Ampacity Rating of Conductor at 60 Degree C rated Termination

[paulengr]

Thanks again for guiding this mere mortal towards a better understanding of the ampacity tables. The Neher-McGrath equation hits the nail on the head Jon.

Here is some info on the development of table 310-16:

Understanding the Neher-McGrath Calculation and the Ampacity of Conductors

Quote from article:

"There are ampacity tables in the National Electrical Code that are sufficient for most installations. However, the tables in the NEC are very crude approximations and therefore include a substantial safety margin."

Never McGrath isn’t hard, just tedious. Lots of variables and calculations. The formula in the NEC is the tip of the iceberg. If you read the Neher-McGrath papers those exact same tables are in the five paper collection. When you use the NEC tables you are using the Neher-McGrath method. The papers are free from Neher-McGrath. So if there is any confusion just download and read them.

It’s 60, 75, or 90 C temperature RISE above an average ambient given in the heading of each table, 30 C. The maximum absolute wire temperature allowed is 120 C (90+30). If the average air temperature is 60 C to begin with you still use the table with the 90 C column in a 30 C temperature, then multiplier THAT by the deratjng factor for 60 C ambient. Do not use the 60 C RISE number in a 60 C space.

You run the calculation twice. Inside the raceway use the maximum rating for the wire. Derate for number of current carrying conductors. Most raceways use the chart a few tables back but tray for instance has its own derating rules. At the terminals run the calculation again but here you are limited by the rise of the terminal if it is lower than the wire which it usually is. However since it’s an open space like a gutter we don’t apply derating for number of conductors.

Now take the lower ampacity number.

We don’t normally actually do it twice in practice and the results will surprise you. For instance using #14 THHN with 75 C terminals we get 20 A in the conduit and 15 A at the terminals with 1-3 conductors. At 4-6 conductors we get 15 A after derating to 80% at 90 C. So contrary to popular belief by the engineering crowd you get 6 conductors “for free” at 90 C (modern, -2 insulation).

You can run the calculations yourself by hand. I’ve done it based on the tantalizing phrase that the tables are “very conservative” and that I’d get higher ampacities by doing the calculation. Lies, lies, lies. I get the same results as in the tables most of the time. Exceptions are buried cable if you deviate from the duct bank and cover specifications.

 

[winnie]

It’s 60, 75, or 90 C temperature RISE above an average ambient given in the heading of each table, 30 C. The maximum absolute wire temperature allowed is 120 C (90+30). If the average air temperature is 60 C to begin with you still use the table with the 90 C column in a 30 C temperature, then multiplier THAT by the deratjng factor for 60 C ambient. Do not use the 60 C RISE number in a 60 C space.

I am pretty sure that for the table formerly known as 310.16 ( *grin*) the column headings are the allowable conductor temperatures, not temperature rise values.

The the paper on Neher McGrath opened my eyes to a misunderstanding of mine. I'd sort of assumed that the ampacity table was derived from the Neher McGrath equations...but they published their paper in 1957 and the ampacity table dates from the beginnings of the electrical code. Obviously the physics applies, and heat transfer equations were known earlier, but I clearly have my history wrong.

-Jon

 

[Carultch]

I am pretty sure that for the table formerly known as 310.16 ( *grin*) the column headings are the allowable conductor temperatures, not temperature rise values.

The the paper on Neher McGrath opened my eyes to a misunderstanding of mine. I'd sort of assumed that the ampacity table was derived from the Neher McGrath equations...but they published their paper in 1957 and the ampacity table dates from the beginnings of the electrical code. Obviously the physics applies, and heat transfer equations were known earlier, but I clearly have my history wrong.

-Jon

It's now back to being known as 310.16 in the 2020 NEC.

We might call them "allowable conductor temperatures", due to the NEC calling the entire wire assembly a conductor. However, it really has nothing to do with the temperature of the conductive material, which Physics would call the conductor. It is the temperature of insulation that is either rated to withstand 60C, 75C, or 90C. The conductor itself can withstand much greater temperatures, before a failure mode such as melting or creep is expected, That's the reason for ASCR cable that has no insulation, can have greater ampacity than insulated wires in Table 310.17 in the otherwise-equivalent application.

 

[wwhitney]

However, it really has nothing to do with the temperature of the conductive material, which Physics would call the conductor. It is the temperature of insulation

Sorry, the two are in good thermal contact. So aren't we discussing the temperature of the inner surface of the insulation / outer surface of the conductor? Meaning it is the temperature of the conductive material?

