Breaker sizing and array configuring in anticipation of 2014 NEC changes?

[solardan]

Hi. I'm a retired engineer getting involved in my first solar PV installation. The initial installation is straightforward: 2 strings of 10 modules each using Enphase microinverters, combined in a Soladeck box, with 20A breakers on each string. This meets the old requirement that the breakers add to 40 or less amps for a 200A service. No problem.

However, in a few years we plan on adding a third string (32A total output). Under the new 2014 NEC (section 705.12(D)), we will be able to simply add a third breaker to the Soladeck and pass the 3rd string through it. However, under previous NECs we would have to jump through some hoops reconfiguring the arrays, with more branches and junction boxes. We could build for that now, complicating the initial installation, or keep things simple in anticipation that the 2014 NEC changes will be widely accepted in a few years.

What would you do? Keep initial installation simple, anticipating NEC 2014 acceptance (i.e., that the old method of adding up breaker sizes will go out of favor), or complicate the initial installation so that it works no matter what? (My inclination is to keep it simple, esp. since it doesn't make any electrical sense to me to be adding up breaker sizes vs. use of the inverter's actual AC output current.) thanks

 

[jaggedben]

If they are all on one array, you could give yourself more flexibility by putting up to 16 or 17 on one circuit (depending on the model), instead of splitting them equally.

If the two 10 module circuits are on separate arrays, I would keep it simple. Note that you could use two 15A breakers in that situation, although it doesn't really give you much extra leeway down the line.

If you don't already know, you should look up when the 2014 will be officially adopted by your state.

 

[solardan]

If they are all on one array, you could give yourself more flexibility by putting up to 16 or 17 on one circuit (depending on the model), instead of splitting them equally.

If the two 10 module circuits are on separate arrays, I would keep it simple. Note that you could use two 15A breakers in that situation, although it doesn't really give you much extra leeway down the line.

If you don't already know, you should look up when the 2014 will be officially adopted by your state.

Hi jaggedben,

Because of voltage drop issues with the Enphase Engage cable (seen as voltage rise by the microinverters), you can't actually string more than about 10 Enphase microinverters together before having to break the string into branches, which then complicates wiring and adds junction boxes. The initial installation has 2 straight rows of portrait-oriented modules of 10 each (2 strings), ending in the Soladeck with 2 x 20A breakers. No junction boxes, no breaks in the Engage cable, no extra Engage connectors - just straight shot into the Soladeck combiner.

The 3rd string (probably 9 to 12 more modules) will sit physically separate from the first array, and not be installed for at least 2-3 years. Following 2014 NEC, I can simply put a 3rd 20A breaker in the Soladeck and run the output from the 3rd string to it for combining with the first two. Couldn't be simpler.

If I stick to use of just 2 x 20A breakers, however, then, because of voltage drop and the 2011 circuit breaker addition rule, you're forced to chop up the system into appropriately sized branches, adding multiple junction boxes, and even changing the optimal position of the Soladeck on the roof. I could anticipate all that by changing the first installation, but it seems kind of foolish given that the experts in the field consider adding up breaker sizes to make no sense, and that the latest code confirms that. See Mike Holt's panel discussion of the issue:

http://www.youtube.com/watch?v=7vD0r_bVjuo

relevant section starts at 53:00
specific reference to no longer adding breaker sizes starts at 56:50

A further consideration, peculiar to the Soladeck, may be the derating of the Soladeck 20A breakers at high roof temperature:

_Ambient Temp Derating_:
30C/86F = 18.7A
40C/104F = 17.3A
50C/122F = 15.8A
60C/140F = 14.1A
70C/158F = 12.2A

So it's obvious from that that you wouldn't want to go near a 15A Soladeck breaker -- another example I think of why the old rule of adding breakers is such a bad idea: it can sucker you into undersizing each string's breakers.

