LFMC rated 60C wet location

[winnie]

The derate calculation is also based on the conductors heating up to their rated temperature. If you derate based on 90C, then you are implying that the conductors will heat to 90C when used at the calculated ampacity.

A #12 has a 90C ampacity of 30A, and there is a 50% derate for 20 conductors.

If you have 20 #12 conductors together in a single conduit, each carrying 15A, then you expect the conductors to heat up to 90C.

Again, I am ignoring the conservative and approximate nature of NEC ampacity calculations (real world vs theory). This is just running the standard calculations backwards.

The standard calculation says 'given this maximum temperature rating and these conditions of use, what is the maximum allowed current. Running the calculation backwards we say 'given these conditions of use and this current level, what is the maximum conductor temperature. '

Jon

 

[wwhitney]

One way to look at it is that when the insulated conduit is touching the temperature limited conduit material, that conduit material is like a second layer of insulation on the conductor. So now you need to treat the conductor as if its insulation temperature rating is the lower of the actual insulation rating, and the conduit material rating.

Cheers, Wayne

 

[wwhitney]

I mean the value shown in the 60°C column of Table 310.16.

Then I would suggest calling that "60C base ampacity" or "60C tabular ampacity," to indicate it is before adjustment or correction. Vs "60C ampacity," which should include adjustment or correction as per the definition of ampacity, and for which the 60C modifier would suggest the computation was based on 60C insulation temperature.

With that terminology, the 60C wet LFMC limit would restrict interior conductor ampacity to the "60C ampacity" rather than just to the "60C tabular ampacity". For the reasons discussed, if you derate starting at an ampacity higher than the 60C tabular ampacity, the conductor operating temperature may exceed 60C.

I seem to recall @david luchini suggesting that "conductor operating temperature" could be calculated by a different procedure than relying on ampacity calculations, but I never saw a procedure spelled out or an NEC citation that would justify such a procedure.

Cheers, Wayne

 

[wwhitney]

insulated conduit

Typo: should be insulated conductor.

Wayne

 

[don_resqcapt19]

Then I would suggest calling that "60C base ampacity" or "60C tabular ampacity," to indicate it is before adjustment or correction. Vs "60C ampacity," which should include adjustment or correction as per the definition of ampacity, and for which the 60C modifier would suggest the computation was based on 60C insulation temperature.

With that terminology, the 60C wet LFMC limit would restrict interior conductor ampacity to the "60C ampacity" rather than just to the "60C tabular ampacity". For the reasons discussed, if you derate starting at an ampacity higher than the 60C tabular ampacity, the conductor operating temperature may exceed 60C.

I seem to recall @david luchini suggesting that "conductor operating temperature" could be calculated by a different procedure than relying on ampacity calculations, but I never saw a procedure spelled out or an NEC citation that would justify such a procedure.

Cheers, Wayne

I don't see the ampacity adjustment or correction entering into this. The ampacities after adjustment and/or correction become the maximum permitted ampacity for the conductor under those conditions.

The rule intends that the operating current of the conductor does not exceed the 60°C ampacity as found in the table.

 

[wwhitney]

The rule intends that the operating current of the conductor does not exceed the 60°C ampacity as found in the table.

The above description matches well the language in, say, 334.80 for NM cable.

The language in 350.10(4) is quite different and requires that "the conductors or cables are not operated at a temperature higher than the listed temperature rating of the LFMC." How are you going to ensure that the conductors or cables are not operated at higher than 60C if you don't stick with the 60C insulation temperature rating for all purposes? What other procedure is there?

If in a combination of conditions and current you can have confidence that the conductors won't exceed 60C, then those conductors could be insulated with 60C insulation and have an ampacity of at least that current. That's the definition of ampacity.

Cheers, Wayne

 

[Eddie702]

I agree with Dennis. Regardless of where you start derating if you are inside LMFC your "operating temp" cannot exceed 60C.

The problem I see is that with sealtight you are usually in a wet location (but not always) so 60C is it

What about the thousands of chillers, Ac units installed with existing conductors sized at 75C inside sealtight?

