Conductor Ampacities

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L

LLTEI

Member
Location
long beach, ca, usa
Hi Guys -

I have a customer who is asking if it is OK to run our equipment at 166 AMPS.

This is oil field equipment specifically I am asking about the Surface Cable - it is a Southwire #2 MC-HL MV-105 Shd. Cable with a rating of 154 amps in air and 190 direct burial. Copied directly from the Southwire Data sheet "+Ampacities are based on the NEC, 2008 Edition. Direct burial ampacities are based on table 310.83 three-conductors within an overall covering directly buried, 105°Cconductor, 20°C earth ambient temperature. In air ampacities are based on table 310.71 three-conductors within an overall covering in free air, 105°C conductor, 40°C
ambient temperature."

I am interested in the in air rating - I understand that the rating was based on a 40C ambient temperature - now my question is - how do I determine what this rating is if my ambient temperature never exceeds 30C?

Is there a simple formula that I can plug the conductor temp (105 C) along with the "new ambient temp" to determine what the ampacity rating should be?


Here's the link to the actual cable I am working with for reference.

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet137

Thank you guys in advance!
 
C

Carultch

Senior Member
Location
Massachusetts
LLTEI said:
Hi Guys -

I have a customer who is asking if it is OK to run our equipment at 166 AMPS.

This is oil field equipment specifically I am asking about the Surface Cable - it is a Southwire #2 MC-HL MV-105 Shd. Cable with a rating of 154 amps in air and 190 direct burial. Copied directly from the Southwire Data sheet "+Ampacities are based on the NEC, 2008 Edition. Direct burial ampacities are based on table 310.83 three-conductors within an overall covering directly buried, 105°Cconductor, 20°C earth ambient temperature. In air ampacities are based on table 310.71 three-conductors within an overall covering in free air, 105°C conductor, 40°C
ambient temperature."

I am interested in the in air rating - I understand that the rating was based on a 40C ambient temperature - now my question is - how do I determine what this rating is if my ambient temperature never exceeds 30C?

Is there a simple formula that I can plug the conductor temp (105 C) along with the "new ambient temp" to determine what the ampacity rating should be?


Here's the link to the actual cable I am working with for reference.

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet137

Thank you guys in advance!
Click to expand...


There is such a formula. It is the square root of the ratio of two temperature differences.

Temperature correction derate factor = sqrt((Tw - T)/(Tw - T0))

Where:
Tw is the temperature rating of the wire
T is the actual ambient temperature
T0 is the temperature off of which the ampacity is based. Usually 30C, if using 310.15(B)(16). I'm not familiar with the other tables in the NEC, as I mostly use the standard one.

A common example I encounter is as follows:
50C ambient due to a rooftop adder
30C basis temperature
90C conductors

sqrt((90 - 50)/(90 - 30)) = 0.816

And if you look this up in the table that determines temperature corrections, you'll see 0.82, which is within a rounding error from what you get by the square root formula.
 
Jraef

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Carultch said:
There is such a formula. It is the square root of the ratio of two temperature differences.

Temperature correction derate factor = sqrt((Tw - T)/(Tw - T0))

Where:
Tw is the temperature rating of the wire
T is the actual ambient temperature
T0 is the temperature off of which the ampacity is based. Usually 30C, if using 310.15(B)(16). I'm not familiar with the other tables in the NEC, as I mostly use the standard one.

A common example I encounter is as follows:
50C ambient due to a rooftop adder
30C basis temperature
90C conductors

sqrt((90 - 50)/(90 - 30)) = 0.816

And if you look this up in the table that determines temperature corrections, you'll see 0.82, which is within a rounding error from what you get by the square root formula.
Click to expand...
But, as in your example, that's for DE-rating a conductor ampacity for ambient OVER the expected ambient. He wants to go the other way, calculate an INCREASED ampacity rating based on a LOWER temperature. I don't believe the NEC allows that.
 
