Ambient Temperature :)

[david luchini]

Doesn't anyone see this as being odd? Triangular stacked cables with proper spacing in an open bottom/top ladder tray are limited to the same ampacity as if they are installed in a conduit? I would suggest that 392.80(a)(2)(d) allows us to use the 75 degree column "in accordance with 310.15(B)."

But what do I know...

Triangular stacked cables with proper spacing are NOT limited to the same ampacity as if they are installed in conduit. For instance, #3/0 would have an ampacity of 245 in cable tray vs. 200 in conduit. What must be coordinated is the terminations. You could not use a #3/0 75deg termination with the higher ampacity. You would need to use a 250mcm termination even though the conductor size is 3/0 to avoid exceeding the operating temperature of the termination.

 

[GoldDigger]

You would need to use a 250mcm termination even though the conductor size is 3/0 to avoid exceeding the operating temperature of the termination.

Are you saying that the allowed ampacity of the wire at the termination can be increased just by using a termination that is capable of accepting larger wire?
That does not seem right to me.
The temperature at which the termination will actually operate is based not only on any heat generated in the termination, but more strongly on the heat contributed by the wire. If the larger termination is able to dissipate heat better than the smaller one, I could see a point, but that is not what the listed temperature rating of the connector is telling you.

 

[larrybartowski]

The cables connect to a piece of copper bus that bolts directly to the Westinghouse SPB65 2000A breaker. How often are the terminals of a 2000 amp 100% rated breaker rated for 90 degrees?

I would think there would be a large market for breakers with terminations rated for 90 degrees.

 

[GoldDigger]

The cables connect to a piece of copper bus that bolts directly to the Westinghouse SPB65 2000A breaker. How often are the terminals of a 2000 amp 100% rated breaker rated for 90 degrees?

I would think there would be a large market for breakers with terminations rated for 90 degrees.

In some sizes, maybe. If the most common wiring attached to the breaker would be NM in residential use, there would be no benefit. And for any method there is still the conductor size limit from the ampacity adjustment and correction en route.
Also, there may be an effect on the thermal portion of the breaker trip if you let the terminations go up to 90C.
My guess is that the unit cost of the redesigned breaker would be more than the sales volume would justify.

 

[david luchini]

Are you saying that the allowed ampacity of the wire at the termination can be increased just by using a termination that is capable of accepting larger wire?
That does not seem right to me.

No, I'm not saying that at all. The allowable ampacity of the wire is increased by installing triangular stacked in a cable tray with maintained spacing instead of installing it in an enclosed raceway.

The temperature at which the termination will actually operate is based not only on any heat generated in the termination, but more strongly on the heat contributed by the wire. If the larger termination is able to dissipate heat better than the smaller one, I could see a point, but that is not what the listed temperature rating of the connector is telling you.

When installed in the triangular stacked method, the 3/0 conductors can supply a 245 amp load and stay within the 75 deg operating temperature, just like 3/0 conductors supplying a 200A load in a conduit will stay withing the 75 deg operating temperature. However, supplyiing a 245A load through a 3/0 75deg termination will cause the termination to exceed its temperature rating. You will need a termination that can carry a 245A load without exceeding the 75deg rating.

 

[JDBrown]

...
However, supplyiing a 245A load through a 3/0 75deg termination will cause the termination to exceed its temperature rating. You will need a termination that can carry a 245A load without exceeding the 75deg rating.

You lost me right here, David. If you have a piece of equipment that draws 245A and has 75 deg terminal lugs, how can you not be allowed to use the 75 deg ampacity of your supply conductors? Is there another NEC table that has allowable ampacities for different sizes of terminations rated at different temperatures, similar to Table 310.16 for conductors?

 

[david luchini]

You lost me right here, David. If you have a piece of equipment that draws 245A and has 75 deg terminal lugs, how can you not be allowed to use the 75 deg ampacity of your supply conductors? Is there another NEC table that has allowable ampacities for different sizes of terminations rated at different temperatures, similar to Table 310.16 for conductors?

Think of it this way. If you had a circuit with 3 #3/0 THWN in a raceway supplying a 200A load, landing on #3/0 75 deg terminations, the conductors and the terminations would not exceed their max operating temperature.

Now suppose you changed the conductors to THHN but left the #3/0 75 deg terminations, and increased the load current to 225A. The conductors would not exceed their max operating temperature, but the increase in current would exceed the operating temperature of the termination.

Now consider 3 #3/0 THWN in the triangular configuration in a cable tray supplying a 245A load. The conductors would not exceed the max operating temperature. But would the #3/0 75deg terminations remain within the max operating temperature with a 245A load, when they would have exceeded the operating temperature when supplying a 225A load?

There is not an NEC table for ampacities of terminations, as far as I know.

 

[Smart $]

You lost me right here, David. If you have a piece of equipment that draws 245A and has 75 deg terminal lugs, how can you not be allowed to use the 75 deg ampacity of your supply conductors? Is there another NEC table that has allowable ampacities for different sizes of terminations rated at different temperatures, similar to Table 310.16 for conductors?

