Charge controlller "80% rule" where is it?

[Smart $]

can I get a code reference on that?

Its actually several sections stitched and condensed: 220PIII/IV and 230.42 to 230.79/80

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!)

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....

It's most likely in listing requirements. I don't have access to the full listing requirements. However, from what I do have access to, there seems to be industry support for the notion as presented. For example...

Conext? MPPT 60 150 Solar Charge Controller (under PV Array Requirements on page 2-2)

You can find a similar caution or warning in the documentation of other such products. Code-wise, it falls under the catch-all 110.3(B).

 

[jaggedben]

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.

I simply don't agree with this.

The charge controller or inverter is the equivalent of a load on the PV modules. There is no 'unwritten concept' that the load has to be capable of using the entire available supply. If there's an unwritten concept (and it's not unwritten in some places), it's the opposite. The wiring and overcurrent device should be rated for at least the load, not less. There are various parts of the code that allow the wiring and overcurrent device to exceed the rating of the load, and there are several parts of 210 which allow multiple load devices with ratings less than the overall current rating of the circuit to be connected to the same circuit.

As for the part about the OCPD, look at 240.3, which refers to 690. 690 says nothing about it, so there you are. There's no requirement here except 110.3(B).

As for 110.3(B), I don't agree that there is any consensus about it. I just looked at the SMA manual for the inverters we most commonly install. It refers ultimately to the 'short circuit current' and contains no admonitions about calculating current according to local codes. The short circuit current is simply what's stated on the modules.

 

[jaggedben]

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?

Not unless the instructions limit what you can do. (110.3(B) again).

For example, the SMA manual I linked to says:

Requirements for the PV Modules:
? The limiting values for the maximum system voltage and the maximum short-circuit current of the
inverter must not be exceeded (see Section 13 "Technical Data", page 82).

It also includes an example with the 7000W inverter:

A maximum of 18 A DC is possible per DC input. The total current may not exceed 30 A DC
though. ... If the maximum input current is exceeded due to the PV array design, the inverter limits the total
current to 30 A DC.

The maximum short circuit current for each input is 19A.

Altogether I take that to mean I can put up to 38A short circuit current rating of modules if divided equally among the inputs. The inverter won't use it, but the instructions 'Requirements' only mention the short circuit current, which is only given per each input. I don't know what Smart$ would say is the 'current rating' of the device in this instance, but I also don't think it matters.

 

[Smart $]

Not unless the instructions limit what you can do. (110.3(B) again).

For example, the SMA manual I linked to says:



It also includes an example with the 7000W inverter:



The maximum short circuit current for each input is 19A.

Altogether I take that to mean I can put up to 38A short circuit current rating of modules if divided equally among the inputs. The inverter won't use it, but the instructions 'Requirements' only mention the short circuit current, which is only given per each input. I don't know what Smart$ would say is the 'current rating' of the device in this instance, but I also don't think it matters.

I agree. 19A is the [short-circuit] input current rating per input. As your quote states, "The limiting values for the maximum system voltage and the maximum short-circuit current of the inverter must not be exceeded (see Section 13 "Technical Data", page 82)."

On page 82...

Maximum input current per MPP tracking input	15 A
Maximum short-circuit current per MPP tracking input	19 A

So how is it that the maximum input current is 15A per input, but yet the maximum short-circuit rating per input is 19A...???

In this case, yes the "electronics" clip the excess over 15A per input... but you still cannot exceed 19A [potential] with your source short-circuit circuit rating... and guess where that rating is established?

 

[jaggedben]

....and guess where that rating is established?

On the module spec sheet.

 

[Smart $]

I simply don't agree with this.

The charge controller or inverter is the equivalent of a load on the PV modules. There is no 'unwritten concept' that the load has to be capable of using the entire available supply. If there's an unwritten concept (and it's not unwritten in some places), it's the opposite. The wiring and overcurrent device should be rated for at least the load, not less. There are various parts of the code that allow the wiring and overcurrent device to exceed the rating of the load, and there are several parts of 210 which allow multiple load devices with ratings less than the overall current rating of the circuit to be connected to the same circuit.

