On 1/26/2014 9:27 AM, Bob Engelhardt wrote:
On 1/26/2014 2:18 AM, bud-- wrote:
... The breaker can be significantly larger
than the rating of the wires, and provides short circuit protection.
...

I don't think so! The breaker is there to protect the wires. If there
is a load on the circuit greater than what the wires can handle, the
breaker opens. You do not want the wires overheating and it doesn't
take a short to do it.

Bob

Under the US NEC the wires have to be 115% of the motor run current. A
time delay breaker can be 175% of the motor run current. (Breakers that
don't have a time delay can be even larger.) As stated, for a motor
circuit, the breaker provides _short circuit protection_. Breakers can
be large so they don't trip on normal motor starts. (I have seen an
example for something like a 1/2HP motor where the breaker was twice the
wire rating.)

_Overload protection_ is usually at the motor (at the load end of the
circuit). One common method is a motor starter with an 'overload' unit
that is closely matched to the run current. Another is an internal
thermal protector that disconnects the motor when it overheats. An
internal thermal protector may be wired to drop out a motor starter.


For welder circuits, the breaker can be even larger with respect to the
wire rating. That can happen if the welder has a low "duty cycle".