2014 NEC Change - OCPD protecting derated ampacity

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Andrew445

Inactive, Email Never Verified
It looks the 2014 code dropped section 690.8(B)(2)(c). In my opinion, this section is still important and I will continue to consider this when sizing DC conductors. The derated ampacity of the conductor must be protected by the required minimum OCPD size. What this means for example is that if you are required to use a 250A fuse, and the derated ampacity brings you down to 220A, you must upsize the conductor until the derated ampacity is greater than 225A.

Does anyone know if this section has moved or what the thinking is here?

Thank you,
 

SolarPro

Senior Member
Location
Austin, TX
The intent does not appear to have changed. Things were just moved around a revised for clarity. The language RE: adjustments and correction factors was added to 690.8(B)(1) and (2). And any content related to OCPDs was moved to 690.9.

If you want to scour the committee comments, you can look up the "Committee Report on Proposals" and the "Committee Report on Comments" related to the 2014 cycle of revisions.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
It looks the 2014 code dropped section 690.8(B)(2)(c). In my opinion, this section is still important and I will continue to consider this when sizing DC conductors. The derated ampacity of the conductor must be protected by the required minimum OCPD size. What this means for example is that if you are required to use a 250A fuse, and the derated ampacity brings you down to 220A, you must upsize the conductor until the derated ampacity is greater than 225A.

If you use 90 degree wire but calculate the ampacity for wire sizing as if it were 75 degree wire because of the terminal ratings, I don't believe that can happen. When you decide if the wire is being protected by the OCPD, you derate the 90 degree ampacity because it's the 90 degree insulation you are concerned about.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
It looks the 2014 code dropped section 690.8(B)(2)(c). In my opinion, this section is still important and I will continue to consider this when sizing DC conductors. The derated ampacity of the conductor must be protected by the required minimum OCPD size. What this means for example is that if you are required to use a 250A fuse, and the derated ampacity brings you down to 220A, you must upsize the conductor until the derated ampacity is greater than 225A.

Does anyone know if this section has moved or what the thinking is here?

Thank you,

I think that the reason it was pulled or revised is that the language "...shall be protected by the OCPD..." was sometimes being interpreted as requiring that the derated ampacity must be equal to or greater than the rating of the OCPD instead of being equal to or greater than the rating of the next OCPD down (if the current is less than 800A).
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
It looks the 2014 code dropped section 690.8(B)(2)(c). In my opinion, this section is still important and I will continue to consider this when sizing DC conductors. The derated ampacity of the conductor must be protected by the required minimum OCPD size. What this means for example is that if you are required to use a 250A fuse, and the derated ampacity brings you down to 220A, you must upsize the conductor until the derated ampacity is greater than 225A.

Just out of curiosity, can you provide an example of this happening? I worked a few test cases and couldn't make it happen.
 

Andrew445

Inactive, Email Never Verified
Just out of curiosity, can you provide an example of this happening? I worked a few test cases and couldn't make it happen.

No problem, it's actually pretty common for me. And everything on the DC side has 90?C terminals so that's not a concern. This calc. is for the PV output circuits.

A 1MW rooftop system has modules with Isc = 9.28A. 18 strings feed into each combiner. The fuse size is 1.25*1.25*9.28*18 = 261 --> 300A.

In this climate, the derate for conduit supported 12" above the roof is 0.76.

690.8(B)(1) ----- 261A
690.8(B)(2) ----- 1.25*9.28*18/0.76 = 274.7A So conductor must be sized for this current.

Aluminum cables places this at 350 kcmil. The derated ampacity of this conductor is 280*0.76 = 212.8A. This is not protected by a 300A fuse. You need to bump up to 500 kcmil so you get 350*0.76 = 266A and you are good.

I can see how the original language might be confusing. My only thinking is that they believe the first sentence of 690.9(A) covers this base. If I were coming into solar design starting with the 2014 NEC then I might not think to check this at first.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
No problem, it's actually pretty common for me. And everything on the DC side has 90?C terminals so that's not a concern. This calc. is for the PV output circuits.

