MLO Panel as a service.

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Ctay005

Member
Location
Orem, Utah, USA
I'm trying to figure out a way to hook 4.5kW (30A OCPD required) of solar to a house that is using a MLO as a service panel. The panel has 2 breakers on it, one 50A breaker for the A/C and a 100A for a subpanel. Since this is a MLO panel I'm wondering how it is protected. Is it the size of the feeders coming from the meter? (Its a meter/main combo.)

The concerns i have is that the sticker with the panel information has long worn off so the rating of the busbars is unknown. Which makes me nervous to suggest using a supply side connection since the feeders to the bus are quite a bit smaller than other houses I've worked with in the past. Any suggestions would be helpful!
 

iwire

Moderator
Staff member
Location
Massachusetts
You can still use a MLO panel as a service panel with up to six (sometimes 7) breakers in it as the service disconnecting means.

The only thing that really protects the conductors from overload is the hope no one adds load without considering the ampacity of the service conductors.

As far as you situation without a tag I would try another direction such as a line side tap.
 

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Just be careful about using the panel as service equipment as many main lug panels will state that they are suitable for use as service equipment only when a main breaker kit or tie down breaker is used. Not sure why but I have read that on some of the smaller 125 amp panels.
 

GoldDigger

Moderator
Staff member
Location
Placerville, CA, USA
Occupation
Retired PV System Designer
If the service disconnect consists of the two breakers (A/C and subpanel), would that not make a backfeed breaker on the same bus a line side tap already?
You may have a problem convincing an inspector of that though. :)

Tapatalk!
 

Pharon

Senior Member
Location
MA
I don't see what the problem is here. The main purpose of OCPD is to protect feeders and bussing from overheating and causing a fire. The OCPD protecting the 30A PV feeder is at the inverter. As long as the panel bussing and service conductors are larger than 30A, there is no hazard.
 

Smart $

Esteemed Member
Location
Ohio
I'm trying to figure out a way to hook 4.5kW (30A OCPD required) of solar to a house that is using a MLO as a service panel. The panel has 2 breakers on it, one 50A breaker for the A/C and a 100A for a subpanel. Since this is a MLO panel I'm wondering how it is protected. Is it the size of the feeders coming from the meter? (Its a meter/main combo.)
The ampacity of the service entrance conductors and the total calculated load.

The concerns i have is that the sticker with the panel information has long worn off so the rating of the busbars is unknown. Which makes me nervous to suggest using a supply side connection since the feeders to the bus are quite a bit smaller than other houses I've worked with in the past. Any suggestions would be helpful!
The service entrance conductors' ampacity must be at least as much as the calculated load, and service equipment must be rated not less than service entrance conductor ampacity. At 30A PV, I don't believe you have to worry about it.

Any chance the service entrance conductors are smaller because they are copper, and you are used to seeing aluminum?
 

Pharon

Senior Member
Location
MA
The 30A PV is not a load, it's a supply. If anything, it would reduce the total calculated load. It's all about how the power flows.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
It's not reasonable to assume that the service is rated only 30A.

You already have two handles, adding the solar will mean three, and you are allowed to have six.

Therefore it's code compliant to add a 30A solar breaker as an additional disconnecting means in the service panel, under 705.12(A).

Anything that's not compliant would be due to some existing code violation.

End of discussion.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
If the service disconnect consists of the two breakers (A/C and subpanel), would that not make a backfeed breaker on the same bus a line side tap already?
You may have a problem convincing an inspector of that though. :)

Tapatalk!

A few months ago I had exactly this discussion with an AHJ. It was a lot of fun because I happened to be hanging in a harness on a 45 degree roof, and I was reciting the code sections from memory. After I repeated the argument about six times he finally got it. ("Okay, I think I see what you're saying, I'll explain that to my boss.")

Now, that job happened to have the same situation that is at issue here, where the panel label was gone. It had 140A of breakers in it already, and we couldn't prove it was more than 100A (like the others on the street). So we had to downsize the existing load breakers to 100A. But we were allowed to put 40A of solar in a 100A service panel because it was a supply side connection and they accepted that.
 
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Pharon

Senior Member
Location
MA
It's not reasonable to assume that the service is rated only 30A.

You already have two handles, adding the solar will mean three, and you are allowed to have six.

Therefore it's code compliant to add a 30A solar breaker as an additional disconnecting means in the service panel, under 705.12(A).

Anything that's not compliant would be due to some existing code violation.

End of discussion.
I don't see anyone suggesting that the service is rated only 30A. I certainly wasn't, if that's what you're implying.

And you're mixing up your sections. The 6 disconnect rule is in 230.71. For this 30A PV breaker install, it's 230.82 that applies.

Again, it's a supply, not a load. And it's located on the supply side of the main disconnects, not the load side. Because of the way the power is flowing.


