So...correct me if I am wrong, but a motor with a FLA of 16A, (16 x 125% =20) could run on 12 ga and be protected by a (16 x 250%=40) 40a breaker AS LONG AS it has overload protection.
Boy that just sounds nuts in my head. I guess I am more used to constant loads, like lights and outlets and such.
CharlieS: You are correct. I said it backwards. Size the breaker to the wire. Except under some special circumstances like these electric motors.
LD1
somewhat correct but code doesnt allow for, and no inspector i know, (myself included) will allow an overfusing scenerio where you overfuse a conductor in a main or sub panel. The proper and safe way is supply amperage through a larger amp circuit and fuse it accordingly at an approved connection point. An SSU on a furnace blower for example by code is required to have fuse protection in accordance with motor overfusing guidelines. so a hypothetical moter with an LRA of 2 amps could legally have a 3 amp fuse fed by a 15 or 20 amp circuit. In the furnace case because of the motor type and application is rated at 150% for start up purposes.
An industrial motor, fed through a contactor with heater coils, or dedicated fuse block, in cases where contactor startup isnt needed, can indeed be over fused sometimes as high as 400% when fed off a properly sized main feeder.
there are a couple of code regs that apply here depending on the application. 25 foot tap rule, and 10 foot tap rule are the most commonly applied.
Though overcurrent protection by code may be rounded up to the next standard fuse (or breaker) size in some cases a slightly higher % overfuse rating may be used. Many inspectors though will not allow this and require the smaller standard size based on application tables.
you are right in your example calculations with the 125% multiplier but that's actually a derate factor. amp draw of any electrical load must be derated to 80 percent, so a load of 16 amps x 125% =20 meaning you can supply a 16 amp load on a 20 amp circuit. Another way to calculate is whats the max load an xx amp circuit can carry. 20 amps x 80% = 16.
the way a breaker or fuse works is pretty basic. they are nothing more than a thermostat that senses overheating in the conductor and opens up when the conductor becomes to warm. larger conducters carry more heat thus larger breakers have a higher temp. rating. (its more complex than that but this is the simple way to explain it) Thus in the 40 amp breaker/#12 wire example the wire doesnt get hot enough, soon enough to trip a breaker or blow a fuse to prevent a melt down.
Wire insulation type as well has a bearing on fusiblity.
THWN rated insulation for example has a lower temp rating than THHN.
physical containment is also an issue. free air power lines for example can be rated at a much higher fused load, than the same conducter/load in conduit.
The number of conductors or "conduit fill" also play's a key part in how circuits must be derated, because of overall heat generation in the raceway.
A conduit at max fill capacity can result in Current carrying conductors having to be derated by as much as 50% over maximum allowable load based on conductor size alone.
Bottom line is there are may variations/combinations allowed by code depending on the application/equipment/working conditions.
Code has many different exceptions that allow for variations depending on specific application and conditions, but in no cases are main feeders allowed to be overfused under conditions discribed in the origional post.