08 NEC 690.8(B)(2)

[inspector23]

2008 NEC 690.8(B)(2)

I am hoping for some insight for this section. For whatever reason, this section makes absolutely no sense to me. I am not sure if it is a senior moment, a blind spot in my code vision, or the result of a Death Star collapsing in another Galaxy, but I cannot fathom:
the intent;
the meaning;
or for that matter, I cannot comprehend what the 125% has to do with anything, because I do not understand anything about this section.

I cannot even tell you why it is located in this section!

If the modules have the ability to internally limit the current are permitted to be rated less than the 125% addition required by 690.8(B)(1), than how can the reduced rating shall be at least 125% of the limited current value.

I have tried ROP?s and ROC?s, I am a member of NEC Plus and I tried there with no luck, and the 2008 NEC handbook did not enlighten me. Before I am committed to a rubber room, I would appreciate any/all help with explaining this code requirement.

I hope I said that well. Thanks in advance for your help!

Mike

 

[WizBandit]

It's simple...

It's simple...

Say I have a PV array that can put out 100amps under certain conditions, I have this fed into a Breaker (OCPD) that feeds an 80 AMP charge controller. The charge controller can never put out more than it's name plate rating no matter how many amps are available at it's input. You need only a 80 x 1.25 = 100 amp breaker on its input and not 100 x 1.25 = 125 amp breaker. Does this make sense? Now the wiring up to the 80 amp breaker needs a 156% adjustment, 125% for 3 hours plus and another 125% for the PV characteristically making more power under certain conditions. 125% x 125% = 156%. As with any interpretation on the NEC this is mine.

 

[WizBandit]

I think you misquoted the NEC...

I think you misquoted the NEC...

Say I have a PV array that can put out 100amps under certain conditions, I have this fed into a Breaker (OCPD) that feeds an 80 AMP charge controller. The charge controller can never put out more than it's name plate rating no matter how many amps are available at it's input. You need only a 80 x 1.25 = 100 amp breaker on its input and not 100 x 1.25 = 125 amp breaker. Does this make sense? Now the wiring up to the 80 amp breaker needs a 156% adjustment, 125% for 3 hours plus and another 125% for the PV characteristically making more power under certain conditions. 125% x 125% = 156%. As with any interpretation on the NEC this is mine.

As in the Exception below one could use a 80A breaker if it were rated at 100% or derate a standard breaker and use it @ 80%

(2) Internal Current Limitation. Overcurrent protection
for photovoltaic output circuits with devices that internally
limit the current from the photovoltaic output circuit shall
be permitted to be rated at less than the value calculated in
690.8(B)(1). This reduced rating shall be at least 125 percent
of the limited current value. Photovoltaic output circuit
conductors shall be sized in accordance with 690.8(B)(1).


Exception: An overcurrent device in an assembly listed for
continuous operation at 100 percent of its rating shall be
permitted to be utilized at 100 percent of its rating.

 

[Marvin_Hamon]

It has a really limited application so that is probably why you are having trouble with it. The example WizBandit gave is great. As you can see, applying this section requires one input to the current limited device and needing an OCPD on that input. I have not found an actual installation where a single circuit would not be run directly from the array to the equipment. There would have to be the possibility of a backfeed current from the device that was greater than the conductor was rated for and I have not run into this. Maybe at one time there was less isolation between the input and output.

If you have two inputs it does not help you. For instance using WizBandit's example with two 50A inputs, each with an OCPD. Each OCPD would have to be sized at 1.25*50 because the battery charger could draw the whole 50A from one input or the other. So the 80A limit makes no difference. If you were really oversizing the array and had two 100A inputs to an 80A device then it would help but there is probably a larger design problem there.