Charge controlller "80% rule" where is it?

[electrofelon]

I keep seeing this or something similar:

To comply with the National Electric Code (NEC), the current rating of the controller for solar charging must be equal or greater than 125% of the solar ISc output

Finally got to looking cant figure out where they are getting this. Is jthis just an assumption/extrapolation from the 690.8 125%?

 

[Smart $]

690.4(B) requires equipment to be listed for the application. Part of equipment listing is its input current rating. That rating has to equal or exceed the Code maximum source circuit current... 690.8(A)(1).

 

[electrofelon]

690.4(B) requires equipment to be listed for the application. Part of equipment listing is its input current rating. That rating has to equal or exceed the Code maximum source circuit current... 690.8(A)(1).

Perhaps......I dont like it....

 

[Smart $]

Perhaps......I dont like it....

In the grand scheme of it all, no one really cares...

 

[electrofelon]

In the grand scheme of it all, no one really cares...

Ok I'll elaborate - even though nobody cares Look at 690.7. It specifically states:

".....this voltage shall be used to determine the voltage rating of cables, disconnects, overcurrent devices, and other equipment...."

690.8 does not have such an analogous statement. The title is "circuit sizing and current" which implies to me it is for conductor sizing. There are many examples I can think of where a conductor needs to be sized with some fudge factor but the equipment supplied by it does not. I understand that unlike a continuous load "fudge factor", the fudge factor in 690.8 is to account for extra current that CAN actually happen, but it does not state that anywhere so one could have no idea what the purpose of that 25% is. Finally, even if that is the intent, I dont know of any off grid equipment that will destroy itself from overloads or overtemp. I know, I know..."so make a proposal, I'm not stopping you."

 

[Smart $]

FWIW, I agree with your assessment. :happyyes:

Now, back to...

 

[jaggedben]

To comply with the National Electric Code (NEC), the current rating of the controller for solar charging must be equal or greater than 125% of the solar ISc output

Is this coming from manufacturer's instructions? Or from AHJs? Or from educational materials?

If from manufacturer's instructions then I guess there's nothing you can do. If from elsewhere, you could consult the manufacturer and see if they agree.

FWIW, whenever I see an inverter's maximum input current rating I just take module Isc without adding 125%. (I don't deal with charge controllers much but it's the same principle.) No one has ever questioned me on this. After all, MPPT inverters and charge controllers 'clip' the current. And then there's the various exceptions for "If a device is listed for continuous use at 100% of it's rating..."

Also, FWIW, there's a section on charge controllers (690.72) and it contains no such requirement.

 

[electrofelon]

Is this coming from manufacturer's instructions? Or from AHJs? Or from educational materials? If from manufacturer's instructions then I guess there's nothing you can do. If from elsewhere, you could consult the manufacturer and see if they agree.

That specific quote is from a morningstar tristar ts-60 manual. The wording before and after that quote implies that they are not saying you "have to" from their stand point, so I dont think its a 110.3(B) issue. I have also heard that statement on some RE forums and other instructions.

FWIW, whenever I see an inverter's maximum input current rating I just take module Isc without adding 125%. (I don't deal with charge controllers much but it's the same principle.) No one has ever questioned me on this. After all, MPPT inverters and charge controllers 'clip' the current. And then there's the various exceptions for "If a device is listed for continuous use at 100% of it's rating..."

I agree, but note that its not the "continuous load" 125% we are talking about, its the "output exceeding STC" 125%. The tristar does specifically state the the ts60 is rated for 60 amps continuous.

I just thought of an implication of this, or more broadly sizing the CC to the array at all: With the low price of PV now, an increasingly common strategy used by off gridders like my self is to "over PV" the system for more output on cloudy days and potentially (if diversion/opportunity loads are not utilized) letting the CC "throw away" the extra on sunny days. Sizing the CC to the array could increase costs and complicate things for someone utilizing this strategy and following code....

 

[zozomike]

Electrofen you can also go to

Electrofen you can also go to

The Northern Arizona Wind Sun Forum for this question. As another completely off grid guy I have had several helpful responses there. The moderators are very experienced with PV. ( and willing to call me on my ignorance)
http://forum.solar-electric.com/

 

[jaggedben]

...I agree, but note that its not the "continuous load" 125% we are talking about, its the "output exceeding STC" 125%. ...

