Sizing Conductors

[Marrselectric]

I know that most equipment breakers, disconnects and other are rated only for 75 C. That means we have to use the 75 C column to size the conductors. If the conductors terminate on lugs mounted to the buss and not the breaker or switch and are rated for 90 C at both ends, could we use the 90 C column to size the conductors?

 

[Dennis Alwon]

I know that most equipment breakers, disconnects and other are rated only for 75 C. That means we have to use the 75 C column to size the conductors. If the conductors terminate on lugs mounted to the buss and not the breaker or switch and are rated for 90 C at both ends, could we use the 90 C column to size the conductors?

Probably not simply because the equipment itself must be rated 90C also. I don't think you will find that in a normal situation

 

[Marrselectric]

I understand your point, but I'm not terminating at the equipment. I know this works when tie buss together. Somewhere on the buss there has to be some piece of equipment. Just not sure.
Don't want seem argumentative just trying me be sure. I have talked to two engineers that say it should be good and one inspector. The inspector kind of hedged his bet by saying "If both points of termination are rated for 90 C it would be a legal installation. I have yet to actually install cable this way and see if he comes up with some other objection.

 

[infinity]

The terminals and all of the equipment that they're attached to would need a 90? C rating, as Dennis stated not very likely.

 

[Carultch]

I know that most equipment breakers, disconnects and other are rated only for 75 C. That means we have to use the 75 C column to size the conductors. If the conductors terminate on lugs mounted to the buss and not the breaker or switch and are rated for 90 C at both ends, could we use the 90 C column to size the conductors?

You can only use the 90C column for this purpose, if you have 90 C connections on both sides of the wire. This is a very rare event.

Even if lugs on manufactured product are marked for 90C (I see that all the time), you still cannot take credit for it. Not unless the manufactured product as a whole is rated for 90C (which is very rare, if it ever is at all).

The most common reason you would use the 90C column, is for your bundling ampacity adjustment and ambient temperature correction calculations. You are permitted to do this with 90C wire, and apply these factors to the 90C column of the NEC. The 75C column gets used with no additional calculation, for terminations and equipment.

 

[Carultch]

I know that most equipment breakers, disconnects and other are rated only for 75 C. That means we have to use the 75 C column to size the conductors. If the conductors terminate on lugs mounted to the buss and not the breaker or switch and are rated for 90 C at both ends, could we use the 90 C column to size the conductors?

There is another application of the 90C column, that is a little obscure and complicated. See page 4 of this white paper:
http://static.schneider-electric.us/docs/Power Management/0110DB9901.pdf

There are only two reasons I can see to do it
1. If I were to screw up, and correct for forgetting about the 75C termination requirement.
2. If I am stuck with an existing conduit that is slightly too small for what I really need, and the 90C column of the NEC would allow me to salvage the situation. Maybe I had someone put a conduit in a long time ago, and the scope of my work expanded to require a greater ampacity.

 

[mikecorlett@hotmail.com]

Wire size

Wire size

What size wire feed for a 400a 3 phase subpanel.

 

[Dennis Alwon]

What size wire feed for a 400a 3 phase subpanel.

How about you tell us first and we can let you know if you are correct. Also aluminum or copper? Parallel feed or not?

 

[wwhitney]

TSee page 4 of this white paper:
http://static.schneider-electric.us/docs/Power Management/0110DB9901.pdf

That paper claims that if the termination rating is 75C, you have to use a conductor with at least 75C rated insulation.

That makes no sense to me, as if you use a conductor at its 60C ampacity, its temperature due to resistive heating should not exceed 60C, which is less the 75C termination rating.

Is equipment with 75C rated terminations really going to product enough heat that combined with the conductor resistive heating the temperature at the termination will exceed 60C, for a conductor used at its 60C ampacity?

Cheers,
Wayne

 

[Carultch]

That paper claims that if the termination rating is 75C, you have to use a conductor with at least 75C rated insulation.

That makes no sense to me, as if you use a conductor at its 60C ampacity, its temperature due to resistive heating should not exceed 60C, which is less the 75C termination rating.

Is equipment with 75C rated terminations really going to product enough heat that combined with the conductor resistive heating the temperature at the termination will exceed 60C, for a conductor used at its 60C ampacity?

Cheers,
Wayne

That's an interesting point, and I think it might explain why I often see terminals rated for "60C / 75C". I thought that the dual rating was redundant, because the 75C would cover you in both cases. Guess not.


In all other cases, it's all a matter of identifying the weakest link, and limiting the feeder ampacity to it.

60C terminals with 75C wire means you have to use the 60C column of the NEC for satisfying the termination ampacity.
60C terminals with 90C wire means you have to use the 60C column of the NEC for satisfying the termination ampacity.

