Temperature rating

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greenawaltmark4

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Why is mc rated for 90 degrees ? when almost all devices are rated 75 or 60 degree . When I read 110.14 (c) (1) it's explains not to go over the device rating.
Thanks in advance

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Ohhhk. My point being electricians typically use the 90c column , even though most breakers and devices are rated at 75c or 60c. That would be a violation ?

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greenawaltmark4 said:
Ohhhk. My point being electricians typically use the 90c column , even though most breakers and devices are rated at 75c or 60c. That would be a violation ?

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Although you may have to, based on termination temperature, use the 75C or 60C column for base ampacity determination, you are still allowed to start from the 90C value when calculating temperature corrections and fill level adjustments.
 
Derating via fill and ambient temperature adjustments apply only to the wire, not the devices, discos, or breakers. For example, 9 #12 THHN CCC in a raceway brings on 70% derating. Since #12 @90*C is good for 30A, 70% x 30A = 21A. Meaning you can still use 20A breakers. If you had 50% derating, you could only use 15A breakers (50% x 30A = 15A).

Note that 240.4(D) doesnt allow #12 on a 30A breaker unless it meets one of the conditions, like a motor or HVAC unit.

It is not a violation to start from the 90*C table for derating purposes.
 
greenawaltmark4 said:
Ohhhk. My point being electricians typically use the 90c column , even though most breakers and devices are rated at 75c or 60c. That would be a violation ?

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Not a violation. You can put 90C wires on 60/75C terminations, you just cannot use the wires' ampacity above the terminal ratings. As stated above, usually for derating and such, but are applications where the wire is used at 90C.
 
jumper said:
Here is an application/install using the full 90C ampacity of a conductor.

View attachment 18524
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How does the NEC (or other documentation) specify that the splicing must take place in a separate enclosure?

I can see that the primary reason one would want to do this, is that they either made a mistake or inherited a mistake, and need to modify the last foot of wire to correct it. I know it could be a value engineering decision known in advance, but I anticipate that to be rare. If it is mandatory to do this in a separate enclosure, this means one would need to take apart the raceway as well, and install a box for the splices. This is major re-work, since the conductors have to be pulled after the raceway system is complete.

It seems at first glance that it would be easier to accomplish the splices inside the destination enclosures (panelboards, disconnects, etc), than to re-work the raceway. Or, to compromise, leave the existing raceway terminations in place, pass existing wire through to an adjacent box, splice there, and pass back to the destination box. Please critique these solutions.
 
Carultch said:
... Please critique these solutions.
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Once you are within the supply or load equipment enclosure, you are subject to the terminal temperature limitation or the maximum operating temperature of the equipment, whichever is lower.
 
I do not see (carefully) cutting a raceway back and installing a box in which the shortened cable is spliced as an event requiring removing and re-pulling all wires.
JMO.

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That diagram implies that the wire is 75c at that the termination and then to splice it with 90c . Why not just run it with 75c the whole run ?

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greenawaltmark4 said:
That diagram implies that the wire is 75c at that the termination and then to splice it with 90c . Why not just run it with 75c the whole run ?

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The reason is that you can run smaller wire sizes for the same current if you are allowed to take the basic ampacity rating at the higher temperature. Depending on the exact load amps involved, you will not always be able to step down one or more wire sizes at the higher temperature rating, but it will often happen.

The assumption is that you will be using similar wire in both cases, so the main cost difference will be the cost of the metal involved.
If you are comparing two different wire types with different insulation to get the different temperature ratings (such as THWN versus THWN-2) then you have to take a harder look at the relative costs.

You may also be able to use smaller raceway or more wires per raceway for an additional cost savings.
 
Back to my initial inquiry, if there is a 75c rated breaker. I need 75c wire on that termination, per nec?

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greenawaltmark4 said:
That diagram implies that the wire is 75c at that the termination and then to splice it with 90c . Why not just run it with 75c the whole run ?

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You asked a question and jumper offered you a diagram to visually represent a place where the full rating of the cable may be used in a current carrying capacity. There is nothing to imply this
application is economical or practical it is just an application. But to give you one scenario, you have an existing conduit extending under a driveway to another building. It isn't large enough to carry the correct 75 degree rated conductors for the amperage you need. It may be cheaper to perform EXACTLY the work jumper specified instead of directional boring or digging up the asphalt.

IMO, the 90 degree rating of the wire is usually used effectively when derating conductors for various reasons in the code. High ambient temperature and the number of current carrying conductors being the top two. Without using the 90 table, for example, you could never run 9 current carrying #12's in a conduit.
 
greenawaltmark4 said:
That diagram implies that the wire is 75c at that the termination and then to splice it with 90c . Why not just run it with 75c the whole run ?

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Because at certain ampacities, that can be a load of extra wire. Take 425A for example, at 75*C you need 750MCM copper. At 90*C, you need 500MCM. Now imagine that's a 3 phase run with full size neutral over 1000'. 4" conduit vs 3". Lot of cash wasted there...

and no, you do not need 75*C wire to land it on a 75*C rated breaker. you can use 90*C rated wire. all day, every day.
 
greenawaltmark4 said:
Back to my initial inquiry, if there is a 75c rated breaker. I need 75c wire on that termination, per nec?

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No, when there is a 75°C rated breaker, the ampacity of the 90°C conductor attached to it is limited to the 75°C column but you can still use the 90° column to apply adjustment and correction factors.
 
greenawaltmark4 said:
Back to my initial inquiry, if there is a 75c rated breaker. I need 75c wire on that termination, per nec?

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75C sizing, not necessarily 75C rated wire. Any wire 75C rated and higher, with 75C sizing, can be used on a terminal in 75C rated equipment. Most single conductors you will find today, are 90C rated.
 
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