Ampacity

[Jody Meulemans]

I have been studying for my test and have noticed. When some people find ampacity they use the 90 degree colom and some use 75 degree colom. What is right I thought you always use the 90 degree colom. Dose the conductor matter . Example THW in the 75 degree colom use that colom for the ampacity. THHN 90 degree colom use that colom. Thanks

 

[infinity]

I would start with reading all of 110.14. It's a good introduction to conductor ampacity.

Welcome to the Forum.

 

[Volta]

Welcome.

Start with 110.14. There is a lot of info. in that little section. The temperature (ampacity) considerations stem from there.

Much of the equipment we connect to is not rated over 75 deg C, so often the final ampacity will be limited to that, or even lower.

 

[Pierre C Belarge]

110.14 is important in the learning process.
Also pay particular attention to Table 310.16 with the 3 temperature columns and the conductor Types.
This will get you started in the process, but by no means is the end to learning the process of sizing conductors and overcurrent protection of conductors.

 

[ibew441dc]

a solid foundation is a must 110.14(C), Table 310.15(B)(2)(A), and Table 310.16

a solid foundation is a must 110.14(C), Table 310.15(B)(2)(A), and Table 310.16

I have been studying for my test and have noticed. When some people find ampacity they use the 90 degree colom and some use 75 degree colom. What is right I thought you always use the 90 degree colom. Dose the conductor matter . Example THW in the 75 degree colom use that colom for the ampacity. THHN 90 degree colom use that colom. Thanks

110.14(C) Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device (weakest link in the chain). Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both(ampacity adjustment= 310.15(B)(2), correction = bottom half of 310.16---ampacity adjustment/correction is often called derating).

Lime Green is my comment not in the NEC

110.14(C)(1)(A) paraphrased by me = use the 60 degree C column unless the equipment is listed otherwise ( in reality most new electrical equipment is rated for 75 degree C, so if it is not marked use the 60 if it says 75 you can use the 75, but remember to read the first sentence of 110.14C )

110.14(C)(1)(B) paraphrased by me = use the 75 degree column unless the equipment is listed for a higher temperature rating (but remember to read the first sentence of 110.14C )

A firm grasp on 110.14 especially 110.14(C) is a must forget about jumping anywhere else until you have a handle on this section:smile:

 

[JohnJ0906]

Table 310.13(A) will tell you the temperature rating of the various conductors.
Some cable temerature ratings are in the specific cable article such as 334 for NM-B (romex)

 

[ZZDoug]

Here it is in simple language - you use the 90 degree column (IF its 90 degree insulation) for derating purposes, but the final ampacity rating cannot be higher than the temp of the terminals etc the wires are landed on or the enclosures they are contained in, which are generally 75 degrees. And as mentioned, there are some wiring types such as romex that you have to use 60 degrees, but remember thats the final ampacity, even for those you can use the 75 or 90 degree column for derating.
Thats the technical answer for the exam, but in the field for a quick size I generally go straight to the 60 degree column. That usually will result in one size larger than legally required but will normally give you the best wire size to use for voltage drop, efficiency etc. This doesnt apply to heavy loads with long circuit runs (more voltage drop) or loads that would be used intermittently where efficiency isnt so important. But for general use the 60 degree column is a quick go to for the right size to use.