What is the real ampacity of 14, 12 and 10 guage wire?

[mbrooke]

Table 310.15 and the older one 310.16 list number 14 for 20 amps, 12 for 25 and 10 for 35, but why the blanket rating of 15, 20 and 30 amps?


What got me wondering was another code violation handy man special:roll: HO window AC breaker kept tripping and someone had replaced the 15 with a 20. The amp draw was around 19 to 20 amps with a clamp on meter but contrary to what some say the wire was barely warm. No molten or burning insulation. Heck I have even seen 14 on 30amp fuses get hot, but not burn up let alone get over 140 degrees.

What is the theory behind the restriction?

 

[charlie b]

The ampacities are what the tables say they are. The ampacities may change, if the installed configuration does not match the description shown in the table. For example, the table speaks of three current-carrying conductors in a raceway. If you put in more, the ampacity changes, and we have a handy table to help us figure out the new ampacities. The 15, 20, and 30 amp values to which we limit AWG sizes 14, 12, and 10 are not ampacity restrictions. They are overcurrent limitations. We limit the overcurrent protection to values that are lower than the ampacity values. Why, you ask? I am not sure. My best guess is that it guards against small differences in manufacturing tolerances. If you short-change the amount of copper than goes into a 1/0 wire, you won't put it at much of a risk of overheating. But a #14 doesn't have much copper to start with, and short-changing it by a small amount might make a significant difference on how it handles heat.

 

[GoldDigger]

Table 310.15 and the older one 310.16 list number 14 for 20 amps, 12 for 25 and 10 for 35, but why the blanket rating of 15, 20 and 30 amps?


What got me wondering was another code violation handy man special:roll: HO window AC breaker kept tripping and someone had replaced the 15 with a 20. The amp draw was around 19 to 20 amps with a clamp on meter but contrary to what some say the wire was barely warm. No molten or burning insulation. Heck I have even seen 14 on 30amp fuses get hot, but not burn up let alone get over 140 degrees.

What is the theory behind the restriction?

The "small wires" ampacity limits are somewhat arbitrary in the sense that they do not correspond to the actual calculated ampacity based on actual conductor insulation and installation details. Just why, I am not sure, except to enforce minimum sizes for mechanical reasons. Maybe somebody else can illuminate that.

As for the wires getting warm but not hot, there can be slow insulation degradation even at lower temperatures, but mostly the NEC has a generous safety factor built in.

One problem with putting in a 20A breaker is that it will allow about 27 amps indefinitely. That is more than the measured 19-20 amps and will generate about 1.8 times as much heat. Now you wires are starting to get hot.
Add additional local heat sources, such as high resistance terminations, reduce the heat loss by throwing in insulation or sheathing, and you may have a real problem.

 

[Smart $]

I believe the small conductors OCPD limitation for non-specific circuits is because 14, 12, and 10 AWG sizes are the most common sizes used in premises wiring. As a result, proper adjustment and correction for ampacity for the conditions of installation have a substantially higher probability of not being performed or adequate.

 

[mbrooke]

The "small wires" ampacity limits are somewhat arbitrary in the sense that they do not correspond to the actual calculated ampacity based on actual conductor insulation and installation details. Just why, I am not sure, except to enforce minimum sizes for mechanical reasons. Maybe somebody else can illuminate that.

As for the wires getting warm but not hot, there can be slow insulation degradation even at lower temperatures, but mostly the NEC has a generous safety factor built in.

One problem with putting in a 20A breaker is that it will allow about 27 amps indefinitely. That is more than the measured 19-20 amps and will generate about 1.8 times as much heat. Now you wires are starting to get hot.
Add additional local heat sources, such as high resistance terminations, reduce the heat loss by throwing in insulation or sheathing, and you may have a real problem.

I do agree that throwing insulation will make a difference. I thought that a 20amp breaker will start its time current curve around 24 to 25?

 

[GoldDigger]

I do agree that throwing insulation will make a difference. I thought that a 20amp breaker will start its time current curve around 24 to 25?

I was remembering 135%, but I might have that wrong.
Don't forget that there is a tolerance band between the "may trip" and the "must trip" curves.

