ampacity

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why is it if you go to table 310.16 allowable ampacity of insulated conductors and go under 90* for thhn it says that 14awg can take 25i 12awg 30i 10awg 40i at the bottom of the page it says*see240.4d were it states that 14 has to be fused at 15i etc...so how is it a hasard if you used 14awg on a 20 amp ocp i never have im just woundering
 

augie47

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One thing you encounter is that running #14 at 20 amps will, over time, create a conductor operating at 75?C and the devices at termination often are rated at 60?C and thus subject to heat damage. (some devices are 60? rated)
IMO, you would see no immediate hazard, but will promote deterioration cause by overheating.
 
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infinity

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With a #14 conductors, a 75? C wiring method, 75? C terminations and a 20 amp circuit breaker the circuit will operate forever without any problems. However the code tells us that we must use the 15 amp ampacity because of 240.4(D). Not really sure why. I could guess that it provides an added safety factor for small conductors.
 

S'mise

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my 2c

my 2c

Motors for example can use the full table current in 310.16, probably because the larger starting current is for a small time duration compared to your average non inductive branch circuit load.
 

charlie b

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This will be a bit of nit picking, but I never liked those little bugs anyway. :roll:

If we keep to an ambient temperature of 30C and limit the number of conductors to three, then the "ampacity" of a #14 THHN is 25 amps. Nothing else in the code changes that. While it is true that we must use a 15 amp breaker for most applications of a #14 wire, what we are doing in those cases is protecting a wire with an ampacity of 25, using a breaker that is rated at 15 amps.

Keep in mind that you don't get to article 240.4(D), without first going through article 240.4. The basic rule is that we must protect all conductors at their ampacity, unless otherwise permitted or required by the subsequent paragraphs. Therefore, what 240.4(D) does is to require us to protect certain wires at values that are below their ampacity.

Article 110.14(C) works in a similar fashion. The ampacity of a #12 THHN is 30 amps. But we must protect it at a value that is lower than that ampacity because of terminal limitations.
 

charlie b

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Just for the record, in the U.S., the term 'nit' refers to the egg of a louse, not the louse itself. Also, the word 'nitpicking' is one word, not two.
Nit picker! And since I am accusing you of taking an action (picking) against a helpless living being (the nit), it is within the rules of our language to express it using two words. So there. :cool::grin:

 

Dennis Alwon

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Does T250.122 have anything to do with it?
Not really since the egc does not have to be larger than the conductors of the circuit. One often sees #10 wire on a 40 amp breaker for a/c units. As long as the load is not larger than the ampacity of the conductor we are allowed this by 240.4(G)
 

ike5547

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14awg can take 25i 12awg 30i 10awg 40i
There's a lot of nit picking in this thread already but...

The "I" is capitalized when it symbolizes current. I suppose you could express it as 25I, 30I, etc., but it looks funny to me. How about 25A, 30A, etcetera? or I = 25?
 
There's a lot of nit picking in this thread already but...

The "I" is capitalized when it symbolizes current. I suppose you could express it as 25I, 30I, etc., but it looks funny to me. How about 25A, 30A, etcetera? or I = 25?
Actually, to be nit picky, the 'i' should not be capitalized since it is not derived from a proper name, like Volt or Ampere. The 'i' comes from the German word for 'intensity'.

It also should not be used as unit of measure. 25A is correct as it means 25 Amperes. 25i is incorrect because intensity is not a unit of measure. Using 25i for 25 Amperes would be like using 120e for 120 Volts or 10r for 10 Ohms.
 
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ike5547

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Chico, CA
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Actually, to be nit picky, the 'i' should not be capitalized since it is not derived from a proper name, like Volt or Ampere. The 'i' comes from the German word for 'intensity'.

It also should not be used as unit of measure. 25A is correct as it means 25 Amperes. 25i is incorrect because intensity is not a unit of measure. Using 25i for 25 Amperes would be like using 120e for 120 Volts or 10r for 10 Ohms.
I guess I could get behind that. For example, I = 25 amps instead of I = 25. Though my old professor would probably have accepted I = 25. Then again, maybe not. It's been awhile.
 
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