Derating question

[teufelhounden91]

An old school journeyman I work with said that you must derate conductors using the 90°C column of T310.15(B)(16)-no matter what the temperature rating of terminations are. 4 CCCs.

I know when you use a 90°C conductor this makes sense, but he's saying for some reason even on a 75°C wire you use the 90°C column for your derate?

Doesn't make sense to me. Even dual-rated wire has 2 temps...one for 90°C the other for 75°C - I've never heard of having to use the 90°C column for 75°C derates.

Code ref anyone?


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[luckylerado]

Balderdash..

You may not use 90° column for 75° wire.

You may derate from 90° column when terminations are 75° but your wire must be 90° rated and your corrected and derated ampacity may not exceed the value in the 75° column

 

[ggunn]

For protecting 75 degree terminals for continuous use you derate in the 75 degree column even if you are using 90 degree wire.

For protecting the insulation on the 90 degree wire you derate in the 90 degree column for conditions of use - ambient temperature, rooftop adder, and current carrying conductor count.

Whichever is the lower ampacity is the real one for your application.

 

[teufelhounden91]

Balderdash..

You may not use 90° column for 75° wire.

You may derate from 90° column when terminations are 75° but your wire must be 90° rated and your corrected and derated ampacity may not exceed the value in the 75° column

Thank you that's what I thought


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[teufelhounden91]

Whichever is the lower ampacity is the real one for your application.

Thank you



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[kwired]

I know when you use a 90°C conductor this makes sense, but he's saying for some reason even on a 75°C wire you use the 90°C column for your derate?

If you are using a 75C conductor then you must make adjustments from values in 75C column. This generally only applies when dealing with existing older conductors. Similar applies if they are only 60C conductors.

There is THHN/THWN that is 90c dry 75C wet, but if it also has a THWN-2 rating then is 90c wet also.

 

[tbartek]

I think that your "old school journeyman" friend might have been thinking about romex using the 90 deg. column for derating even though the 60 deg is for ampacity.

 

[teufelhounden91]

I think that your "old school journeyman" friend might have been thinking about romex using the 90 deg. column for derating even though the 60 deg is for ampacity.

Do you have a code reference for this?


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[infinity]

Do you have a code reference for this?

Look at 334.80.

 

[kwired]

Applies to more then just NM cable. NM just happens to be required to be treated like a 60 amp conductor for termination but can have 90C as a base for ampacity adjustments - it has to be newer NM-B style cable that has 90 C insulation, if you have a bunch of old existing cables you very well may need to adjust from 60C.

For all conductors 110.14(C) says we must determine ampacity based on the termination temperature of what it attaches to. For 600 volts and below you almost never find anything with 90C rated terminations. (individual lugs may be marked 90C but the listed assembly is not) This means minumum size of conductor will be based on either 60C or 75C terminations.

Now step to 310.15(B) in particular sub parts 2 and 3 - we get to use 90C column of tables to make ampacity adjustments.

From those last two items mentioned we must still select the largest conductor determined by those items because we must satisfy both the terminal temp requirements and the conductor insulation requirements.