Cheers, Wayne

 

[Carultch]

Sorry, the two are in good thermal contact. So aren't we discussing the temperature of the inner surface of the insulation / outer surface of the conductor? Meaning it is the temperature of the conductive material?

Cheers, Wayne

They are in thermal contact, true. But just like there is a fraction of an Ohm when wire meets a termination, there is a fraction of a Kelvin/Watt when two materials are in thermal contact. It might be modeled as a strict zero in the theory behind Neher McGrath for simplicity, but contact thermal resistance applies in reality, and there will be a fraction of a Celsius difference.

 

[ramsy]

At the terminals ..you are limited by the rise of the terminal..

if we see a terminal marked 75°C can we use the 75° ampacity column in 310.16, or must we take the temperature of the enclosure?

 

[Carultch]

if we see a terminal marked 75°C can we use the 75° ampacity column in 310.16, or must we take the temperature of the enclosure?

Good question. The answer is that the manufactured product as a whole needs to be listed for use with the termination temperature in question, if you'd like to take credit for it being anything other than the default. It is very common that you find lugs marked AL9CU in a piece of equipment, which implies a 90C rating, but the equipment itself is only listed for 75C. You'd then have to go by the 75C rating in that installation. Similarly with the 60C rating when the listing doesn't otherwise specify, but this is rare.

If you have an otherwise-empty enclosure (e.g. junction box), which carries a 90C rating or greater itself, where all products within it are listed for 90C, that is when you get to take credit for the 90C rating of connectors. Such as split bolts, Polaris blocks, and insulated tap connectors.

 

[paulengr]

if we see a terminal marked 75°C can we use the 75° ampacity column in 310.16, or must we take the temperature of the enclosure?

Use the 75 C column. Then if the enclosure is warmer than 30 C derate with the temperature multipliers.


Sent from my iPhone using Tapatalk

 

[ramsy]

The engineering & manufacturing industry appear to prohibit fixed assumptions for ambient temperature.

Sent: Friday, February 5, 2021 12:42 PM
To: Sales at Online Electronics <Sales@onlineelec.com>
Subject: On-Line Electronics: Someone asked a question about product

WAGO 221-485 specifies 105°C operating temp.

Please advise, is this temperature of the environment, or conductor temperature for ampacity calculations?

Thanks for your consideration
---------

From: "Jim Blair" <jblair@onlineelec.com>
To: "rruhle@gmail.com" <rruhle@gmail.com>
Subject: RE: On-Line Electronics: Someone asked a question about product
Date sent: Fri, 5 Feb 2021 21:32:12

Hello Roger,

Re: WAGO 221-485 specifies 105°C operating temp.

The temperature rating shown is the combination of ambient temperature and temperature rise at 100% ampacity.

Hope this answers your questions…

Thank you for this opportunity,

75°C + 30°C (NEC Table assumption) = 105°C. 90°C rated equipment is not adequate for 75°C ampacity from NEC tables.

Contractors detrimentally rely on NEC Ampacity tables, such as 310.16, which assume 30°C ambient. Never heard of installation contractors checking temperature of enclosures or conduit, even when baking in the sun without ventilation or circulation.

While a contact temperature probe would be simple and accurate, the only trade that carries a temperature probe are HVAC techs for refrigerant performance. The only cottage industry that attempts to match radiant emissivity of materials with reports of over heated wire is thermal imaging, best known for relative temperature rather than actual temperature.

How can anyone trust contractors relying on 75°c NEC table values to install anything without overheating the wire & equipment burning at 105°c or higher at enclosures warmer than 30°c?

 

[retirede]

How can anyone trust contractors relying on 75°c NEC table values to install anything without overheating the wire & equipment burning at 105°c or higher at enclosures warmer than 30°c?

I’d say because 99.999% of installed circuits never see anything close to the NEC table ampacity for any length of time.
Factor into that how many operate at 30C or less most of the time and the conservative nature of the NEC ratings....

 

[ramsy]

And for circuits relying on equipment nameplate?

..product as a whole needs to be listed for use with the termination temperature in question

75+30 or more =105. Are motor controllers, lugs, or enclosures listed for 105°C or more.

Use the 75 C column. Then if the enclosure is warmer than 30 C derate with the temperature multipliers.

That is brilliant, but where does NEC 310.15 or tables direct the knuckleheads to do this?

 

[Carultch]

75+30=105. Are motor controllers, lugs, or enclosures listed for 105°C or more.

Where are you getting the idea of adding 30 to the aforementioned 75C temperature rating?

