AC discos needed per NEC?

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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I am designing a PV system consisting of (40) Enphase micros, (12) on a house and (28) on a detached garage. The output of all the micros is combined in an AC combiner at the house and interconnected in the house MDP. The micros on the garage feed two sets of underground conductors in a trench between the garage and the house.

Question: Do I need to install a pair of AC discos on the garage? 690.13 only governs DC conductors and so far I haven't been able to find any similar language governing AC conductors.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
By a pair of discos I assume you mean one for each circuit on the garage, within sight of the inverters.

The answer depends if your AHJ accepts each of the connectors that plug into the trunk cable as meeting the requirements for the disconnect within sight of inverters mounted in not-readily-accessible locations. The connectors are load break rated and meet 690.33 requirements, for what that's worth. Most AHJs don't require it around here, but a couple do because the plug connectors require a tool to disconnect, or for less logical reasons.

Personally, if I were an AHJ, would consider the connectors good enough. The purpose of the code section is to allow servicing to happen with equipment de-energized. There are no serviceable parts or even anything to open on these inverters. The only 'servicing' is replacement. For that, you either have the tool or you don't, but you are not doing any other servicing if you don't.
 

shortcircuit2

Senior Member
Location
South of Bawstin
I am designing a PV system consisting of (40) Enphase micros, (12) on a house and (28) on a detached garage. The output of all the micros is combined in an AC combiner at the house and interconnected in the house MDP. The micros on the garage feed two sets of underground conductors in a trench between the garage and the house.

Question: Do I need to install a pair of AC discos on the garage? 690.13 only governs DC conductors and so far I haven't been able to find any similar language governing AC conductors.


The micro string circuits should be combined at the detached garage and then run a feeder back to your AC combiner at the house. I would look to 225.30(A)(5) for allowing the additional feeder to the garage, (assuming it already has a power to the garage). Also follow 225.31, 225.32, 225.33, 225.34...and also label the 225.31 disconnect as the 690.12 Rapid Shutdown for the PV system on the detached garage. Then you need a grounding electrode system.
 

mwm1752

Senior Member
Location
Aspen, Colo
The micro string circuits should be combined at the detached garage and then run a feeder back to your AC combiner at the house. I would look to 225.30(A)(5) for allowing the additional feeder to the garage, (assuming it already has a power to the garage). Also follow 225.31, 225.32, 225.33, 225.34...and also label the 225.31 disconnect as the 690.12 Rapid Shutdown for the PV system on the detached garage. Then you need a grounding electrode system.

Emphase inverters are mounted on panels & will never be past 10' away from array ---- rapid shutdown does not apply 690.12(1)
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
My apologies -- rapid shutdown has been addressed
What has not been addressed is my original question, which is this:

If I have microinverters on a detached garage with their output trenched over to the MDP on the house, do I need an AC disco on the garage?
 

GoldDigger

Moderator
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Location
Placerville, CA, USA
Occupation
Retired PV System Designer
What has not been addressed is my original question, which is this:

If I have microinverters on a detached garage with their output trenched over to the MDP on the house, do I need an AC disco on the garage?

My take would be that if there is only one branch circuit to the garage then a snap switch at the house on that circuit should be sufficient. But if you really have a feeder, then a local disconnect would be required.
But there is a lot of room for AHJ judgement there.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
My take would be that if there is only one branch circuit to the garage then a snap switch at the house on that circuit should be sufficient. But if you really have a feeder, then a local disconnect would be required.
But there is a lot of room for AHJ judgement there.
Here's what I have: about 30A AC coming off the garage roof to an AC combiner panel on the garage exterior wall and going through UG conduit to a disco on the house 30-40 feet away. I'm sure that the garage has power but we are not interconnecting out there. Do I need to put a disco on the output of the AC combiner per NEC or is up to the discretion of the AHJ?

We had a similar case in Austin on a much larger system and the AHJ called for the disco.
 

david

Senior Member
Location
Pennsylvania
The micro string circuits should be combined at the detached garage and then run a feeder back to your AC combiner at the house. I would look to 225.30(A)(5) for allowing the additional feeder to the garage, (assuming it already has a power to the garage).

your thinking of going that route to avoid calling the sub at a garage a feeder from the primary source. and the PV connection a load side connection.

Isn't that very gray. I'm not sure I could use that design to justify two feeders at the garage.

Shouldn't transfer equipment be installed to prevent the inadvertent interconnection of the primary sources of supply .

"Transfer equipment and electric power production systems installed to permit operation in parallel with the normal source shall meet the requirements of Article 705."
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
your thinking of going that route to avoid calling the sub at a garage a feeder from the primary source. and the PV connection a load side connection.

Isn't that very gray. I'm not sure I could use that design to justify two feeders at the garage.

Shouldn't transfer equipment be installed to prevent the inadvertent interconnection of the primary sources of supply .

"Transfer equipment and electric power production systems installed to permit operation in parallel with the normal source shall meet the requirements of Article 705."

