breaker size

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lmchenry

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I have always been told that breakers were designed to protect conductors. That is not there only purpose though, is it? What I am getting at is can you put a 15 amp breaker on a #10 cu that supplies equipment that pulls 15 or more amps? Not that you would want to, but does the code not allow it?
 
lmchenry said:
I have always been told that breakers were designed to protect conductors. That is not there only purpose though, is it? What I am getting at is can you put a 15 amp breaker on a #10 cu that supplies equipment that pulls 15 or more amps? Not that you would want to, but does the code not allow it?
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Yes, you could. Because the breaker would trip before the wire fails isn't it protecting the conductor?
 
there are pieces of equipment that have minimum circuit ratings though. So there would be some exceptions. Where does the code say that most breakers are rated at 80%?
 
lmchenry said:
there are pieces of equipment that have minimum circuit ratings though. So there would be some exceptions. Where does the code say that most breakers are rated at 80%?
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Start with 210.20 and also look at 210.23.
 
lmchenry said:
So it is quite alright to have 20 amp noncontinuous load on a 20 amp breaker.
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Yup look at this:

210.23 Permissible Loads.
In no case shall the load exceed the branch-circuit ampere rating. An individual branch circuit shall be permitted to supply any load for which it is rated. A branch circuit supplying two or more outlets or receptacles shall supply only the loads specified according to its size as specified in 210.23(A) through (D) and as summarized in 210.24 and Table 210.24.
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lmchenry said:
What I am getting at is can you put a 15 amp breaker on a #10 cu that supplies equipment that pulls 15 or more amps?
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Maybe, it depends on the equipment, check out 422.11

As an example, 422.11(E)

A single Non-motor operated appliance, the over current protection shall

1) Not exceed that marked on the appliance

2) Not exceed 20 amps if not marked and less then 13.3 amps or

3) Not exceed 150% if not marked and over 13.3 amps

(Please read it right out of the code book, don't count on my transcribing skills)

IMO the NEC and UL expect the over current protection to provide some level of protection for the equipment as well as the wiring on the way.


Edit, sorry I misread the question.
 
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General design philosophy acording to carl (recognized as no more valid than anyone else's). Some exceptions apply.

Basic premise: Something broke or the fuse wouldn't have blown.

OPC doesn't protect the device. The device is protected by design. If the device goes bad and draws too much current, what is there to protect? - remember, "the device goes bad".

So if the device goes bad and draws too much current, and the OCP trips, the OCP protected the conductors from over heating.

If the conductors go bad (backhoe attack for example) and draw extra current, and the OCP trips, it is not protecting the conductors, they are dead, it is protecting the personnel or structure.

Purpose of the OCP:
1. Protect the conductors from overheating in the event the design did not prevent device from overloading.

2. Protect the personel/structure/conductors in the event of device failure ending in a fireball/molten slag.

3. Protect the personel/structure in the event of conductor failure ending in conductor overheat/fireball/molten slag.

carl
 
lmchenry said:
... What I am getting at is can you put a 15 amp breaker on a #10 cu that supplies equipment that pulls 15 or more amps? Not that you would want to, but does the code not allow it?
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Yes for 15A. No for "or more".
Take a look at 210.20.A (2005)

carl
 
Where does the code say that most breakers are rated at 80%?

Where does the code say that most breakers are rated at 80%?

The NEC does not specifically state that breakers are rated at 80% that is true. Section 210.23(a) states “The rating of any one cord and plug connected utilization equipment shall not exceed 80 percent of the branch circuit ampere rating.” Section 210.23(a)(b) are referring to cords and plugs connecting to utilization equipment such as window mounted air conditioning units, washers and dryers, portable air compressors, power hand tools, microwave ovens, ect. I do not interpret 210.23(a)(b) as applicable to the wire conductors connected from the overcurrent device (circuit breaker) to the outlet where the cord is plugged into.

The section that I adhere to in my designs is 210-22(c). The equipment that I design all have continuous loads, therefore I apply the 125% rule when sizing a circuit breaker. I determine the maximum continuous load current, multiply by 1.25 and choose a circuit breaker as close the calculated current that I can. Always go up to the next available standard current rating. This process appears to be the 80% rule. The circuit breaker is not rated at 80%, I am only choosing a device that is 125% greater that my maximum continuous load current.
 
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coulter said:
General design philosophy acording to carl (recognized as no more valid than anyone else's). Some exceptions apply.

Basic premise: Something broke or the fuse wouldn't have blown.

OPC doesn't protect the device. The device is protected by design. If the device goes bad and draws too much current, what is there to protect? - remember, "the device goes bad".

So if the device goes bad and draws too much current, and the OCP trips, the OCP protected the conductors from over heating.

If the conductors go bad (backhoe attack for example) and draw extra current, and the OCP trips, it is not protecting the conductors, they are dead, it is protecting the personnel or structure.

Purpose of the OCP:
1. Protect the conductors from overheating in the event the design did not prevent device from overloading.

2. Protect the personel/structure/conductors in the event of device failure ending in a fireball/molten slag.

3. Protect the personel/structure in the event of conductor failure ending in conductor overheat/fireball/molten slag.

carl
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I don't agree, you can have a motor become overloaded not due to it "going bad" and the OCP trip and save the motor. In industrial applications it wouldn't be the breaker that mostlikely trips but in a residential setting it would be.
 
I think that is covered under item 1 of "Purpose of the OCP".

Give me an example of a residential motor that could overload under normal operation, that the OCP would save - I'm sure there are cases, I just can't think of any.

carl
 
lmchenry said:
I have always been told that breakers were designed to protect conductors. That is not there only purpose though, is it? What I am getting at is can you put a 15 amp breaker on a #10 cu that supplies equipment that pulls 15 or more amps? Not that you would want to, but does the code not allow it?
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See 210.3 (nec 2008)

Where Conductors of higher ampacity are used for any reason, the ampere rating or setting of the specified overcurrent device shall determine the circuit rating.

Consider derating, voltage drop and such. Wire size not the isuue.
 
Assuming my 2005 240.1 FPN reads the same as the 2008, if you read all of the FPN, the concern is for excessive temp in conductors or conductor insulation - not the equipment (paraphrased)

carl
 
coulter said:
Assuming my 2005 240.1 FPN reads the same as the 2008, if you read all of the FPN, the concern is for excessive temp in conductors or conductor insulation - not the equipment (paraphrased)

carl
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You are right, that is the concern. I was trying to respond to oringinal post about the purpose of the OCPD as being other than just protecting the circuit conductors. The FPN also refers to 110.9 and 110.10. Read last paragraph of 110.9 and 110.10 ...clear a fault to do so without extensive damage to the electrical components of the circuit. No changes noted in 2008.

Meditate on both for awile. :grin:

(nec 2008) Thanx for listing the code year you are using. We should get in the habit of that.
 
All UL listed overcurrent protective devices (breakers and fuses) are tested at 100% in open air. But, as soon as they are put into any general enclosure they effectively become 80% rated. this 80% effect matches exactly with the NEC rule for 125% conductor protection.

The only 100% rated protective devices require "special" heavily ventilated enclosures.
 
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