Ambient Temperature and Termination Rating

[JeffCH]

I have been informed that the 2014 NEC code has changed (in section 110.14C) and now we must do an ambient temperature correction to the value in the 75C column (from 310.15 tables) as a result of termination/equipment rating even if we have a 90C rated cable. I completely disagree with this individual's new interpretation but am having a difficult time convincing of this. Can someone confirm that if a 90C cable has terminations/equipment rated at 75C then we do all derating from 90C amperage in the table and then compare that to value in 75C (unchanged due to ambient temperatue correction) and then use the lower of the two values. I tried watching numerous Mike Holt videos on this, but I could only find videos where at the end he said "we'll get into that later in the next video" but could never find the next video. Also, please attempt to explain why an ambient temperature correction is not done to the value in the 75C column. I know this is possibly answered in various ways in other posts but wanted to ensure nothing has changed for 2014. Thanks!

 

[david luchini]

I have been informed that the 2014 NEC code has changed (in section 110.14C) and now we must do an ambient temperature correction to the value in the 75C column (from 310.15 tables) as a result of termination/equipment rating even if we have a 90C rated cable. I completely disagree with this individual's new interpretation but am having a difficult time convincing of this. Can someone confirm that if a 90C cable has terminations/equipment rated at 75C then we do all derating from 90C amperage in the table and then compare that to value in 75C (unchanged due to ambient temperatue correction) and then use the lower of the two values. I tried watching numerous Mike Holt videos on this, but I could only find videos where at the end he said "we'll get into that later in the next video" but could never find the next video. Also, please attempt to explain why an ambient temperature correction is not done to the value in the 75C column. I know this is possibly answered in various ways in other posts but wanted to ensure nothing has changed for 2014. Thanks!

I don't see any change in 110.14(C) in 2014 NEC. It still says "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both."

 

[charlie b]

I confirm your description of the derating process. To answer your other question, start by noting that ampacity limits are all about protecting the conductor?s insulation system from damage due to overheating. A conductor with 90C insulation can handle a higher current (i.e., without risk of damage) than a conductor with 75C insulation. However, the current also passes through the terminations, and they can also be damaged by overheating. That is why we use the lower of the two numbers, as you mentioned.

We derate for having large numbers of conductors in the same raceway because in addition to the ambient temperature, and in addition to the heat rise from current passing through the conductor, you also have heat contribution from the nearby conductors. But since a 90C rated conductor can handle a higher temperature, we are allowed to use the tabulated 90C ampacity value as the starting point for derating.

 

[JeffCH]

Thanks Charlie and David for your responses to my questions.

So, the bottom line is, there is no ambient correction factor applied at a termination? I agree with this but am having a bit of difficulty understanding why.

 

[charlie b]

We don't calculate (or otherwise determine) the ampacity of a termination. You can't derate the ampacity of a termination, because there is no starting point for their ampacity in the first place.

 

[winnie]

I've always understood this as 'At each point in the circuit, you use the lowest temperature rating _at that point_, and the conditions _at that point_ to determine the ampacity of that point of the circuit.'

So if you have 90C wire connected to a 75C termination in a panel, then the ampacity of the wire in the panel where it is connected is limited to the 75C ampacity of the wire. But when that 90C wire continues into a conduit with a bunch of other wires, and the only thing present in the conduit is 90C wire, then the ampacity _in the conduit_ is the 90C ampacity of the wire (as adjusted and derated...)

If, for whatever reason, you had a 90C wire terminated with a 75C termination in a situation with high ambient temperature, then _at that location_ you would need to adjust from the 75C ampacity of the wire.

To answer Charlie's comment "You can't derate the ampacity of a termination, because there is no starting point for their ampacity in the first place." I am specifically talking about the ampacity of the _wire_ on the termination. The wire may have a 90C rating, but you can't allow it to self heat up to 90C if it is connected to a 75C termination.

-Jon

 

[david luchini]

If, for whatever reason, you had a 90C wire terminated with a 75C termination in a situation with high ambient temperature, then _at that location_ you would need to adjust from the 75C ampacity of the wire.

This seems contrary to what the Code says.

110.14(C) says "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity correction." ("Ampacity correction" = adjustment for ambient temperature.)