Yup, I should find out what Illinois is up to. At this point, however, I haven't been able to find anyone in the field (once they know and understand NEC 2014 section 705.12(D)) that has attempted to defend using the old method of summing OCPD sizes for busbar calc.s. I suspect that the fact that the old rule made little sense (see Mike Holt's video), and the new rule simplifies and reduces installation costs, will lead it to become more quickly followed than one might have thought.

But this area is still pretty new to me, and I don't know much about how long it takes for code changes to take hold, so thought it would be prudent to ask what others thought.

 

[Smart $]

"Derating" of breaker rating is not peculiar to Soladeck (but derating values are specific to manufacturer as published).

Without looking up the actual Code wording, I believe you can use the "derated" value for your breaker sum. For example, if the derated breaker rating is 15.8, that would have to be the minimum value for 125% of inverter output, so you'd be permitted to use the 15.8 value in summing multiple breakers. Of course, however, 3 would still put you over 40A on 200A service equipment rating... :weeping:.

And on Code adoption, AFAICT Illinois adoption is local, not statewide.

 

[solardan]

"Derating" of breaker rating is not peculiar to Soladeck (but derating values are specific to manufacturer as published).

Without looking up the actual Code wording, I believe you can use the "derated" value for your breaker sum. For example, if the derated breaker rating is 15.8, that would have to be the minimum value for 125% of inverter output, so you'd be permitted to use the 15.8 value in summing multiple breakers. Of course, however, 3 would still put you over 40A on 200A service equipment rating... :weeping:.

Yeah, I was highlighting the Soladeck because of its rooftop location, and thus an example of exposure to extreme temp.s (vs. for example, doing the combining as part of a ground level disconnect box). Seems like use of derated breaker ratings in the calculation would be yet another layer of unnecessary complexity, all flowing from the original mistake of linking busbar calc.s to OCPD sizing, rather than rated inverter output.

 

[jaggedben]

Hi jaggedben,

Because of voltage drop issues with the Enphase Engage cable (seen as voltage rise by the microinverters), you can't actually string more than about 10 Enphase microinverters together before having to break the string into branches, ....

It's good you're aware of this issue, but you're making it out to be more restrictive than it is. Just use #10 for your run from the Soladeck, and you'll have no issues putting 16-17 inverters on a single cable, at least up to about 150' of wiring (then go to #8). With that said...

The initial installation has 2 straight rows of portrait-oriented modules of 10 each (2 strings), ending in the Soladeck with 2 x 20A breakers. No junction boxes, no breaks in the Engage cable, no extra Engage connectors - just straight shot into the Soladeck combiner.

The 3rd string (probably 9 to 12 more modules) will sit physically separate from the first array, and not be installed for at least 2-3 years.

...that's a nice simple set up, so as I said above, I'd probably just do it that way.

Good point about watching out for the derated breaker on the roof.

http://www.youtube.com/watch?v=7vD0r_bVjuo

Thanks for posting that, I hadn't noticed it yet. Most of it is review for me, but I've already learned a couple things even though I haven't finished watching. Also, Mike is a riot. :lol:

 

[solardan]

It's good you're aware of this issue, but you're making it out to be more restrictive than it is.

Ahh, yes, you may be right. I probably got to the 10 or so module limit per string based on derating of the Soladeck 20A breakers on a hot roof. For anyone interested, the voltage drop stuff for M250s is described here,

http://enphase.com/global/files/EnphaseTechBrief_Vdrop_M250.pdf
"Regardless of the application, Enphase recommends that the total percentage of voltage rise in the AC wiring be less than 2%, with (an inclusive) less than 1% voltage rise in the Engage Cable."

From the table in the pdf, at 10 panels there is a 0.32% drop/rise, and at 16 it is 0.80%, from the Engage cable alone (for my portrait-oriented system). So it just depends on how you've sized the rest of the system as to whether the exponential rise in voltage drop with panel number matters. But I think you're right in this case, that I was reacting to the Soladeck breaker derating, and not so much to the Engage cable voltage drop. thanks