Breaker with 90? terminations

[Smart $]

It was already proposed, but the CMP rejected the idea.
EDIT: Nevermind, I see the proposal also would've removed the ability to use the 90? for derating which is no bueno.

However, I still don't understand the CMPs response that someone might actually need to termination at 90? rated equipment.

In some cases, transitions from 75?C to 90?C terminations are made remote to the 75?C-rated equipment... using all 90?C equipment... just for the purpose of using a conductor at its 90?C ampacity (i.e. smaller, less expensive).

For example, 75?C equipment requires a 500kcmil conductor. Routed via cable tray, ten feet away from equipment terminal enclosure, a 90?C-rated compression barrel splice transitions to a 400kcmil conductor for the bulk of the run... then another but reversed transition is made at the other end.

It's not a common scenario, but it does occur...

 

[templdl]

In some cases, transitions from 75?C to 90?C terminations are made remote to the 75?C-rated equipment... using all 90?C equipment... just for the purpose of using a conductor at its 90?C ampacity (i.e. smaller, less expensive).

For example, 75?C equipment requires a 500kcmil conductor. Routed via cable tray, ten feet away from equipment terminal enclosure, a 90?C-rated compression barrel splice transitions to a 400kcmil conductor for the bulk of the run... then another but reversed transition is made at the other end.

It's not a common scenario, but it does occur...

That makes sense. However, at what length from the 75degC termination is it allowable to make the transition? Breakers for example are tested by using strict testing requirements including the use of a specific length of 75deC rated wire on the line and load ends which off hand is 3' but that's not for sure. If were to make a termination based upon 75degC rated wire and were to make a transition at a length equal to or greater that the length of wire that the UL489 testing requires it would be acceptable.
So, simply stating that a transion to 90degC rated wire and then applying it per the 90degC column may not be correct.

 

[templdl]

That makes sense. However, at what length from the 75degC termination is it allowable to make the transition? Breakers for example are tested by using strict testing requirements including the use of a specific length of 75deC rated wire on the line and load ends which off hand is 3' but that's not for sure. If were to make a termination based upon 75degC rated wire and were to make a transition at a length equal to or greater that the length of wire that the UL489 testing requires it would be acceptable.
So, simply stating that a transion to 90degC rated wire and then applying it per the 90degC column may not be correct.

I looked at my copy of NEMA AB4 and it states the length to be 4'. As such I would have to conclude that any transition that is made is OK as long as it isn't made within 4' of a breaker. Other devices may have similar requirements.

 

[big john]

That makes sense. However, at what length from the 75degC termination is it allowable to make the transition...?

I know 4' is the magic number when testing UL489 breakers, but the exception to 310.15(A)(2) seems to suggest that 10' feet would be needed in a real-world installation:

(2) Selection of Ampacity. Where more than one ampacity applies for a given circuit length, the lowest value shall be used.
Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length ?gured at the higher ampacity, whichever is less.

 

[templdl]

I know 4' is the magic number when testing UL489 breakers, but the exception to 310.15(A)(2) seems to suggest that 10' feet would be needed in a real-world installation:

I believe we made the point in that there is more to making a transition. Without this additional info one could conclude that a transition could be made at any distance as soon as practical from the breaker. The inconvenience may make it uneconomical.
Thanks for pointing that NEC reference out.

 

[Smart $]

I believe we made the point in that there is more to making a transition. Without this additional info one could conclude that a transition could be made at any distance as soon as practical from the breaker. The inconvenience may make it uneconomical.
Thanks for pointing that NEC reference out.

I absolutely agree 4' would be the minimum. Others say 10' because of the exception noted by Big John. Obviously 10' should be more acceptable. However the principle concept is not the ampacity of the wire, but rather the temperature along its length. Can you look in your copy of NEMA AB4, or UL489 if you have access to it, and see what is at the other end of the 4' length of wire, or anything relating to the conductor temperature along its length.

 

[jim dungar]

For breaker tests there is usually a huge, oversized, bus bar of some type, eventually connected to the desired load, maybe even be a short circuit.

 

[Smart $]

For breaker tests there is usually a huge, oversized, bus bar of some type, eventually connected to the desired load, maybe even be a short circuit.

Helps in understanding the test method... but little in assessing a minimum distance for the mentioned transition. Have a feeling someone well versed in thermodynamics would be required. :happyyes:

As far as Code goes, I think we could make a transition right next to, but not in the 75?C equipment enclosure. Code is considered in general to be a "permissive" text, correct? Do NEMA and UL testing methods even enter the picture for code compliance? Aren't we only concerned with the requirements of the listing itself, e.g. consumer requirements mentioned in the White Book or the product documentation?

 

[jim dungar]

Aren't we only concerned with the requirements of the listing itself, ...

The requirements of UL489 involves 4' of 'rated conductor' after that the test, and therefore the listing, does not care what is used in the circuit.

 

[Smart $]

The requirements of UL489 involves 4' of 'rated conductor' after that the test, and therefore the listing, does not care what is used in the circuit.

Not sure what you mean. Does UL489 affect actual "in use" conditions? For example, what if in actual use there's only 2' of rated conductor between breaker and load? I don't see the length of conductor specified in any manner under Code or listing.

