Conductor ampacity

[david luchini]

It depends on the size conductor that is connected to it. That's why we use the 75 degree column; it is the current through the conductor that heats the terminal.

Why would it depend on the conductor size connected to it? If the termination can carry 200A without exceeding it's 75deg rating,, then the rating is 200A.

 

[wwhitney]

Why would it depend on the conductor size connected to it? If the termination can carry 200A without exceeding it's 75deg rating,, then the rating is 200A.

If you run 200A into that terminal with a smaller wire, that wire will have a higher resistance (per unit length), and hence more heat per unit time will be generated from I^2**R from the wire near the termination. [Plus the contact area within the termination may be lower, so the termination resistance may be higher.] And there is less copper at the connection to aid in redistributing the heat.

So with the smaller wire the terminal temperature will rise. I don't believe the listing standards require testing such a combination, and so there will be no guarantee that the higher terminal temperature will not negatively impact the equipment.

Cheers, Wayne

 

[ggunn]

Why would it depend on the conductor size connected to it? If the termination can carry 200A without exceeding it's 75deg rating,, then the rating is 200A.

A terminal is rated for heat, and the heat comes from the wire that is connected to it, not the terminal itself. 75A in a #6 copper conductor will get hotter than 75 degrees, but 75A in a #4 will not.

 

[david luchini]

A terminal is rated for heat, and the heat comes from the wire that is connected to it, not the terminal itself. 75A in a #6 copper conductor will get hotter than 75 degrees, but 75A in a #4 will not.

And yet the Code permits you to run a conductor hotter than 75degC and connect it to a 75deg termination when it allows you to use a conductor with temperature ratings than the specified termination for ampacity adjustment, correction, or both.

 

[wwhitney]

And yet the Code permits you to run a conductor hotter than 75degC and connect it to a 75deg termination when it allows you to use a conductor with temperature ratings than the specified termination for ampacity adjustment, correction, or both.

The NEC doesn't require us to do ampacity adjustment or correction at equipment terminations, so it is presumably up to the manufacturers to design accordingly. E.g. provide enough space in the termination compartment so that adjustment is not an issue. And specify what the acceptable ambient temperatures are for the installing the equipment and design accordingly. And/or test for performance per the listing standard.

For example, if you run conductors in conduit into a panelboard with breakers in 40C ambient, you'll need to do temperature correction for the conductors in the conduit. But the breakers have already been tested at 40C ambient per the listing standard.

Cheers, Wayne

 

[david luchini]

The NEC doesn't require us to do ampacity adjustment or correction at equipment terminations, so it is presumably up to the manufacturers to design accordingly.

Exactly... So "the heat comes from the wire connected to it" isn't an answer for what would require you to limit the current of a #3, 100A, 75degC rated termination to 65A when a #6 wire is connected to it, if the code will allow a conductor with a load of 100A to operate at 90degC and still connect to that same #3, 100A, 75degC rated termination.

 

[wwhitney]

Exactly... So "the heat comes from the wire connected to it" isn't an answer for what would require you to limit the current of a #3, 100A, 75degC rated termination to 65A when a #6 wire is connected to it, if the code will allow a conductor with a load of 100A to operate at 90degC and still connect to that same #3, 100A, 75degC rated termination.

You seem to be conflating the conductor operating temperature outside the termination enclosure with the conductor operating temperature at the termination itself. The latter will clearly affect the temperature of the termination and thus possibly the equipment; the former can only do so by the thermal conduction of the length of intervening conductor. They are not equivalent, and that the former is allowed to exceed 75C doesn't imply that the latter may do so as well.

It would be interesting to actually know to what extent the conductor temperature outside the termination enclosure does affect the termination temperature via thermal conduction. Obviously it would depend on length.

BTW, are you actually arguing that the wording of 110.14(C) permits your interpretation, or just that the physics works and so the wording of 110.14(C) should be changed?

Cheers, Wayne

 

[david luchini]

You seem to be conflating the conductor operating temperature outside the termination enclosure with the conductor operating temperature at the termination itself.

Well...no.

You seem to not realize that I am quoting somebody else.

 

[wwhitney]

You seem to not realize that I am quoting somebody else.

No, I'm clear on who said what.