Yes, but to your original point... Where does the code state that an inverter or CC needs to be rated for this number? For that matter, where does it state that an inverter or CC needs to be rated greater than the Isc before this 125% multiplier?

My take is that these types of equipment control the amount of current in the circuit and clip it at the limit they are designed for. Unless the instructions for a CC state that the module Isc cannot exceed a given number, I don't see where the code requires it. I guess I simply disagree with the Morningstar quote. Again, it's not in 690.72, which is where it ought to be if such a requirement existed.

 

[electrofelon]

Yes, but to your original point... Where does the code state that an inverter or CC needs to be rated for this number? For that matter, where does it state that an inverter or CC needs to be rated greater than the Isc before this 125% multiplier?

Right I agree with you. I was/am reluctant to agree with Smart's case.

 

[Smart $]

Right I agree with you. I was/am reluctant to agree with Smart's case.

JB's assessment about "the electronics" is correct to a degree. They CAN limit current under certain conditions... they do not under all conditions. Under certain other conditions, they will allow as much current as the array will put out.

Now combine that with the fact that the Isc rating is determined under STC. It is possible for real world irradiation to exceed that of STC, whereby current can exceed Isc. That is the reason why 690.8(A)(1) factors Isc at 125% and it has nothing to do with continuous loading.

Need I say more...

 

[Smart $]

... Where does the code state that an inverter or CC needs to be rated for this number? For that matter, where does it state that an inverter or CC needs to be rated greater than the Isc before this 125% multiplier? ...

I do agree it is not stated explicitly in 690. I believe it to be one of the electrical trade concepts that go unwritten in Code simply because it is such a basic and universally accepted concept, the Code-writing process has not deemed it necessary.

The concept is: Equipment current ratings must equal or exceed the maximum possible current under nominal conditions.

In most cases, a load or other restrictive component does not factor into the maximum current possible. An OCPD is typically considered as the only current limiting component... and maximum current possible is that of the OCPD rating. However, in the case of PV source circuits, array output current is self limiting. So the sum of paralleled Isc values times 125% is used as the determining basis.

 

[electrofelon]

JB's assessment about "the electronics" is correct to a degree. They CAN limit current under certain conditions... they do not under all conditions. Under certain other conditions, they will allow as much current as the array will put out.

First, I think the NEC has thought very little about off grid charge controllers, but I think the same question can be asked of GTI's. Do you see it as a violation to use a GTI that has an input current rating less than the array?

Every battery charge controller spends much of its time with more current than the batteries need - That IS how battery based charge controllers work. 690.72(B) addresses diversion charge controllers and gives a bunch of requirements for the diversion LOAD but no statement is made about the diversion controller rating relative to the array.

 

[Smart $]

....

The concept is: Equipment* current ratings must equal or exceed the maximum possible current under nominal conditions.

....

*Distribution and control equipment.

 

[Smart $]

First, I think the NEC has thought very little about off grid charge controllers, but I think the same question can be asked of GTI's. Do you see it as a violation to use a GTI that has an input current rating less than the array?

Yes.

Every battery charge controller spends much of its time with more current than the batteries need - That IS how battery based charge controllers work. 690.72(B) addresses diversion charge controllers and gives a bunch of requirements for the diversion LOAD but no statement is made about the diversion controller rating relative to the array.

Output current has no bearing on input current rating and source circuit compliance.

The nature of the equipment is variable, controlled let-through current. With distribution and control equipment, the maximum let-through current is considered to be limited solely by the OCPD.

 

[electrofelon]

I do agree it is not stated explicitly in 690. I believe it to be one of the electrical trade concepts that go unwritten in Code simply because it is such a basic and universally accepted concept, the Code-writing process has not deemed it necessary.

The concept is: Equipment current ratings must equal or exceed the maximum possible current under nominal conditions.

That would seem to be a good requirement and I do think that should be stated clearly in the code, but as you know, it is not. AFAIK, the closest general statement to stating that is ye old 110.3(B).

In most cases, a load or other restrictive component does not factor into the maximum current possible. An OCPD is typically considered as the only current limiting component... and maximum current possible is that of the OCPD rating.

