[General] ? on mixing wire sizes

I know there was a recent question here on the subject. My question is a) whether there are certain specific instances when it is permissible to mix wire sizes (according to the NEC, anyhow), and b) what some of those specific instances are.
I'm talking, of course, about downgrading the "last inch" of wiring, say a short run to a device like a switch or light fixture. For example, running a short leg of 14 ga. wire on a 20 A circuit. I'm pretty sure such exemptions exist to the otherwise ironclad rule of sizing conductors according to the "ampacity" of the circuit.
If you think about it, *all* our circuits are actually wired that way, or at least connected that way. In fact, it's way worse than that: a typical 20 A outlet circuit has lots of devices plugged into it with, say #18 cord sets (maybe even smaller). (Yes, I fully understand why you don't want to have any #18 wire *inside* your walls, thank you very much.)
I will say that in practice, I never do this (run smaller wire to part of a circuit), since I'm not sure of these exemptions; even if I do learn of specific cases, I may still never do so.
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David Nebenzahl wrote:

Some cut.
One example unrelated to household use. We bury wire to irrigation well motors and irrigation systems. It might be a 1300' run or longer. We oversize the underground wire due to voltage drop. We run the wire to a terminal box, then run the regularly sized THWN to the pump and system panels. We might bury 4/0 AL, then run #2 and #10 CU to the well motor and irrigation system. Would someone with a large home shop do something similar?
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On 3/18/2011 6:59 PM, David Nebenzahl wrote:

...
As long as it is stationary equipment, afaik for household wiring, no exceptions.
Unless the most recent Code is different, the Code refers to loads as permanently connected and cord- and plug-connected. The Code wiring refers to the wiring to permanently connected devices and the outlets for cordset appliances, not the appliances themselves. The protection of circuit breakers in those circuits is for the protection of the wiring not the loads so it is with respect to that sizing that wiring sizes are limited.
_Within_ the device of a connected load the wiring may be smaller just a cordset may be but it's the responsibility of the device to protect that wiring, not the supplying circuit.
If there are exceptions, they're pretty arcane in household application anyway...
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On 3/18/2011 7:59 PM, David Nebenzahl wrote:

Not to mention, you never know when some dumb SOB 20 years later will hang something else off the end of your branch circuit.
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David Nebenzahl wrote:

You can, of course, use larger wire.
You can't use smaller wire for the branch circuit.
There are specialized rules for taps to feeders which usually take me about 5 minutes to understand again.

You can use smaller fixture wires and extension cords. The basic rule is 240.4.
240.5 has size and length limits for smaller fixture wires. Also for extension cords and the cords that comes on equipment.
Ovens and cooktops can be smaller wire to connect to the branch circuit.
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On Fri, 18 Mar 2011 15:59:22 -0800, David Nebenzahl

You can't install smaller wire that 240.4(D) specifies on the line side of an outlet (receptacle or lighting outlet) The actual fixture or plug in equipment can use the "fixture wire rules" that allow smaller wire, based on the load of that equipment. You still have rules about what fixture wires can be connected to what size breaker but that is based on short circuit protection, not overload protection. They assume the load is limited by the design of the equipment. 240.4(D) is the way the code keeps you from overloading the wire in the wall. If you look at the ampacity of 12 and 14 gauge wire in 301.16 you see a number significantly higher than 20 and 15a, safely at least 125% higher. That is your 80% safety factor. Once you stick a plug in the receptacle, you are on your own. It should be noted, most electrical fires are started on the customer side of the outlet.
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I am not one of the electricians. There used to be a provision in the code that said something like, "if the portion in question can never pull more than the ampacity of the smaller wire then it is okay to use."
To be honest, I never really figured out how that could be. A switch run on the end of a circuit was the closest example I ever conceived of where some dufus could not add on to the end of the run in the future.
Lamp cords and fixture wire in the box do not apply here.
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