Some questions concerning wire temperature ratings. Briefly, 100 amp service, and due to on going remodeling and ever changing plans, a service disconnect was placed within five feet of the meter, and the new load center several feet away, using EMT raceway.

Table 310.15(B)(6) indicates #4 copper for 100 amp residential, which was used between the disconnect and the load center. However, after reading Section 110 and referencing table 310.16, the wire ampacity is limited to the temperature rating of the equipment. The wire is #4 copper THWN-2/THHN 90degree Centigrade, and the equipment is conductor rated at 75 degree Centigrade. Per the table, this limits the ampacity to 85 amps at 75 degrees Cent.

Q- Is the #4 undersized based on the equipment temperature rating? Should I have used #3 copper? Does this make a difference in residential wiring as it is not exposed to industrial heat conditions? Also, I replaced all the aluminum wiring with copper, and all new cooper use only swithes & outlets. The branch wiring is all 90 degree C-again any problems with this? What temp. rating is normally used in residential wiring?

BTW, all the wire sold here, both Southwire and Cerro, no matter the gauge are all rated 90 degreesC, yet all of the equipment and breakers I see are rated 75 degreesC. I was told all “new” wire is rated 90 degreesC, or maybe that's all that is sold at the "improvement" stores.

I see you have been researching, and trying to connect the dots in the Code.

No wonder electricians can be a bit moody.

The wire sizes listed in Table 310.15(B)(6) are smaller than what is normal simply because of the typical low total actual demand in a residential application by taking this low demand along with the temp ratings of equipment.

So educated judgments were made to create the list of reduced wire sizes for residential services entrance and feeder conductors based on statistically low current demand, temp ratings of equipment, and the unlikelyhood of equipment ever being subjected to the actual temperature, and the 80% derating rule of circuit breakers.

Just to be clear, the temp ratings listed in Table 310.16 is for the terminals of equipment, devices, etc. The most popular is the 75 Deg C for panels, meters, disconnects.

The temp rating of equipment will limit the ampacity of the wire connected to it.

Outside of a residential application:

For example, your #4 THHN is rated 95 Amps when connected to equipment with 90 Deg C rated terminals. But 90 Deg C rating is rare, found only in heavy commercial and industrial equipment. I do believe in many cases it is special order, and is standard for 100 % rated equipment, which again is special order.

The typical temp rating of most equipment is 75 Deg C, as you noted. In this case, other than the residential feeder allowance of Table 310.15(B)(6), the amp rating of #4 will be 85 amps.

Kind of trying to draw a circle with no end, isn't it?

I will say this, Table 310.15(B)(6) is only permitted if it applies to the installation. The conductors must be rated to handle the load served. Trying to think of an example, not coming up with a real world application, but there can be a situation that just because Table 310.15(B)(6) states a wire/insulation can be rated the listed ampacity, if the actual load dictates a larger wire be used, then so be it, and Table 310.15(B)(6) must be ignored.

Branch wiring is different. You state your branch wiring is rated 90 Deg C. What type of cable are you using?

NM-B cable , AKA Romex, is automatically rated 60 Deg C, regardless of temp rating of equipment. MC cable will have 90 Deg C rated wire, but limited to lower temp rating of the devices it serves. A breaker at one end may be 75 Deg C rated, but the device at the load end may only be 60 Deg C, so the 60 Deg C column will prevail.

Residential devices such as switches, receptacles, etc will all have a temp rating of 60 Deg C. so even MC cable will need to use the 60 Deg C column of Table 310.16.

So, back to the service, your #4 can remain and be protected by a 100 Amp circuit breaker.

I hope I made the dots easier to connect without too much rambling on.

Very good question, but thanks, now I am going to be moody all day.

TK, thanks for the info, the more I get, the better... but yeah, I tend to read too much, and read more into it than there is. Just sorry that I put you "mood funk" for the rest of the day:p

As to the branch wiring, it is Southwire THHN/THWN all rated at 90 deg C. The house uses conduit. Like I said all the "improvement" stores sell the same thing. It's cheaper for me to buy there since I don't have a commercial account.

Using the reasoning in your post, and Section 240.4(D), it appears this should (hopefully) not be a problem. The Square D breakers are rate 60/75 deg C, but I have never seen a temp. rating on a switch or outlet. Everyone seems to use this stuff, so hopefully I'm not missing something here.

