Fun with Ampacity

[ggunn]

Every once in a while I have a disagreement with a plan reviewer about PV ampacities where they want to include continuous usage derate in the conditions of use calculation. It's been a long time since I have had to do it, so here is what my response will be for (18) #10 90 degree conductors in a single conduit to a SolarEdge inverter. Note that SolarEdge optimizers are current limited to 18.0A, so the 1.25 factor for excess insolation does not apply. Also note that I realize that the technically correct calculation for continuous use multiplies Imax by 1.25 rather than the ampacity by 0.8, but both get you to the same result and this way is easier for most people to understand.

Conditions of use derate calculation, which ensures that the 90 degree conductor insulation is not compromised:
(90 degree ampacity)(derate for ambient conditions)(derate for conduit fill) = Ampacity 1

Continuous use derate calculation, which ensures that the 75 degree rated terminals are not compromised:
(75 degree ampacity)(derate for continuous use) = Ampacity 2

For a 90 degree conductor, both Ampacity 1 and Ampacity 2 must be greater than the maximum current in the conductor (Imax).

In this case:
Imax = 18.0A
90 degree ampacity of #10 = 40A
75 degree ampacity of #10 = 35A
Derate for ambient conditions = 0.91
Derate for conduit fill = 0.5
Derate for continuous use= 0.8

Conditions of use derate calculation:
(40A)(0.91)(0.5) = Ampacity 1 =18.2A

Continuous use derate calculation:
(35A)(0.8) = Ampacity 2 =28.0A

Ampacity 1 and Ampacity 2 are both greater than 18.0A.

 

[ggunn]

Every once in a while I have a disagreement with a plan reviewer about PV ampacities where they want to include continuous usage derate in the conditions of use calculation. It's been a long time since I have had to do it, so here is what my response will be for (18) #10 90 degree conductors in a single conduit to a SolarEdge inverter. Note that SolarEdge optimizers are current limited to 18.0A, so the 1.25 factor for excess insolation does not apply. Also note that I realize that the technically correct calculation for continuous use multiplies Imax by 1.25 rather than the ampacity by 0.8, but both get you to the same result and this way is easier for most people to understand.

Conditions of use derate calculation, which ensures that the 90 degree conductor insulation is not compromised:
(90 degree ampacity)(derate for ambient conditions)(derate for conduit fill) = Ampacity 1

Continuous use derate calculation, which ensures that the 75 degree rated terminals are not compromised:
(75 degree ampacity)(derate for continuous use) = Ampacity 2

For a 90 degree conductor, both Ampacity 1 and Ampacity 2 must be greater than the maximum current in the conductor (Imax).

In this case:
Imax = 18.0A
90 degree ampacity of #10 = 40A
75 degree ampacity of #10 = 35A
Derate for ambient conditions = 0.91
Derate for conduit fill = 0.5
Derate for continuous use= 0.8

Conditions of use derate calculation:
(40A)(0.91)(0.5) = Ampacity 1 =18.2A

Continuous use derate calculation:
(35A)(0.8) = Ampacity 2 =28.0A

Ampacity 1 and Ampacity 2 are both greater than 18.0A.

Anyone see any problem with that?

 

[wwhitney]

Not with the math. On the presentation you might want to reference the relevant NEC sections. Also I'm not sure your terminology exactly matches the NEC wording, but that may not matter.

If you want a further argument for your position, you can point out that the definition of "ampacity" means it's inherently a continuous rating, and so for the wire itself, there is no need for any "derate for continuous use." It's only the equipment at the terminations that may require it (and in particular, the OCPD).

Cheers, Wayne

 

[ggunn]

Not with the math. On the presentation you might want to reference the relevant NEC sections. Also I'm not sure your terminology exactly matches the NEC wording, but that may not matter.

If you want a further argument for your position, you can point out that the definition of "ampacity" means it's inherently a continuous rating, and so for the wire itself, there is no need for any "derate for continuous use." It's only the equipment at the terminations that may require it (and in particular, the OCPD).