Transfer equipment? Why? The interconnection is just a simple load side connection through a breaker in the MDP like we build every day; it's just that the PV is on one building and the interconnection is on the other. The conductors from the PV do not interact with whatever power the garage has except that they are both tied to the same service.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Do I need to put a disco on the output of the AC combiner per NEC ... ?

I think you technically have a feeder to the building that requires a disconnecting means per 225.32. But you could meet this requirement by putting a main breaker in the combiner instead of having a separate disco.

Neither article 690 nor 705 require a disco there, especially if you have a disco elsewhere on the premises meeting 705.22.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I'm not sure I could use that design to justify two feeders at the garage.

See 225.30(A)(5).

Shouldn't transfer equipment be installed to prevent the inadvertent interconnection of the primary sources of supply .

With all due respect, I think you still don't understand how interactive inverters work. Get it out of your head that they are dedicated to any particular loads whatsoever. That's not how they work.
 

shortcircuit2

Senior Member
Location
South of Bawstin
I think you technically have a feeder to the building that requires a disconnecting means per 225.32. But you could meet this requirement by putting a main breaker in the combiner instead of having a separate disco.

I agree with this. But you should be OK with the 2 breakers in your combiner panel at the garage for the 2 micro-inverter strings as your discos. (6-switch rule)
 

shortcircuit2

Senior Member
Location
South of Bawstin
The way I see it its a 2-way street. The Solar on the garage won't work without the Utility AC power from the house. So you need to feed the garage with AC power for the micro-inverters and the power feed to the garage is subject to article 225. Article 225 says you need a disco.

I say you need to combine the 2 micro-inverter strings at the garage so you won't have more than 1 additional power feed to the garage. This design is good practice also to deal with voltage rise as the micro-inverters are sensitive to this within 2%.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Tell me more about that please!

The 'within 2% part is not strictly true but is a good design rule-of-thumb, especially when considering the voltage rise at the end of a micro-inverter trunk cable. Simply put, if the voltage measured by the inverter is not within the IEEE standard of +10%/-12% of nominal then the inverters will stop.

Enphase has published several application notes on this over the years. I think you'll have no trouble grasping the concepts.
https://enphase.com/sites/default/files/EnphaseTechBrief_Vdrop_M250.pdf
https://enphase.com/sites/default/files/EnphaseAppNote_Vdrop_Calculations.pdf
 

david

Senior Member
Location
Pennsylvania
Transfer equipment? Why? The interconnection is just a simple load side connection through a breaker in the MDP like we build every day; it's just that the PV is on one building and the interconnection is on the other. The conductors from the PV do not interact with whatever power the garage has except that they are both tied to the same service.

This is my reading of this.
705.12 Point of Connection.
The output of an interconnected electric power source shall be connected as specified in 705.12(A), (B), (C), or (D).

(A) Supply Side. An electric power production source shall be permitted to be connected to the supply side of the service disconnecting means as permitted in 230.82(6).

I read this as a simple statement an electric power production source shall be permitted to Connect (shake hands with ) the primary source at the supply side of the service disconnecting means as permitted in 230.82(6).

B , C and D says you are permitted to connect at a point elsewhere on the premise from what was stated in A. You are making a load side connection of the inverter output to the primary source
• (D) Utility-Interactive Inverters. The >>output of a utility-interactive inverter<< shall be permitted to be connected to the load side of the service disconnecting means of the other source(s) at any >>distribution equipment<<on the premises.
• Where >>distribution equipment<< including switchboards and panel boards is fed simultaneously by a primary source(s) of electricity and one or more utility-interactive inverters
• where this >>distribution equipment<< is capable of supplying multiple branch circuits or feeders or both, the interconnecting provisions for the utility-interactive inverter(s) shall comply with (D)(1) through (D)(7).

Distribution equipment for what the primary source or solar source, define it one or the other but the subject of distribution equipment is the same in all three points above, at least thats how I read it and apply it. I read it that you are connecting to primary source feeder distribution equipment. Build the system than tear out the solar you still can distribute the primary source through the primary source distribution equipment.

I think it is a very gray area to claim you are running two primary source feeders to a building and claim one is a different animal. When the section states it is a load side (of the primary source line side connection point) to any point at any primary source distribution equipment

In my way of thinking it would be correct to run the inverter output circuit to the main building unless you want to define a feeder connection point at the garage a load side connection

keep in mind im still learning i do not think every statement i make will be with out correction
 
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GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
...
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Enphase has published several application notes on this over the years. I think you'll have no trouble grasping the concepts.
https://enphase.com/sites/default/files/EnphaseTechBrief_Vdrop_M250.pdf
https://enphase.com/sites/default/files/EnphaseAppNote_Vdrop_Calculations.pdf
I would feel much better about both papers if they did not totally confuse voltage drop as a function of position in the string with end voltage drop as a function of number of modules in the string.
Worse, there is a fair chance that some of the calculated numbers are incorrect as a result.
 
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