As for the part about the OCPD, look at 240.3, which refers to 690. 690 says nothing about it, so there you are. There's no requirement here except 110.3(B).

As for 110.3(B), I don't agree that there is any consensus about it. I just looked at the SMA manual for the inverters we most commonly install. It refers ultimately to the 'short circuit current' and contains no admonitions about calculating current according to local codes. The short circuit current is simply what's stated on the modules.

You're entitled to your opinion and your disagreement is so noted. Frankly I don't care if you want to resort to using 110.3(B) vs. my earlier 690 references and comments, simply because I've yet to see a listed PV 'converter' that didn't have in effect the same in its instructions. Granted you may have to read between the lines sometimes to extract it, but its in there all the same.

 

[Smart $]

On the module spec sheet.

In part, yes... overall, no.

690.8 Circuit Sizing and Current.
(A) Calculation of Maximum Circuit Current. The
maximum current for the specific circuit shall be calculated
in accordance with 690.8(A)(1) through (A)(5).

Informational Note: Where the requirements of 690.8(A)(1)
and (B)(1) are both applied, the resulting multiplication factor
is 156 percent.​

(1) Photovoltaic Source Circuit Currents. The maximum
current shall be the sum of parallel module rated short-
circuit currents multiplied by 125 percent.

(2)...

 

[jaggedben]

There is nothing in the inverter instructions or the code that says I have to apply that to the inverter input.

 

[Smart $]

There is nothing in the inverter instructions or the code that says I have to apply that to the inverter input.

I will continue to disagree with you no matter how many times or ways you say it...

 

[SolarPro]

Manufacturers provide various dc input current values that mean different things.

For example, the SMA SB 3000 TL has a maximum input current rating of 18 A. This is the dc input maximum current that the device can put to work. As an example, if the device is working at its lowest MPPT voltage, which is 175 V, and it is using 18 A of current from the array, it is basically operating just below its 3,200 W maximum dc input rating: 18 A x 175 V = 3,150 Wdc. (In this case each individual MPPT channel is only rated for a maximum input current of 15 A, but the inverter can process 18 A total from both input channels.)

Obviously, if the array is going to operate at 18 A, then the Isc input from the array must be higher than 18 A. That's why you can load inverters beyond the maximum input current without violating the manufacturer's instructions, the product listing or the NEC. The maximum dc input value is not a hard stop design limit, but rather an operation limit imposed by the software.

However, there most definitely is an un upper limit for the dc input for these devices in practice. Manufacturers identify this value as the maximum short circuit current. For this particular SMA inverter, the maximum short circuit current is provided per MPPT input. In this case, you could put up to 19 A Isc into each MPPT channel. This max Isc rating is basically the amount of fault current that the device is rated to withstand. That being the case, this max Isc rating is a hard stop design limit. You find this Isc value for the array simply by looking at the module nameplate. No other safety factors or deratings apply.

 

[SolarPro]

So how is it that the maximum input current is 15A per input, but yet the maximum short-circuit rating per input is 19A...???

In effect you could have 19 A Isc into each of the MPPT input, for a total of 38 A Isc. However, the most current an individual input channel can process is 15 A. And the most power the device can process across both inputs is 18A.

(You can see how these operational limits scale at each successive capacity rating. But the hard stop design limit remains the same 19 A per MPPT input.)

 

[Smart $]

Manufacturers provide various dc input current values that mean different things.

For example, the SMA SB 3000 TL has a maximum input current rating of 18 A. This is the dc input maximum current that the device can put to work. As an example, if the device is working at its lowest MPPT voltage, which is 175 V, and it is using 18 A of current from the array, it is basically operating just below its 3,200 W maximum dc input rating: 18 A x 175 V = 3,150 Wdc. (In this case each individual MPPT channel is only rated for a maximum input current of 15 A, but the inverter can process 18 A total from both input channels.)

