AC combiner box circuit breaker sizing question

solardan

Given the limit of 40A of added solar power to a 200A service, must the total rated amperage of the circuit breakers in an upstream AC combiner box also be limited to 40A? For example, if I have 3 Enphase M250 strings, each with a 15 or 20A dual pole circuit breaker in an AC combiner box, is that alone a violation of the 40A rule? Or is it okay as long as the maximum expected combined amps does not exceed about 32A (32A x 1.25 = 40A limit), regardless of the circuit breakers used???

Cariboocoot

Re: AC combiner box circuit breaker sizing question

Welcome to the forum.

It's a matter of not exceeding the current rating on the circuit protected by the breaker. The "40 Amp for a 200 Amp service" rating refers to limiting the current from both power sources to the bus bars of the service panel so that their maximum is not exceeded.

In the example you give, two 15 Amp or even two 20 Amp branch circuits feeding into the 40 Amp 'main' solar breaker is not a problem. There is no need for "double derating" as it were.

Unfortunately some inspectors don't understand this.

solardan

Re: AC combiner box circuit breaker sizing question

Cariboocoot wrote: »

In the example you give, two 15 Amp or even two 20 Amp branch circuits feeding into the 40 Amp 'main' solar breaker is not a problem. There is no need for "double derating" as it were.

Thanks for the quick response Cariboocoot. Just to be clear, my example had 3 (not 2) branch circuits, each with a 15 or 20A breaker (45 or 60 total), but actual amps produced would not exceed about 32A. Is that okay? I've got an installer/electrician trying to tell me that I can't use 3 x 20, or even 3 x 15 amp circuit breakers for my branch circuits since that would "add up" to more than 40 amps, even though the system will not produce more than about 32 amps.

BB.

Re: AC combiner box circuit breaker sizing question

Also, you can drop the AC main rating too... Say you only need 125 amp main breakers. Then:

200 Amp Bus+panel * 1.2 = 240 total supply breakers

240 total - 125 actual main = 115 Amps of "GT Solar" feed.

However, in many locations, your maximum is around 10kW of solar power:

10,000 W * 1.25 NEC branch circuit derating / 240 VAC = 52 amps minimum branch circuit for 10kW install

So, for many places, unless you want a whole 'nother level of Utility+State regulations, ~60 amps or 3x 20 amp of Enphase branch circuits is going to be your limit.

And the typical maximum array would be around:

10,000 watts * 1/0.77 typical GT derating = 12,987 watt array (approximate) cost effective max DC array rating

-Bill

Cariboocoot

Re: AC combiner box circuit breaker sizing question

solardan wrote: »

Thanks for the quick response Cariboocoot. Just to be clear, my example had 3 (not 2) branch circuits, each with a 15 or 20A breaker (45 or 60 total), but actual amps produced would not exceed about 32A. Is that okay? I've got an installer/electrician trying to tell me that I can't use 3 x 20, or even 3 x 15 amp circuit breakers for my branch circuits since that would "add up" to more than 40 amps, even though the system will not produce more than about 32 amps.

Well, he's wrong.
The output from the inverters will never exceed their maximum. Current only flows from them, not to them (except for the mA needed for detecting Voltage and frequency on qualifying).

Enphase inverters are roughly 1 Amp maxim. So if you have five inverters on each of three circuits that's 15 Amps total being fed to the 40 Amp main. It does not matter that each branch is connected by 14 AWG wire protected by a 15 Amp breaker: the feed to the 40 Amp breaker would be the total of all the inverters, not the total of the individual circuit ratings. In effect you can have any combination of inverters on any number of circuits providing the total output of the inverters doesn't exceed the 40 Amp 'main' solar breaker.

You could in fact use 12 AWG wire protected by 20 Amp breakers on the three circuits so long as the total inverter output did not exceed the rating of that breaker to the main service panel. The current will not be "20 Amp breakers * the number of circuits". If someone comes along later and maxes out each of those circuits so that the output of the combined inverters exceeds 40 Amps guess what happens? The breaker at the service panel will trip and shut the whole thing down.

Just because a circuit has wiring and over-current protection capable of 'X' Amps does not mean that's how many Amps will be on that circuit.

solardan

Re: AC combiner box circuit breaker sizing question

Cariboocoot wrote: »

If someone comes along later and maxes out each of those circuits so that the output of the combined inverters exceeds 40 Amps guess what happens? The breaker at the service panel will trip and shut the whole thing down.