Cheers, Wayne

Thanks for the backup. It all comes down to the inspector saying we can't load a conduit with (18) #10 90 degree conductors carrying 18A each. It's not the end of the world if I lose the fight, but he's wrong. In his response he is throwing 110.14(C)(1) at me. The joys of dealing with inspectors...

 

[wwhitney]

BTW, you're in luck that (2017) 690.8(B) is more well written than 215.2(A)(1), for example.

690.8(B)(1) is titled "Before Application of Adjustment and Correction Factors" and includes the 125% continuous use factor. While 690.8(B)(2) is titled "After Application of Adjustment and Correction Factors" and does not include any 125% factor.

So that should make it clear that you don't need to simultaneously apply adjustment, correction, and continuous use factors in any calculation. Either the first two or the last one, separately. 110.14(C) then tells you need to use the termination temperature limit when not using adjustment and correction factors; but that when using adjustment and correction factors, you can use the insulation temperature limit.

Cheers, Wayne

 

[pv_n00b]

For completeness, you do three calculations:

1. Conditions of use derating @ the conductor's rated temperature, no continuous current derating
2. Continuous current derating @ the conductor's rated temperature, no conditions of use derating
3. If required, continuous current derate of the conductor using the conductor's rated temperature at the terminal temperature if less than the conductor's rated temperature

In PV the voltage drop usually requires conductors larger than the minimum current rated conductor anyway.

 

[wwhitney]

For completeness, you do three calculations:

Seems like (2) and (3) can be easily combined into one check using the minimum of the termination temperature limit and the conductor temperature limit. Which is basically always the termination temperature, yes?

Cheers, Wayne

 

[pv_n00b]

Seems like (2) and (3) can be easily combined into one check using the minimum of the termination temperature limit and the conductor temperature limit. Which is basically always the termination temperature, yes?

Cheers, Wayne

If you have to do terminal temperature correction then the two collapse together. I listed all 3 for completeness and since there are situations where there may not be a temperature-limited terminal on both ends.
In PV applications we are typically using 90C conductor and 75C terminals so the terminal temperature check will always reduce the ampacity of the conductor, but in my experience, it is rarely the dominant reduction. Here's my ranking for things that drive up the conductor size:

1. Voltage drop
2. Conditions of use
3. Continuous current derating @ the conductor's rated temperature or terminal temperature derating.

 

[wwhitney]

If you have to do terminal temperature correction then the two collapse together

That phrase make it sound like doing temperature correction at the terminal ampacity check, based on the ambient temperature at the terminals. [Which correction I understand is generally not considered to be required, but the last time I checked I though the relevant code sections were a bit ambiguous and could be read to require it.]

But I don't think that's what you mean. Regardless, (2) and (3) can be combined by just using min(termination temperature, insulation temperature).

Cheers, Wayne

 

[pv_n00b]

That phrase make it sound like doing temperature correction at the terminal ampacity check, based on the ambient temperature at the terminals. [Which correction I understand is generally not considered to be required, but the last time I checked I though the relevant code sections were a bit ambiguous and could be read to require it.]

But I don't think that's what you mean. Regardless, (2) and (3) can be combined by just using min(termination temperature, insulation temperature).

Cheers, Wayne

Of the two the terminal temperature correction will usually reduce the ampacity of 90C rated conductor more, so if you don't have to do it then take the higher ampacity.
I don't follow what you are trying to describe so I can't say if that's what I meant or not. The terminal temperature correction is only required if the terminal landed on has a temperature rating lower than the rating of the conductor. If the terminal is not on an OCPD it's probably not limited to 75C, and a 90C rated terminal and 90C rated conductor requires no correction for terminal temperature.

 

[ggunn]

Of the two the terminal temperature correction will usually reduce the ampacity of 90C rated conductor more, so if you don't have to do it then take the higher ampacity.
I don't follow what you are trying to describe so I can't say if that's what I meant or not. The terminal temperature correction is only required if the terminal landed on has a temperature rating lower than the rating of the conductor. If the terminal is not on an OCPD it's probably not limited to 75C, and a 90C rated terminal and 90C rated conductor requires no correction for terminal temperature.

