90C in Conduit AND 75C in Free Air

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beanland

beanland

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I want to run a 200A 3-wire cicuit using aluminum conductor. 4/0 THHN AL is rated 205A @ 90C. So that works for the conduit.

But, I must land on the terminations at 75C. 4/0 THHN is rated 280A @ 75C in free air at the terminations. Since I am only loading it to 200A, I am good.

Right or wrong?
 
Whether you are good or not I can’t say – that’s a moral judgment. However you aren’t correct. See the last sentence of 110.14 (C)(1). (You must Table 310.16)
 
Wrong.

I assume your taking about THHW Al. Your landing on 75C, free air has nothing to do with this, your good for 180A.
 
110.14(C)(1)(b)

110.14(C)(1)(b)

That was the section I needed. per (b)(1) I can use the 75C rating.

But, (2) is unclear. It seems to say use whatever temperature rated conductor you want but limit the ampacity to that at 75C. But then it says "or up to their ampacity if the equipment is listed and identified for use with such conductors"

Does this mean that if the equipment comes with 90C-rated terminals, I can use the 310.15(B)(16) 90C ampacity or not?
 
beanland said:
Does this mean that if the equipment comes with 90C-rated terminals, I can use the 310.15(B)(16) 90C ampacity or not?
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Almost.

The key is the "terminations" (i.e. lug and mounting pad) not just the terminal (lug) itself.
 
You may be good but not because of the reasons you state. You need to use the 75 degree column which gives you 180 but you get to use the "next size up rule" and protect it with a 200 ocpd - if your load is less than 180. You said loading it to 200 but i assume you mean that sort of generically? What is this feeder (assuming?) for?
 
More details...

More details...

Here is the situation I am trying to get straight: 225kVA 480-208Y transformer, 800A secondary breaker, 400A primary breaker, parallel 200A feeders, circuit breaker has 75C terminals so 75C raceway ampacity is required by 110.14(C)(1)(b) but transformer has 90C-rated terminals.

If I use 2-sets of 3-250kcmil AL, each rated 205A @ 75C, I can land them on the circuit breaker. All rules met.

If I use 4/0 AL which is 90C 205A rated, I can land on the transformer but not at the circuit breaker.

However, if I splice a short section of 250kcmil AL to the 4/0 AL at the breaker, I can then land the 250kcmil on the breaker.

Just trying to find all the rules...
 
beanland said:
Here is the situation I am trying to get straight: 225kVA 480-208Y transformer, 800A secondary breaker, 400A primary breaker, parallel 200A feeders, circuit breaker has 75C terminals so 75C raceway ampacity is required by 110.14(C)(1)(b) but transformer has 90C-rated terminals.

If I use 2-sets of 3-250kcmil AL, each rated 205A @ 75C, I can land them on the circuit breaker. All rules met.

If I use 4/0 AL which is 90C 205A rated, I can land on the transformer but not at the circuit breaker.

However, if I splice a short section of 250kcmil AL to the 4/0 AL at the breaker, I can then land the 250kcmil on the breaker.

Just trying to find all the rules...
Click to expand...
Correct. But the big question is how short of a section will "sink" approximately 90?C on the 4/0 to 75?C on the 250 at the breaker.
 
4-feet

4-feet

UL 489 uses a 4-foot piece of conductor. Therefore, to met the listing, the wire attached must be at least the minimum size permitted by the terminals, at least 4-feet in length, and rated for 75C at the current in question. After 4-feet, UL489 listing and the 75C terminal listing does not apply.

In fact, it would be an interesting scientific study to determine how long of what size conductor is required to match the UL 489 testing regimine.

So, once I have 4-feet of 75C wire on the breaker, I can drop to 90C wire and comply with UL listing and NEC.

I love the NEC. This is like having to oversize the EGC to match the oversized current-carrying conductors, unless the conduit is the EGC. Or, maybe this is more like tax code "Always follow the rules except for the exceptions and special cases listed below....."
 
beanland said:
UL 489 uses a 4-foot piece of conductor. Therefore, to met the listing, the wire attached must be at least the minimum size permitted by the terminals, at least 4-feet in length, and rated for 75C at the current in question. After 4-feet, UL489 listing and the 75C terminal listing does not apply.

...

So, once I have 4-feet of 75C wire on the breaker, I can drop to 90C wire and comply with UL listing and NEC.

...
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I agree with you that the conductor connected to the breaker (75?C termination) must be at least 4 feet in length, but I disagree that it can be as short as 4 feet, and it cannot be 75?C-rated wire.

When UL tests this, the load-end terminations temperature are also 75?C or less. This will not be the case if you transition to a smaller 90?C-rated conductor. At the current level used for size determination, the smaller 90?C-rated conductor will be operating at a temperature somewhere less than 90?C, but substantially over 75?C. Some of this generated heat will be transferred to the larger conductor connected to the breaker, and thus push its temperature at the transition above 75?C. The conductor connected to the breaker will thus have to be rated for 90?C operation, but sized as if 75?C rated.

For the purpose of discussion, let's say the temperature at the transition ends up being half the difference at 82.5?C. What is the likelihood the temperature at the breaker will remain 75?C or less? Under the UL test I assume at the rated current the conductor temperature is 75?C throughout the length of the conductor. I don't see how this conductor can dissipate extra heat at one end to the point the other end remains at the same temperature (75?C). It would have to be longer (or larger). But how much longer (or larger) is still the question...???

In fact, it would be an interesting scientific study to determine how long of what size conductor is required to match the UL 489 testing regimine.
Click to expand...
Yes it would!
 
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Smart $ said:
.... It would have to be longer (or larger). ...
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Speaking of larger, what if the 4 feet of breaker-connected wire were sized per 60?C ampacity while using 75?C or higher rated conductor? Would this not allow for at least a 15?C rise in temperature at the transition end and as such, the breaker-connected end would be operate at a temperature less than 75?C?
 
hardworkingstiff said:
Aren't you also supposed to make the transitions in an enclosure that is rated for 90c terminations/splices?

I'm not sure what the temperature ratings of enclosures are.
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All the transition equipment that carry current, i.e. lugs, terminal blocks, wire connectors, etc., must be a minimum 90?C rated. Enclosures do not carry current, so they are not rated in this manner. They will, however, either aid or hinder dissipation of the heat generated by the current passing through the conductors.

I found this article by Hoffman. A little much for a splice box, eh? :blink:
 
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