Ampacity Adjustment Factors and Temperature Rating of Terminations

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I?ve done a fair amount of searching in this forum and other resources and have not been able to understand why the NEC appears to ignore the temp rating of terminations when applying ampacity adjustment factors to conductors. Under conditions where ampacity adjustments aren?t required, the current flowing though a 90 degree conductor connected to 75 degree termination must not exceed the value shown in the 75 degree column in the conductor ampacity table. In this situation it appears the operating temperature of the conductor is limited to the rating of the termination, which makes perfect sense.

In the case where adjustment factors apply, it?s possible to have conductors operating at temps over 75 degrees if the starting point for the ampacity calculation is the 90 degree ampacity rating of the conductor. This would seem to overheat the 75 degree termination. Stated alternatively, if 200 Amps is flowing through each of six #4/0 copper conductors sharing a conduit, the insulation temp rating would need to be 90 degrees. If this operating condition would overheat 75 degree insulation then why wouldn?t it overheat 75 degree terminations?

Is there a flaw in my logic or am I just overthinking the issue?
 

david luchini

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I don't think you've given us enough information to follow your logic all the way through. How much of the 200A load is continuous, and how much is non-continuous?
 

don_resqcapt19

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... Stated alternatively, if 200 Amps is flowing through each of six #4/0 copper conductors sharing a conduit, the insulation temp rating would need to be 90 degrees. If this operating condition would overheat 75 degree insulation then why wouldn?t it overheat 75 degree terminations?

Is there a flaw in my logic or am I just overthinking the issue?
Because the terminations are not in the conduit. They are in more open air where the heat can dissipate better than it can in the conduit.
 
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In the example above the 200 Amps is non-continuous and there are 2 parallel sets of conductors per phase.

The above example is arbitrary, pick any situation where the application of adjustment factors results in a required ampacity that?s near the 90 degree ampacity rating of the proposed conductor. I don?t see why it?s ok to land these conductors on terminations rated for 75 degrees.

Don?s point about the conductors not being in the conduit at the termination point seems valid for some cases, but maybe not all. It doesn?t seem to be as valid where adjustments are made due to high ambient temps. If the nature of the termination area provides a significant cooling effect at the end of the conductor, then I?d think we could use the 90 degree column all the time.
 

elohr46

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In the example above the 200 Amps is non-continuous and there are 2 parallel sets of conductors per phase.

The above example is arbitrary, pick any situation where the application of adjustment factors results in a required ampacity that?s near the 90 degree ampacity rating of the proposed conductor. I don?t see why it?s ok to land these conductors on terminations rated for 75 degrees.

Don?s point about the conductors not being in the conduit at the termination point seems valid for some cases, but maybe not all. It doesn?t seem to be as valid where adjustments are made due to high ambient temps. If the nature of the termination area provides a significant cooling effect at the end of the conductor, then I?d think we could use the 90 degree column all the time.

I would think that the termination would be the hottest point, not the coolest. Run the vacuum cleaner for 10 minutes, then feel the plug, it's always hotter than the rest of the cord.
 

david luchini

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In the example above the 200 Amps is non-continuous and there are 2 parallel sets of conductors per phase

So you have a 400A, non-continuous load, and you are using a feeder with parallel conductors instead of single conductors. Per 215.2(A)(1), your minimum parallel conductor size would be 3/0 (200A in the 75deg column.)

Since you have six CCCs, you need to derate 80%. Using a 75 deg cable, you can see that 3/0 and 4/0 would not have sufficient ampacity for the load, so you would need two sets of 250mcm. Using a 90 deg cable, 3/0 would not have sufficient ampacity, but 4/0 would.

If this operating condition would overheat 75 degree insulation then why wouldn?t it overheat 75 degree terminations?

If the termination is rated for 200A @ 75deg, it doesn't matter if the load is supplied by the two sets of 250mcm (75deg) or two sets of 4/0awg (90 deg,) the termination doesn't see more than 200A.
 
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T.M.Haja Sahib

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I’ve done a fair amount of searching in this forum and other resources and have not been able to understand why the NEC appears to ignore the temp rating of terminations when applying ampacity adjustment factors to conductors. Under conditions where ampacity adjustments aren’t required, the current flowing though a 90 degree conductor connected to 75 degree termination must not exceed the value shown in the 75 degree column in the conductor ampacity table. In this situation it appears the operating temperature of the conductor is limited to the rating of the termination, which makes perfect sense.