Now you replace a unit with like for like amps and now you have a violation because the conductors are sized at 75C which is very common over 100 amps.

So now you have to pull larger conductors? What if the pipe is too small?

Or am I making a mountain out of a molehill as I sometimes do?

 

[wwhitney]

I agree with Dennis. Regardless of where you start derating if you are inside LMFC your "operating temp" cannot exceed 60C.

Say I take (6) #8 AWG Cu conductors and put them in a conduit at 30C ambient. Say the terminations are 75C so the terminations are not a limit in the following.

Now if the conductors have 60C insulation, their ampacity will be 40A * 0.8 = 32A. So under "worst case" conditions (for the various factors not specified that may influence the ultimate thermal equilibrium) the actual insulation temperature for a 32A continuous load could get up to 60C. In practice certainly less than 60C, assuming the NEC is suitably conservative in its ampacity tables. Maybe it turns out to be 45C or 55C, we don't know. We know it's definitely above 30C, and should be no more than 60C.

Now suppose the conductors have 75C insulation, their ampacity will be 50A * 0.8 = 40A. So under "worst case" conditions, the actual insulation temperature for a 40A continuous load could get up to 75C. In practice certainly less, and certainly more than with 32A. Maybe it turns out to be 55C, or 65C. We don't know, we just know it's definitely above 30C, definitely above the previous case, and should be no more than 75C.

So if the conduit is LFMC, and the conductor operating temperature is limited to 60C, but the conductor insulation rating is 75C, which ampacity value should you start derating at? The 60C base ampacity, which will give you the usual guarantee that the insulation temperature will stay below 60C? Or the 75C base ampacity, for which the insulation temperature may exceed 60C, the operating temperature limit?

And if you want to derate starting at the 75C base ampacity, because your ampacity is too low if you start at the 60C base ampacity, what other measures will you take to comply with the 60C operating temperature limit?

Cheers, Wayne

 

[Eddie702]

350-10 (4) says you can install 75C or 90C wire just can't operate it where it will exceed 60C taking ambient temp, # of conductors and load on the conductors.

I think the whole thing is dumb but that's what it says.

It would be a lot easier if someone came out with 75C rated LMFC for a wet location. Then it would match the wire rating which it needs to do.

 

[LarryFine]

So if the conduit is LFMC, and the conductor operating temperature is limited to 60C, but the conductor insulation rating is 75C, which ampacity value should you start derating at? The 60C base ampacity, which will give you the usual guarantee that the insulation temperature will stay below 60C? Or the 75C base ampacity, for which the insulation temperature may exceed 60C, the operating temperature limit?

Logically, I'd have to say the first answer; that which "assures" not exceeding 60C..

And if you want to derate starting at the 75C base ampacity, because your ampacity is too low if you start at the 60C base ampacity, what other measures will you take to comply with the 60C operating temperature limit?

A 30-amp breaker. As has been said, I think derating has nothing to do with this.

 

[Eddie702]

I think all they are saying is 75 & 90C conductors are permitted in a 60C raceway, but they cannot be used over 60C in a wet location. There is plenty of LMFC rated higher than 60C just not wet

 

[wwhitney]

Here's a much simpler NEC path to my argument:

(2017) Table 310.15(B)(16) says just above the 60C/75C/90C columns "Temperature Rating of Conductor [See Table 310.104(A)]."

Table 310.104(A) is titled "Conductor Applications and Insulations Rated 600 Volts" and has a column "Maximum Operating Temperature" which has the usual values such as 75C for THHW in wet locations and 90C for THHW in dry locations.

So what we typically call the "temperature rating of conductor" or the "insulation temperature" is the same thing as "operating temperature". If there's an operating temperature limit of 60C, that means you must only ever use the 60C column of Table 310.15(B)(16) and can never use the 75C or 90C columns.

Cheers, Wayne

 

[wwhitney]

A 30-amp breaker. As has been said, I think derating has nothing to do with this.

Not sure if I agree with that, but can you give me an example of an application where 30A OCPD is OK but you are required to have a conductor ampacity in excess of 32A? Since we got to a 32A ampacity in my example just using the 60C base ampacity.

