Understanding 110.14(C) - Temperature Limitations

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sirdle

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I have a lot of trouble understanding NEC 110.14(C) because I don't really understand where the heat comes from and what we are trying to protect from what. Are we worried about (1) the heat generated in the conductor, (2) the heat generated by the equipment at it's terminals, or (3) the ambient heat?

Example #1 - a 90 deg conductor ( #8 THHN) connected to a 75 deg equipment terminal, pulling 55 amps.

The heat generated by the current flowing through the conductor will not damage the conductor insulation, but will damage the equipment terminals? So I can still use 90 deg THHN as long as I increase the wire size to #6 (75 deg rating of 65 amps)?

Example #2 - a 60 deg conductor (#8 TW) connected to a 75 deg equipment terminal, pulling 40 amps.

The heat generated by the current through the conductor will not damage the conductor insulation or the equipment terminals... but the heat generated by the equipment at the terminals could rise to 75 deg and that heat can damage the conductor insulation? So a 60 deg conductor cannot be used at all no matter what the actual current flow is?

Example #3 - (this is what I'm leading up to) - An immersion heating element with no equipment markings, connected to some power SCRs with no termination markings. An equipment drawing from the manufacturer requires a minimum of #6 90 deg copper wire. The heating elements are rated at 41.7KW at 480VAC, 3 ph.

It seems to me that the load is 50.2A ( = 41,700 watt / 480 vac / 1.73) and requires an ampacity of at least 63A (at 125%). Except when I shoot the temperature inside the heating element termination enclosure, I get a reading of 200 degF.

So if the SCR had a 75 deg termination rating (I'm making this up just to understand the concept) and the conductor had at least a 75 deg rating, then the current and the SCRs aren't the source of the heat I should be worrying about - it is the ambient heat inside the heating element termination enclosure. But the temperature correction factors for THHN don't go higher than 158deg. So I can't use THHN and I would need to bump up the insulation to a Type Z (Table 310.18)?

If I did, the conductor would be rated at 302deg... but not the mechanical connectors.And in any event, I can't connect the Type Z to the 75deg terminations at the SCR because Table 310.18 doesn't have a 75deg column.

So at this point my head explodes.

Any clarification would be greatly appreciated.
 
sirdle said:
I have a lot of trouble understanding NEC 110.14(C) because I don't really understand where the heat comes from and what we are trying to protect from what. Are we worried about (1) the heat generated in the conductor, (2) the heat generated by the equipment at it's terminals, or (3) the ambient heat?
Click to expand...

All three.



Example #1 - a 90 deg conductor ( #8 THHN) connected to a 75 deg equipment terminal, pulling 55 amps.

The heat generated by the current flowing through the conductor will not damage the conductor insulation, but will damage the equipment terminals? So I can still use 90 deg THHN as long as I increase the wire size to #6 (75 deg rating of 65 amps)?
Click to expand...

Assuming pipe and wire not NM ...... yes.



Example #2 - a 60 deg conductor (#8 TW) connected to a 75 deg equipment terminal, pulling 40 amps.

The heat generated by the current through the conductor will not damage the conductor insulation or the equipment terminals... but the heat generated by the equipment at the terminals could rise to 75 deg and that heat can damage the conductor insulation? So a 60 deg conductor cannot be used at all no matter what the actual current flow is?
Click to expand...

No, unless the equipment requires 75 C conductors.

(Some large breakers require 90 C conductors but the ampacity has to be based on 75 C or less for the reason you describe, the heat the equipment will generate)


Example #3 .................

So at this point my head explodes.

[/QUOTE]


Mine did as well, I did not follow you.
 
Example #1 - check!

Example #2 - How can I tell the difference between equipment that will permit a 75 deg connection and one that requires a 75 deg connection?

Example #3 - let me simplify the situation by making up some stuff I don't know.

1) Immersion heater termination lugs rated at 90 deg.
2) SCR termination lugs rated at 90 deg.
3) Conductors rated at 90 deg.
4) Measured temperature inside the immersion heater junction box - 100 deg.

This seems to be a case of ambient temperature being a source of heat that could damage the conductor insulation. I think I need wire rated at least 100 deg. But if I install this wire, then I have a temperature mis-match at the SCR between the conductor insulation (100 deg) and the SCR lug rating (90 deg).

From the answer you gave to #2 above, I'm guessing this is not a problem.

So the real problem is in trying to size this wire.

Within Table 310.16 I can adjust the ampacity downward for a conductor by using a 75 deg column instead of a 90 deg column. (Like we did in example #1.) But Table 310.16 doesn't list wire with a 100 deg rating. So do I have to use at a different table, say 310.18? And how do I derate the ampacity to make sure the conductor doesn't generate too much heat for the 75 deg SCR terminations?

I'm not sure I made my question any clearer, but at least I think I understand examples #1 and #2. Thanks!
 
You are talking ambient temp. which is different from conductor insulation temp.

Also you use the 90C rating and then derate from there not from 75C. The end result cannoy be any higher than the weakest link. Thus a 90C wire can be derated and if the end result calls for a conductor higher then the 75C ampacity of the wire then we must go down to the 75C rating.
 
Dennis Alwon said:
You are talking ambient temp. which is different from conductor insulation temp.

Also you use the 90C rating and then derate from there not from 75C. The end result cannoy be any higher than the weakest link. Thus a 90C wire can be derated and if the end result calls for a conductor higher then the 75C ampacity of the wire then we must go down to the 75C rating.
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Okay. I'm with you.

But doesn't the conductor insulation rating still have to exceed the ambient temperature?

