air compressor

[220/221]

Short circuits are very different then overload, overload is another issue all together.

The only way for motor circuit conductors to be overloaded is if the motor starts to malfunction.

If we are taliking about "dead" shorts, I agree but I can see cases where the conductors can be overloaded WITHOUT a motor malfunction. I have seen it MANY times.

It starts with a poor connection in a JB before the motor. The connection gets worse with use resulting in more resistance. Eventually the wire gets so hot it starts melting the insulation.

It seems like the 30 amp breaker would trip befor the 60 but I guess that the short is what eventually trips it anyway huh?


Hmmmm.

 

[iwire]

It starts with a poor connection in a JB before the motor. The connection gets worse with use resulting in more resistance. Eventually the wire gets so hot it starts melting the insulation.

No matter where the loose connection might be in the circuit the circuit can not continuously draw current in excess of the motor overload rating.

But a loose connection can cause conductor melting from the heat and that can happen even when you have a 30 amp breaker protecting 10 AWG.

Also something to keep in mind is that the instantaneous trip portion of breakers from 15 to 100 amps is pretty close to the same, it is only the thermal trip part (long term overload) that is really different from one breaker to another

 

[220/221]

No matter where the loose connection might be in the circuit the circuit can not continuously draw current in excess of the motor overload rating.

I am probably just not seeing it correctly. I'm just a mechanic and have a more simplified version of how stuff acrtually works. In my head the electron flow in a short circuit BEFORE the motor would not go thru the overloads....hence the term "short" circuit.

 

[resistance]

I'm just a mechanic

Really! Interesting!

 

[coulter]

..In my head the electron flow in a short circuit BEFORE the motor would not go thru the overloads....hence the term "short" circuit.

Maybe this will help - I have often been asked a similar question about installations where a transformer secondary CB is 500 feet away from the transformer. Ususlly it starts out with:
What protects the secondary conductors from overcurrent?

The secondary CB. The current that goes into the transformer end on the conductors comes out of the cb end of the conductors.

But what if there is something wrong in the middle and the overcurrent doesn't come out the CB end of the conductors?

Ah - you're talking about a backhoe attack. In that case, you don't have any conductors to protect, they're toast. You just have a fire to put out. And the xfm primary will do that. Protecting against a conductor overcurrent in the middle on the run is not the job of a secondary CB. To protect against this failure mode, one uses concrete and steel.

This is similar to what you are discussing - an overcurrent/short circuit between the CB and the overload. You are dealing with a pick, hacksaw, forklift, .... Conductors are dead/gone. You just want the fire out.

A normal installation for me is a 40Hp, 480V, 3ph, FLA 49A, 3 - #6, OL set at 68A, CB set to 500A. (Listed combination starter with a mag only CB) Meets code and it's a good installation.

carl