 

[iceworm]

... a 20A breaker is that it will allow about 27 amps indefinitely. ....

... a 20amp breaker will start its time current curve around 24 to 25?

Gold is right on. UL489 CB spec is

From UL-489
Table 7.1.2.2.1Automatic tripping time ? 200 percent rated maximum trip time
0 - 30A ------- 2 minutes

7.1.2.3.1 A circuit breaker carrying 135 percent of its rated current in accordance with Table 7.1.1.2 shall
trip within 1 hour for a device rated at 50 A or less,

7.1.2.4.1 A circuit breaker shall be capable of carrying 100 percent of its rated current without tripping
until temperatures become constant.

At 134%, 26.8A, if it never trips, it meets spec

at the other end of the spec

At 101%, 20.2A, if it trips in 1 minute, it meets spec

ice

 

[winnie]

The tabulated ampacity of conductors will almost always be less than the _actual_ ampacity of the conductors.

Ampacity is that current which a conductor can carry on a continuous basis without overheating its insulation. It depends on the rate of heat production and the rate of heat rejection to the environment. Without using the tables, you would have to do a fairly complex calculation on each small wire; it makes far more sense to have a very conservative table.

If you are running a large underground feeder, then having an engineer do the calculations might save money For a lighting circuit, not so much.

The trip curve of a breaker is usually defined by 'must not trip at the rated current' and a 'must trip in a certain amount of time at some multiplier of the rated current', with a pretty large grey area in between where the breaker might, or might not, trip.

I believe that one of the reasons for the 'small conductor' limits is to prevent damage to the wire during a short circuit event.

-Jon

 

[mbrooke]

Gold is right on. UL489 CB spec is




At 134%, 26.8A, if it never trips, it meets spec

at the other end of the spec

At 101%, 20.2A, if it trips in 1 minute, it meets spec

ice

Thanks! Forgot about that. You are spot on, the maximum limit isn't the same as the 'general limit', ie where most manufactured breakers will trip.

 

[mbrooke]

The tabulated ampacity of conductors will almost always be less than the _actual_ ampacity of the conductors.

Ampacity is that current which a conductor can carry on a continuous basis without overheating its insulation. It depends on the rate of heat production and the rate of heat rejection to the environment. Without using the tables, you would have to do a fairly complex calculation on each small wire; it makes far more sense to have a very conservative table.

If you are running a large underground feeder, then having an engineer do the calculations might save money For a lighting circuit, not so much.

The trip curve of a breaker is usually defined by 'must not trip at the rated current' and a 'must trip in a certain amount of time at some multiplier of the rated current', with a pretty large grey area in between where the breaker might, or might not, trip.

I believe that one of the reasons for the 'small conductor' limits is to prevent damage to the wire during a short circuit event.

-Jon

But here is where I get confused. The ampacity for #14 at 75*C is listed at 20amps yet even though all devices are rated at 75*C the blanket rating still holds. But for say #8 MC, even if its inside a wall, I can go from 40amps to 50amps and use the 75 degree column without issue. Code lets me in this case.

I agree with you on the breakers, but for #8, the maximum time limit for a breaker to open at 135% is around the same as #14 in terms of % of overload.


135% of 20=27amps while 135% of 50 amps= 67.5. At this point even on a code complaint cable (#8MC) I am going over the 75 degree column. Technically such a sustained overload isn't code allowed, but the restriction to the smaller wires is confusing.

 

[petersonra]

Some applications routinely use wire much smaller than we commonly see in NEC applications. they often have 200Deg C insulation.

 

[GoldDigger]

A more commonly seen instance under NEC is fixture wire.

 

[iceworm]

... Technically such a sustained overload isn't code allowed, but the restriction to the smaller wires is confusing.

Yep, you got that part right. And it gets really strange if one considers a motor application. What is max motor load for a #14?

ice

 

[mbrooke]

Yep, you got that part right. And it gets really strange if one considers a motor application. What is max motor load for a #14?

ice

I think 20 amps? I have to look. but OCPD for short circuit protection can be a lot higher. A 30 amp breaker would be fine as long as overload protection is present for the motor itself.