Using the 90C column in the Ampacity Table

[augie47]

From the UL Whitebook: Panelboards QUEY
Unless the panelboard is marked to indicate otherwise, the termination provisions are based on the use of 60°C ampacities for wire sizes 14–1 AWG, and 75°C ampacities for wire sizes 1/0 AWG and larger as specified in Table 310.15(B)(16) of ANSI/NFPA 70, ‘‘National Electrical Code’’

 

[Vines41]

Looking through the datasheets I cannot find any ratings, so I started a chat with Schneider and will see what they have to say.

Schneider said the lugs are rated for 75C, which I know isn't true as I have seen the lugs themselves. I'll bet it is true enough that likely the enclosure isn't rated more than 75C.

Do others agree "you probably wont find that (90C rated electrical equipment) anywhere?" When I reached out to Solar Edge, they said the 100kW inverters have 105C lugs. I wonder if that is the whole story though?

 

[Vines41]

Interesting couple links here that were passed along to me. According to the information posted there are basically no distribution equipment rated to 90C. I guess I will stop trying to make this work lol.

https://www.se.com/us/en/faqs/FA176152/
https://www.se.com/ww/resources/sit...2/en_US/Wire Terminations 0110DB9901R2-02.pdf

 

[wwhitney]

If someone says "lug rating" that typically refers to the rating as a standalone part. And that would be the meaning of any rating stamped on the lug itself. But once that lug is incorporated into a piece of equipment, you don't care about the lug rating anymore, you care about the overall equipment rating, as the lugs will conduct heat to the rest of the equipment.

Cheers, Wayne

 

[acolella]

I have recently had several discussions about only using the 90°C column for conductor sizing when fill and/or ambient require derating.

Tell me if I've got this right.

Here's an example scenario: A piece of equipment has an MCA of 72 amps. The unit and supply conductors will be in a 125°F ambient. If ambient temperature did not require a derate, #4 THHN would be selected since it has a 75°C ampacity of 85 amps. However, the ambient is 125°F and a derate is going to be required. The derate factor for THHN is .76 in a 125°F ambient. Going by the 90°C column and applying a .76 derate, the same #4 THHN has an ampacity of 72.2 amps. The #4 THHN is still adequate because going by the 90°C column to derate, the conductor ampacity exceeds the MCA by 0.2 amps. A small margin, but still acceptable, correct?

One point of confusion that I had is when the derated ampacity falls below that of the 75°C column such as in this example.

Does the derated ampacity need to meet or exceed the 75°C column ampacity or can it be utilized up to its derated ampacity below the 75°C column ampacity.

If it falls below the 75°C column ampacity and is utilized up to that derated ampacity, will it exceed the temperature limitations of 75°C rated terminations?

What if the wire is derated from the 90°C column and the derated ampacity is above that which is in the 75°C column? Can the wire then still only be utilized up to it's 75°C rating despite the derated ampacity being higher than that of the 75°C column ampacity?

Thanks!

 

[augie47]

You derate based on the insulation rating. If the derated value is equal to or greater than the load the conductor is correct.
You then size the overcurrent for the conductor based on 240.4 unless there is another Code section or manufacture requirement in effect.
Does that answer yiru question ?

 

[Jon456]

For 100A and less, you have a burden of proof if you want to use the 75C terminations, but most of the time, you will find 75C terminals in practice. The default is 60C for 100A and less. It is usually academic examples where you encounter the need for 60C termination sizing, since equipment listed otherwise for 75C is most common.

I assume that "burden of proof" means you must be able to provide a manufacture's listing that shows the terminals are rated at 75C. But what if an installed piece of equipment bears no such label?

I have an existing 3-phase 240VAC "Heavy Duty Safety Switch" (fused disconnect) that is rated at 100A (Sylvania/GTE RHD323SN). There is nothing on the nameplate that states 75C terminations, and there are no internal labels. The existing installation has #1 AWG THW aluminum conductors which would be sufficient for 100A if the terminals are rated at 75C, but insufficient if we must assume 60C terminals.