Sent from my iPad using Tapatalk
 

Pharon

Senior Member
Location
MA
In short, it is a parallel power source, but it does not reduce the calculated load.
Yes, you're correct. My bad. What I meant was that it would reduce the load being supplied by the utility - but the calculated load should be unchanged.


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jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I don't see anyone suggesting that the service is rated only 30A. I certainly wasn't, if that's what you're implying.

I was responding to the OP, and the thread in general, and not implying anything about your response.

And you're mixing up your sections. The 6 disconnect rule is in 230.71. For this 30A PV breaker install, it's 230.82 that applies.

It's certainly debatable whether 230.71 applies, and how, but with only three handles it's moot.

Again, it's a supply, not a load. And it's located on the supply side of the main disconnects, not the load side. Because of the way the power is flowing.

Yup. I get that. :cool:
 

Ctay005

Member
Location
Orem, Utah, USA
Thanks for the help guys. Now its just a matter of convincing the AHJ in this city who hates supply side "taps" of any kind that this is ok. I think having a breaker will be enough to convince him since it seems that he mainly is concerned with tapping conductors.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I don't see what the problem is here. The main purpose of OCPD is to protect feeders and bussing from overheating and causing a fire. The OCPD protecting the 30A PV feeder is at the inverter. As long as the panel bussing and service conductors are larger than 30A, there is no hazard.
A couple of things?


Why are we discussing the load breakers? The size of the load breakers does not figure into the 120% rule in the panel.

Also, OCPD for an inverter feed needs to be at the service end of the conductors. If the system is designed correctly the conductors cannot be endangered by the inverter. The OCPD is to protect the conductors from fault current flowing from the service.
 

Pharon

Senior Member
Location
MA
My position is that the 120% rule doesn't come into play because the 30A PV circuit would be connected to the supply side of the two main breakers (100A and 50A). As for the 30A OCPD that protects the PV feeder, that needs to be on the load side of the inverter, not in the panel. Any 30A breaker in the panel connected to the PV feeder would be acting as a disconnect only, not as an OCPD.
 

Smart $

Esteemed Member
Location
Ohio
My position is that the 120% rule doesn't come into play because the 30A PV circuit would be connected to the supply side of the two main breakers (100A and 50A).
Correct.

As for the 30A OCPD that protects the PV feeder, that needs to be on the load side of the inverter, not in the panel. Any 30A breaker in the panel connected to the PV feeder would be acting as a disconnect only, not as an OCPD.
A single 30A breaker in the panel would be all that is required (depending on installation details*). However, you could technically tap the service entrance conductors (which include the panel bus :happyyes:) and install the 30A breaker remote to the panel.

*Without looking it up, aren't inverter output OCPD's required to be within sight of the inverter?

If you install a 30A breaker in the panel and a 30A inverter OCPD remote thereto, wouldn't the feeder be subject to the 120% rule?
 

Pharon

Senior Member
Location
MA
A single 30A breaker in the panel would be all that is required (depending on installation details*). However, you could technically tap the service entrance conductors (which include the panel bus :happyyes:) and install the 30A breaker remote to the panel.

*Without looking it up, aren't inverter output OCPD's required to be within sight of the inverter?

If you install a 30A breaker in the panel and a 30A inverter OCPD remote thereto, wouldn't the feeder be subject to the 120% rule?
Funny you should bring this up, because I've seen several installations with no OCPD at the inverter, and for the life of me I can't figure out what's protecting the feeder. Is it something internal in the inverter itself? Maybe something on the DC side? I'm sure there's something about it in article 690 or 705 but don't have a Code handy right now.

Regarding the 120% rule, even with OCPD at the inverter and a 30A disconnect in the MLO panel, aren't we technically still on the supply side of all building loads? And if so, I don't see how the 120% rule applies.
 

SolarPro

Senior Member
Location
Austin, TX
Funny you should bring this up, because I've seen several installations with no OCPD at the inverter, and for the life of me I can't figure out what's protecting the feeder.

The OCPD needs to be located at the source of the fault currents, and a utility-interactive inverter itself is not a potential source of higher than expected currents.

Here's a relevant discussion from a recent article Understanding the NEC 2014 and Its Impact on PV Systems:

Section 690.9 ?Overcurrent Protection?

Section 690.9(A) now provides additional clarification regarding the proper location of overcurrent protection devices (OCPDs). Since PV modules and utility-interactive inverters are inherently current limited, overcurrent risk in PV systems is associated with parallel-connected circuits. On the dc side of the system, the risk comes from PV source or output circuits that are connected in parallel. On the ac side of the system, the risk of encountering significantly higher currents comes from the utility grid or inverter output circuits aggregated in parallel. Per Section 690.9(A), all of the ac and dc circuits in a PV system must be protected at the source ?of significantly higher current.? Thus PV source circuit series fusing is located in combiner boxes, PV output circuit fusing is required in subarray combiners or recombiners, and overcurrent protection for ac circuits is required at the point of interconnection, or where multiple inverters are connected in parallel.
 
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