A 1MW rooftop system has modules with Isc = 9.28A. 18 strings feed into each combiner. The fuse size is 1.25*1.25*9.28*18 = 261 --> 300A.

In this climate, the derate for conduit supported 12" above the roof is 0.76.

690.8(B)(1) ----- 261A
690.8(B)(2) ----- 1.25*9.28*18/0.76 = 274.7A So conductor must be sized for this current.

Aluminum cables places this at 350 kcmil. The derated ampacity of this conductor is 280*0.76 = 212.8A. This is not protected by a 300A fuse. You need to bump up to 500 kcmil so you get 350*0.76 = 266A and you are good.

I can see how the original language might be confusing. My only thinking is that they believe the first sentence of 690.9(A) covers this base. If I were coming into solar design starting with the 2014 NEC then I might not think to check this at first.

Thank you.
 

Zee

Senior Member
Location
CA
No problem, it's actually pretty common for me. And everything on the DC side has 90?C terminals so that's not a concern. This calc. is for the PV output circuits.

A 1MW rooftop system has modules with Isc = 9.28A. 18 strings feed into each combiner. The fuse size is 1.25*1.25*9.28*18 = 261 --> 300A.

In this climate, the derate for conduit supported 12" above the roof is 0.76.

690.8(B)(1) ----- 261A
690.8(B)(2) ----- 1.25*9.28*18/0.76 = 274.7A So conductor must be sized for this current.

Aluminum cables places this at 350 kcmil. The derated ampacity of this conductor is 280*0.76 = 212.8A. This is not protected by a 300A fuse. You need to bump up to 500 kcmil so you get 350*0.76 = 266A and you are good.

I can see how the original language might be confusing. My only thinking is that they believe the first sentence of 690.9(A) covers this base. If I were coming into solar design starting with the 2014 NEC then I might not think to check this at first.

That looks good.

It may be just late in the evening here....or am i seeing that you applied the derate factor of 0.76 to increase the amperage of the Isc (by dividing), and also to downsize the ampacity of a given wire size (by multiplying) ?
Either method seems fine, but both?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
That looks good.

It may be just late in the evening here....or am i seeing that you applied the derate factor of 0.76 to increase the amperage of the Isc (by dividing), and also to downsize the ampacity of a given wire size (by multiplying) ?
Either method seems fine, but both?
That's what I thought upon first reading, but I don't believe he was double derating anything.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
That looks good.

It may be just late in the evening here....or am i seeing that you applied the derate factor of 0.76 to increase the amperage of the Isc (by dividing), and also to downsize the ampacity of a given wire size (by multiplying) ?
Either method seems fine, but both?

If you know the current and need to know the required (non-derated) ampacity of the wire, then you divide.

If you know the non-derated ampacity of the wire and need to know the derated ampacity, then you multiply.

He is doing the first one to choose the wire, and the second one to choose the fuse; two different tasks. Since the actual (non-derated) ampacity of the wire chosen is greater than what is required, they aren't the same exact numbers.
 

Smart $

Esteemed Member
Location
Ohio
It looks the 2014 code dropped section 690.8(B)(2)(c).

...

Does anyone know if this section has moved or what the thinking is here?
Still covered. Article 690 doesn't establish any requirement which takes precedence over 240.4. Possibly removed because it was a redundant requirement.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
In a related story...

A fellow engineer pointed out to me this morning the new in 2014 exception noted in 310.15(B)(3)(c) that exempts XHHW-2 (cross linked polyethylene) wire from the rooftop adder for temperature ampacity derating.
 

Smart $

Esteemed Member
Location
Ohio
In a related story...

A fellow engineer pointed out to me this morning the new in 2014 exception noted in 310.15(B)(3)(c) that exempts XHHW-2 (cross linked polyethylene) wire from the rooftop adder for temperature ampacity derating.
Yep... it almost seems like a hidden-in-plain-sight NEC secret. ;)
 
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