This disconnect appears to be discontinued and I have not been able to locate a spec sheet for it. But even if I did, would a spec sheet be sufficient without a manufacturer's printed label inside the enclosure listing the terminal ratings?

 

[Carultch]

I assume that "burden of proof" means you must be able to provide a manufacture's listing that shows the terminals are rated at 75C. But what if an installed piece of equipment bears no such label?

I have an existing 3-phase 240VAC "Heavy Duty Safety Switch" (fused disconnect) that is rated at 100A (Sylvania/GTE RHD323SN). There is nothing on the nameplate that states 75C terminations, and there are no internal labels. The existing installation has #1 AWG THW aluminum conductors which would be sufficient for 100A if the terminals are rated at 75C, but insufficient if we must assume 60C terminals.

This disconnect appears to be discontinued and I have not been able to locate a spec sheet for it. But even if I did, would a spec sheet be sufficient without a manufacturer's printed label inside the enclosure listing the terminal ratings?

Indeed, that is what I mean by burden of proof. It means that the manufacturer's product documentation needs to specify that the 60C default is overridden by a product listing for 75C. The NEC currently requires marking this on the equipment itself, which is why it uses the word "labeled" in addition to "listed". I'm not familiar with what this rule used to require, but I would guess that factory-labeling wasn't always a requirement to qualify for overriding the defaults. That would explain why there could exist a device with paperwork that specifies a 75C rating, without factory labeling of the product to corroborate it. Requiring product labeling simplifies the issue, so it can be inspected without needing to chase product paperwork.

110.14(C)(1)(b) also specifies that provisions for wires larger than #1 (i.e. #1/0 and larger), make for a 75C terminal rating default. This doesn't necessarily mean that you are using 1/0 or larger to qualify, but that the equipment is built with terminals that can accept 1/0 or larger, which is common for 100A equipment. This leaves open a narrow window where NEC110.14(C) is ambiguous. Because it draws a line between 100A and 101A, while it also draws a line with the same conclusions between max terminal sizes of #1 and #1/0. Equipment can exist that is on opposite sides of these two lines, like a 100A device with 1/0 terminals (which is common), or a 125A device with #1 max terminals (which is rare). I would expect that most likely it would be rated for 75C, as it is rare to encounter equipment requiring 60C terminal sizing. It is more of an academic rule, than a rule that impacts what you do in practice, because most equipment is rated for 75C anyway.

 

[winnie]

I have recently had several discussions about only using the 90°C column for conductor sizing when fill and/or ambient require derating.

Here is my response to your question from a different thread:

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

Jon

 

[acolella]

You derate based on the insulation rating. If the derated value is equal to or greater than the load the conductor is correct.
You then size the overcurrent for the conductor based on 240.4 unless there is another Code section or manufacture requirement in effect.
Does that answer yiru question ?

I appreciate the reply. I think it partially answers my question. I'm still not 100% clear on how the conductor is correct if the derated ampacity is greater than the 75°C column. So, #4 THHN has an insulation rating of 90°C and going by the 90°C column the ampacity is 95 amps. If the ambient is 95°F the derate factor is 0.96 which puts the ampacity at 91.2 amps. If the load is, lets say, 91 amps and the disconnect, conductors and utilization equipment are all in the 95°F ambient and the disconnect and equipment have 75°C terminals, I can use this conductor because the derated value is equal to or greater than the load? Wouldn't I be limited to the 85 amps of the 75°C column?

 

[acolella]

Here is my response to your question from a different thread:

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

Jon

I appreciate the reply. So, if the derated ampacity of the conductor is above the ampacity of the 75°C terminals, the circuit is still limited to the ampacity of the 75°C terminals, correct? If the derated ampacity is below that of the 75°C terminals, the conductor then becomes the weakest link and the circuit is limited to the derated ampacity of the conductor, correct?

 

[augie47]

correct. Some waht a two-step process... make sure your conductor has the ampacity to carry the load (after derating, etc) then check the Table and eb sure you are not exceeding the ampacity per 1109.14 rules.