I would partially disagree with that (because you did say "most cases"). Conductors and panelboards are the only things I can think of that must be protected at their rating by an OCPD. Of course many things are allowed to be protected at somewhat over their rating like motors, phase converters and transformers. Switchboards and meter sockets have no requirements to be protected by an OCPD of any size. Then there is the issue of equipment that does not have a specific NEC article: Where is the OCPD requirement for those?

 

[Smart $]

That would seem to be a good requirement and I do think that should be stated clearly in the code, but as you know, it is not. AFAIK, the closest general statement to stating that is ye old 110.3(B).

If you are looking for explicitly stated, you are correct. However, it is inferred throughout Code. Half the problem with Code is it is written for the most part in pseudo-legalese form, as opposed to explicit, direct, concise technical form. I surmise if written in the latter form, Code could be reduced to roughly half its current size and less misunderstood.

I would partially disagree with that (because you did say "most cases"). ...

Quit using my own 'brand' of textual logic against me... :angel:

... Conductors and panelboards are the only things I can think of that must be protected at their rating by an OCPD. Of course many things are allowed to be protected at somewhat over their rating like motors, phase converters and transformers. Switchboards and meter sockets have no requirements to be protected by an OCPD of any size. Then there is the issue of equipment that does not have a specific NEC article: Where is the OCPD requirement for those?

I don't want to get into specific requirements for each and every type of equipment, but here is where some of the ratings are inferred. For example, switchgear is typically used as service equipment. While Article 408 don't require OCPD protection, Article 230 requires it to be rated not less than the ampacity of the service conductors. Minimum meter socket rating is typically specified by the utility provider (yet what of customer-owned meter sockets?). Then we have items such as a 15A receptacle permitted on a 20A-rated circuit.

You are in error regarding motors. Yes, SC/GFP rating is greater than conductor rating, but you have to consider motor circuits have additional protection in the form of overload protection. It is generally limited to 115% of rated current, while conductors must be sized at 125%.

Phase converters and transformers fall into the category of non-load equipment, as do inverters, charge controllers, and other distribution and control equipment.

The easiest way to look at the required rating for anything that is not a source, OCPD, or load... and not specified explicitly... is as if it is a conductor. This equipment must have a current rating not less than the minimum required ampacity of its circuit conductors... but there will always be those items which fall through Code 'cracks'.

 

[ggunn]

Originally Posted by electrofelon
"First, I think the NEC has thought very little about off grid charge controllers, but I think the same question can be asked of GTI's. Do you see it as a violation to use a GTI that has an input current rating less than the array?"

Yes.

Maybe I am misunderstanding you, but GT inverters are overloaded quite frequently where the maximum input current the inverter can take is significantly lower than the STC Isc of the array. As long as the DC conductors are rated at 1.25 X 1.25 X Isc (or 1.25 X Isc derated if that is greater) and the maximum temperature corrected Voc of the array is below the max voltage allowed by the inverter there is no violation; the inverter will take what current it can and clip the rest.

 

[electrofelon]

For example, switchgear is typically used as service equipment. While Article 408 don't require OCPD protection, Article 230 requires it to be rated not less than the ampacity of the service conductors.

can I get a code reference on that?

The easiest way to look at the required rating for anything that is not a source, OCPD, or load... and not specified explicitly... is as if it is a conductor. This equipment must have a current rating not less than the minimum required ampacity of its circuit conductors... but there will always be those items which fall through Code 'cracks'.

yes makes sense, but until the code says that my widget machine needs to be rated at or greater to the circuit rating......(we will keep going around in circles!)

Output current has no bearing on input current rating and source circuit compliance.

What I meant was that if a widget machine or charge controller can control and vary its power transfer than it may not need to be sized to the circuit capacity. Sure, in theory just because a CC can vary its power transfer, does not necessarily mean it is "smart" enough and has the appropriate heat sensors, programming, etc to not destroy itself, but I have never known of such a thing to be that dumb. I concur with ggun that this is done all the time in the PV world. As an example, I worked on a 1 MW system last December and it had 30 24KW inverters which totals .72 MW. I dont know what the specific breakdown of that difference between STC nameplate and inverter capacity is, but its some combination of voltage, current, losses, clipping, inverter able to exceed rating in lower temps, etc....