310.16 ampacities

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Sharpie

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If this topic has already been discussed, please point me in the direction of that thread.

In table 310.16 it shows 14AWG Cu as having ampacities ranging from 20-25A depending on temp ratings.

In 240.4(D)(3) (referring to over current protection) it says "14 AWG Copper. 15 amperes"

There are some situations that I would like to use 14AWG on a 20A breaker.

Can any one give me a good explanation of why the extra buffer is put in there?:confused:


The same question would apply to 12 and 10AWG respectively.
Thanks,
 

stjohnbarleycorn

Senior Member
In most cases #14 is only good for 15 amps. motors are an exception. You use the higher amps when you have to take into account (de-rate) for the ambient temperature.Look on the bottom of 310-16
I am sure you will get clearer examples and explanations here.
 

augie47

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Note the initial wording in 240.4(D) which tells you that the 15 amp limitation is applicable with possible exceptions from 240.4(E) or (G).
If your application falls in the venue of (E) or (G) you might be able to exceed the 15 amp limitation, if your application does not, then you abide by the 15 amp limit.
 

charlie b

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In table 310.16 it shows 14AWG Cu as having ampacities ranging from 20-25A depending on temp ratings.
Point in fact, if it has 90C insulation, the ampacity of #14 is 25 amps! Because of that other section you cited, we are required to protect a #14 conductor with no more than a 15 amp breaker. But that does not change its ampacity value. The same thing applies to the #12 and #10.


Take a look at the Article 100 definition of ampacity. It has to do with the amount of current a conductor can handle, under the conditions of use, without getting so hot that the insulation can be damaged. It is all about the ability of the insulation system. What you used to protect the conductor does not change anything about the insulation system. You can use a 1 amp fuse to protect a #14 THHN, and the wire will still have an ampacity of 25.
 

steve66

Senior Member
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Illinois
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Can any one give me a good explanation of why the extra buffer is put in there?:confused:

I think there are many reasons:

- A 15 amp breaker doesn't trip right at 15 amps.
- You have an unknown amount of load that can be plugged into receptacle circuits.
- The wire might be burried in insulation where it can't dissipate heat as well.
- The wire might be ran through an attic or somewhere that has an elevated temperature.
- Statistics from years and years of installations generally indicate that limiting #14 to a 15 amp breaker generally provides a reasonably safe installation.

Steve
 

480sparky

Senior Member
Location
Iowegia
........- Statistics from years and years of installations generally indicate that limiting #14 to a 15 amp breaker generally provides a reasonably safe installation......


The "Small Conductor Rule" was initiated in the 1956 NEC. At that time, what is now T310-16 was Table 1, Chapter 10.

This table included a new type of insulation, type RHH. At the bottom of the table, the following was added:

"The current-carrying capacities for Type RHH conductors for sizes 14, 12 and 10 shall be the same as designated for Type RH conductors in this Table." This put 14, 12 and 10 RHH at 15, 20 and 30 amps respectively.

Similar wording bounced around the Codebook, along with the table. In 1999, it was removed from the bottom of the table and relocated to 240-3. It's current address is 240.4(D).
 

Cold Fusion

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way north
...Can any one give me a good explanation of why the extra buffer is put in there? ...
I've never seen an explanation. I even doubt the code panel members can give you one. I think the code panel follows the "double dumbs..t" rule.

"That's the way we have always done it. That's the way everyone else does it."

The 15A rule for #14 is the law of man. And as many in the electrical field believe, that always trumps the laws of God and Physics:roll:.

That being said, 15A on #14 isn't a bad idea. It keeps the non-thinking house ropers (none of which are part of the exceptionally sharp forum members here:)) out of trouble.

cf
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
Since there are situations where #14 can be used above the 15 amp limit, it seems obvious that currents higher than 15 amps don't damage the conductor.
The 15 amp overcurrent device does provide a high degree of protection for cords and end use utilization equipment.
A number of electricians I know prefer 15 amp B/Cs for that reason
 

winnie

Senior Member
Location
Springfield, MA, USA
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Electric motor research
Table 310.16 tells you one thing: the ampacity of the conductor.

Ampacity is the maximum current that a conductor can carry, on a continuous basis, given the specified ambient conditions and surroundings, without overheating.

In general you are required to use OCPD that protects the conductors from exceeding their ampacity.

My understandingis that the rule for 'small conductors' is to provide protection from damage during short circuit conditions, rather than overload conditions. However thinking about this, it doesn't make sense when you consider that in the situations where we can use the conductors to their full ampacity, the OCPD is usually only providing short circuit protection.

-Jon
 

Sharpie

Senior Member
Location
PA
Thanks for the replies so far. This is a good discussion. There are obviously different opinions on this topic. Keep 'em coming!

For the most part, I would agree from this statement from CF:

I've never seen an explanation. I even doubt the code panel members can give you one. I think the code panel follows the "double dumbs..t" rule.

"That's the way we have always done it. That's the way everyone else does it."

The 15A rule for #14 is the law of man. And as many in the electrical field believe, that always trumps the laws of God and Physics:roll:.

That being said, 15A on #14 isn't a bad idea. It keeps the non-thinking house ropers (none of which are part of the exceptionally sharp forum members here:)) out of trouble.

cf
 

stjohnbarleycorn

Senior Member
If you are talking residential, which probably most of the #14's reside, you may consider the loads that are on the circuit. Lamps with tinsel cords, and many cheaply made products with no internal protection. Putting it on a larger CB would not make the circuit safer. OF course now they are all AFCI so that changes things also.
 

kwired

Electron manager
Location
NE Nebraska
If you are talking residential, which probably most of the #14's reside, you may consider the loads that are on the circuit. Lamps with tinsel cords, and many cheaply made products with no internal protection. Putting it on a larger CB would not make the circuit safer. OF course now they are all AFCI so that changes things also.

I kind of have to disagree with that. The same circuits would be allowed on a 20 amp circuit which would allow more current through the same tinsel cords, and other cheaply made products. Not only the increase in overload protection here but the increased conductor size would allow more current to flow during a short circuit or ground fault.
 

stjohnbarleycorn

Senior Member
maybe we are saying the same thing, I think. I am saying that you are better off with 15 OP, on #14 not 20 even though the nmb can take it the other stuff would not.
 
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