One final question. A #4 wire has a set cross sectional size, so what determines the temp rating, the insulation?

Here is a typical Leviton Decora receptacle:

5325 > Decora > Duplex > Straight Blade Receptacles (Residential Grade) > Straight Blade Receptacles > All Leviton Products from Leviton Electrical and Electronic Products (http://www.leviton.com/OA_HTML/ibeCCtpItmDspRte.jsp?item=3517&section=10971)

Note the max temp is 60 Deg C. And this will be very typical no matter the brand.

Every product out there will have a datasheet listing the various ratings.

Since the melting point of copper metal is 1984 Deg F or 1084 Deg C:
Copper - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Copper), the insulation burning temp is the issue.

It has been determined that the optimum maximum operating temp of THHN/THWN is 190 Deg C.

Sec 240.4(D) limits any insulation for #14,#12, and #10 to those ampacities of 15, 20, and 30 amps respectively, and thus supersedes the ampacities listed in Table 310.16.

So, and #14,12 or 10 wire must be protected with those amp ratings, no matter what the temp rating is of the wire, or terminals. Larger than #10 is then subjected to

Note, the code is written in a logical order, or better, I should say the Code is to implemented in the order it is written.

Sq D is only noting the range of 60-75 Deg C to reinforce the 75 is the max of the range from 60 to 75 Deg C.

No sweat on my mood, only my wife has to put up with it, and she ignores me anyway. She did office work in my contracting business for 10 years, so she knows all about electricians.

As TKRussell suggests, you are limited to the 75 degree rating, because most terminations are rated 75 degrees. Square D dual lists their connections at 60/75 degrees because you may be upgrading a service in an old building that was wired with 60 degree insulation. (So the electrician knows he can land a 60 degree wire on the termination. In this case, the 60 degree temperature rating of the insulation is the limiting factor.)
He also correctly points out the limitations placed on No. 14, 12, and 10 AWG conductors by article 240.4


The 90 degree table is used only for de-rating, either for high ambient temperature, or more commonly, for more than three current carrying conductors in a conduit. This is more common in industrial installations, and is hardly ever seen in residential work.

For example, a No. 12 Type TW insulation, rated 60 degrees, is rated 25 amps. If you put 4 wires in a pipe, you must de-rate to 80%, or 20 amps. But if you use 90 degree type THHN, you de-rate 80% of the 40 amps it is allowed to carry, to 32 amps (Remember according to 240.4, you must still protect the wire at 20 amps maximum.
The derating progresses very quickly, 4-6 wires must be de-rated to 80%, 7-9 wires to 70%, and 10-20 wires to 50%. So, if you had 10 No. 12 AWG wires installed in a conduit, you could still use a 20 amp breaker if you used 90 degree wire, if you used 60 or 75 degree wire, you would have to protect the wire at 15 amps.

The other reason for using 90 degree wire is the insulation is thinner, and you can fit more wires inside a conduit. Refer to the conduit fill tables in Chapter 9

I will say this, Table 310.15(B)(6) is only permitted if it applies to the installation. The conductors must be rated to handle the load served. Trying to think of an example, not coming up with a real world application, but there can be a situation that just because Table 310.15(B)(6) states a wire/insulation can be rated the listed ampacity, if the actual load dictates a larger wire be used, then so be it, and Table 310.15(B)(6) must be ignored.



The only real world application I can think of is a centrifuge in a sugar refinery where I used to work. It was a 50 HP two-speed motor, operating at 480 volts, and the supply wires were 2/0! (Normally, a 50 HP motor would be wired in No. 4 AWG)

I didn't engineer the job, but some of the things the engineer had to take into consideration were:
- This was in the southern US, the ambient temperature inside the refinery might be 110 degrees F in the summer
- this was a two speed motor, there were six wires in the conduit
-the cycle time was three minutes. (Every cycle, the motor had to be completely stopped, then started. Starting current can be up to six times the normal running current)
-The motor was "down-shifted" to reduce wear on the mechanical brake. We did this by suddenly engaging the low speed starter.

The engineer had to de-rate the wire for each of these conditions. Most people on this board are asking questions about residential applications, and don't get into something as complex as this.