derate conductors to 60C in LFMC?

[coop3339]

Hi Everyone,

What are your thoughts on derating conductors to the raceway rated temperature? Most LFMC is only rated to 60C wet. Does this mean we have to use the 60C ampasity for conductors in LFMC where in wet location? It would seem the conductors should not be allowed to exceed the temperature limitation of the raceway.

 

[luckylerado]

Hi Everyone,

What are your thoughts on derating conductors to the raceway rated temperature? Most LFMC is only rated to 60C wet. Does this mean we have to use the 60C ampasity for conductors in LFMC where in wet location? It would seem the conductors should not be allowed to exceed the temperature limitation of the raceway.

I think you use the rating of the insulation to derate from which may be 60° when in a wet location depending on the insulation. I was not aware that LFMC had a temperature limitation.

 

[don_resqcapt19]

From the UL Guide Information (white book) for "Flexible Metal Conduit, Liquidtight (DXHR)

Liquid-tight flexible metal conduit not marked with a temperature designation or marked "60 C" is intended for use at temperatures not in excess of 60°C (140°F).

If the LFMC is not marked with a temperature rating or is marked 60°C, I would agree that you could only use the 60°C ampacities for the conductors installed in that LFMC.

 

[mwm1752]

From the UL Guide Information (white book) for "Flexible Metal Conduit, Liquidtight (DXHR)

If the LFMC is not marked with a temperature rating or is marked 60°C, I would agree that you could only use the 60°C ampacities for the conductors installed in that LFMC.

How does the raceway effect the conductor amperage rating? I would think the 60 C marking is reference to installation of the LFMC only -- location the installation in an area of 60C or less seem the criteria for LFMC not to limit a 90C conductor amp rating install inside to 60C. not similar to 110.14 IMO

 

[Carultch]

From the UL Guide Information (white book) for "Flexible Metal Conduit, Liquidtight (DXHR)

If the LFMC is not marked with a temperature rating or is marked 60°C, I would agree that you could only use the 60°C ampacities for the conductors installed in that LFMC.

Just because you have a wire inside the conduit that operates with part of it at 90C, does not necessarily mean that the entire raceway is operating at 90C. There is a temperature fall-off curve, when you have a cylindrical heat source. The metal core of the wire may be 90C, but that is going to decrease with radius as you progress through the insulation. And decrease even more as you progress through the walls of the conduit.

 

[don_resqcapt19]

How does the raceway effect the conductor amperage rating? I would think the 60 C marking is reference to installation of the LFMC only -- location the installation in an area of 60C or less seem the criteria for LFMC not to limit a 90C conductor amp rating install inside to 60C. not similar to 110.14 IMO

It doesn't effect the ampacity of the conductor...it just says you cannot use a conductor that can operate at a temperature that exceeds 60°C in that raceway. While not spelled out in the code for LFMC, it works that same as 352.10(I) for rigid PVC.

 

[mwm1752]

It doesn't effect the ampacity of the conductor...it just says you cannot use a conductor that can operate at a temperature that exceeds 60°C in that raceway. While not spelled out in the code for LFMC, it works that same as 352.10(I) for rigid PVC.

The wording of both section are certainly different -- So, If I get the jest of the section
Where any combination of ambient and conductor temperature produces an operating temperature in excess of that for which the material is approved
then, LFMC @ 60 C on a ambient temp roof of 35 C with the (Adjustment 33 C) could not be used.
& if the adjustment was 14C then the conductor draw would be limited to 11 C. or 8# thwn = 90C @ 55A --- 11 C @ ? or 11 C/ 90 C = ? / 55 a ? = 6.7 amps

PVC is more of a reference to temp limit due to the PVC --where if the PVC was marked @ 75 C the conductor rating would be @ 75 C.

 

[don_resqcapt19]

The wording of both section are certainly different -- So, If I get the jest of the section
Where any combination of ambient and conductor temperature produces an operating temperature in excess of that for which the material is approved
then, LFMC @ 60 C on a ambient temp roof of 35 C with the (Adjustment 33 C) could not be used.
& if the adjustment was 14C then the conductor draw would be limited to 11 C. or 8# thwn = 90C @ 55A --- 11 C @ ? or 11 C/ 90 C = ? / 55 a ? = 6.7 amps

PVC is more of a reference to temp limit due to the PVC --where if the PVC was marked @ 75 C the conductor rating would be @ 75 C.

Where did the words you have in bold come from?

 

[mwm1752]

350.12(2) I was surprised also

 

[Carultch]

350.12(2) I was surprised also

So what exactly does that sentence mean? Does that sentence mean that the LFMC temperature rating of 60C implies that the conductor ampacity is limited to the value calculated from the NEC's 60C column?

 

[don_resqcapt19]

So what exactly does that sentence mean? Does that sentence mean that the LFMC temperature rating of 60C implies that the conductor ampacity is limited to the value calculated from the NEC's 60C column?

That is how is I read it.

 

[mwm1752]

So what exactly does that sentence mean? Does that sentence mean that the LFMC temperature rating of 60C implies that the conductor ampacity is limited to the value calculated from the NEC's 60C column?

