Breaker/Wire Size

A

Alwayslearningelec

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NJ
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Is there a problem( or danger) in having a 40A breaker with #10 wire? Wouldn't that let more current in and possibly overheat the wire although it is good for 40A at 90 degree.
 
Keep in mind there are some specific instances in the NEC where that is allowed such as motors or HVAC (and other 240.4 G appl;cations) due to the conductor being protected by other means.

edit (posted with Larry)
 
Thanks. So what is the rule( or general idea) for breaker ampacity compared to the wire ampacity? I know it need ot be at least the ampacity of the wire. But not more than?
 
The conductor must have an ampacity equal to or greater than that of the OCPD, unless the installation is covered by 240.4(E) through (H).
The most common cases where the conductor does not have an ampacity equal or greater than the rating of the OCPD are tap conductors, motor circuit conductors, and HVAC conductors.
 
Alwayslearningelec said:
Thanks. So what is the rule( or general idea) for breaker ampacity compared to the wire ampacity? I know it need ot be at least the ampacity of the wire. But not more than?
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Read through 240.4 and the other sections and articles referred.

We don't "just know" the answers, we know how to find them.
 
don_resqcapt19 said:
The conductor must have an ampacity equal to or greater than that of the OCPD, unless the installation is covered by 240.4(E) through (H).
The most common cases where the conductor does not have an ampacity equal or greater than the rating of the OCPD are tap conductors, motor circuit conductors, and HVAC conductors.
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But if you have a conductor with an ampacity that's larger than the OCPD ampacity won't the breaker possibly trip when it shouldn't?
 
Alwayslearningelec said:
But if you have a conductor with an ampacity that's larger than the OCPD ampacity won't the breaker possibly trip when it shouldn't?
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Ampacity is the ability to carry current. The conductor ampacity must be greater than the current drawn by the load. Breakers do not limit current, they open when too much current is drawn by the load or during a fault.

Having an oversized conductor does not affect the breaker operation at all.
 
Alwayslearningelec said:
But if you have a conductor with an ampacity that's larger than the OCPD ampacity won't the breaker possibly trip when it shouldn't?
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No, because breakers respond to actual current flow, not wire size.

A fuse or breaker is an intentional weak link inserted into a circuit.

Increasing the wire size makes the weak link a relatively weaker link.
 
Alwayslearningelec said:
But if you have a conductor with an ampacity that's larger than the OCPD ampacity won't the breaker possibly trip when it shouldn't?
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The size of the wire does not affect how much current it draws.

How much current is pulled through a breaker is determined by the load you turn on. For example, if you run a light off of a 20 amp circuit, and the light pulls 1 amp at 120 volts, only one amp will be pulled through the circuit breaker, even though the wire is capable of safely carrying 20 amps. If you have a wire that can safely carry 20 amps, it can also carry 1,000 amps if the "load" is increased to pull that much - it just won't carry it long before the wire melts.

It's not like domestic water in plumbing where the size of the pipe determines how much flow is available, it's all about the load. The breaker is there to shut the system down if the load starts pulling too many amps for the wire.

You can get killed just as fast by a small wire as a big wire if they are the same voltage because the resistance of your body is what creates the load and drives the amount of current flowing through you, not the size of wire delivering the current.
 
NEC Inspector said:
If you have a wire that can safely carry 20 amps, it can also carry 1,000 amps if the "load" is increased to pull that much - it just won't carry it long before the wire melts.
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Just to clarify, our wire ratings are based on avoiding insulation damage, including over time.

NEC Inspector said:
It's not like domestic water in plumbing where the size of the pipe determines how much flow is available, it's all about the load.
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Well, it kinda is when you compare current flow to water flow. Having a 1" pipe does not force more water through a valve than with a 1/2" pipe, but the capacity is greater if the demand is made.

NEC Inspector said:
You can get killed just as fast by a small wire as a big wire if they are the same voltage because the resistance of your body is what creates the load and drives the amount of current flowing through you, not the size of wire delivering the current.
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Technically, the voltage drives the current, limited by the body resistance. That it takes so little current to harm the human body is what makes safety critical.
 
don_resqcapt19 said:
The conductor must have an ampacity equal to or greater than that of the OCPD, unless the installation is covered by 240.4(E) through (H).
The most common cases where the conductor does not have an ampacity equal or greater than the rating of the OCPD are tap conductors, motor circuit conductors, and HVAC conductors.
Click to expand...
What is the reasoning behind say motor circuit conductors not having to follow that rule?
 
Alwayslearningelec said:
What is the reasoning behind say motor circuit conductors not having to follow that rule?
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Besides what Jim mentions above ......depending on the type of starter used the motor may need a much larger ampacity C/B to allow the motor to start than it does to run after it is started. An across the line starter can allow 6 times the motors full load when the motor starts which would cause smaller breakers to trip thinking there is an overload.
 
What about the fuse size in a disconnect? Needs to be sized according to what? If you have an OCPD with a fused disconnect switch and the load.
 
Alwayslearningelec said:
What about the fuse size in a disconnect? Needs to be sized according to what? If you have an OCPD with a fused disconnect switch and the load.
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It depends, are the fuses being used to protect the conductors? For example you can have a 150 circuit breaker with #1/0 going to the equipment and at the equipment you have a 200 amp disconnect with 200 amp fuses.
 
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