupting the circuit and preventing damage.

Why is this so important? Fire Prevention

Wires that are overloaded can overheat, melting insulation and potentially igniting nearby materials.

Equipment Protection

Overcurrent can damage equipment connected to the circuit.

NEC (National Electrical Code) Compliance

The NEC mandates that OCPDs be sized to protect the wires they serve.

In summary



Wire Ampacity >= Load Current (The wire must be able to handle the expected current) OCPD Rating <= Wire Ampacity (The OCPD must be small enough to protect the wire)Example

Let's say you have a circuit with a continuous load of 16 amps.1.

Calculate the minimum required ampacity

16 amps 1.25 (125% for continuous loads) = 20 amps.
2.

Choose a wire size

Based on common wire ampacity tables (like those in the NEC), you might choose 12 AWG copper wire, which is typically rated for 20 amps.3.

Choose an OCPD

You would use a 20 amp circuit breaker or fuse to protect the 12 AWG wire. You cannot use a 25 amp breaker, as that would exceed the wire's ampacity and defeat the purpose of the OCPD. You can use 10 AWG copper wire, which is rated for 30 amps and still use a 20 amp breaker.

Important Considerations



NEC (National Electrical Code)

Always consult the latest version of the NEC for specific requirements and exceptions. Wire ampacity tables, derating factors, and OCPD selection rules are all detailed in the NEC.*

Qualified Electrician: Electrical work can be dangerous. If you're not comfortable working with electricity, always hire a qualified and licensed electrician to perform the work.Keep asking questions! It's the best way to learn and understand these important concepts. Good luck with your electrical endeavors! Flag for review