For instance, if you had a 100A non-continuous load on a feeder, the minimum conductor size and OCPD from 215.2 and 215.3 would be #3awg Cu, and 100A OCPD. If the ambient is 30degC, then #3 THWN would be sufficient to carry the load. You could use #3 THHN, which has an ampacity of 115, but you can only put 100A through the THHN since the terminations are rated 75degC.

If the same feeder was in an ambient of 45degC, then the #3 THWN would only have an ampacity of 82A and wouldn't be able to carry the load. You would have to upsize to a #1 THWN to get a conductor with sufficient ampacity to carry the load at that ambient. But if you used #3 THHN in the 45degC ambient, the ampacity of the conductor is 100A, and can be used without upsizing.

 

[stephena]

310.15B

 

[kwired]

As others have been trying to point out, there are two concerns

1. the temperature developed at a termination.

2. the temperature of the conductor insulation.

When determining minimum size of a conductor one always must use a conductor that fulfills the termination temperature requirement.

When you get away from the termination - all that really matters is making sure conductor insulation rating is not exceeded, the conductor itself can operate much higher temp but keeping insulation integrity is important and is a limiting factor. We are allowed to make adjustments based on 90 C (for most common applications 90 C insulation is what is present) but no matter what size conductor those adjustments allow you still must use a minimum conductor size based on the termination rating.

Example - say I determine I need a 6 AWG conductor based on termination rating.
I then calculate that for ambient temp, number of conductors in raceway - that a 8 AWG is capable of fulfilling requirements at 90C - I still need to use the 6 AWG because of the termination rating. But on the other extreme say I determined I needed a 6 AWG for termination rating, but did have sufficient ambient temp or number of conductors that 4 AWG was deemed necessary - then I would be running the 4 AWG even though the termination would be fine with 6 AWG.

 

[Smart $]

... So, the bottom line is, there is no ambient correction factor applied at a termination? I agree with this but am having a bit of difficulty understanding why.

Terminations are metal to metal. No insulation involved. Rather than correct for ambient temperature, continuous loads are factored by 125% as padding. We do not apply the 125% continuous load factoring to the conductor ampacity determination.

 

[kwired]

Terminations are metal to metal. No insulation involved. Rather than correct for ambient temperature, continuous loads are factored by 125% as padding. We do not apply the 125% continuous load factoring to the conductor ampacity determination.

With terminations they also depend on the conductor to sink some of the heat away from the termination, and this is part of why you sometimes see when a termination has failed or is in the process of failing, or maybe conductor was undersized, that only a few inches of the conductor appear to have been overheated right near the termination but not the entire length of the conductor.

 

[JeffCH]

Sorry to everyone for "beating a dead horse" but our a senior electrical engineer who I work with sent this email out to all the electrical engineers in our group and I just wanted to get your take on it...

"Here?s the NFPA response to the subject issue. As discussed below termination ambient temperature ratings need to be derated.

The latest NFPA ? 70 handbook 2014 edition relating to Article 110.14C on page 34 states ?Conductor terminations, as well as conductors, must be rated for the operating temperature of the circuit.? For example, the load on a 8 AWG THHN, 90Deg. C copper conductor is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60 deg. C. The same conductor is limited to 50 amperes where connected to a fusible switch with terminals rated at 75 Deg. C. Not only do termination temperature ratings apply to conductor terminations, but the equipment enclosure marking must also permit terminations above 60 Deg. C. This is a change from the 2011 Code.

The question posed to NFPA. Is it required to adjust conductor terminations for ambient temperatures?

I contacted {name removed} of NFPA on 8/18/14, 1-800-344-3555 pertaining to obtaining an interpretation of the aforementioned code requirement. His interpretation was YES, that adjustments for conductor terminations for ambient temperatures has to be accounted for.

The temperature correction and adjustment factors apply to the ampacity for the temperature rating of the conductor, provided the corrected and adjusted ampacity doesn?t exceed the ampacity for the temperature rating of the termination per the provisions of 110.14(C). Put another way, this change clarifies that, after applying these adjustments and corrections, the resulting ampacity still can?t exceed the temperature limitations of the equipment termination.