I was referring more specifically to your comparison "if the code will allow a conductor with a load of 100A to operate at 90degC and still connect to that same #3, 100A, 75degC rated termination" Just because the conductor's operating temperature outside of the termination enclosure is elevated doesn't mean that its operating temperature at the termination is elevated.

Cheers, Wayne

 

[david luchini]

No, I'm clear on who said what.

I was referring more specifically to your comparison "if the code will allow a conductor with a load of 100A to operate at 90degC and still connect to that same #3, 100A, 75degC rated termination" Just because the conductor's operating temperature outside of the termination enclosure is elevated doesn't mean that its operating temperature at the termination is elevated.

Cheers, Wayne

So someone else is conflating the conductor operating temperature outside the termination enclosure with the conductor operating at the termination itself. And I'm responding to that.

 

[wwhitney]

Why would it depend on the conductor size connected to it? If the termination can carry 200A without exceeding it's 75deg rating,, then the rating is 200A.

Perhaps I have been misinterpreting the above. What exactly do you mean by the "rating is 200A"? What information does this rating convey, and how is that different from saying "the termination rating is 75C and up to 3/0 Cu; you can connect any size conductor between the (unspecified) minimum and 3/0 and use that conductor at its 75C table 310.16 ampacity, no higher"?

Cheers, Wayne

 

[wwhitney]

So someone else is conflating the conductor operating temperature outside the termination enclosure with the conductor operating at the termination itself. And I'm responding to that.

I don't believe anyone else has commented on the conductor operating temperature outside the termination enclosure in relation to termination considerations (at least since you joined the conversation), while I take "a conductor with a load of 100A to operate at 90degC" as referring to the conductor outside the termination enclosure.

Perhaps we have different interpretations of "the heat comes from the wire that is connected to it"? I took that to mean the I²*R heating of the conductor at and in the immediate vicinity of the termination, and not as any reference to the conductor outside the termination enclosure.

Cheers, Wayne

 

[solarken]

What i am talking about is what i said from the beginning. Many electricians here have tried to sqeeze more ampacity out of wires based on their particular wiring method.

For example, removing wire from conduit to place it in free air and using the higher ampacity for the same wires while now using table 310.17. Then they do this without considering ampacity ambient adjustment or conductor bundling ampacity correction and table 310.16.

If i did not read 110.14 C 1, I also would have thought using other tables ampacity without matching table 310.16 after ampere derating would have been correct.

As the picture i sent from the nec book shows, device temperature termination points are tested using table 310.16 and using other tables without reducing ampacity to correspond to this table could result in higher temperatures than allowed

I agree the code as written is confusing. 110.14(C)(1) states "unless the equipment is listed and marked ... conductor ampacities shall be based on Table 310.16 as ... modified by 310.12." 110.14(C) is intended to specify temperature limitations when selecting conductor ampacities, as I read it in order to protect against exceeding rated terminal limits. It really should have nothing to do with the sizing of the conductor, which is addressed in 310. Yet it directly references Table 310.16 which specifies ampacities for a specific situation, which is conductors in conduit or in a cable. I don't think it is about Electricians trying to squeeze more ampacity out, but there are applications where we have single conductors in free air, and that should use Table 310.17, not 310.16.

It is common in Solar PV and battery storage systems to use single conductors in free air, and maybe also other applications. The 75C ampacity of 1AWG copper with 3 CC in conduit from Table 310.16 is 130A, while in 310.17 it is 195A. We should be able to use 195A when using single conductors in free air, don't you agree?

It seems that 110.14(C)(1) could instead just reference conductor ampacities per 310, and not specifically reference Table 310.16, and that would allow the use of Table 310.17 for appropriate instances.

 

[wwhitney]

It seems that 110.14(C)(1) could instead just reference conductor ampacities per 310, and not specifically reference Table 310.16, and that would allow the use of Table 310.17 for appropriate instances.

Terminations means at equipment, like a PV inverter or a breaker or utilization equipment, yes? Then typically those terminations will be in an enclosure, so referencing Table 310.16 as the default seems perfectly reasonable to me. If you have some atypical equipment where the terminations are actually in free air, the language allows the equipment to listed for other termination ampacities.

So I'm not seeing the problem. For your outdoor free air PV application, you can use say an inverter with a conduit riser and weatherhead, and splice at the weatherhead. The conductors between the splice and the inverter would likely be governed by the termination ampacity within the inverter. While the other conductors may be in free air and governed by Table 310.17.

