334.80 Provided the final final derated ampacity

[bgelectric]

Provided the final final derated ampacity does not exceed that for 60 C rated conductor.

Can someone explain this statement and how it relates to the statement previous to it. Thanks

 

[augie47]

It means that you can use the 90? insulation rating to derate for the fill, but the number you obtain, if higher than the 60? rating, can not be used for final ampacity.
In other words if you have (3) 10/2 cables for a total of 6 current carrying conductors and you apply a derating factor of 80% to the 90? rating of 40 amps and end up with 32 amps, the actual ampacity is limited to 30 amps because of the 60? rating.

 

[ceb58]

This is something an inspector tried to get me on for having 4 14-2' in a fire caulked hole.

So the 4 14-2's would have a total of 8 ccc (Current Carrying Conductors) The conductors for derating are by the 90deg column in table 310.16 which puts the conductors at a ampacity of 25amp. Then by table 310.15 B 2 a I would have to derate the conductors by 70%.

25*.70= 17.5

The ampacity for #14 in the 60deg. column is 20amp BUT the Astrix leads you to 240.4 (D) which states #14 ( unless specifically permitted) be protected by a 15amp OCP.

So after derating the conductors are good for an ampacity of 17.5 amp. The over-current protection is 15 amp. So the amp value of the wire is still larger than the OCP.

 

[kwired]

If you have several NM cables bundled together in a way that ampacity adjustments are required you are allowed to derate starting from the 90 degree columns of T310.16.

Ok, I thought I had a good answer but now I am confused with how this is worded. I think the intention is that the minimum size of conductor allowed must still meet the requirements of the 60 degree column. Which I think is required even if this statement were not there.

I'm racking my brain trying to come up with an example of how one could derate a 90 degree conductor and have a final ampacity less than that of a 60 degree conductor of the same size, which is minimum size required anyway before this statement comes into play.

 

[kwired]

Augie47 and Ceb58 - your examples are true for conductors for general wiring also, why is the text in question in the OP in that section applying to NM cable?

In general conductors must have an ampacity equal or greater than the load applied to them. This is no different than calculating ampacity of a conductor with 90 degree insulation but it is terminated to a 60 degree terminal. (maybe that there is the answer to the OP question). You can use the higher insulation temperature rating to allow more conductors or cables to be bundled but the minimum size of conductor used must still be at least the same as if you were running a conductor that is marked with a 60 degree rating on it.

 

[augie47]

Augie47 and Ceb58 - your examples are true for conductors for general wiring also, why is the text in question in the OP in that section applying to NM cable?

In general conductors must have an ampacity equal or greater than the load applied to them. This is no different than calculating ampacity of a conductor with 90 degree insulation but it is terminated to a 60 degree terminal. (maybe that there is the answer to the OP question). You can use the higher insulation temperature rating to allow more conductors or cables to be bundled but the minimum size of conductor used must still be at least the same as if you were running a conductor that is marked with a 60 degree rating on it.

I agree....same basics 90? used derating but final number must still be based on 60?

 

[kwired]

I guess I find that particular wording to be redundant.

I have always been taught to size the conductor to the lowest temperature rating that applies first - no matter what any derating calculations leave you with you can never be lower than this first value on conductor size.

So if you have a 42 amp load and you know you have a 60 deg conductor, don't even bother looking at a #8 because it is already too small for 60 deg even if you are allowed to use 90 deg values for derating a portion of the circuit.

 

[ceb58]

Augie47 and Ceb58 - your examples are true for conductors for general wiring also, why is the text in question in the OP in that section applying to NM cable?

In general conductors must have an ampacity equal or greater than the load applied to them. This is no different than calculating ampacity of a conductor with 90 degree insulation but it is terminated to a 60 degree terminal. (maybe that there is the answer to the OP question). You can use the higher insulation temperature rating to allow more conductors or cables to be bundled but the minimum size of conductor used must still be at least the same as if you were running a conductor that is marked with a 60 degree rating on it.

The OP's title concerns 334.80. 334.80 is for the ampacity of NM,NMC and NMS installed without maintaining spacing through the same hole that is to be caulked.
Reading the code section gives you a headache but it tells you the 90deg rating can be used to derate the wire but in the end the conductor rating after derating must be of equal or larger ampacity in the 60deg collum than the ocp.

 

[kwired]

The OP's title concerns 334.80. 334.80 is for the ampacity of NM,NMC and NMS installed without maintaining spacing through the same hole that is to be caulked.
Reading the code section gives you a headache but it tells you the 90deg rating can be used to derate the wire but in the end the conductor rating after derating must be of equal or larger ampacity in the 60deg collum than the ocp.

This was true before they decided to make us derate for cables in sealed holes. If deration applied for any reason that was the rule.

The difference with the sealed hole situation was before we could treat 2 feet or less bundled together like a raceway 2 feet or less. Now they specifically have excluded this situation.

 

[Little Bill]

I guess I find that particular wording to be redundant.

I have always been taught to size the conductor to the lowest temperature rating that applies first - no matter what any derating calculations leave you with you can never be lower than this first value on conductor size.

So if you have a 42 amp load and you know you have a 60 deg conductor, don't even bother looking at a #8 because it is already too small for 60 deg even if you are allowed to use 90 deg values for derating a portion of the circuit.

The way derating is explained in my studies is that it's not so you can use a smaller size wire but in certain circumstances using the derating factors may keep you from having to use a larger wire.

 

[ceb58]

This was true before they decided to make us derate for cables in sealed holes. If deration applied for any reason that was the rule.

The difference with the sealed hole situation was before we could treat 2 feet or less bundled together like a raceway 2 feet or less. Now they specifically have excluded this situation.

That is true. This is just another change in the 08 code that had most scratching their head.