And if this rating were, say 150C, how can I derate from 150C to 75C when they are in different tables?

Or does ambient temperature correction have its own rules completely separate from conductor insulation rating?

Thanks.
 
sirdle said:
Okay. I'm with you.

But doesn't the conductor insulation rating still have to exceed the ambient temperature?

And if this rating were, say 150C, how can I derate from 150C to 75C when they are in different tables?

Or does ambient temperature correction have its own rules completely separate from conductor insulation rating?

Thanks.
Click to expand...


Well 90C is 194 degrees Farenheit--- That's pretty hot but there are some cables rated higher usually for higher voltages.
 
I'm still confused.

I have always used Table 310-16 to determine ampacity. But it occurs to me that it can also be used to illustrate current-induced temperature, as well. For example, if I push 55 amps through a #8 THHN in an ambient temperature of 30?C... then I will drive the temperature in that conductor to 90?C. And this is why that conductor can't be terminated to a piece of equipment rated at 75?C?

But if I increase the wire size to #6 THHN and still push 55 amps through it, then I am only generating 60?C of heat... which is lower than the rating of equipment, so I'm okay?

Which brings me back to my earlier question - how can I tell the difference between a piece of equipment that permits 75?C connections and one that requires 75?C connections? Is this identified by labeling on the equipment? And if so, what would a typical label say?
 
Confusion about ambient/ terminal temperature corrections

Confusion about ambient/ terminal temperature corrections

sirdle said:
I'm still confused.

I have always used Table 310-16 to determine ampacity. But it occurs to me that it can also be used to illustrate current-induced temperature, as well. For example, if I push 55 amps through a #8 THHN in an ambient temperature of 30?C... then I will drive the temperature in that conductor to 90?C. And this is why that conductor can't be terminated to a piece of equipment rated at 75?C?

But if I increase the wire size to #6 THHN and still push 55 amps through it, then I am only generating 60?C of heat... which is lower than the rating of equipment, so I'm okay?

Which brings me back to my earlier question - how can I tell the difference between a piece of equipment that permits 75?C connections and one that requires 75?C connections? Is this identified by labeling on the equipment? And if so, what would a typical label say?
Click to expand...

You have the correct understanding. The code is confusing to the point of being wrong when it calls the 60, 75 and 90 degree columns of ampacity table 310.16 (NEC2008) as "conductor temperature rating". Those columns really mean how hot the conductor will get when you pass the indicated amperage throiugh it. A 10AWG wire passing 40 amps is going to reach 90c whatever type of insulation it has. Of course if the insulation is only good for 60c, you won't want to let it get that hot.

This causes confusion when checking terminal temperature limitations. For example, if you are using 10AWG THHN 90C cable attached to 75C terminals, how many amps are you allowed to put through it? The correct answer is to look up the ampacity of 10AWG that will keep its temperature down to75C. This is the value of 35 amps given in the 75 degree column. You use that column even though THHN is not listed in that column. That's the problem with the NEC as written!

In the 2011 code, a new problem has appeared in art. 310.15B(2).
There is a formula for ambient temperature correction factor that uses a value Tc, called "temperature rating of the conductor"

So, if I use THHN do I put 90 degrees there? NO NO NO!

What you have to plug in there is the temperature that you want the wire to run at, e.g. 75 degrees if connected to a 75 degree terminal.
It's got nothing to do with what the wire CAN tolerate, but, as electricians and inspectors are going to read the code like the devil reads the Bible, they are plugging 90 degrees in there for THHN or THWN-2 because the variable Tc is mis-named "conductor temeperature rating". This variable should be renamed "desired conductor operating temperature". That might be -5c if it was running though a freezer, for example, even if it was THHN capable of 90c.
 
If this is listed equipment and the instructions call for the use of 90? conductors, you don't have to do anything else for the conductors at the equipment.
 
PaulWDent said:
Those columns really mean how hot the conductor will get when you pass the indicated amperage through it.
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Not quite. The ampacity in the (for example) 90C columns mean that if you pass that amount of current through the wire, the temperature will not exceed 90C.

PaulWDent said:
In the 2011 code, a new problem has appeared in art. 310.15B(2). There is a formula for ambient temperature correction factor that uses a value Tc, called "temperature rating of the conductor."
Click to expand...
PaulWDent said:
So, if I use THHN do I put 90 degrees there? NO NO NO!
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I just want to mention here that I disagree. But you have another thread that is discussion that question, so let's not duplicate that discussion here. Anyone interested can participate in the other discussion here:
http://forums.mikeholt.com/showthread.php/137245-Ambient-Temperature-Correction-Factors-art.-310.15-B(2)
 
Can I get a clarification here? :?

My understanding is that the 90C column (and the others as well) is talking about a temperature rise of 90C due to I2R heating caused by current flow, and is in addition to the 30C ambient, so that the max temperature seen by a wire loaded to that ampacity value will be 120C. True or false? Reference?
 
charlie b said:
Can I get a clarification here? :?

My understanding is that the 90C column (and the others as well) is talking about a temperature rise of 90C due to I2R heating caused by current flow, and is in addition to the 30C ambient, so that the max temperature seen by a wire loaded to that ampacity value will be 120C. True or false? Reference?
Click to expand...

I always understood it as that the maximum temperature at which the insulation will safely maintain it's insulating value. Conductor temperatures beyond the insulation's rating will cause the insulation to breakdown and fail.
 
I also see the temperature rating of the wire as an absolute maximum. I think you can infer this from the ampacity temperature correction factors.
 
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