Standard Circuit Breaker Sizes

[fifty60]

NEC 240.6 lists the smallest "fuse or inverse time breaker" as 15A. It then lists some smaller standard "fuse" sizes. Does this mean that a heating load does not have less than a 15A circuit breaker as it branch OCPD?

For example, I have a heater that has a 5.7A load. I can do the 1.25X's rule, and get a 7.125. Can I round this up to 15A, as long as I am using 14AWG? Is the branch OCPD on resistive heating loads meant to only be short circuit protection, and not overload protection for the heater?

 

[ritelec]

The heater load @ 1.25 determines the circuit size.
Yes it would be 15 amp.
The breaker protects the wire, the heater's on it's own.

how's that...??

 

[Dennis Alwon]

The heater overload is for overcurrent protection. The overcurrent protective device is for short circuit and ground fault protection. This is why you could have a 40 amp breaker on an a/c unit and feed it with #12 in some case. The a/c has the overload protection built in.

 

[ritelec]

The heater overload is for overcurrent protection.

don't think there is one...
just a 4' or 8' baseboard....

op read through 424......

jeez, looks like you can gang them up and stick them on 30 amp if you'd like (residentially). non residential can gang them to 50 amp. 424.3(A)
then reading through III control and protection , it looks that way there to..

please correct if wrong good fellas

 

[Jraef]

The heater load @ 1.25 determines the circuit size.
Yes it would be 15 amp.
The breaker protects the wire, the heater's on it's own.

how's that...??

So to that point fifty60, think about this:
How does one "overload" a resistive heater, in a way that does NOT involve a short circuit or ground fault?

I have this argument with people all the time with regard to motor controllers.

User: "I want the breaker to protect the motor windings against short circuits"
Me: "So you want the breaker to read into the future and turn off if there is GOING to be a short circuit?"
User: "No, that's stupid. I want the breaker to protect the motor AGAINST a short circuit."
Me: "Protect against a short circuit WHERE?"
User: "In the motor windings of course!"
Me: "If there is a short circuit already IN the motor winding, what am I protecting it against?"

 

[GoldDigger]

How about: "I would like to limit any additional damage to the motor when any kind of short circuit (line to line, coil to ground or turn to turn) happens in the motor in the future."
And your answer, of course, is: "Oh, you want short circuit protection for the wire and overload protection for the motor!"

Tapatalk!

 

[fifty60]

The heaters do have thermal cutouts, thermostats, or limit controllers to protect against overloads. But even if they did not have this, I can't use an OCPD as an overload? The only way I could see an overload happening is if the voltage increased to the heater or the heater resistance somehow became less.

I am satisfied with the thermal cutouts, thermostas, or limit controllers providing the overvoltage protection. But what if they are not available? For instance, a 25W strip window heater. It is on the secondary of a control transformer. 115V and 25W, so there is not much current. The entire secondary of the control transformer is protected by a 10A circuit breaker. If 10A went through the window heater. Should I be comfortable with the 10A breaker as the only protection on a heater that has a relatively low watt output?

So, for resistance loads like heaters, I should only look at the circuit breaker as short circuit protection. Size the conductors to the circuit breaker and don't worry about the 1.25X's rule?

 

[fifty60]

The heater load @ 1.25 determines the circuit size.
The heater overload is for overcurrent protection. The overcurrent protective device is for short circuit and ground fault protection. This is why you could have a 40 amp breaker on an a/c unit and feed it with #12 in some case. The a/c has the overload protection built in.

Are you referring to the Circuit Breaker as the overload protection? Or are you referring to some kind of thermal cut-out device other than a circuit breaker? You say the overload is the overcurrent protection, but the overcurrent protective device is the short circuit and ground fault protection. Does there need to be overload protection for heaters?

 

[Jraef]

How about: "I would like to limit any additional damage to the motor when any kind of short circuit (line to line, coil to ground or turn to turn) happens in the motor in the future."
And your answer, of course, is: "Oh, you want short circuit protection for the wire and overload protection for the motor!"

Tapatalk!

The thing is, that was already there. They wanted ANOTHER breaker, in between the VFD and the motor. I had already explained that the VFD had overload protection for the motor and short circuit protection for the conductors To the motor. They insisted on having ANOTHER breaker between them because they didn't trust that the VFD trip value was low enough to protect the 18ga wire inside of the motor windings.

 

[Jraef]

The heaters do have thermal cutouts, thermostats, or limit controllers to protect against overloads. But even if they did not have this, I can't use an OCPD as an overload? The only way I could see an overload happening is if the voltage increased to the heater or the heater resistance somehow became less.

I am satisfied with the thermal cutouts, thermostas, or limit controllers providing the overvoltage protection. But what if they are not available? For instance, a 25W strip window heater. It is on the secondary of a control transformer. 115V and 25W, so there is not much current. The entire secondary of the control transformer is protected by a 10A circuit breaker. If 10A went through the window heater. Should I be comfortable with the 10A breaker as the only protection on a heater that has a relatively low watt output?

So, for resistance loads like heaters, I should only look at the circuit breaker as short circuit protection. Size the conductors to the circuit breaker and don't worry about the 1.25X's rule?

Size the conductors to the load x 1.25, but field wiring must be 14ga minimum, protected by a 15A breaker. So on the 25W heater, your field wiring will be 14ga anyway and a 15A breaker will protect it. It will NOT protect the 25W heater, but what it the failure mode? Resistance can't "somehow change" without having it open, in which case current STOPS, or short, in which case you have Short Circuit protection.

 

[GoldDigger]

Or you have a winding to winding short or mid-winding to ground short which does not trip the 15A breaker.
But there is a good chance that a smaller breaker would do that either.

Tapatalk!

 

[kwired]

Thermal cutouts, limits, or whatever you want to call them are for overheating protection, not overload protection. Only possible way to overload a resistive item is to apply overvoltage to it, otherwise it is a fixed load as long as voltage is constant. If something breaks down, you don't have "overload" you have short circuit or ground fault conditions, that my cause additional "overload damage" but at that point the damage is already done and no "overload" protection is going to save anything anyway.

Motors are able to be overloaded because if the driven load is demanding enough torque to slow the motor down it will draw more in an attempt to try to maintain speed. We can get more technical as to why if you wish, but a basic approach is a motor is always trying to achieve it's rated speed, and if it must draw more power to try to get there it will and of course power is a product of volts and amps.