table 310.15(B)(16) wire size

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askDano

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When sizing wire using table 310.15(B)(16) what temperature column do I use? I understand that these columns are the equipment terminal temperature rating, is this explained somewhere in the code. I'm feeding a magnetek VFD drive on a 20 hp motor and Magnetek has never been asked about their terminal rating.

Also on a crane application is table 610.14(A) used the same way as table 310.15(B)(16)?
 
The columns are not the equipment terminal rating. They are the rating of the conductor's insulation system. The conductor types are listed at the top of the columns. For example, if you are using type TW, then you are limited to the ampacity shown in the 60C column. If you are using type THHN, then the ampacities in the 90C column become part of the design process. However, most terminations are only rated for 75C, and that will limit the ampacity to the values shown in the 75C column. This is explained (though not very well) in 110.14(C).

I believe the same concept applies to the crane-related table you mentioned.
 
How does the terminal temperature affect wire size

How does the terminal temperature affect wire size

Why does the terminal temperature limit the size of the wire connected to it. I will run this wire 50+ feet and because of a lug on the disconnect the wire will need up sized.

Is this because of heat sinking or contact area between wire and lug? Is there more details (data, research) to why this is in the code this way?
The columns are not the equipment terminal rating. They are the rating of the conductor's insulation system. The conductor types are listed at the top of the columns. For example, if you are using type TW, then you are limited to the ampacity shown in the 60C column. If you are using type THHN, then the ampacities in the 90C column become part of the design process. However, most terminations are only rated for 75C, and that will limit the ampacity to the values shown in the 75C column. This is explained (though not very well) in 110.14(C).

I believe the same concept applies to the crane-related table you mentioned.
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Think of it this way-- if you had a piece of #8 wire running 200' and you added a short piece of #12 to it then the circuit itself is only capable of the current afforded by it's weakest link. Thus 20 amp is all that circuit can handle since the #12 is the weak link.

Also in 110.14(C) it basically states if the terminal is not know then we must use 60C for anything of 100 amps or less and 75C for greater than 100 amps. If the terminals are marked 75C and you have a circuit that is 100 amps or less then you may use the 75C rating.
 
Dennis Alwon said:
Think of it this way-- if you had a piece of #8 wire running 200' and you added a short piece of #12 to it then the circuit itself is only capable of the current afforded by it's weakest link. Thus 20 amp is all that circuit can handle since the #12 is the weak link.
Click to expand...

Let me add to what you said, that same circuit with a short piece of #12, will still carry the current, but the 12 AWG segment will operate at a higher temperature. The 12 AWG copper is likely to be able to carry the load, the insulation may not be able to handle the additional heat. The different columns are for insulation temperature more so than conductor temperature considerations.
 
kwired said:
Let me add to what you said, that same circuit with a short piece of #12, will still carry the current, but the 12 AWG segment will operate at a higher temperature. The 12 AWG copper is likely to be able to carry the load, the insulation may not be able to handle the additional heat. The different columns are for insulation temperature more so than conductor temperature considerations.
Click to expand...

That is correct. It would not be compliant and as you stated the insulation will break down over time.
 
kwired said:
Let me add to what you said, that same circuit with a short piece of #12, will still carry the current, but the 12 AWG segment will operate at a higher temperature. The 12 AWG copper is likely to be able to carry the load, the insulation may not be able to handle the additional heat. The different columns are for insulation temperature more so than conductor temperature considerations.
Click to expand...
True...

However, 110.14(C) uses Table 310.15(B(16) values to establish a conductor operating termperature (or heat sinking capability, if you wish) based on conductor size and the termination temperature rating.
 
askDano said:
Why does the terminal temperature limit the size of the wire connected to it. I will run this wire 50+ feet and because of a lug on the disconnect the wire will need up sized.

Is this because of heat sinking or contact area between wire and lug? Is there more details (data, research) to why this is in the code this way?
Click to expand...
Think about it. If my terminal is rated at 60C, but I have a conductor that I am expecting to reach 75C, what temperature do you think the terminal is going to reach since the wire stuck in it is going to be at 75C?

We will need to make the conductor bigger so it will only reach 60C not because of the conductor, but because of the terminal.

The reverse is true for a 75C terminal and a 60C conductor. Just because the terminal can handle 75C...
 
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