What is the purpose of the 4' of 'rated conductor' in testing?

What is the pass/fail criteria related to this 4' of conductor?

It is commonly said that the rated conductor (in use) is a heat sink for the termination... but that would mean the current cannot cause self-heating of the conductor to the point it is at the same temperature along its length as the rating. Should it be, it will not sink any heat away from the termination.

 

[templdl]

I believe the objective is that the wire doesn' t add to the heating of a breaker as the wire is said to be a heat sink. As such the objective then would be that the breaker respond as designed and tested. But, this may be a case of splitting hairs and really not affect the installation at all being the the average breaker certainly isn't loaded to its rating. If it were then any heating that would occur coulkd cause the breaker to posibly nuisance trip because of the wire terminated on the load end of the breaker which is close to the trip unit.

 

[jim dungar]

What is the purpose of the 4' of 'rated conductor' in testing?

What is the pass/fail criteria related to this 4' of conductor?

It is commonly said that the rated conductor (in use) is a heat sink for the termination... but that would mean the current cannot cause self-heating of the conductor to the point it is at the same temperature along its length as the rating. Should it be, it will not sink any heat away from the termination.

The conductor has length to it for several reasons, among them:
to better simulate real world installed conditions
to test the effectiveness of the device to hold the conductor in its termination during short circuits
to test that the conductor insulation is not damaged during overloads (consistent heating over the length of the conductor is not part of this test).
to make sure the device time current characteristics perform consistently when connected to conductors sized per the NEC.

By the way enclosed fusible devices also are tested with 4' of conductor.

 

[big john]

Anyone have any actual history behind the selection of that UL 4' conductor length? I've seen it come up a number of times, and there's always good arguments that it's for heat-sink purposes or that it represents real-world conditions, but the length still seems arbitrary and I'm sure it's not.

As an interesting aside, the tests can be with longer lengths of wire as long as the circuit is calibrated to that longer length prior to the breaker being tested.

 

[templdl]

Anyone have any actual history behind the selection of that UL 4' conductor length? I've seen it come up a number of times, and there's always good arguments that it's for heat-sink purposes or that it represents real-world conditions, but the length still seems arbitrary and I'm sure it's not.

As an interesting aside, the tests can be with longer lengths of wire as long as the circuit is calibrated to that longer length prior to the breaker being tested.

Are you getting into theory or an actual application?
Should you or others find a need for deviation from the 4' length I would think it would be doable should one go through the expense for testing and certifying the application and have it UL listed as such which I don't believe it would be practical. Just stick with the 4'

 

[Smart $]

Are you getting into theory or an actual application?
Should you or others find a need for deviation from the 4' length I would think it would be doable should one go through the expense for testing and certifying the application and have it UL listed as such which I don't believe it would be practical. Just stick with the 4'

But there is no indication we must go 4', nor any indication it voids the breaker's listing if we don't... that I'm aware of. Sums to no requirement to get application listed. AHJ blessing is all that is necessary.

 

[big john]

But there is no indication we must go 4', nor any indication it voids the breaker's listing if we don't....

I don't think it really applies in the real world: I think it's probably just a way of ensuring that all the breaker tests are reproducibly accurate.

But that's why I asked, because we're all speculating. I suppose it would be helpful to see if I can find the substantiation for the 10'/10% rule in 310.15.

 

[Smart $]

...we're all speculating. I suppose it would be helpful to see if I can find the substantiation for the 10'/10% rule in 310.15.

I don't believe that'll help. As I mentioned earlier, that is for conductor ampacity. The termination temperature limitation is relative to conductor size. You cannot go lower than the minimum SIZE at the termination regardless of AMPACITY.

 

[templdl]

It may be appropriate to establish what an anticipated load would be and then see what affect that would have on the various configurations. One would also consider if it is a continuous or non continuous load. This may be better than just shooting at shadows.
Go packers!!

 

[Smart $]

It may be appropriate to establish what an anticipated load would be and then see what affect that would have on the various configurations. One would also consider if it is a continuous or non continuous load. This may be better than just shooting at shadows.
Go packers!!

I think the continuous factoring required, coupled with non-continuous time frame, is a means of assuring the conductor will always sink heat away from the termination.

However, when we do a transition to 90?C, the temperature at the 'splice' may exceed that at the 75?C terminals. This means the 75?C conductor could be sinking heat from both directions. The shorter that conductor is, the higher the temperature will be at the 75?C termination. How do we determine a minimum distance???

 

[templdl]

I think the continuous factoring required, coupled with non-continuous time frame, is a means of assuring the conductor will always sink heat away from the termination.

However, when we do a transition to 90?C, the temperature at the 'splice' may exceed that at the 75?C terminals. This means the 75?C conductor could be sinking heat from both directions. The shorter that conductor is, the higher the temperature will be at the 75?C termination. How do we determine a minimum distance???

Yes, but provide an actual real life load example and then evaluate the installation. Opinions so far have not provided anything based upon an actual load. If you have a 600a breaker where the normal load may be 450a how would that affect this discussion as an example?