Thanks you really help me understand the tap rule .why does the nec allow you to size the conductors less than the Ocp , if you stay under 25' & 10' .how does this not result in the insulation being burnt up
No problem as that is what we are here for.
Just to add a little to what others have said, we have two types of protection that we use to protect conductors, and we see this when we apply the rules in 240.4(D) that allow us to follow different rules for tap conductors (e) and motors or other installations listed in (G) such as air conditioner condensers where we can have a #12 awg with a 40 amp breaker on it where we wire for the minimum and protect for the max circuit on the A/C unit label, this is because motors and A/C units being a single fixed load will have overloads at the end of the circuit that will protect the conductors from over loads, while the breaker at the supply end will protect the conductors from short circuits whether they are a line to line or ground fault, we use this allowance so the the start up current of motors which is the lock rotor current many times the normal run current don't trip the breaker when a motor starts but are of such short duration that the conductors are not damaged.
Breakers have two protection modes, one is for overloads which are a long term over loading of the conductors when to much current is being drawn from the end loads which can cause the conductor to exceed the insulation safe temperature rating and can cause the insulation to fail.
The second mode if called the instantaneous rating which is a magnetic pick up that is fast acting when the circuit exceeds this pickup level, this pick up can be hundreds of amps higher then the rating on the breaker, but is designed to open the breaker within a few cycles so the conductors do not have time to heat up to the point of damaging the insulation which is up to 5 cycles, we can see this protection when we look at the protection curve for a given breaker, a bolted fault line to line or to ground will produce hundreds of amps if available, the only thing that limits this is the impedance or resistance of the circuit as resistance is current limiting, this is why if the circuit is to long the time for the breaker to open will be longer and can exceed the safe rating of a conductor so all this must be taken into account when selecting the right size conductor for a given circuit, the NEC tends to error on the side of safety to prevent this from being exceeded but there are some cases you must do the calculations yourself to make sure the length of the conductor will still allow the breaker to be able to protect it in the case of over loads or short circuit ground fault.
Some online voltage drop calculators will warn you if you exceed this when you use it, here is one such calculator that does:
http://www.electrician2.com/calculators/vd_calculator.html
A little side effect of this instantaneous rating of breakers is why we some times see the 100 amp main on a house trip when a 30 amp circuit is faulted when available fault current is high enough to exceed both breakers instantaneous ratings.
so knowing this helps us understand why the NEC doesn't allow us to use undersized conductors on circuits that the end load can not be controlled such as receptacles or lightning circuits or other loads such as motors that don't have overload protection, it only allows it on the loads in 240.4 (E) or (G)'s list where the articles in the list allow it, because these loads are a fixed current amount and protected by overloads at the load end of the circuit that will not allow the circuit to be over loaded.
