# Thread: Question on ampacity

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## Question on ampacity

I have SE 14.4k inverter 208V three phase output. On the DC inverter side I have EMT condit with 6 #10 Thwn-2 + #8 Gnd from inverter which is inside the building to outside to roof. On the roof under solar panels I have junction box which transtion cable to 6 #10 PV wire + #8 Gnd which go to solar panel 3 strings without conduit.

Now when I do my calculation for cable sizing Thwn-2 copper is 90 degree C only column ampacity table ampacity of 40A.

My more than 3 current carrying derating is 0.8

My roof temp correction factor is 0.76

My optimizer worst case Isc is 18A.

40x0.76x0.8 = 24

18x1.25 = 22

24>22.

My questions are following is in regarding 110.14(C) temprature rating:

1. Do my terminals, device have to be rated 90C?

2. If terminals or device are 75C then 40A would not be the ampacity and 75C would apply? If yes then Thwn-2 is not in 75C. What would be new amapcitty be?

3. Is their such a thing inverter rated 90C or inverter terminal rated 90C?

4. How does 110.14(C) apply to inverter terminals or inverter temp rating?

I appreciate your input. Not trolling but trying to clear things up please if possible forgive my ignorance.

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Originally Posted by hhsting
I have SE 14.4k inverter 208V three phase output. On the DC inverter side I have EMT conduit with 6 #10 Thwn-2 + #8 Gnd from inverter which is inside the building to outside to roof. On the roof under solar panels I have junction box which transtion cable to 6 #10 PV wire + #8 Gnd which go to solar panel 3 strings without conduit.

Now when I do my calculation for cable sizing Thwn-2 copper is 90 degree C only column ampacity table ampacity of 40A.

My more than 3 current carrying derating is 0.8

My roof temp correction factor is 0.76

My optimizer worst case Isc is 18A.

40x0.76x0.8 = 24 (ampacity after derating)

18x1.25 = 22 (what is needed)

24>22. So #10 awg THWN-2 is good given 40A at 90C insulation.

My questions are following is in regarding 110.14(C) temprature rating:

1. Do my terminals, device have to be rated 90C in above example for #10 THWN-2 to be good?

2. If terminals or device are 75C then 40A would not be the ampacity and 75C would apply? If yes then Thwn-2 is not in 75C. What would be new amapcity be?

3. Is their such a thing inverter rated 90C or inverter terminal rated 90C or are inverter DC rated for 90C?

4. How does 110.14(C) apply to inverter terminals or inverter temp rating?

I appreciate your input. Not trolling but trying to clear things up please if possible forgive my ignorance.

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No one knows or have any input?

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Originally Posted by hhsting
No one knows or have any input?

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Are you asking about DC or AC circuits? Are your inverter output circuits in rooftop conduit? What is the nameplate current rating of your inverter? What NEC code cycle is the AHJ on?

In general, you derate the 90 degree ampacity of your conductors for conditions of use (temperature and conduit fill) and the 75 degree ampacity for continuous use (X 0.8), and both results must be greater than Imax, which is 125% of Isc in "normal" DC strings, 15A with SolarEdge optimizers, or the nameplate AC rating of your inverter(s). The rooftop adder to the ambient temperature is mostly gone in the 2017 NEC.

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Originally Posted by ggunn
Are you asking about DC or AC circuits? Are your inverter output circuits in rooftop conduit? What is the nameplate current rating of your inverter? What NEC code cycle is the AHJ on?

In general, you derate the 90 degree ampacity of your conductors for conditions of use (temperature and conduit fill) and the 75 degree ampacity for continuous use (X 0.8), and both results must be greater than Imax, which is 125% of Isc in "normal" DC strings, 15A with SolarEdge optimizers, or the nameplate AC rating of your inverter(s). The rooftop adder to the ambient temperature is mostly gone in the 2017 NEC.
I am asking in regards to 110.14(C) DC side. I am using THWN-2 #10 awg conductor up to juntion box on roof right at the solar panels. THWN-2 is found only in 90C column so amapcity #10 AWG is 40A Table 310.15(B)(16).

I am asking do the terminals, devices have to be rated 90C or can they be 75C or 60C for 40A ampacity of THWN-2?

However, I just maybe found the answer NEC 2014 Article 110.14(C) Temprature Limitations:

"...Conductors with temprature rating higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both...."

No?

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Originally Posted by hhsting
I am asking in regards to 110.14(C) DC side. I am using THWN-2 #10 awg conductor up to juntion box on roof right at the solar panels. THWN-2 is found only in 90C column so amapcity #10 AWG is 40A Table 310.15(B)(16).

I am asking do the terminals, devices have to be rated 90C or can they be 75C or 60C for 40A ampacity of THWN-2?

However, I just maybe found the answer NEC 2014 Article 110.14(C) Temprature Limitations:

"...Conductors with temprature rating higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both...."

No?

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90C rated wire does not necessarily mean you need 90C terminals/equipment. It just means that 60C or 75C rated terminals become the "weak link" in the terminal-wire-terminal assembly. Most equipment in practice will be rated for 75C. This is the default for equipment over 100A, and while it is most common for equipment 100A and less, you do have a burden of proof in that case to use the 75C values. Because 100A and less, the default is 60C rated.