Note that 60C terminals are uncommon on new equipment. Officially, 100A and less is by default rated 60C terminations unless listed and marked otherwise. But this is primarily an academic rule. In practice, most terminals are rated otherwise for 75C.

One place you still see 60C terminations in practice, is any time you use NM cable, aka ROMEX. It has a special rule that all of its terminations are rated at 60C, no matter what equipment is on the end. Even if the wire itself has a 90C rating.

75C terminals with 75C wire means you have to use the 75C column of the NEC for satisfying the termination ampacity.
75C terminals with 90C wire means you have to use the 75C column of the NEC for satisfying the termination ampacity. This is the most common situation in my scope of work.
90C terminals with 75C wire means you have to use the 75C column of the NEC for satisfying the termination ampacity. This is rare.

90C terminals (field-installed, not part of a manufactured product) with 90C wire, means you get to use the 90C column of the NEC. This is rare.

 

[Carultch]

What size wire feed for a 400a 3 phase subpanel.

Assuming copper wire.........

 

[Dennis Alwon]

What do you say about this mike holt graphic

 

[Smart $]

What do you say about this mike holt graphic

I have to wonder if the incomplete depiction of the enclosure was intentional. :blink:

If all that is in the enclosure is bussing, should be okay to use 90?C rating. If there are disconnects, breakers, etc. within the enclosure, connected to the bussing, there would likely be a 75?C limitation imposed.

 

[Marrselectric]

That Buss has to attach to equipment at some point. That equipment will be rated for 75 C. If the graphic is correct, would a pull section work? Is a pull section or termination section part of the equipment? How far away from the equipment do we need to go?

 

[Carultch]

What do you say about this mike holt graphic

When they talk about that in the video, Eric suggests that one place you might see this is at a motor control center line-up. And there is no space left for a new breaker, so you connect from the bus to an external disconnect.


Mike Holt did not say that the problem with taking credit for the 90C rating in that example is not that you have a field-installed connector on the busbar of manufactured product (MCC line-up). He says the problem is when you terminate to the disconnect.

 

[Dennis Alwon]

I have to wonder if the incomplete depiction of the enclosure was intentional. :blink:

If all that is in the enclosure is bussing, should be okay to use 90?C rating. If there are disconnects, breakers, etc. within the enclosure, connected to the bussing, there would likely be a 75?C limitation imposed.

Are we assuming the bussing is rated 90C????

 

[Marrselectric]

When they talk about that in the video, Eric suggests that one place you might see this is at a motor control center line-up. And there is no space left for a new breaker, so you connect from the bus to an external disconnect.


Mike Holt did not say that the problem with taking credit for the 90C rating in that example is not that you have a field-installed connector on the busbar of manufactured product (MCC line-up). He says the problem is when you terminate to the disconnect.

I still don't understand when it would be correct to use the 90 C wire to connect the bus as shown. At some point that buss will terminated at some piece of equipment. In some instance the enclosure might be too small. If it was tested base on the conductors being rated at 75 C, it's possible there is not enough cubic inches to dissipate the heat. If we are taking about something large like 2500a bolted pressure switch or a pull section containing only the bus and the lugs, I don't think it would be a factor. I have talked to rep from Eaton and SQD, the manufactures don't have a rating for enclosures.
Thoughts?

 

[Dennis Alwon]

I still don't understand when it would be correct to use the 90 C wire to connect the bus as shown. At some point that buss will terminated at some piece of equipment. In some instance the enclosure might be too small. If it was tested base on the conductors being rated at 75 C, it's possible there is not enough cubic inches to dissipate the heat. If we are taking about something large like 2500a bolted pressure switch or a pull section containing only the bus and the lugs, I don't think it would be a factor. I have talked to rep from Eaton and SQD, the manufactures don't have a rating for enclosures.
Thoughts?

In some cases you can run a smaller conductor if it is rated 90 C. Thus one part of the run may take advantage of the 90C while the other part will not be able to use the 90C

For instance if the buss bar is connected to a 75C terminal further down the line then the ampacity of the bus itself will be rated at the 75C but the connection to the buss can be 90C at the part as shown above

 

[Smart $]

Are we assuming the bussing is rated 90C????

Bare busbar has no insulation temperature rating.

 

[Smart $]

In some cases you can run a smaller conductor if it is rated 90 C. Thus one part of the run may take advantage of the 90C while the other part will not be able to use the 90C

For instance if the buss bar is connected to a 75C terminal further down the line then the ampacity of the bus itself will be rated at the 75C but the connection to the buss can be 90C at the part as shown above

It all has to do with how much heat the conductor can sink away from the pertinent temperature rated equipment... without overheating itself while conducting current. Too many variables enter the thermodynamics for anyone to give hard-line go, no go factors without a case-by-case study.