Good, that makes much more sense. I had this system planned for months, and then he says it won't work since the total of the circuit breakers can't exceed the 40A (btw, the phone help at Enphase said the same thing). Didn't sound right, but didn't know if there was some funky NEC rule behind it. This seems to be a common misconception. If believed, it will lead you to do some strange things in terms of sizing your strings just to get the right combination of circuit breaker sizing to stay within the 40A. Thanks again!

BB.

Re: AC combiner box circuit breaker sizing question

If you change out the main breaker to 175 amps, then:

240 Amp total - 175 Amp main = 65 amps for GT solar...

Should be "good enough" for what you need.

There may be some localities that specify the minimum main breaker size for the panel (i.e., if you have A/C, electric stove/hot water/etc.)--The breaker change over may need a little talking with the AHJ to ensure they will accept that as a plan.

-Bill

vtmaps

Re: AC combiner box circuit breaker sizing question

solardan wrote: »

and then he says it won't work since the total of the circuit breakers can't exceed the 40A (btw, the phone help at Enphase said the same thing).

It doesn't matter what your installer says, or what we say or what Enphase phone help says. What matters is what your AHJ inspector says.

--vtMaps

solardan

Re: AC combiner box circuit breaker sizing question

vtmaps wrote: »

It doesn't matter what your installer says, or what we say or what Enphase phone help says. What matters is what your AHJ inspector says.--vtMaps

Whatever my upstream combination of circuit breakers is, I could always "fix" the issue by inserting a 40A breaker after combining the circuits (who would object to that?). But there's already such a breaker at the panel, making such a fix redundant. As cariboocoot first noted, "It's a matter of not exceeding the current rating on the circuit protected by the breaker". In other words, the upstream breakers are there to protect the circuit they are on, not the panel. Anyway, that's my latest understanding of the issue.

solar_dave

Re: AC combiner box circuit breaker sizing question

BB. wrote: »

If you change out the main breaker to 175 amps, then:

240 Amp total - 175 Amp main = 65 amps for GT solar...

Should be "good enough" for what you need.

There may be some localities that specify the minimum main breaker size for the panel (i.e., if you have A/C, electric stove/hot water/etc.)--The breaker change over may need a little talking with the AHJ to ensure they will accept that as a plan.

-Bill

+1 on this one! The 175 amp derate on a 200 amp panel is exactly what I had to do to get 2 X 23 amp inverters past my AHJ. They feed an AC combiner containing a pair of 30 amp breakers and the main panel is 60 amps. With even 46 amps feeding the 60 amp breaker I think then NEC 1.25 rule for breaker loading would kick in if I add much more to the combiner. The 60amp divided by 1.25 would be 48 amp max I suspect. I do have my combiner wired heavy enough to go up to 80 amps, but the the main breaker would need to come down to 150 amps or I might get an actual buss bar rating from the manufacturer.

BTW some manufacturers will actually document the panel buss bar rating and many are higher than the 200 amp service panel rating. The back feed rating is for the buss bar but I also suspect that would be a AHJ battle in the brewing as they can be very arrow minded.

vtmaps

Re: AC combiner box circuit breaker sizing question

solardan wrote: »

I could always "fix" the issue by inserting a 40A breaker after combining the circuits (who would object to that?)

Not anyone with brain, but that category of people does NOT include all AHJ inspectors. --vtMaps

Cariboocoot

Re: AC combiner box circuit breaker sizing question

What vtMaps says is regrettably true: too many inspectors do not understand the concept of two power sources feeding to the same load distribution point and instead view the connected GTI source as a 'load'.

In reality the 120% rule is for protecting the circuit that consists of the two bus bars in the main service panel and all loads attached thereto.

Say you have a service with bus bars rated for 250 Amps and a main service breaker of 200 Amps. Everything is fine. Add a 40 Amp input from solar to those bus bars and the total potential current on them becomes (200 + 40) 240 Amps. Since this is less than the 250 Amp bus bar rating it is safe.

Now suppose you put in a 60 Amp input from solar. The total potential current on the bus bars would then be (200 + 60) 260 Amps, which is more than the bus bars are rated for and therefor not safe.