I

Ingenieur

Senior Member
Location
Earth
Using the formula from above 1.075 x 154 = 165
is the 166 the actual load?
or the calculated ampacity?

not sure if a mv cable with a 40c ambient rating can be adjusted though
 
I

Ingenieur

Senior Member
Location
Earth
C

Carultch

Senior Member
Location
Massachusetts
Jraef said:
But, as in your example, that's for DE-rating a conductor ampacity for ambient OVER the expected ambient. He wants to go the other way, calculate an INCREASED ampacity rating based on a LOWER temperature. I don't believe the NEC allows that.
Click to expand...

In the tables where the NEC pre-evaluates this formula for you, there are values greater than unity in the cases of less than 30C ambient. I don't understand why the NEC would include these values, if you are never supposed to use them. If you are never supposed to have a greater-than-one temperature correction factor, I'd expect the pre-evaluated tables to have a value of 1 override all of the cases of less than 30C.

What you do need to keep an eye out for, is the termination ampacities prior to temperature correction, because the NEC doesn't specify that you can adjust termination ampacities in your favor. And often, this will be the limiting factor before the greater-than-unity temperature correction will help you.
 
I

Ingenieur

Senior Member
Location
Earth
Carultch said:
In the tables where the NEC pre-evaluates this formula for you, there are values greater than unity in the cases of less than 30C ambient. I don't understand why the NEC would include these values, if you are never supposed to use them. If you are never supposed to have a greater-than-one temperature correction factor, I'd expect the pre-evaluated tables to have a value of 1 override all of the cases of less than 30C.

What you do need to keep an eye out for, is the termination ampacities prior to temperature correction, because the NEC doesn't specify that you can adjust termination ampacities in your favor. And often, this will be the limiting factor before the greater-than-unity temperature correction will help you.
Click to expand...

Aren't those ratings (75, 90, etc) for the operating temp of the cond/insul?
so if a 75C cond is uprated 10% due to lower ambient I would think the appropriate termination would follow?
 
C

Carultch

Senior Member
Location
Massachusetts
Ingenieur said:
Aren't those ratings (75, 90, etc) for the operating temp of the cond/insul?
so if a 75C cond is uprated 10% due to lower ambient I would think the appropriate termination would follow?
Click to expand...

You would think so, but I don't think you can have it both ways with this rule. The same rule that applies at high ambient temperatures, should be consistent with the one applying at lower ambient temperatures.

And for high ambient temperatures, 110.14(C) states that you do not apply derate factors to termination ratings, but instead apply them to conductor ampacity as a whole.

I agree that it would make more sense if ambient temperature corrections applied to terminations, and bundling adjustments didn't have to. It is probably an oversight when this rule was originally written, and no one has demonstrated the need to change the rule.
 
I

Ingenieur

Senior Member
Location
Earth
NEC 2014
table 310.60(C)(4) Ambient Temp Correction Factors (other than 40 C)
appears to allow adjustment for a 5 kv cable
for example: 26-30 C ambient (40 C ambient rated cable) 105C conductor
correction is 1.07 x rating (154 in this case) ~165

if the ambient was 11-15 C cf is 1.22 with 95C conductor
etc

bottom line: cable seems marginal at best
 
L

LLTEI

Member
Location
long beach, ca, usa
Carultch said:
There is such a formula. It is the square root of the ratio of two temperature differences.

Temperature correction derate factor = sqrt((Tw - T)/(Tw - T0))

Where:
Tw is the temperature rating of the wire
T is the actual ambient temperature
T0 is the temperature off of which the ampacity is based. Usually 30C, if using 310.15(B)(16). I'm not familiar with the other tables in the NEC, as I mostly use the standard one.

A common example I encounter is as follows:
50C ambient due to a rooftop adder
30C basis temperature
90C conductors

sqrt((90 - 50)/(90 - 30)) = 0.816

And if you look this up in the table that determines temperature corrections, you'll see 0.82, which is within a rounding error from what you get by the square root formula.
Click to expand...


sqrt((105-27)/(90-40)) = 1.095

Tw = 105C (rating of the conductor)
T = 27C Actual Ambient Temperature
T0 = 40C (according to the Southwire Spec sheet the rating was based on this number)

Am I supposed to take this 1.095 and multiple it by the current ampacity rating of the cable which is 154 amps?

