Why 15A for #14, 20A for #12, and 30A for #10?

[winnie]

This was discussed somewhere else recently, but I'll be damned if I can find the reference.

My hunch on this topic:

Insulation damage is caused by excessive temperature. When current flows in the conductor, heat is generated, and that causes the temperature to rise.

The thermal ampacity of a conductor (given in table 310.16, for example) is set by the combination of heat being produced in the conductor and heat being dissipated at the surface of the conductor. At the 'thermal ampacity' current level, the temperature of the conductor will rise to its design limit, with heat being produced equal to the heat being dissipated.

Conductors can tolerate a time limited overload, in that it takes _time_ for the generated heat to raise the temperature. The rate of temperature rise is set by how much heat is being produced, and the thermal mass of the conductors being heated. The greater the current density, the faster the heating. In the limit of severe overload, we can ignore the heat dissipation capability of the conductor, and simply consider the current density.

Now compare table 310.16 with the 'conductor properties' table. You will note that as conductors get larger, their permitted current density goes down. Examples:
14ga 20A 4100cmil 205 cmil/amp
10ga 30A 10380cmil 346 cmil/amp
6ga 55A 26250cmil 477 cmil/amp
2ga 95A 66370cmil 699 cmil/amp

For the same _percentage_ overload, smaller conductors will heat up and overheat faster than larger conductors.

My _hunch_ is that smaller conductors will reach thermal damage faster during overload, and thus breakers with lower trip settings are used to provide the necessary overload protection.

-Jon

[kingpb]

I think we are getting away from the intended question. We can easily verify that a #12 THHN conductor has an ampacity, at 90C, of 30 amps. However, 240.4(D) limits its overcurrent protection to 20 amps. Why? What is the historical basis for 240.4(D)?

Kingpb: Did I summarize your question correctly? If so, I do not know its answer.

Yes, thanks, it was starting to go a drift.

[kingpb]

I may be mistaken, but I believe it has to do with the fact that most 15 and 20 amp devices being rated at 60 deg. In accordance with 110.14(C)(1) . If the termination device is 60, then you have to use the wire at 60.

I understand your point, however, Table 310.16 states the same limits of current for #14 and #12 regardless of the 60 deg or 75 deg C rating.

Also, by checking manufacturers catalogs, I believe you will find that there is no, or limited equipment sold that has a rating of 60 deg C, everything seems to have gone to 75 deg C.

[kda3310]

Response to Winnie

Yes, that’s what I was thinking. I just can not explain it that well. People will loud up a circuit over time keeping the amps close to the conductors limit with out tripping the breaker. Witch will keep the conductor warm. Now, take nine wires I a half inch peace of EMT and six of them wires are current carrying conductors. Companies grow and that means more stuff. When they moved in you set up the place to the minima requirement of the NEC just to suet there needs at a low cost. That means you gave the minima requirements for what they paid you for. That does not usually include for expansion. If the company does well they will grow and will soon need a bigger building. However, they will use the building they are in to the max before they will move. The code will allows you to load a breaker to a 100% of a non-continues load. So, to help out the company we will set up there loads at 80% to help out for expansion. That is not in the code. That is something we do as a common knowledge thing witch people will mistake as code. There are many things like this that people think are in the code but, they are not. Just like the line side of a disconnect or breaker will always be on top. This is not code but we all kwon to do it. The code does not care witch is line side or load unless the device is marked line and loud.

[kingpb]

winnie/Jon has a plausible explanation. The only issue I see is that if the cable itself has a rating of 90 Deg C, then according to Table 310.16 that means #14 - 25A, #12 - 30A, #10 - 40A continuously without damage. So I can put 16A continuously on a #14 AWG, rated for 90 Deg C, and the cable will not be damaged. On a 20A breaker, I could easily carry the 20A as well without damage. Over 20A, I don't care beacause the breaker will trip. I understand the terminals may only be rated for 60 or 75 deg C, which will limit the load I can put on the circuit, but it does not change the fact that the cable will not be damaged for higher currents.

I plotted #12 wire damage curve against a 30A circuit breaker and you can see it is well protected -



So, using the principles allowed for other cable sizes, if I start with #12, 90 deg C, is good for 30A, now load to 80% which comes out to 24A. The cable at 75 deg C is good for 25A, and therefore is acceptable. According to the damage curve I can run 30A continuously.