Obviously, if the array is going to operate at 18 A, then the Isc input from the array must be higher than 18 A. That's why you can load inverters beyond the maximum input current without violating the manufacturer's instructions, the product listing or the NEC. The maximum dc input value is not a hard stop design limit, but rather an operation limit imposed by the software.

However, there most definitely is an un upper limit for the dc input for these devices in practice. Manufacturers identify this value as the maximum short circuit current. For this particular SMA inverter, the maximum short circuit current is provided per MPPT input. In this case, you could put up to 19 A Isc into each MPPT channel. This max Isc rating is basically the amount of fault current that the device is rated to withstand. That being the case, this max Isc rating is a hard stop design limit. You find this Isc value for the array simply by looking at the module nameplate. No other safety factors or deratings apply.

In effect you could have 19 A Isc into each of the MPPT input, for a total of 38 A Isc. However, the most current an individual input channel can process is 15 A. And the most power the device can process across both inputs is 18A.

(You can see how these operational limits scale at each successive capacity rating. But the hard stop design limit remains the same 19 A per MPPT input.)

I think you're reading that wrong.

The STC-rated input current is 15A per channel. The MPPT maximum input current is 18A per channel. However, the total input current is 30A (approximately)... that is with both channels being driven. Input current to each channel will vary and quite often be unequal. What the MPPT rating per channel means is if one channel is processing 18A under MPPT, it is because the other channel is only experiencing a 12A input current... and both channels combined are a total of 30A (approximately).

The maximum short-circuit input current rating of 19A is just the max MPPT point padded. Module Isc rating is provided at STC to standardize the rating. Module current output in the field can exceed STC value under more favorable conditions. The STC levels were developed by industry so as to not give a false impression of realistic output power. This is where the MPPT maximum current rating stems from. In a way, it is proof the Isc@STC is technically not the maximum short-circuit current for the module. Industry representatives are aware of it and this is why Code requires PV source output circuits are to be rated at 125% Isc@STC. Guess what 15A times 125% is?

The downside is there are locations on the planet which will never exceed in reality the summed parallel Isc@STC rating... yet they still must meet the requirements currently set forth. The upside is, not all locations on the planet are under NEC purview. :angel:

 

[jaggedben]

The STC-rated input current is 15A per channel. The MPPT maximum input current is 18A per channel.

Your confusing different inverters and making up a non-existent specification difference. The 3000-6000W inverter max is 15A per channel, and the 7000 and 7700 is 18A. These are the same spec, just for different models.

The manual says nothing about STC. This is simply the most actual current then inverter will throughput per channel under any conditions, STC or otherwise.

Module Isc rating is provided at STC to standardize the rating. Module current output in the field can exceed STC value under more favorable conditions.

Really this is only the case if the load (charge controller, inverter, etc.) resistance allows this current through.

... In a way, it is proof the Isc@STC is technically not the maximum short-circuit current for the module.

The short-circuit current is the spec on the module. Nobody anywhere uses the term 'short-circuit current' for anything else. The NEC term for the Isc*125% which you are so fond of is 'PV Source Circuit Current'. For the life of me, I can't figure out how 'short-circuit current' in the SMA manual can plausibly refer to what you think it does and not simply the module short-circuit current spec.

The maximum short-circuit input current rating of 19A is just the max MPPT point padded. ... ... Industry representatives are aware of it and this is why Code requires PV source output circuits are to be rated at 125% Isc@STC. Guess what 15A times 125% is?

Considering that 18A/19A for the 7000-7700W inverters is only 105%, this explanation makes no sense whatsoever.