154 X 1.095 = 168.63
 
L

LLTEI

Member
Location
long beach, ca, usa
Julius Right

Julius Right

Senior Member
Occupation
Electrical Engineer Power Station Physical Design Retired
According to art. 310.60 Conductors Rated 2001 to 35,000 Volts.
Ampacities for solid dielectric-insulated conductors
shall be permitted to be determined by tables or under engineering
supervision, as provided in 310.60(B) and (D).
[Table 310.60(B)(4) does not exist, indeed,
however, in B (4) Ambient Temperature Correction it is stated:
Ampacities for ambient temperatures other than those specified in the ampacity tables shall be corrected in accordance with
Table 310.60(C)(4) or shall be permitted to be calculated using
the following equation:
I'=I*SQRT((Tc-Ta')/(Tc-Ta))
In my opinion, it means this rule permits higher ampacity for lower ambient temperature.
(1) Selection of Ampacity. Informational Note: See 110.40 for conductor temperature limitations due to termination provisions.
110.40 Temperature Limitations at Terminations. Conductors
shall be permitted to be terminated based on the
90°C (194°F) temperature rating and ampacity as given in
Table 310.60(C)(67) through Table 310.60(C)(86), unless
otherwise identified.
So, from Table 310.60(C)71 only 140 A is permitted for 40oC ambient.
For 30oC ambient will be then 1.1*140=154 A
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Carultch said:
...

And for high ambient temperatures, 110.14(C) states that you do not apply derate factors to termination ratings, but instead apply them to conductor ampacity as a whole.

I agree that it would make more sense if ambient temperature corrections applied to terminations, and bundling adjustments didn't have to. It is probably an oversight when this rule was originally written, and no one has demonstrated the need to change the rule.
Click to expand...
Bingo!!! You cannot exceed the "termination ampacity". In this case, for 2AWG, that's 115A for copper conductors and 75°C-rated terminations. At best one could interpose a 90°C-rated transition and get up to 130A.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Julius Right said:
According to art. 310.60 Conductors Rated 2001 to 35,000 Volts.
Ampacities for solid dielectric-insulated conductors
shall be permitted to be determined by tables or under engineering
supervision, as provided in 310.60(B) and (D).
...
Click to expand...
Those sections do not override 110.14(C) termination temperature limitations, as it stipulates the use of Table 310.16.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
Smart $ said:
Bingo!!! You cannot exceed the "termination ampacity". In this case, for 2AWG, that's 115A for copper conductors and 75°C-rated terminations. At best one could interpose a 90°C-rated transition and get up to 130A.
Click to expand...
PS: I just realized from reading additional posts that we are talking MV cable, and 90°C-rated terminations apply from the get go.
 
W

wwhitney

Senior Member
Location
Berkeley, CA
Occupation
Retired
Julius Right said:
110.40 Temperature Limitations at Terminations. Conductors shall be permitted to be terminated based on the 90°C (194°F) temperature rating and ampacity as given in
Table 310.60(C)(67) through Table 310.60(C)(86), unless otherwise identified.
Click to expand...
"Shall be permitted" is not mandatory language. So does this mean one may choose to terminate based on the 105°C temperature?

Cheers, Wayne
 
I

Ingenieur

Senior Member
Location
Earth
Apples and oranges
the termination ratings are not affected
the operating temperture does not change

all the re-rating does is allows an increase in ampacity due to lower ambient/better cooling
but the 75/90/etc rating is still maintained
 
I

Ingenieur

Senior Member
Location
Earth
LLTEI said:
I am not sure if this is the same cable or even similar. The cable in this chart is if I am reading it correctly is for 2000 volts or below.

The cable that I am dealing with is 5000 volts rated and will be ran at 4500 volts.
Click to expand...

the chart applies for 2001 to 35000 vac
2011
310.60 (C) tables
chart referenced 310.60(C)(4)

1.25 x 166 = 208

the cable is too small
 
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