Cheers, Wayne

 

[david luchini]

Then typically those terminations will be in an enclosure, so referencing Table 310.16 as the default seems perfectly reasonable to me.

I don't believe 110.14 mentions "enclosure." Nor, I don't believe, does 310.14, 310.15 or 310.16.

Nor does the Code require that terminations be in enclosure.

 

[david luchini]

110.14(C)(1) states "unless the equipment is listed and marked ... conductor ampacities shall be based on Table 310.16 as ... modified by 310.12."

You left out a couple of words, which change the meaning.

110.14(C) is intended to specify temperature limitations when selecting conductor ampacities, as I read it in order to protect against exceeding rated terminal limits. It really should have nothing to do with the sizing of the conductor, which is addressed in 310.

Yes.

 

[wwhitney]

I don't believe 110.14 mentions "enclosure." Nor, I don't believe, does 310.14, 310.15 or 310.16.

No, so my statement was an inference about the reason. The NEC doesn't provide reasons.

Nor does the Code require that terminations be in enclosure.

No, but surely you would agree that the listing standard for most equipment would? Obviously there are exceptions, e.g. a plug-in wall wart 24V transformer with open screw terminals for the LV connections.

You left out a couple of words, which change the meaning.

I left out "used in determining equipment termination provisions". I don't see that omission as changing the meaning, when from context we are discussing termination ampacity.

Cheers, Wayne

 

[david luchini]

No, so my statement was an inference about the reason. The NEC doesn't provide reasons.

The inference doesn't make sense if the same rules apply to both terminations within and outside of enclosures.

No, but surely you would agree that the listing standard for most equipment would?

Obviously not. "Most" isn't really relevant. I've seen exposed electrical installation. Not all equipment, terminations, conductors are required to be within enclosures.

I left out "used in determining equipment termination provisions".

You didn't leave out anything in this case. This response was to someone else's post.

 

[wwhitney]

Why would it depend on the conductor size connected to it? If the termination can carry 200A without exceeding it's 75deg rating,, then the rating is 200A.

You have made a couple statements like this which seem to imply that you think it is NEC compliant to use a 2/0 Cu 90C conductor at its full 90C ampacity of 195A if it is connected to any piece of equipment designed for at least 200A and having 75C terminals accepting at least 3/0 Cu, but reviewing your posts in this thread you haven't quite explicitly stated that you believe that.

So here is a series of hopefully fairly simple statements that should explain my understanding of these ampacity matters, and I would be interested to know which if any you disagree with:

1) Ampacity is a quantity that is initially calculated for a particular point in a circuit, like a particular point in a conduit run with a given number of CCCs and a given ambient temperature, or a terminal in a piece of equipment.

2) Other than as allowed by 310.14(A)(2) Exception, the ampacity of a circuit (feeder, branch circuit, etc) will be lowest of the ampacities of all the points along that circuit, including the terminations.

3) For the wire portion of the circuit, ampacity is determined by using the applicable table in Article 310 and applying any applicable adjustment and correction factors. This is the value referred to by sections such as 215.2(A)(1)(b) (or rather the minimum over just the wire portions of the circuit, excluding the terminations).

4) For the terminations of the circuit, 110.14(C) tells us to use the value from Table 310.16 directly (unless the equipment is listed and marked otherwise) (and ignoring the whole 310.12 fiasco), no adjustment or correction is applied. This is the value referred to by sections such as 215.2(A)(1)(a).

5) In determining the ampacity at a termination, if the conductor insulation rating exceeds the terminal temperature rating, that higher rating is of no value. In selecting the termination ampacity from Table 310.16, we are limited to the lower terminal temperature rating in all cases.

Cheers, Wayne

 

[wwhitney]

The inference doesn't make sense if the same rules apply to both terminations within and outside of enclosures.

I don't see why not. 110.14(C) is just telling us Table 310.16 is the default. Equipment with terminations outside of enclosures for which it makes sense to allow termination ampacities from other Tables simply needs to so "listed and marked" to allow that to happen.

You didn't leave out anything in this case. This response was to someone else's post.

Yes, thank you, in this case I did get mixed up on who said what. : - )

Cheers, Wayne