My read is that the combination of temperature location with conductor current will determine where it can be installed -- it is not the same as PVC is more of a reference to temp limit due to the PVC --where if the PVC was marked @ 75 C the conductor rating would be @ 75 C.
I am just not sure how to apply the factors, my example in the earlier post is a hypothesis -- back to the archives to see if they discussed intent

 

[Carultch]

My read is that the combination of temperature location with conductor current will determine where it can be installed -- it is not the same as PVC is more of a reference to temp limit due to the PVC --where if the PVC was marked @ 75 C the conductor rating would be @ 75 C.
I am just not sure how to apply the factors, my example in the earlier post is a hypothesis -- back to the archives to see if they discussed intent

My point is that it is an oversimplification to assume the entire raceway system operates at 90C, if you intend to take credit for the 90C conductor rating in the wire ampacity calculation. This calculation is based off keeping the copper or aluminum core of the wire below 90C, so that the insulation is entirely less than this. Obviously, there is a falloff of temperature with radius through the insulation, because it also acts as thermal insulation. So if the inner core of the wire is limited to 90C, I expect the outer edge to be significantly less.

Now making this oversimplification may very well be what you have to do, despite the fact that it is a Murphy's law oversimplification to avoid a more complicated thermal model.

 

[mwm1752]

Archives have minimal comments starting @ 1993 some history
1965 NEC 351-2 there is no references to “not used”
1968 NEC 351-2(b)(3)- Under conditions such that its temperature, with or without enclosed conductor carrying current , is above 60C (140F)

1971 NEC 351-2(b)(2)- Where any combination of ambient and/or conductor temperature will produce an operating temperature in excess of that for which the material is approved

1993 NEC 351-2(b)(2)- Where any combination of ambient and conductor temperature will produce an operating temperature in excess of that for which the material is approved

2104 NEC - Where any combination of ambient and conductor temperature produces an operating temperature in excess of that for which the material is approved

Found these specs online Temperature Rating -80°C to –30°C Dry - 60°C Wet - 70°C Oil resistant

 

[gadfly56]

My point is that it is an oversimplification to assume the entire raceway system operates at 90C, if you intend to take credit for the 90C conductor rating in the wire ampacity calculation. This calculation is based off keeping the copper or aluminum core of the wire below 90C, so that the insulation is entirely less than this. Obviously, there is a falloff of temperature with radius through the insulation, because it also acts as thermal insulation. So if the inner core of the wire is limited to 90C, I expect the outer edge to be significantly less.

Now making this oversimplification may very well be what you have to do, despite the fact that it is a Murphy's law oversimplification to avoid a more complicated thermal model.

It is, I think, a bridge too far to go from wire core at 90C to suggesting that you're OK to use that credit in LFMC that is limited to 70C based on "the outer edge to be significantly less". While we can all agree that if the wire core is 90C the LFMC will see less than that, I'd like to see some calculations that it is going to be 70C or less. I'd be amazed if anyone on this forum had access to heat transfer modeling software that could be applied to this situation. Perhaps some Masters or PhD candidate in Mechanical or Chemical Engineering has used this example for their thesis but I doubt it.

 

[luckylerado]

Where any combination of ambient and conductor temperature produces an operating temperature in excess of that for which the material is approved

So what exactly does that sentence mean? Does that sentence mean that the LFMC temperature rating of 60C implies that the conductor ampacity is limited to the value calculated from the NEC's 60C column?

That is how is I read it.

This is not limiting the temperature rating of of the conductor in any way, rather it limits the operating temperature of the raceway. I do not know how to calculate operating temperature of a conductor under load in a raceway but I would not think that you can use the ampacity tables to get there. As long as the circuit conductors are not creating operating temps above the rating within the raceway and you are not installing the raceway in an ambient above rating you are compliant 350.12

I would expect language similar to other articles if the intent was to limit the temperature rating of the conductor...."The allowable ampacity
shall not exceed that of a 60°C" or something similar.

 

[shortcircuit2]

Question on this thread.

The basis of the answers on this thread reflect an premise that LFMC is limited to 60°C according to the UL 360 standard.

In researching this subject, I found the UL 360 standard has reference to a higher temperature of 80°C for a dry location rated LFMC product.

UL360...https://standardscatalog.ul.com/stan...standard_360_7

LFMC high temp product...http://www.afcweb.com/liquid-tuff-co...uit-type-lfmc/

With this high temperature rated product used in a dry location circuit installation, I would be able to install a circuit rated at 75°C provided my terminations are rated equally.

Agree?

 

[infinity]

Looks like if it's not labeled then it's a 60° C raceway, if so labeled then, (using Southwire as an example) it's 80° C dry and 60° C wet.

 

[shortcircuit2]

Thanks for the feedback...

Below is a LFMC product for up to 105°C

http://www.afcweb.com/liquid-tuff-conduit/ul-hi-low-temperature-type-lfmc/

I would like to know why all LFMC products have their rating cut to 60°C when wet.

 

[retirede]

Thanks for the feedback...

Below is a LFMC product for up to 105°C

http://www.afcweb.com/liquid-tuff-conduit/ul-hi-low-temperature-type-lfmc/

I would like to know why all LFMC products have their rating cut to 60°C when wet.

When I was doing OEM machinery, we used CSA-only rated LFMC to avoid having to size conductors for 60C.
We never had a problem getting our machines accepted in the US - virtually everybody accepted the CSA approval.