110.14(C) doesn't care about the ambient temperature when determining your termination temperature rating. Counter-intuitive as it may be, a 100A device with 75C termination ratings, only requires #3Cu wire, regardless of it the ambient temperature is 30C or 40C. The wire in the raceway gets a derate for ambient temperature, but the terminations do not.

 

[ramsy]

Where are you getting the idea of adding 30 to the aforementioned 75C temperature rating?

Forgot 30°C ambient was built into 75°C column of ampacity tables.

310.16 Table assumption of 30°C max ambient + (by deduction) 45°C ampacity rise = 75°C max at ampacity

So, for enclosures warmer than 30°C, with less than 10ft of wire or 10% of run, the NEC requires no derating of wire, nor probing actual temperature of lugs or breakers that may exceed 75°C, since most lugs are rated for 90°C ?

 

[Carultch]

Forgot 30°C ambient was built into 75°C column of ampacity tables.

310.16 Table assumption of 30°C max ambient + (by deduction) 45°C ampacity rise = 75°C max at ampacity

So, for enclosures warmer than 30°C, with less than 10ft of wire or 10% of run, the NEC requires no derating of wire, nor probing actual temperature of lugs or breakers that may exceed 75°C, since most lugs are rated for 90°C ?

The NEC doesn't require derating for the terminations, regardless of ambient temperature. The NEC only requires derating of the wire once it leaves the terminations. The 10%/10ft rule is only about conductor ampacity, and not termination ampacity.

Where you often see the 10 ft/10% rule, is for a stub-up on a rooftop, where there is a short section of conduit that connects to serve an HVAC unit, while the majority of the conductor length is inside a building, where it is not subject to an ambinet temperature adder of direct sunlight. Another example where you see it, is if you use a trough to distribute wire, where the wire would otherwise be subject to bundling derates. But due to the vast majority length of each of those circuits being in a dedicated conduit where bundling derates do not apply, the 10 ft/10% rule allows you to neglect the reduced ampacity in that trough, provided it is less than 10 ft long and less than 10% the circuit length.

Most lugs might be rated for 90C, but you don't necessarily get to take credit for that. The entire manufactured product that contains the lugs, needs to have a termination temperature rated 90C, for you to take credit for 90C terminations. Either that, or you would have to have those lugs in an otherwise-empty enclosure, like an enclosure with Polaris splices, where all products within the enclosure are 90C rated.

 

[ramsy]

The entire manufactured product that contains the lugs, needs to have a termination temperature rated 90C

I'm seeing 60/75°C, and "75°C only" marked on typical 15-100A molded case circuit breakers (MCCB).

Can these breakers still protect wire sized from 75°C table column, when enclosures get warmer than 30°C ?

The only temperature mark I'm finding on 200A+ breakers is AMB. Compensated 40°C.

 

[Carultch]

I'm seeing 60/75°C, and "75°C only" marked on typical 15-100A molded case circuit breakers (MCCB).

Can these breakers still protect wire sized from 75°C table column, when enclosures get warmer than 30°C ?

The only temperature mark I'm finding on 200A+ breakers is AMB. Compensated 40°C.

What this particular rating means, is whether or not it approves the use of 60C wiring. It still has nothing to do with ambient temperature.

If it indicates "75C only", that means no matter how large you make the wire, it doesn't permit the use of 60C rated wire, such as UF or TW. That is, unless it carries additional ratings to make it 75C+ rated. Even if you connected #1 Cu on a 50A breaker with a 75C only rating, it would still not be permitted by the manufacturer's instructions. It is rare that this would impact your work, because 90C rated wire is the most common of what is currently available.

If it indicates 60/75C, that means you have options. You can either use 75C or 90C rated wire, with the 75C termination sizing, or you can use 60C wire with 60C sizing.

Breakers do have ambient temperature ratings, commonly 40C. You are allowed to use the breaker in ambient temperatures up to 40C, with wire sized according to the 75C column (assuming it is rated for it). At that temperature, they test the breaker with the 75C-sized wire to confirm it is safe at the trip rating of the breaker. This doesn't necessarily mean that the termination gets to 75C, it just means that the termination is tested with enough wire mass that corresponds to 75C sizing, to safely carry the nameplate amps.

 

[ramsy]

Thank you for clarifying the application of 310.15(A)(2), equipment ratings, and temperature marking, beyond the Informational note: at bottom of 110.14(C). This was a most productive Sunday.

 

[don_resqcapt19]

if we see a terminal marked 75°C can we use the 75° ampacity column in 310.16, or must we take the temperature of the enclosure?

Almost all terminals are marked for 90°C. That does not mean you can use 90°C. It is the temperature rating of the equipment that counts, not what is marked on the termination device.