Still trying to figure out why the limit in the NEC :-?

[kda3310]

Article 240.4(D) Small Conductors. Unless specifically permitted in 240.4(E) or 240.4(G), the over current protection shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15 amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum [COLOR="Red"]after any correction factors for ambient temperature and number of conductors have been applied.[/COLOR]

You can only run 30 amps through a number 12 AWG before the insulation starts to melt. Amps are a measurement of heat. Ambient temperature is a measurement of heat. Rise the ambient temperature and combine the heat of the load and that wire will be hot. Ambient temperature can be raised by other conductors in the same conduit. I believe the over current rating is to help not to rise the temperature up so high it melts the insulation of the conductor because of how much load gets put on general use outlet.

[bkludecke]

I've seen here a lot of good speculation and good reasons not to change the rule. But I wonder if all this was the thinking that actually created the rule.

[kingpb]

You can only run 30 amps through a number 12 AWG before the insulation starts to melt. ... I believe the over current rating is to help not to rise the temperature up so high it melts the insulation of the conductor because of how much load gets put on general use outlet.

I think I understand the point you are trying to make, but according to Table 310.16, #12 is good for 30A continuously, with a temp rise of 90 deg C, so I'm not sure why the insulation will start to melt. Please refer to the damage curve on previous post. Also, a general use outlet circuit load is limited by the circuit breaker rating, so as long as the breaker is sized properly, then the cable is protected.

As stated by bkludecke, this maybe a requirement that dates back to when there was nothing other then 60 deg C, and to build in additional safety these limits were applied. As the residential market costs tighten further, all potential cost savings need to be reviewed.

Still looking for engineering back-up to show why the limit........

[haskindm]

As others have said, a 20-amp breaker will carry substantially more than 20-amps for a fairly long time before tripping. I am not old enough to have been in on the early code making decisions but I suspect that it was recognized that these smaller conductors are more likely to be "abused" via overcurrent, etc, since they are most likely to be used in general use circuits where the load is variable and there is no control over what the user may "plug in" or install. That, added to the fact that the smaller conductors (having less "metal") are less able to dissipate heat, especially at termination points would seem to justify the reduced ampacity. My copy of the 1897 Code indicates the "Rubber-covered Wires" will have the following ampacities:
#14 - 12 amps
#12 - 17 amps
#10 - 24 amps
These wire sizes are B&SG which I assume is equivalent to AWG.
It appears that we have gotten less conservative over time, but the exact thought process that was used to determine these ampacities may be lost.

[iwire]

Just wondering how it would be if 240.4(D) was removed and we used T310.16?

Would we see 15 amp circuits disappear?

Would we see a smaller AWG allowed for 15 amp circuits?

[Pierre C Belarge]

Just wondering how it would be if 240.4(D) was removed and we used T310.16?

[COLOR=blue]Would we see 15 amp circuits disappear?[/COLOR]
[COLOR=blue][/COLOR]
[COLOR=blue]Would we see a smaller AWG allowed for 15 amp circuits[/COLOR]?

Those questions are interesting.

[kda3310]

Oops, Let me try to Quote again.

[kda3310]

I think I understand the point you are trying to make, but according to Table 310.16, #12 is good for 30A continuously, with a temp rise of 90 deg C, so I'm not sure why the insulation will start to melt. Please refer to the damage curve on previous post. Also, a general use outlet circuit load is limited by the circuit breaker rating, so as long as the breaker is sized properly, then the cable is protected.

As stated by bkludecke, this maybe a requirement that dates back to when there was nothing other then 60 deg C, and to build in additional safety these limits were applied. As the residential market costs tighten further, all potential cost savings need to be reviewed.

Still looking for engineering back-up to show why the limit........

[kingpb]

Understand that ambient temperature plays a role in final sizing of conductor. Adjustments should be made according to temperature correction factors at the bottom of T310.16. Increases in wire size may be required to accomodate temperatures in excess of 30 deg C. But derating factors will be required no matter what wire size you start with, and would seem independent of the issue at hand, which is why the limit was imposed on #14, #12, and #10 to begin with.

[Pierre C Belarge]

Here is a bulletin from SQD that may shed some light on the topic.

http://ecatalog.squared.com/pubs/Ele...B9901R2-02.pdf