#12 thhn on a 25a breaker, why not?
- Thread starter Saywatt
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iMuse97
Senior Member
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- Chicagoland
If you are asking why the code prohibits greater than 20A OCPD on #12AWG conductors, that's information that would be available from the code-making panel when the 240.4(D) was first written in the code. I don't know where to find that. [looking at 2008 code, sorry
]However, if you are asking, rhetorically, why such a code is written, just ask yourself how many times people overload wires? I think that sometimes the code-making panels simply seek to build into the code some minimum protections against ignorance.
On the other hand, if you read carefully, you can see that the code-making panel allows exceptions in 240.4(E) and 240.4(G), for those who know how to protect the wires properly.
broadgage
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- London, England
The simple answer is that #12 can not be used on a 25 amp breaker because to so do would be a violation (in most circumstances)
In all common circumstances, #12 is limited to 20 amps by the NEC not by the laws of physics.
In other countries with different codes or regulations, but the same laws of physics, it would be allowed.
The NEC takes a very cautious view of current carrying capacity compared to codes and regulations in other nations.
In practice quite a bit of un-permitted DIY work goes on in the USA. Whilst such is not to be encouraged, it must be said that "no more than 20 amps on a #12" is usually understood even by the DIY lot.
De-rating for grouping, high ambient temperatures, or thermal insulation, is unlikely to be understood by un-licensed persons.
In practice though "no more than 20 amps on #12" tends to ensure a big enough margin of ignorance in all but extreme cases.
In all common circumstances, #12 is limited to 20 amps by the NEC not by the laws of physics.
In other countries with different codes or regulations, but the same laws of physics, it would be allowed.
The NEC takes a very cautious view of current carrying capacity compared to codes and regulations in other nations.
In practice quite a bit of un-permitted DIY work goes on in the USA. Whilst such is not to be encouraged, it must be said that "no more than 20 amps on a #12" is usually understood even by the DIY lot.
De-rating for grouping, high ambient temperatures, or thermal insulation, is unlikely to be understood by un-licensed persons.
In practice though "no more than 20 amps on #12" tends to ensure a big enough margin of ignorance in all but extreme cases.
kingpb
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- SE USA as far as you can go
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- Engineer, Registered
I asked the same question to Southwire back in 2007, this is the response I got...............
"The Circuit Breakers - 15, 20 and 30 amp- were tested 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 with the #12 on a 30 amp breaker. The heat of the wire would cause the breaker to over heat which would cause the wire temperature to rise causing the breaker temperature to rise....... so, the NEC limited the conductor ampacity to help solve the problem.
It is what it is.............
"The Circuit Breakers - 15, 20 and 30 amp- were tested 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 with the #12 on a 30 amp breaker. The heat of the wire would cause the breaker to over heat which would cause the wire temperature to rise causing the breaker temperature to rise....... so, the NEC limited the conductor ampacity to help solve the problem.
It is what it is.............
Because 240.4(D) says we can't.
- Location
- Illinois
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- retired electrician
I asked the same question to Southwire back in 2007, this is the response I got...............
"The Circuit Breakers - 15, 20 and 30 amp- were tested 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 with the #12 on a 30 amp breaker. The heat of the wire would cause the breaker to over heat which would cause the wire temperature to rise causing the breaker temperature to rise....... so, the NEC limited the conductor ampacity to help solve the problem.
It is what it is.............
That's a great answer. I have tried over the years to find a good reason for the limits and this makes sense to me. I believe this is a similar reason we handle continuous loads as we do.
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- Location
- Illinois
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- retired electrician
hurk27
Senior Member
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- Portage, Indiana NEC: 2008
not for me
Well how about this one:
While in almost all cases the wire itself will handle 25 amps without a problem, but the terminations won't, the problem is the heating at terminals and transferring to the wire melting the insulation back, and also damaging the terminal, the wire will act as a heat sink and if it is not large enough will not dissipate enough heat away from the terminal, most circuit failures are at terminations, this would also explain the problems at breakers.
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glene77is
Senior Member
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- Memphis, TN
Thank You , Hurk,
Remarkably clear description from a Physics viewpoint. :happyyes:
Most Electricians simply don't see these concepts. :happysad:
- Location
- Illinois
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- retired electrician
As always the terminations are a factor, but most breakers and some other items are rated at 75?C. If the 75?C wire and terminations cannot handle the 75? ampacity, then those ratings not real...just made up numbers not backed by physics and testing.
glene77is
Senior Member
- Location
- Memphis, TN
Hurk,
Then there is the case of my brother in Alaska.
In the dead of winter,
he says that they can run 67 Amps through a #12,
and the conductor temp is only slightly warm to hold.
Sounds "safe" in terms of terminal temp ratings,
but obviously not up to code ! :happysad:
Then there is the case of my brother in Alaska.
In the dead of winter,
he says that they can run 67 Amps through a #12,
and the conductor temp is only slightly warm to hold.
Sounds "safe" in terms of terminal temp ratings,
but obviously not up to code ! :happysad:
Sierrasparky
Senior Member
- Location
- USA
- Occupation
- Electrician ,contractor
I bet that is outside at the breaker where it's Coooooooooooold
I would think that in those circumstances the breakers would need to be placed in an area that is room temp ( 68) so they will operate properly.
Theres a fire waiting to happen.
Hey maybe that actually heats the walls up like heat trace cables?
The technical reason for limiting the current is the effect on insulation degradation over time due to elevated temperature.
It may be unlikely that a current overloaded wire at 200 deg C would actually start a fire due to its temperature but temperature degraded insulation will eventually breakdown allowing for shorts causing sparks that will be much more likely to start a fire.
Wire insulation has evolved and modern insulation can take higher temps for longer periods of time without degradation.
The NEC spec's for wire sizing is in the ballpark of 2 watts of dissipation per foot of wire with modification consideration based on its surface area, ambient temp surrounding and ability of wire to dissipate heat based on its insulation and surrounding enclosure (conduit) which restricts heat dissipation. For example, SOOW cable is reduced in maximum carrying amperage compare to THHN for same wire gauge due to the thick multi-layer casing of SOOW which restricts ability of wire to dissipate heat.
Each current carrying wire in an enclosure adds to the heating per unit length so there is derating for multiple current carrying conductors.
It may be unlikely that a current overloaded wire at 200 deg C would actually start a fire due to its temperature but temperature degraded insulation will eventually breakdown allowing for shorts causing sparks that will be much more likely to start a fire.
Wire insulation has evolved and modern insulation can take higher temps for longer periods of time without degradation.
The NEC spec's for wire sizing is in the ballpark of 2 watts of dissipation per foot of wire with modification consideration based on its surface area, ambient temp surrounding and ability of wire to dissipate heat based on its insulation and surrounding enclosure (conduit) which restricts heat dissipation. For example, SOOW cable is reduced in maximum carrying amperage compare to THHN for same wire gauge due to the thick multi-layer casing of SOOW which restricts ability of wire to dissipate heat.
Each current carrying wire in an enclosure adds to the heating per unit length so there is derating for multiple current carrying conductors.
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