Generally speaking, 90C rated equipment is rare. Even if terminals are marked AL9CU, they have to be installed either in a separate enclosure from the equipment, or in equipment that is rated 90C as an assembled product.

The most common reason you get to take credit for the 90C rating, is for your derate calculations that go with bundling adjustments and ambient temperature corrections. These factors do not apply to terminations, they apply to the wire as a whole. So your ampacity is ultimately limited by the raw value of ampacity of the terminations, but you can take credit for the wire's 90C rating as your starting point for these calculations.
Last edited by Carultch; 06-25-19 at 09:51 AM.

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The terminal temperature rating usually relates to conductors terminated on an OCPD or an insulated terminal. Often if you land on a busbar with an uninsulated "9" rated lug you can use the full 90C rating of 90C conductor. For instance I've seen DC combiners where the fuse terminal are 75C rated and the output terminals that land on the busbar are 90C rated.

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Originally Posted by Carultch
90C rated wire does not necessarily mean you need 90C terminals/equipment. It just means that 60C or 75C rated terminals become the "weak link" in the terminal-wire-terminal assembly. Most equipment in practice will be rated for 75C. This is the default for equipment over 100A, and while it is most common for equipment 100A and less, you do have a burden of proof in that case to use the 75C values. Because 100A and less, the default is 60C rated.

Generally speaking, 90C rated equipment is rare. Even if terminals are marked AL9CU, they have to be installed either in a separate enclosure from the equipment, or in equipment that is rated 90C as an assembled product.

The most common reason you get to take credit for the 90C rating, is for your derate calculations that go with bundling adjustments and ambient temperature corrections. These factors do not apply to terminations, they apply to the wire as a whole. So your ampacity is ultimately limited by the raw value of ampacity of the terminations, but you can take credit for the wire's 90C rating as your starting point for these calculations.
Ok here is my question:
18A is Isc of the optimizer.
If i do 18x1.25x1.25= 28A then i need 28A.

Now without any temperature correction or any derating since my circuit is less than 100A I would take 60C but 60C does not have THWN-2 column so what would be the size if THWN-2 is provided?

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Originally Posted by hhsting
I am asking in regards to 110.14(C) DC side. I am using THWN-2 #10 awg conductor up to juntion box on roof right at the solar panels. THWN-2 is found only in 90C column so amapcity #10 AWG is 40A Table 310.15(B)(16).

I am asking do the terminals, devices have to be rated 90C or can they be 75C or 60C for 40A ampacity of THWN-2?
Virtually all terminations are rated at 75 degrees C. In most of Texas under the 2014 NEC, ambient plus rooftop adder (>3.5 inches off the roof) is 0.71. Conduit fill derate for 4-6 current carrying conductors is 0.8. #10 THHN-2 has a 40A ampacity @90 degrees and 75 degree #10 is rated at 35A @75 degrees. You must consider both the 90 degree number for conditions of use (heating of the insulation) and the 75 degree number for continuous use (heating of the terminals).

For 3 DC circuits (6 CCC's) in a single conduit >3.5 and <12 inches off the roof:

Conditions of use: (40A)(0.71)(0.8) = 22.7A
Continuous use: (35A)(0.8) = 28.0A

You must "beat" both those numbers. To account for excess insolation, your string Isc must be less than (0.8)(22.7A) = 18.16A

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Originally Posted by ggunn
Virtually all terminations are rated at 75 degrees C. In most of Texas under the 2014 NEC, ambient plus rooftop adder (>3.5 inches off the roof) is 0.71. Conduit fill derate for 4-6 current carrying conductors is 0.8. #10 THHN-2 has a 40A ampacity @90 degrees and 75 degree #10 is rated at 35A @75 degrees. You must consider both the 90 degree number for conditions of use (heating of the insulation) and the 75 degree number for continuous use (heating of the terminals).

For 3 DC circuits (6 CCC's) in a single conduit >3.5 and <12 inches off the roof:

Conditions of use: (40A)(0.71)(0.8) = 22.7A
Continuous use: (35A)(0.8) = 28.0A

You must "beat" both those numbers. To account for excess insolation, your string Isc must be less than (0.8)(22.7A) = 18.16A
Where do you see THWN-2 @ 75C in ampacity table?

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Originally Posted by hhsting
Where do you see THWN-2 @ 75C in ampacity table?

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You don't. You find it in the 90 degree column and look to the left. The temperature rating of a conductor is actually the rating of its insulation (copper is copper), but you don't want to overheat your 75 degree rated terminals, either.

The number in the 75 degree column tells you the maximum current in a conductor to keep the temperature below 75 degrees, but it has nothing to do with the rating of the insulation. A #10 copper wire with 35A in it gets it to 75 degrees irrespective of the rating of the insulation.

The COU derating of the conductor @ 90 degrees keeps the wire below 90 degrees to protect the insulation. The CU @ 75 degrees derating of the conductor keeps it below 75 degrees to protect the terminals. You run both calcs and the lesser of the two is your "real" ampacity.
Last edited by ggunn; 06-25-19 at 07:56 PM.

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