Cheers, Wayne

 

[xguard]

"provided the conductors or cables are not operated at a temperature higher tan the listed temperature rating of the LFMC per 110.14(C)."

I have a chiller with an MCA of 174.1 amps with 3/0 awg XHHW-2.

3/0 - 60 deg C ampacity 165 amps
3/0 - 75 deg C ampacity 200 amps

Based off the MCA I couldn't use 3/0 conductors in LFMC rated for 60 degree C in a wet location.

But if I combine the nameplate current of each component (adding up the compressors and motors of the chiller) I get 161 amps and then this conduit/conductor combination would be okay.

Thoughts?

 

[don_resqcapt19]

"provided the conductors or cables are not operated at a temperature higher tan the listed temperature rating of the LFMC per 110.14(C)."

I have a chiller with an MCA of 174.1 amps with 3/0 awg XHHW-2.

3/0 - 60 deg C ampacity 165 amps
3/0 - 75 deg C ampacity 200 amps

Based off the MCA I couldn't use 3/0 conductors in LFMC rated for 60 degree C in a wet location.

But if I combine the nameplate current of each component (adding up the compressors and motors of the chiller) I get 161 amps and then this conduit/conductor combination would be okay.

Thoughts?

The MCA is a fitcious current required by the rules of the product standard and the NEC and is not the operating current. It is my opinion that you can use the total running current without the 25% adder that is part of the MCA for this as that is the operating current.

With a total running current of 161 and an MCA of 174.1, it does not appear that the MCA was calculated in accordacne with the rules in the product standard. Those rules require 125% of the compressor current plus the sum of the full load currents of the other motors.

 

[wwhitney]

The MCA is a fictitious current required by the rules of the product standard and the NEC and is not the operating current. It is my opinion that you can use the total running current without the 25% adder that is part of the MCA for this as that is the operating current.

How would you reconcile that with the logic of post #32?

350.10(4) imposes a maximum operating temperature on the conductors of 60C for wet location LFMC so listed. Maximum operating temperature is precisely the meaning of the column headings on (2020) Table 310.16, so 350.10(4) limits the conductors to using the 60C column when determining the ampacity. 165 < 174.1, a violation.

Cheers, Wayne

 

[xguard]

The MCA is a fitcious current required by the rules of the product standard and the NEC and is not the operating current. It is my opinion that you can use the total running current without the 25% adder that is part of the MCA for this as that is the operating current.

With a total running current of 161 and an MCA of 174.1, it does not appear that the MCA was calculated in accordacne with the rules in the product standard. Those rules require 125% of the compressor current plus the sum of the full load currents of the other motors.

There are six compressors and six fans. They don't all have the same current rating. Looks like they multiplied one compressor by 1.25. (Even still it doesn't exactly work out right.)

 

[xguard]

How would you reconcile that with the logic of post #32?

350.10(4) imposes a maximum operating temperature on the conductors of 60C for wet location LFMC so listed. Maximum operating temperature is precisely the meaning of the column headings on (2020) Table 310.16, so 350.10(4) limits the conductors to using the 60C column when determining the ampacity. 165 < 174.1, a violation.

Cheers, Wayne

161 < 165.

 

[don_resqcapt19]

How would you reconcile that with the logic of post #32?

350.10(4) imposes a maximum operating temperature on the conductors of 60C for wet location LFMC so listed. Maximum operating temperature is precisely the meaning of the column headings on (2020) Table 310.16, so 350.10(4) limits the conductors to using the 60C column when determining the ampacity. 165 < 174.1, a violation.

Cheers, Waynec

The operating current is 161. The 60°C ampacity of the conductor is 165. The conductor will be operating at less than 60°C.

 

[wwhitney]

The operating current is 161. The 60°C ampacity of the conductor is 165. The conductor will be operating at less than 60°C.

May the 3/0 Cu in the LFMC be operated continuously at 171.4A, or would that violate 350.10(4)?

If the conductor is disallowed from continuously carrying 171.4A, then its ampacity is below 171.4A. Meaning its ampacity is below MCA, a violation.

Cheers, Wayne