So you change the main breaker to 175 Amps and the total potential on the bus bars becomes (175 + 60) 235 Amps. Below the current rating and safe.

For some reason inspectors think a breaker rating is the same thing as the amount of current available, and so in their minds a 40 Amp breaker on the GTI circuit means 40 Amps absolutely will be flowing there. They do not comprehend that the actual current is limited to the output of the GTI and can never be any more than that.

In the off-grid world we see inverter outputs capable of only 30 Amps divided up into multiple 15 Amp circuits even though the total potential could only supply two such circuits at maximum power. This is normal distribution practice, and the point of it is if any one of those circuits gets overloaded to the (30 Amp) potential of the inverter the 15 Amp breaker will trip before the 14 AWG wire ignites. In reality the inverter faults first, but you never know.

This install with three 15 Amp branch circuits picking up three separate groups of inverters is much the same as that, only the branching is turned around the other way.

Another way to look at it is that the service to your house is fed by multiple megawatt power plants each of which have over-current safeguard in the circuitry along the way, but the main breaker limits the feed to the service panel to 200 Amps no matter how many paralleled generators are behind it.

solardan

Re: AC combiner box circuit breaker sizing question

Well, we've managed to piss off my installer with this discussion. He insists that the issue is a matter of NEC code and that they are not going to violate code (i.e., insists the upstream circuit breakers add up to 40A or less). I guess the next step is to look at the relevant code. Any thoughts on how to proceed with that, or where we can get a more authoritative opinion that might satisfy the installer? thanks

BB.

Re: AC combiner box circuit breaker sizing question

Exactly how many enphase inverters (Enphase Model #, and 20 amp AC circuits) do you want to install?

-Bill

solardan

Re: AC combiner box circuit breaker sizing question

Thanks to peakbagger on SolarPanelTalk forum: latest issue of Home Power (Apr/May 2014, p. 86) refers to code change that will allow use of 125% of inverters output rather than OCPD (breaker) size. Yeah!

"Section 705.12(D)
The most significant changes in Article 705 are in 705.12(D),
which further clarifies the ways a PV system can be connected
on the load side of the AC service disconnecting means,
particularly 705.12(D)(2), “Bus or Conductor Ampere Rating.”
Under earlier editions of the Code, busbar calculations were
performed using the rating of the overcurrent protection
devices (OCPDs) that were supplying power to the busbar
(the main, or supply breaker, plus any back-fed breakers
connected to grid-tied inverters).
One significant change is that 125% of the inverter’s rated
AC output current, rather than the actual back-fed breaker
size, is used for busbar calculations. The advantage is that
the installed OCPD size is usually larger than the calculated
minimum OCPD size (since, if the calculation doesn’t result
in a standard size, it has to be rounded up), but 125% of the
inverters’ rated output current (typically a smaller value than
the installed OCPD size) can be used in busbar calculations,
potentially allowing larger PV systems to be interconnected
on the load side of a service."

imho, that is a pretty complete admission by the experts in the field that adding up circuit breaker sizes never made much electrical sense, as outlined so clearly by cariboocoot in previous posts.

solardan

Re: AC combiner box circuit breaker sizing question

And here is Mike Holt's related discussion with other experts, including one who was involved in writing 705.12(D):

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

Holt: "you prevent the roundup [of OCPDs] from making it bigger, when there was no reason for making it bigger"

Cariboocoot

Re: AC combiner box circuit breaker sizing question

Didn't I say that? Why doesn't anyone ever listen to me? Am I speaking Polish again? I'm never sure with what goes on in my brain these days.

solardan

Re: AC combiner box circuit breaker sizing question

Another reason I can see to get away from simply adding OCPD sizes is that it seems likely to lead you to undersize breakers in situations where they are exposed to high roof temp.s, as in a rooftop Soladeck box. In the following post, for example,

http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/2013/039027.html

it is pointed out that Soladeck's 20A breakers can easily derate to 12A on a hot summer roof (like my south-facing asphalt shingle roof),

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

so that, for example, if you've "cleverly" used even smaller 15A breakers for a string of 10 or so Enphase M250 (1.04A each) microinverters (to help meet 2011 code), you're likely to see nuisance trips of the breaker whenever the roof gets very hot. Much smarter to put their 20A breaker on that string, and to not have to worry about that circuit-related decision getting caught up in unrelated busbar calculations.