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Temperature Rating

Posted by wxtrender (My Page) on
Sun, Nov 10, 13 at 7:11

My building inspector is asking I provide a drawing of the the little bit of electrical work I am doing.

In my barn I want to add a circuit feeding 4 outlets from the panel that is already in the barn. The inspector asked me to identify the "type/size/temperature rating for each circuit"

Any idea what he means by type?

Size is 12/2 20 amps

Any idea what temperature rating means?

Is there a chart or table that tells you temperature ratings based on circuit size?


Follow-Up Postings:

RE: Temperature Rating

Type is the kind of conductor. For example NM (commonly called ROMEX) UF, THHN, THWN, etc... are all wire types.

Each wire type has a temperature rating (it will be 60, 75, or 90 degrees C). You can figure out the temperature rating from the wire type.

The reference most used is the table 310-16 of the NEC. I've put a link to a copy that's available on line below (you can dig it out of the free version of the NEC at the NFPA website as well).

Here is a link that might be useful: Wire Types/Temperature Ratings/Ampacities

RE: Temperature Rating

Although it not shown on table 310-16, the temperature rating of NM-B cable is 60 degrees Celsius.

RE: Temperature Rating

"Although it not shown on table 310-16, the temperature rating of NM-B cable is 60 degrees Celsius."
How did you reach that conclusion? What about Article 334.112
in the 2011 NEC? And what is the meaning of the -B suffix on the NM?

RE: Temperature Rating

According to the Wire and Cable Reference Guide by Essex (a wire & cable manufacturer), these are the following definitions...

NM - Non metallic sheathed cable, braid or plastic covered. For dry use, 60C.

NM-B - Non metallic sheathed cable, plastic covered. For dry use, 90C.

As I recall,
Type NM cable included the cloth-jacketed version which had problems in high-temp areas, like attics. It had a 60 degree C specification. Type NM-B was established to specify higher temp materials which increased the limit to 90 degrees C. I'm sure that no NM is currently being manufactured, only NM-B.

RE: Temperature Rating

Yes, the suffix -B in NM-B denotes that the conductors within the sheath are rated at 90c, as required by 334.112. However, it is my understanding that the 90 degree C rating of NM-B shall be used for derating purposes, provided the final de-rating ampacity does not exceed that for a 60degrees C conductor (Article 334.80). As I'm not up to date with the 2011 NEC, feel free to correct me if I'm wrong.

RE: Temperature Rating

When would one derate NM-B cable? Have you ever encountered a situation where that is necessary?
And does this project sound as if it will encounter such a requirement?

RE: Temperature Rating

Would they be derated if put inside a protective cover (conduit)? Which might be required in a barn? Just asking...

RE: Temperature Rating

[Quote] "When would one derate NM-B cable? Have you ever encountered a situation where that is necessary? And does this project sound as if it will encounter such a requirement?" ... Oh now I get it, the inspector wants to know the temperature rating of each circuit to see if it meet the requirements of 334.112. Well then in that case I should have not responded about table 310.16 and article 334.80. I made a mistake.

This post was edited by Stevie51 on Sun, Nov 10, 13 at 19:55

RE: Temperature Rating

It appears that temperature rating and amperes have been mixed or confused by some parties. NM-B is rated for 90 deg.C. But the overcurrent protection for it cannot exceed that of the table values for 60 deg C conductors. But for instances where adjustments must be made- such as derating-- the calculations may begin using the values in the table for 90 deg C. And the FINAL overcurrent device in those instances cannot be larger than that used for 60 deg C even if the derating calculations results in higher amperages than the 60 deg C table. .
It is regrettable that the posts have sidetracked so far from the original request. My hope has been to counteract bad information.
And the content of the original post indicates by inference that the person has no prior knowledge of calculating derating nor of knowing when it is required. It was counterproductive and a disservice to the original poster to even mention it.
Those who brought up the subject of derating unwittingly reveal that they are not knowledgeable about the subject and process. For #14 and #12 conductors, after the calculations AND correct application of the rules of Article 240, the result is almost always no change in the rating of the specified overcurrent device.

This post was edited by bus_driver on Mon, Nov 11, 13 at 7:34

RE: Temperature Rating

'...The inspector asked me to identify the 'type/size/temperature rating for each circuit'

Any idea what he means by type?...'

Type may mean general receptacle or dedicated to a certain piece of equipment. If those four receptacles are for certain pieces of equipment a load calculation would have to be done.

Size is as you said or would be determined by the load calc.

Temperature rating could also mean the ambient temperature. For instance a greenhouse could be kept at a constant 100 degrees F.

'...Is there a chart or table that tells you temperature ratings based on circuit size?'

I don't understand the question. Amperage causes heating. The tables we use restrict amperage based on how much heat the wire's insulation can handle, ambient temperature and bundling.

Here is a link that might be useful: Table 310.16 ampacity and temperature adjustments

This post was edited by glennsparky on Mon, Nov 11, 13 at 5:07

RE: Temperature Rating

I would ask the inspector for clarification before providing all kinds of detailed info. He may just want to make sure you know what kind of wire you are using and how to install it. In an unfinished barn space I think most cities would allow you to use NM-B stapled to the framing. However, if you decided to protect it with conduit (which many folks seem to like) he may want to ensure you to understand the installation differences.

If you run individual wires in conduit you can fit larger wires and/or more circuits than you could trying to run NM-B through the conduit. Some cities also have specific rules for running NM-B in conduit.


RE: Temperature Rating

First of all, it is a barn. Therefore it will have to be a GFCI
TYPE of circuit. It will also be dedicated circuit for a specific piece of equipment or purpose, or it will be general purpose. Temp rating would deal with the expected ambient temperature in the environment where the circuit actually exists, passes through, occupies. (knowledge or not derating is a subject that needs to be taken into account for either grouping, temp, distance(technically not derating but voltage drop) or ampacity. If in doubt support your local electrician as I am sure there is someone nearby with the knowledge and expertice to advise the proper selection for a small fee. Do your own surgey lately? Sorry about that, but electricity will kill you faster than botched surgery, and might even burn down your place as a bonus. One of your cronies has a kid that is a qualified, certified electrician, or one of the guys at the Elks or Moose or Native Sons. A wise man knows his limitations, especially when the stakes are ultimately high.

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