The ambient temperature ratings of all devices need to be verified with the manufacturers to ensure that ambient temperatures are accounted for. Most electrical devices such as breakers have been rated for 40 deg. C ambient however other ambient temperatures may be used and as a result the most conservative approach for the limiting device for the circuit is to be used. So if you have a device with terminations rated for 30 Deg. C ambient and the circuit will be in a 40 deg. C ambient the termination must also be derated as well.

As an example, if you had a 40 deg. C ambient condition and a circuit breaker back feeding another circuit breaker say in an MCC and the circuit was greater than 100 amps you would use the 75deg. C rating of the termination. The ambient temp. rating of the circuit breaker would typically be rated for 40 deg. C. Per Article 110.14C you would use table 310.15(B). According to John for conservatism, this temperature rating table are for cable ampacities in a 30deg. C ambient even though the ambient temperature of the circuit breaker is at 40 deg. C you need not adjust for the increased ampacity. So you could select a #2 Awg. 90 deg. C cable and if it were in a 40 deg. C ambient would have to be derated by .91 or the cable ampacity would be 130 amps X 0.91 = 118.3 amps. The termination would be rated for 75 Deg. C so For a #2 cable the 75 deg. C ampacity is 115 amps. Since the termination at the circuit breaker is rated for 40Deg. C no other adjustment is required as noted by John, the 115 amp would be the maximum rating of this circuit to be considered.

If the circuit breaker termination was rated for another ambient i.e. 30 deg. C, ampacity adjustments for the termination must be used to account for a different ambient temperatures.

Hope this clears this issue."

 

[david luchini]

The latest NFPA ? 70 handbook 2014 edition relating to Article 110.14C on page 34 states ?Conductor terminations, as well as conductors, must be rated for the operating temperature of the circuit.? For example, the load on a 8 AWG THHN, 90Deg. C copper conductor is limited to 40 amperes where connected to a disconnect switch with terminals rated at 60 deg. C. The same conductor is limited to 50 amperes where connected to a fusible switch with terminals rated at 75 Deg. C. Not only do termination temperature ratings apply to conductor terminations, but the equipment enclosure marking must also permit terminations above 60 Deg. C. This is a change from the 2011 Code.

The language of 110.14(C) in 2014 appears to be exactly the same as the language of 110.14(C) in 2011.

How does that equate to a "change from the 2011 Code"?

 

[winnie]

Again, the question is: for _which_ ambient temperature must the termination temperature be accounted for.

I stand firm that the termination temperature rating is relevant only at the location of the termination, not somewhere down in the conduit.

Thus if you have a 90C conductors on 60C terminations in an enclosure, then you calculate the 60C ampacity as determined by the conditions in the enclosure.

If the conductor then goes into a conduit with a bunch of other wires, you calculate the 90C ampacity as determined by the conditions in the conduit.

Finally, you select the _smaller_ of all of these calculations as the ampacity of the conductors as installed.

-Jon

 

[Smart $]

Again, the question is: for _which_ ambient temperature must the termination temperature be accounted for.

I stand firm that the termination temperature rating is relevant only at the location of the termination, not somewhere down in the conduit.

Thus if you have a 90C conductors on 60C terminations in an enclosure, then you calculate the 60C ampacity as determined by the conditions in the enclosure.

If the conductor then goes into a conduit with a bunch of other wires, you calculate the 90C ampacity as determined by the conditions in the conduit.

Finally, you select the _smaller_ of all of these calculations as the ampacity of the conductors as installed.

-Jon

The only problem, if you can even call it that, is the NEC has no prescribed method for determining conductor ampacity within an enclosure. It is easier to understand and correlate with NEC requirements to simply state the termination temperature limitation determination is for minimum conductor size. The sizing process just happens to use an ampacity table, which may or may not even correlate with the conditions of use beyond the enclosures [there's usually one at each end of the conductor :happyyes:]. A common example is where conductors beyond the enclosure are installed in cable tray and that ampacity determination doesn't even use Table 310.15(B)(16).

 

[stephena]

You do adjust your ampacity from the 90C ampacity. 310.15B second paragraph.

 

[stephena]

310.15B second paragraph

 

[Smart $]

You do adjust your ampacity from the 90C ampacity. 310.15B second paragraph.

310.15B second paragraph

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