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

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kda3310

Senior Member
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 after any correction factors for ambient temperature and number of conductors have been applied.

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

Senior Member
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.:roll:
 

kingpb

Senior Member
kda3310 said:
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

Senior Member
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

Moderator
Staff member
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

Senior Member
iwire said:
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?
Those questions are interesting.
 

kda3310

Senior Member
kingpb said:
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........
Ok, I see this problem a lot. I am working in Iraq and the ambient temperature here gets up to 50 Deg. C. out side. Just to clarify the ambient temperature is the air temperature. How hot is the air around the wire or the outside air around the conduit? The loud alone may bring the temperature of the conductor up to 60 – 75 Deg. C. Now add in 50 Deg. C. air temperatures. If the conductor alone is 75 Deg. C., how hot will 50 Deg. C. air temperature make that cable. I have burned my hands on conduit that was so hot you could not touch it. My panels are in the shade and do not get as hot and so the drakes are not tripping. In some office buildings in the states I have pulled out cable that the insulation on the current carrying conductors has melted to gather. Not because the circuit was over laoded but because there was too many current carrying conductors in one ? in conduit then was allowed. That made the ambient temperature in the conduit rise witch made the insulation melt with out over loading the breakers. The ? in conduit was hot to the touch even in a building with A/C. The only resin we found that one was because my boss told me to add one more circuit in that run.
 
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kingpb

Senior Member
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.
 

kingpb

Senior Member
Pierre C Belarge said:
Here is a bulletin from SQD that may shed some light on the topic.

http://ecatalog.squared.com/pubs/Electrical%20Distribution/0110DB9901R2-02.pdf
Thanks for the article, it has some good info. Unfortunately, they do not discuss why the smaller conductors are limited in current carrying capacity lower then the value provided in NEC T310.16.

For conservatism, I will accept that I should use the 60 deg C column for ampacity on #14, #12, and #10 wire, regardless of the actual temp rating of the insulation. With that said, I have a 20A breaker, and I am going to only load it to 80%. T310.16 says #14 is good for 20A, and there cannot be more then 16A load on the circuit, which matches my breaker. (I can always size up to the next standard size = 20A) I meet all code requirements, "EXCEPT" that the NEC says I have to limit #14 AWG to 15A, WHY, WHY, WHY, WHY, WHY...????????????????????

This makes no sense!
 

haskindm

Senior Member
Sometimes rules is rules.
I may not agree with the 30 mph speed limit in an area of open road, but it is still the speed limit.
 

kda3310

Senior Member
kingpb said:
Thanks for the article, it has some good info. Unfortunately, they do not discuss why the smaller conductors are limited in current carrying capacity lower then the value provided in NEC T310.16.

For conservatism, I will accept that I should use the 60 deg C column for ampacity on #14, #12, and #10 wire, regardless of the actual temp rating of the insulation. With that said, I have a 20A breaker, and I am going to only load it to 80%. T310.16 says #14 is good for 20A, and there cannot be more then 16A load on the circuit, which matches my breaker. (I can always size up to the next standard size = 20A) I meet all code requirements, "EXCEPT" that the NEC says I have to limit #14 AWG to 15A, WHY, WHY, WHY, WHY, WHY...????????????????????

This makes no sense!
The world may never know.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
kingpb said:
. . . there cannot be more then 16A load on the circuit . . .
Actually, there certainly can. There may not be more than 80% load continuously applied, but the rules have no more power over abuse than a stop sign has over a moving vehicle.

As Jon said back in: http://forums.mikeholt.com/showpost.php?p=731244&postcount=16,
winnie said:
For the same _percentage_ overload, smaller conductors will heat up and overheat faster than larger conductors.
Think of which circuits in your home you could fully load, or overload, without a malfunction of equipment, through normal use.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
kingpb said:
. . . WHY, WHY, WHY, WHY, WHY...????????????????????
Agent Smith said:
"Why, Mr. Anderson? Why do you do it? Why get up? Why keep fighting? Do you believe you're fighting for something? For more that your survival? Can you tell me what it is? Do you even know? Is it freedom? Or truth? Perhaps peace? Yes? No? Could it be for love? Illusions, Mr. Anderson. Vagaries of perception. The temporary constructs of a feeble human intellect trying desperately to justify an existence that is without meaning or purpose. And all of them as artificial as the Matrix itself, although only a human mind could invent something as insipid as love. You must be able to see it, Mr. Anderson. You must know it by now. You can't win. It's pointless to keep fighting. Why, Mr. Anderson? Why? Why do you persist?"
~ The Matrix Revolutions
 

kingpb

Senior Member
May be on to something.......

May be on to something.......

persist - To hold firmly and steadfastly to a purpose, state, or undertaking despite obstacles, warnings, or setbacks.

So, in that regard the quest continues, and low and behold, under the right rock-

According to the technical folks at Southwire, although they do not have anything in writing, the word is UL tested circuit breakers - 15, 20 and 30A as a system using #14, 12 and 10 Awg wire. UL found that when testing a #14 on a 20 amp breaker, there were problems with the breaker not operating properly. The same thing occurred with the #12 on a 30 amp breaker.

Supposedly the heating of the wire would cause the breaker to over heat, which in turn would cause the wire temperature to rise, leading to the breaker temperature rising....... so, to combat this oddity, UL simply put a limit on the conductor ampacity to resolve the problem, instead of trying to understand what was really happening.

I will continue to try and confirm this, and possibly get something in writing.
 
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