In addition to David 's reply, you have to remember the respective Tables [i.e. 310.15(B)(16) thru (21)] are for the conditions of use... external to the enclosure where they are terminated. Heat dissipation is different in an enclosure than it is in raceway, through free air, on a messenger, etc. Termination temperature relative to a conductor's insulation and its condition(s) of use beyond the enclosure are [considered] insignificant to determining its maximum operating temperature at the termination... in the enclosure. Termination temperature considerations for bare, covered, and insulated is the same.

To get a better understanding of the principle involved, consider how you might size a bare conductor for a specific load with 75?C-rated terminations...

 

[JDBrown]

Think of it this way. If you had a circuit with 3 #3/0 THWN in a raceway supplying a 200A load, landing on #3/0 75 deg terminations, the conductors and the terminations would not exceed their max operating temperature.

Now suppose you changed the conductors to THHN but left the #3/0 75 deg terminations, and increased the load current to 225A. The conductors would not exceed their max operating temperature, but the increase in current would exceed the operating temperature of the termination.

Now consider 3 #3/0 THWN in the triangular configuration in a cable tray supplying a 245A load. The conductors would not exceed the max operating temperature. But would the #3/0 75deg terminations remain within the max operating temperature with a 245A load, when they would have exceeded the operating temperature when supplying a 225A load?

There is not an NEC table for ampacities of terminations, as far as I know.

I don't know if I'm having an off day here or what, but there's something about this that's just not clicking. Using your example above, I guess my problem is, how do I know that 200A won't exceed the max operating temperature of 75 deg #3/0 terminations? Similarly, how do I know that 225A or 245A will exceed the max operating temperature of 75 deg #3/0 terminations?

I guess my real question is, How do you know how much current a given termination will be able to handle without exceeding its maximum operating temperature? If there's no NEC table, is there something in the product's listing? Do I need to dust off my old textbooks and solve some thermodynamic equations? I feel like there's some key piece of information right in front of me that I'm just not seeing.

 

[kwired]

Are you saying that the allowed ampacity of the wire at the termination can be increased just by using a termination that is capable of accepting larger wire?
That does not seem right to me.
The temperature at which the termination will actually operate is based not only on any heat generated in the termination, but more strongly on the heat contributed by the wire. If the larger termination is able to dissipate heat better than the smaller one, I could see a point, but that is not what the listed temperature rating of the connector is telling you.

I don't know if I'm having an off day here or what, but there's something about this that's just not clicking. Using your example above, I guess my problem is, how do I know that 200A won't exceed the max operating temperature of 75 deg #3/0 terminations? Similarly, how do I know that 225A or 245A will exceed the max operating temperature of 75 deg #3/0 terminations?

I guess my real question is, How do you know how much current a given termination will be able to handle without exceeding its maximum operating temperature? If there's no NEC table, is there something in the product's listing? Do I need to dust off my old textbooks and solve some thermodynamic equations? I feel like there's some key piece of information right in front of me that I'm just not seeing.

Many, maybe even most lugs are rated for 90deg C. The problem is usually the rating of overcurrent device terminations is only 60 or 75 deg C, even though the attached lug may be rated 90 deg if it were not attached to the overcurrent device.

Standard overcurrent devices sink heat into the attached conductor and is the reason we have to add 25% to the load when we have a continuous load.

Breakers that are rated for loading to 100% continuously do not sink heat into the conductor, or at least not as much as standard devices do.

Have you seen lugs with markings like AL9CU? That means they are rated for aluminum and copper which may be somewhat obvious, but the "9" means they are rated 90 deg C. I would like to think it can handle the current of the largest conductor it is rated for if sized according to 90 deg ampacity table, without overheating.

 

[Smart $]

I don't know if I'm having an off day here or what, but there's something about this that's just not clicking. Using your example above, I guess my problem is, how do I know that 200A won't exceed the max operating temperature of 75 deg #3/0 terminations? Similarly, how do I know that 225A or 245A will exceed the max operating temperature of 75 deg #3/0 terminations?

I guess my real question is, How do you know how much current a given termination will be able to handle without exceeding its maximum operating temperature? If there's no NEC table, is there something in the product's listing? Do I need to dust off my old textbooks and solve some thermodynamic equations? I feel like there's some key piece of information right in front of me that I'm just not seeing.

The temperature at the termination is relative to the size of wire, its composition (cu or al), and the amount of current through the connected equipment, the termination, and the wire. Given the equipment and terminal temperature ratings are set in stone, so to speak, the variables in the determination are the size of wire, its composition, and the maximum current.

Its fairly obvious that given any sustained amount of current through a conductor, one with a greater cross-sectional area will not get as warm as conductor of the same composition with less cross-sectional area. So regardless of whether it provides absolutely correct temperature at the termination, the NEC directs us to use Table 310.15(B)(16) values to make the determination.