Type NM-B Ampacities

[scottam99]

If type NM-B has insulation which is THHN, then why can't the 75 degree table be used?
Per 334.80 there is no mention of the 75 degree table. Only to use 60 degree for ampacity rating and 90 degree for ampacity derating. Or am I missing something???

 

[Dennis Alwon]

If type NM-B has insulation which is THHN, then why can't the 75 degree table be used?
Per 334.80 there is no mention of the 75 degree table. Only to use 60 degree for ampacity rating and 90 degree for ampacity derating. Or am I missing something???

Well why would you need to use the 75C if it is suitable at 90C. The final OCPD cannot be larger than the 60C but for derating you can start at 75 but why when you can start at 90C

 

[Sierrasparky]

I think he is reffering to the Max OCPD from the 75 table

 

[charlie]

If type NM-B has insulation which is THHN, . .

Some manufacturers do use unmarked THHN without the outer nylon jacket, others do not. The product standard only requires 90? C insulation to be used.

. . . then why can't the 75 degree table be used? . .

The simple answer is that the code says you can't. However, the reason is that NM-B was developed to prevent overheated cable in attics.

 

[Dennis Alwon]

I think he is reffering to the Max OCPD from the 75 table

You cannot use the 75C col. with NM cable for the OCPD. If he is asking why? I cannot answer that other than 334.80

 

[Sierrasparky]

I think he was asking why. I too wonder why. You can have the same conductors in MC/ AC and in emt , RNMC, IMC, RMC and you can use the 75C table. GO figure
The cable is required to have 90 conductors but yet does not offer the ratings of other methods.

If it is simply because of the old wire out there. I think that is a crock.

 

[ivsenroute]

1) The code states that you can't
2) The individual conductors of NM are not marked as being THHN
3) The outer jacket retains more heat than loose THHN conductors inside conduit
4) You can use the 90 column to derate but you still cannot exceed the 60 column rating
5) Stick to the 60 deg column and you will be fine

 

[Sierrasparky]

1) The code states that you can't
2) The individual conductors of NM are not marked as being THHN
3) The outer jacket retains more heat than loose THHN conductors inside conduit
4) You can use the 90 column to derate but you still cannot exceed the 60 column rating
5) Stick to the 60 deg column and you will be fine

Thanks Very Much for the code .
This still does not answer why the decision made.
What was the reasoning behind the rule. I know the code.

 

[brian john]

What is the temperature rating of the terminals on the devices you are terminating this conductor too?

 

[Sierrasparky]

What is the temperature rating of the terminals on the devices you are terminating this conductor too?

I think what the OP was looking for is an answer to why the 75c table cannot be used. I too wonder why. And I'm not reffering to " because it's the code"
There must be reason behind this. Codes are not just arbitrary.

 

[brian john]

I think what the OP was looking for is an answer to why the 75c table cannot be used. I too wonder why. And I'm not reffering to " because it's the code"
There must be reason behind this. Codes are not just arbitrary.

What are the temperature rating of the terminals?

 

[Dennis Alwon]

If the temp. ratings of the terminals are rated 75 c then the entire run of cable must be protected at it's weakest link. That weakest link is the 60 C rating of nm. NM is rated 60 C simply because of it's contact with thermal insulation that abounds in a hme. It has been determined that nm breaks down when encases in themal insulated and used at a higher temp than 60C

 

[ivsenroute]

Read #3 of my post.

 

[electricmanscott]

Wow, way to muck up a good question guys.

 

[LarryFine]

One way to investigate the decision would be to study the more-recent decision to treat SE cables the same way.

 

[Sierrasparky]

What are the temperature rating of the terminals?

Is there a parrot in the house.

 

[Sierrasparky]

One way to investigate the decision would be to study the more-recent decision to treat SE cables the same way.

Larry
Could you be so kind to direct us in the right direction . A link maybe.
I am interested in the what was behind this decision also.

 

[nakulak]

I have never seen thhn in romex. All I have seen is what appears to be hhn.

 

[Sierrasparky]

I have never seen thhn in romex. All I have seen is what appears to be hhn.

Ok then Why are they not putting THHN and calling it same ampacity as THHN. What's the deal and why so much cockyness for what might be a simple answer.

 

[iwire]

Thanks Very Much for the code .
This still does not answer why the decision made.
What was the reasoning behind the rule. I know the code.

You got the answer in post 4, Charlie is pretty sharp about these things.

However, the reason is that NM-B was developed to prevent overheated cable in attics.