True, but most of the terminals I encounter are 75 degree rated.

 

[JEFF MILLAR]

Assume terminals rated 75 C. Use the 75 C rated conductor amps.
Conduit fill, can not be more than 40 %.
Conductors installed must be derated according to the number of current carrying conductors installed in the same conduit.
For Conductor derating, use the 90 C conductor amp rating. Thats what i understand.

 

[pv_n00b]

True, but most of the terminals I encounter are 75 degree rated.

That's typical. Usually, one end or the other of a conductor is going to land on an OCPD terminal where the temperature is limited. But I have had instances where I have a conductor going from bus to bus with no terminal temperature limit. So I just wanted to point out that the extra step is there even if it does not usually drive the conductor size.

 

[electrofelon]

That's typical. Usually, one end or the other of a conductor is going to land on an OCPD terminal where the temperature is limited. But I have had instances where I have a conductor going from bus to bus with no terminal temperature limit. So I just wanted to point out that the extra step is there even if it does not usually drive the conductor size.

My understanding is that pretty much all "equipment" has terminals that are rated 75°. Not sure about something like a bussed gutter, never thought to check. For example an mlo panel board or NF safety switch would still be limited to 75 degree terminals even if the lugs say 90 on them (which they probably do). Can you point to a piece of equipment that is not limited to 75°?

 

[wwhitney]

For example an mlo panel board or NF safety switch would still be limited to 75 degree terminals even if the lugs say 90 on them (which they probably do).

Are you saying that's always the case? Is there some listing or technical reason that a panelboard couldn't be made with 90C terminations at the MLO lugs or at feedthru lugs?

Cheers, Wayne

 

[electrofelon]

Are you saying that's always the case? Is there some listing or technical reason that a panelboard couldn't be made with 90C terminations at the MLO lugs or at feedthru lugs?

Cheers, Wayne

I believe it is always the case, but I am all ears if you can find some equipment that is okay with 90°. It must be something in the product standard.

 

[ggunn]

I believe it is always the case, but I am all ears if you can find some equipment that is okay with 90°. It must be something in the product standard.

FWIW, I always assume 75 degree terminals when I am designing a PV system.

 

[pv_n00b]

My understanding is that pretty much all "equipment" has terminals that are rated 75°. Not sure about something like a bussed gutter, never thought to check. For example an mlo panel board or NF safety switch would still be limited to 75 degree terminals even if the lugs say 90 on them (which they probably do). Can you point to a piece of equipment that is not limited to 75°?

As an example, most DC combiner inputs where the string conductors land on touch-safe fuse holders are 75C terminals. The output bus, on the other hand, has no fuses attached and does not require terminal temp derating, so if you use 90C conductor then the bus terminal is 90C. I've had some interesting arguments with combiner manufacturers over the years over the manufacturers writing on the datasheet that the combiner is 90C rated but that really only applies to the output bus. This leads people to make the incorrect assumption that the string terminals are 90C rated too. See attached datasheet where it says "75/90C output terminals" for the correct way to show it.

 

[wwhitney]

The output bus, on the other hand, has no fuses attached and does not require terminal temp derating, so if you use 90C conductor then the bus terminal is 90C.

I have not looked into the product standards. But if the above is true, why couldn't it similarly be true in a regular panelboard, for MLO lugs or feed-thru lugs? (Come to think of it, doesn't the product above meet the definition of panelboard?)

Cheers ,Wayne

 

[electrofelon]

As an example, most DC combiner inputs where the string conductors land on touch-safe fuse holders are 75C terminals. The output bus, on the other hand, has no fuses attached and does not require terminal temp derating, so if you use 90C conductor then the bus terminal is 90C. I've had some interesting arguments with combiner manufacturers over the years over the manufacturers writing on the datasheet that the combiner is 90C rated but that really only applies to the output bus. This leads people to make the incorrect assumption that the string terminals are 90C rated too. See attached datasheet where it says "75/90C output terminals" for the correct way to show it.

I am skeptical you can use those lugs at 90. I suspect the writer of the sales literature doesn't really know much. In not sure what product standard applies to that.