 

[ggunn]

In some locations and orientations it is fairly routine to load inverters to 150% DCW STC to ACW. Since voltage is what it is, that overload is all Isc. In some AHJ's that ratio is limited (California, for example), but my impression was that the rule exists to keep rebates that are based on installed STC DC ratings under control.

 

[SolarPro]

I think you're reading that wrong.

To paraphrase: No matter how many times you disagree, you'll still be wrong.

PV systems operate across a wide range of conditions, both in terms of loading, available solar resource and environmental conditions. The operational limits of power electronics take these extremes into account, and the implications these conditions have regarding the array MPPT curve. The operational limits have nothing to do with STC values, which are ratings at factory test conditions. Operational limits are ratings in real world conditions.

I agree that it can be confusing at first. I had to have multiple conversations with product vendors and NRTL reps to wrap my head around this issue and many others. In this case, if you think you know better than the NRTLs, the CMP and the product manufacturer, I'm afraid you're alone on an island.

 

[ggunn]

To paraphrase: No matter how many times you disagree, you'll still be wrong.

PV systems operate across a wide range of conditions, both in terms of loading, available solar resource and environmental conditions. The operational limits of power electronics take these extremes into account, and the implications these conditions have regarding the array MPPT curve. The operational limits have nothing to do with STC values, which are ratings at factory test conditions. Operational limits are ratings in real world conditions.

I agree that it can be confusing at first. I had to have multiple conversations with product vendors and NRTL reps to wrap my head around this issue and many others. In this case, if you think you know better than the NRTLs, the CMP and the product manufacturer, I'm afraid you're alone on an island.

Right. An array on a rooftop in Canada with a 5 degree tilt and pointed due west, for example, is never going to produce anywhere near its STC DC rated Isc. There is no reason to limit the overloading of a system like that to some arbitrary value of STC Isc.

Finding the "sweet spot" for overloading an inverter by an array at a given location and orientation can be a significant challenge in systems design. Some clipping at peak times is acceptable if it is more than offset by increased production on the shoulders of the output curve.

 

[ggunn]

Right. An array on a rooftop in Canada with a 5 degree tilt and pointed due west, for example, is never going to produce anywhere near its STC DC rated Isc. There is no reason to limit the overloading of an inverter in a system like that to some arbitrary value of STC Isc.

Addendum: Of course, this does not relieve the designer of the requirement to size DC conductors to the correct ampacity with respect to STC Isc for the array.

 

[Smart $]

To paraphrase: No matter how many times you disagree, you'll still be wrong.

PV systems operate across a wide range of conditions, both in terms of loading, available solar resource and environmental conditions. The operational limits of power electronics take these extremes into account, and the implications these conditions have regarding the array MPPT curve. The operational limits have nothing to do with STC values, which are ratings at factory test conditions. Operational limits are ratings in real world conditions.

I agree that it can be confusing at first. I had to have multiple conversations with product vendors and NRTL reps to wrap my head around this issue and many others. In this case, if you think you know better than the NRTLs, the CMP and the product manufacturer, I'm afraid you're alone on an island.

The concept you guys are trying to force feed me with is not all that hard to comprehend. I think its more a matter of terminology, or rather misuse of terminology. Refer to 90.3, the definition of short-circuit current rating in Article 100, and 110.10, which covers short-circuit current ratings and listed equipment.

Beyond that, I'm done debating the matter and leave it to the AHJ to properly enforce Code as they see fit. :bye:

 

[ggunn]

Beyond that, I'm done debating the matter and leave it to the AHJ to properly enforce Code as they see fit. :bye:

So far I am batting 1000 for AHJ's not even questioning how much I overload inverters. As long as my conductor sizes are correct for the derated Isc of my array, they are happy.

There is nothing to debate.

 

[Smart $]

So far I am batting 1000 for AHJ's not even questioning how much I overload inverters. As long as my conductor sizes are correct for the derated Isc of my array, they are happy.

There is nothing to debate.

The majority of inspectors I've run across do not understand the concept of short-circuit current rating.