Puzzling Wire Size Question

I was getting ready to install my new bath fan when something struck me and now I need to have some answers...

I am installing a fan on an existing 20 Amp line w/12 gauge, but the small wires from the fan are 14 gauge... not suppose to let 20 Amp go on 14 gauge. This goes against what we're taught???

What about when we put a lamp w/thin lamp cord (16 or less) into a 15 Amp outlet. Why don't the wires melt away???

Are you electricians playing some cruel joke on us???

In all seriousness, why are we allowed to go down to a smaller gauge when we power our stuff?

 

The breaker will provide short circuit interuption regardless of wire size, in other words, a short circuit will draw hundreds of amps for an instant and trip the breaker even on a smaller wire. The wires supplying your fan are supplying a fixed load, in other words again that it will only draw so much current and it will not overload the incoming wire. The breaker system is designed to protect the wiring in the structure, not neccesarily everything plugged in to it. It will provide short circuit interuption but it is the users responsibility to use extension cords responsibly and not overload them, should be taught in school.

 

The rules for wiring in the wall are different than the rules for fixture wiring. In part, this is due to the fact that concealed wires in the wall are a greater hazard and dissipate heat less efficiently.

 

Also, it is very common for a 20A circuit to have many "outlets" on it. This is why it is legal to have 15A receptacles on a 20A circuit as long as there is more than one outlet. (Outlet refers to any electrical using tap (light, receptacle, fan, etc.)

 

All of this begs a question I have also wondered...

When a 15 Amp outlet is the first ficture on a 20 Amp circuit, will the fixtures after the 1st receptacle be receiving 20 Amps, or drop down to 15 Amp???

My questions may seem odd to some, but these are just little things that I have thought of. I appreciate the help.

 

All correctly-working fixtures receive the amps they ask for (via their internal impedence) regardless of what else is on the circuit -- until the breaker blows. The order of the outlets on the circuit makes no difference.

 

Most people dont know this, but there is smoke in the wires and when it gets too much electric it comes out.

 

The fixture wires only have to carry enough current for the particular fixture. Chandeliers are a great example. The wire is in the air, generally can dissipate heat, and is rated for the size bulbs the manufacturer specifies. The limit on using to small a wire is that the breaker needs some time to open, and the wire should support an overload to allow the breaker to trip on a short circuit without being damaged. The 'fusing current' is the number used to determine where the wire itself melts. Small wires can carry surprising currents without damage if the duration is held to the millisecons a breaker needs to trip.

 

Article 240.5(B)(2) of the National Electrical Code permits the connection of fixture wire to a 20 amp branch circuit as follows: Up to 50 feet of #18, up to 100 feet of #16, and no limit to the length of #14. The manufacturer knows how much current the fixture will draw. That #14 will not be called upon to carry more than it is capable of carrying. A #14 fixture wire is rated for 17 amps (Table 402.5), and your fan is likely to use less than half of that value.