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

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kingpb

Senior Member
I never really put much thought to it that the NEC states that #14 is limited to 15A, #12 is limited to 20A, and #10 is limited to 30A. Amazingly then, through divine intervention, suddenly #8 is good for, not 40, not 50, but 60-70A or more, depending on the temperature rating. The limit on #14, #12, and #10 has been in the NEC as long as I can remember (early 80's, anyway). But another post regarding the 80% limit on breakers got me thinking about it.

In a nutshell, why? Where does this come from, and what documentation substantiates this limit? The conductor is by calculation capable of carrying more then the NEC limit of Table 310.16. I looked on manufacturers website, and I looked through IEEE 835. The tables in IEEE 835 appear to have some similarity between them and the NEC, but not exactly, such as ambient temperatures are 40 deg C not 30 deg C. In reviewing IEEE 835, the tables are much more detailed, and specific, whereas the NEC tables seem to be more of a summary, one size fits all.

So, the question is, can anyone provide some insight as to the engineering validity of the NEC cable ampacity tables? Otherwise it seems #14 should be able to be used on a 20A breaker for house wiring. After all, you can only load the circuit to 16A, and the conductor is actually good for 20A.

I hope this brings some good discussion.
 

Mike01

Senior Member
Location
MidWest
20A rated for 20A

20A rated for 20A

Correct me if I?m wrong but a 20A breaker is rated for 20A, the 16A rating is for continuous loads of more than three hours but the breaker itself is tested and rated at its full current carrying capacity for 20A.
 

haskindm

Senior Member
Location
Maryland
Remember that conductors #1 and smaller are limited to the 60 degree ampacity unless you know that the terminations are rated for 75 degrees. See 110.14(C). Also keep in mind that NM cables are always limited to the 60 degree ampacity - see 334.80. This may be due, in part, to the fact that these conductors are often installed where we have little control over the loads to be placed on the conductors, such as residential general use receptacles. It may also be due, partly, to the fact that small conductors are less likely to disapate heat, especially at the terminations.
 

72.5kv

Senior Member
UL test breakers in open air, but when they are installed they are placed in an enclosure. This enclusure has other breaker in it which are a source of heat. This is why the 80 percent limit come to place
 

etszap

Member
A 100%-rated CB and the end use equipment have been tested to verify that the additional heat generated by the 100% continuous loading conditions is safely dissipated. Other equipment specifications also are driven by the need to dissipate the heat associated with the level of heat rise achieved during 100% rated testing. In cases where the temperature at the CB wiring terminals exceeds 50 [degrees] C during 100% rated testing, UL 489 requires the use of 90 [degrees] C insulated wire (sized at the 75 [degrees] C ampacity) with these CBs, and the CB must be marked as such by the manufacturer. UL 489 also specifies minimum enclosure size and venting requirements if needed for heat dissipation. A CB that successfully has passed these additional tests is still not listed for application at 100% of its rating for continuous loading unless it's marked as such by the manufacturer.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
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.
 

kda3310

Senior Member
Could it be over caution? I mean these are the most popular wire sizes for every day uses. We know people in general do not know anything about electricity and they keep over loading circuits with plugging in to many devices.
 

stickboy1375

Senior Member
Location
Litchfield, CT
kda3310 said:
Could it be over caution? I mean these are the most popular wire sizes for every day uses. We know people in general do not know anything about electricity and they keep over loading circuits with plugging in to many devices.

Isn't that why we install overcurrent protection?
 

andinator

Senior Member
Location
Lilburn Georgia
I want to get back to the whole 80% thing. I know what the code says. Is it true that a 20A breaker loaded to exactly 20 amps will eventually trip? I asked this because i'm being pulled both ways in school!
 

kda3310

Senior Member
I have seen so many devices on one circuit that it was reading 22 amps on a 20 amp breaker that was not tripping in an office. The office is open over eight hours and was currying the load for days and was not tripping. I am just saying there are something's that can happen when you use something that often.
 
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LarryFine

Master Electrician Electric Contractor Richmond VA
kda3310 said:
Could it be over caution? I mean these are the most popular wire sizes for every day uses. We know people in general do not know anything about electricity and they keep over loading circuits with plugging in to many devices.
This sounds most plausible. General-purpose receptacle circuits are the easiest type to abuse, and are the circuits that use these smaller conductor sizes.

Anything larger than #10 is almost always used for specific-purpose circuits, such as those described in Art.'s 240.3 and 240.4 ('02 NEC), and easy to predict.
 

eric stromberg

Senior Member
Location
Texas
LarryFine said:
This sounds most plausible. General-purpose receptacle circuits are the easiest type to abuse, and are the circuits that use these smaller conductor sizes.

Anything larger than #10 is almost always used for specific-purpose circuits, such as those described in Art.'s 240.3 and 240.4 ('02 NEC), and easy to predict.
I have to agree. The other day i inspected an (industrial) MCC room. The electricians were pulling 12 AWG circuits for the receptacles and lighting. I looked in a box and it had three circuits in the same conduit. I said to the foreman "OK, let's see. THHN; 30 Amps. 6 current carrying conductors; derated to 80%, that's 24 Amps Ampacity. 20 Amp breakers; good to go."

IMHO, it's back to the fudge factor again. There is simply, probably, a good historical basis for keeping these at 15,20, and 30. :-? :roll:
 

augie47

Moderator
Staff member
Location
Tennessee
rating

rating

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.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Is it true that a 20A breaker loaded to exactly 20 amps will eventually trip?
The trip curve for QO 20 amp breaker shows that it will trip between 300 seconds and never with a 20 amp load. It also shows a trip time of 80 seconds to never for a 24 amp load.
Don
 

winnie

Senior Member
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

Senior Member
charlie b said:
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

Senior Member
augie47 said:
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

Senior Member
Response to Winnie

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.
 
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kingpb

Senior Member
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 -

View attachment 813

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 :-?
 
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