Amperage for secondaries

[ronball]

When pulling secondaries from power co, to switch gear, can I use the 90 degree rating chart.( for amperage)
or does it have to be the 75 chart. Thx

 

[kwired]

What kind of conductor insulation and what is termination temp rating in your gear?

THWN is only 75C. THWN-2 is 90C.

Almost all low voltage gear has 75C terminals.

That said, typically if you have THWN-2 you need minimum size based on 75C table, but can use 90C tables for any necessary adjustments.

 

[ronball]

Thx, lugs rated 75. just was wondering since you can use 90 chart on thhn when in EMT.
Ron

 

[david luchini]

Thx, lugs rated 75. just was wondering since you can use 90 chart on thhn when in EMT.
Ron

Only if the equipment was rated 90C. That would be rare.

 

[texie]

Thx, lugs rated 75. just was wondering since you can use 90 chart on thhn when in EMT.
Ron

This statement indicates that you don't understand how to use and apply the rules for conductor ampacity. If your terminals and device is rated for 90C you could use the 90C rating, but as David said, that would be extremely rare. You can use the 90C ampacity as the basis for derating for CCC fill or ambient temperature correction but the final ampacity can not exceed the value shown in the 75C column for a 75C terminal/device. Also note that, for example, a breaker that has 90C terminals does not mean it is rated for 90C use. You have to look at the breaker conductor temp. rating which today is almost always 75C.

 

[kwired]

Plus you said this is conductors from utility transformer to switchgear. So likely at least a portion of it needs to be wet location rated and if so THWN designation is needed, which is only 75C unless it is also THWN-2 rated. In that case you can use 90C ampacity for ampacity adjustments, but in no case can the conductor be smaller than what is required by the 75C ampacity table.

 

[winnie]

Here is a post that I wrote on how the 90C ampacity applies in a run:

Conductor rating question

The General Contractor sent this request-for-information: "The engineer is saying the wire is rated for 90 degrees which is correct, but per NEC we can only use wire sizes off the 75 degree chart. 90 degrees is only good for de-rating purposes." Is this sentence true? Can you please provide...

forums.mikeholt.com

 

[Carultch]

Thx, lugs rated 75. just was wondering since you can use 90 chart on thhn when in EMT.
Ron

Think of it like a chain rated for 90 pounds, connected to hooks that are rated for 75 pounds. What governs the strength of this hook-chain-hook assembly? The 75 pound rated hooks. You would need hooks at both ends that are also rated for 90 pounds, if you wanted to take credit for the chain's 90 pound rating.

This is also the case with the 90C vs 75C ampacity ratings. You need both wire and terminations to be 90C rated, if you want to use the full 90C rating. It is common for wire to be 90C rated, but rare for terminations to be 90C rated. Separately-installed connectors like split bolts and insulated splice blocks (e.g. Polaris) commonly carry a 90C rating which you can use in an otherwise-empty enclosure, but for lugs that are built as part of manufactured equipment, the equipment usually has a 75C rating, or possibly a 60C rating. Even if the lug is marked for 90C, the equipment it is in does not necessarily allow you to take credit for this. You can value-engineer your way around this rule if you connecting 90C-rated/75C-sized wire locally at the equipment on both ends, and then splice it in separate enclosures to 90C-sized wire for the majority of the run. I'm not sure when that value-engineering would be cost-effective enough to justify the separate enclosures/splices/extra complexity, but it exists as a solution in theory.

It is difficult to fit the following point in the chain/hook analogy, but the primary value in having 90C rated conductors, is that it gives you some headroom for your the ampacity adjustment and temperature correction calculations. These calculations don't need to be applied to terminations, just to the wire itself once it leaves the equipment and travels in the raceway or cable. You get to use the 90C rating, as opposed to the 75C rating, as the starting point for these calculations, which reduce the ampacity from the way the conductor is tested with not more than 3 conductors bundled together in a 30C ambient temperature. A 4th wire requires a bundling derate (now called ampacity adjustment). An ambient temperature higher than 30C also requires a temperature correction factor. The 90C rating gives you some headroom for these calculations, which you can use even if terminations require a 75C rating.