In the case where adjustment factors apply, it’s possible to have conductors operating at temps over 75 degrees if the starting point for the ampacity calculation is the 90 degree ampacity rating of the conductor. This would seem to overheat the 75 degree termination. Stated alternatively, if 200 Amps is flowing through each of six #4/0 copper conductors sharing a conduit, the insulation temp rating would need to be 90 degrees. If this operating condition would overheat 75 degree insulation then why wouldn’t it overheat 75 degree terminations?

Is there a flaw in my logic or am I just overthinking the issue?

My conjecture is the application of ampacity correction factors will result higher size conductor, the operating current of which will fall under 75 degrees column.Check and report,if possible.
 
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don_resqcapt19

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I would think that the termination would be the hottest point, not the coolest. Run the vacuum cleaner for 10 minutes, then feel the plug, it's always hotter than the rest of the cord.
That is because of the poor quality of the connection either in the male cord end or between the male prongs and the female socket of the receptacle. That is not normal for a properly made wire termination. In fact, if there are not other heat sorces, the wire termination point will normally be cooler than the wire itself.
 

charlie b

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Is there a flaw in my logic or am I just overthinking the issue?
You are overthinking the issue.

. . . if 200 Amps is flowing through each of six #4/0 copper conductors sharing a conduit, the insulation temp rating would need to be 90 degrees. If this operating condition would overheat 75 degree insulation then why wouldn?t it overheat 75 degree terminations?
This operation would not overheat a 75 degree termination because the termination can handle a current of 230 (i.e., the 75C rating of the 4/0 copper conductor). Your scheme would only push 200 amps through that conductor.
 

elohr46

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That is because of the poor quality of the connection either in the male cord end or between the male prongs and the female socket of the receptacle. That is not normal for a properly made wire termination. In fact, if there are not other heat sorces, the wire termination point will normally be cooler than the wire itself.

Well, that's not the case here. 3 year old 20 amp P&S receptacle and a 1 year old appliance. The termination is almost always the weakest link in the circuit, that's why we have to go back and retighten connections from time to time.
 

don_resqcapt19

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Well, that's not the case here. 3 year old 20 amp P&S receptacle and a 1 year old appliance. The termination is almost always the weakest link in the circuit, that's why we have to go back and retighten connections from time to time.
Re-tightening is something that never should be done with a mechanical connection that applies direct pressure to the conductor. Most of the excessive heat I have seen at a cord/receptacle is from a bad connection in the cord end or poor contact between the blades of the cord end and the receptacle.
 
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You are overthinking the issue.
This operation would not overheat a 75 degree termination because the termination can handle a current of 230 (i.e., the 75C rating of the 4/0 copper conductor). Your scheme would only push 200 amps through that conductor.

I don't think we can simply look at the current flowing through the termination to determine if it will overheat. I could drop the current from 200 to 195 amps and overheat it by using a #2/0 wire.
 

Smart $

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I don't think we can simply look at the current flowing through the termination to determine if it will overheat. I could drop the current from 200 to 195 amps and overheat it by using a #2/0 wire.
But that again assumes a conductor temperature rise of 45?C is when the #00 is not more than three conductors in raceway, cable, or earth at an ambient temperature of 30?C.

The #00 in the enclosure likely has a lot more separation than it does in most common raceway installations, and definitely nowhere close to as constrained as in cable or earth. By comparison, it will have plenty of "breathing" room.

Addtionally, the code just says the termination temperature limitation shall be based on this value, not that it will actually be that temperature. I imagine if experiments were conducted and temperature data were published, some conservatism would be revealed... and unless you can by empirical data and/or several event-causing failure reports prove the method as faulty under a substantial number of normal conditions, it is doubtful the CMP is going to make this more difficult to implement.
 
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david luchini

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I don't think we can simply look at the current flowing through the termination to determine if it will overheat. I could drop the current from 200 to 195 amps and overheat it by using a #2/0 wire.

I don't quite understand your point here. Using 2/0 for a 195A load would violate 215.2.
 
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T.M.Haja Sahib

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May be the reason for choosing 90 degree rated cable which has 75 degree rating also and after applying ampacity adjustment factors,deciding size of that cable based on the 75 degree rating of it to suit 75 degree cable termination instead of choosing a 75 degree rated cable directly is one of economy:The 90 degree rated cable is a lower size than a 75 degree rated cable.
 
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Addtionally, the code just says the termination temperature limitation shall be based on this value, not that it will actually be that temperature. I imagine if experiments were conducted and temperature data were published, some conservatism would be revealed... and unless you can by empirical data and/or several event-causing failure reports prove the method as faulty under a substantial number of normal conditions, it is doubtful the CMP is going to make this more difficult to implement.

Well said, I think this is a good place to end the conversation. Thanks.
 
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