turning a subpanel to a "service panel" using a line side tap

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We are inputting a PV inverter(s) into a garage subpanel using a 50 A CB (to code. The subpanel (100A main CB/125 A Bus) connects to the main service panel via a 100 A CB in the main service panel (200 A bus /200 A bus). This is incorrect. Hence the problem : We should be connecting to a 50 A CB in the Main service panel, but cannot because the max value of the CB cannot exceed 40 A. So,can I connect this garage subpanel between the service main CB and the utility meter using a line side tap ? Of course,we would use a fused AC disconnect. We do this all the time for a subpanel which only contains inverter CBs. This is a new situation where the subpanel not only contains our inverter CB but ALSO load CBs which supply current to the garage loads.

QUESTION : can we disconnect the garage subpanel from the 100 A CB in the main service panel and reroute to a line side tap ??
 

Carultch

Senior Member
Location
Massachusetts
We are inputting a PV inverter(s) into a garage subpanel using a 50 A CB (to code. The subpanel (100A main CB/125 A Bus) connects to the main service panel via a 100 A CB in the main service panel (200 A bus /200 A bus). This is incorrect. Hence the problem : We should be connecting to a 50 A CB in the Main service panel, but cannot because the max value of the CB cannot exceed 40 A. So,can I connect this garage subpanel between the service main CB and the utility meter using a line side tap ? Of course,we would use a fused AC disconnect. We do this all the time for a subpanel which only contains inverter CBs. This is a new situation where the subpanel not only contains our inverter CB but ALSO load CBs which supply current to the garage loads.


QUESTION : can we disconnect the garage subpanel from the 100 A CB in the main service panel and reroute to a line side tap ??

The NEC permits a supply-side PV system interconnection ahead of the service disconnecting means. But I don't anticipate that it allows expanding the loads with additional service disconnects, when the sum of the service disconnects for loads, exceeds the rating of the service.

Suppose if the main house panel draws up to 200A, and there are going to be 50A of loads (not sources) added to the PV system panelboard, then that is 250A. And your service may only be 200A. This is a problem.

The way that I see it, service sizing for a site with on-site loads and sources is an either/or problem. The service needs to be large enough for either the total loads assuming the sources are zero, or the total sources assuming the loads are all zero.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
QUESTION : can we disconnect the garage subpanel from the 100 A CB in the main service panel and reroute to a line side tap ??

I think the answer to your question is yes, in theory you could do it. There are a lot of potential pitfalls when you get to the details though, especially if the existing service panel is a meter-main combo.

To be clear, the main service panel has a 200A main breaker and other branch breakers in it?
Why not just downsize the main breaker in the service panel to 175A?
 
I dont see how this could be done. I think the "mixed use" of the panel makes it not in the spirit of a PV supply side connection. I would have to check the specific wording, but it seems like a super long shot to me; perhaps I am misunderstanding the proposal.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I dont see how this could be done. I think the "mixed use" of the panel makes it not in the spirit of a PV supply side connection. I would have to check the specific wording, but it seems like a super long shot to me; perhaps I am misunderstanding the proposal.

I thought more or less the same thing until I checked 230.40 Exception 2. It's open to interpretation though. I certainly would look at all other options first. Main breaker downsize would be my first consideration.

The original post also prompts the question: if a supply side tap can be done, why not just do it for the solar only?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I don't see this as a supply side tap for the purpose of connecting the solar. It just takes the solar out of the existing main service panel where the solar breaker would be limited to 40 amps and puts it into the panel with a 100 amp main and 125 amp busing where the solar can be a 50 amp breaker.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I don't see this as a supply side tap for the purpose of connecting the solar. It just takes the solar out of the existing main service panel where the solar breaker would be limited to 40 amps and puts it into the panel with a 100 amp main and 125 amp busing where the solar can be a 50 amp breaker.

I won't disagree, but I think that's mostly semantic.

The question comes down to: does the code allow multiple service disconnecting enclosures for one dwelling that all serve regular loads?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
I won't disagree, but I think that's mostly semantic.

The question comes down to: does the code allow multiple service disconnecting enclosures for one dwelling that all serve regular loads?
The code allows six means of disconnect for any service.
 
I don't see this as a supply side tap for the purpose of connecting the solar. It just takes the solar out of the existing main service panel where the solar breaker would be limited to 40 amps and puts it into the panel with a 100 amp main and 125 amp busing where the solar can be a 50 amp breaker.

OK I understand now. The OP calling it a supply side threw me off. Then I say sure except that the two service disconnects would need to be grouped. If one of them was PV only, then they would not need to be grouped.
 

david

Senior Member
Location
Pennsylvania
We are inputting a PV inverter(s) into a garage subpanel using a 50 A CB (to code. The subpanel (100A main CB/125 A Bus) connects to the main service panel via a 100 A CB in the main service panel (200 A bus /200 A bus). This is incorrect. Hence the problem :

I don't see this as a supply side tap for the purpose of connecting the solar. It just takes the solar out of the existing main service panel where the solar breaker would be limited to 40 amps and puts it into the panel with a 100 amp main and 125 amp busing where the solar can be a 50 amp breaker.

I was thinking this was already done in the garage 100 amp subpanel, load side connection.
The 100 amp sub panel in the garage is in series with the 200 amp rated service panel.
Sure 200 amp overcurrent for the utility source plus 50 amp overcurrent for solar source exceeds the rating.

Why not just move the 100 amp breaker in the service panel to opposite end of the main in the 200 amp service panel?
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I was thinking this was already done in the garage 100 amp subpanel, load side connection.
The 100 amp sub panel in the garage is in series with the 200 amp rated service panel.
Sure 200 amp overcurrent for the utility source plus 50 amp overcurrent for solar source exceeds the rating.

Why not just move the 100 amp breaker in the service panel to opposite end of the main in the 200 amp service panel?
That won't fly. There is only 40A of electrical space in the main panel, and either 50A or 125% of the inverter output current (depending on the code cycle) counts toward the 120% rule in the main panel as well as in the sub if he does it that way. If 125% of the inverter output current were less than 40A he would be using a 40A breaker instead of a 50A CB.

Converting the sub to a second main might work, but it's subject to the tap rules.

His best option might be to install a fused disco within 10' of the service, tap the service conductors on the supply side of the main, and trench his PV output to the disco.

Or he could replace the busbar in the main with a 225A bus, leave the main breaker at 200A, and connect it the way you suggest.

Or he could downsize the main breaker and connect as you suggest, but he runs the risk of nuisance trips on the main.
 
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david

Senior Member
Location
Pennsylvania
That won't fly.


Never mind I see they flipped flopped the language about panels connected in series between article 690 and article 705

Article 690
(2) Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor. In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors.

Article 705
2) Bus or Conductor Rating. The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor shall not exceed 120 percent of the rating of the busbar or conductor.

Article 690
(7) Inverter Output Connection. Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent devices supplying it, a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220. A permanent warning label shall be applied to the distribution equipment with the following or equivalent marking:

Article 705
(7) Inverter Output Connection. Unless the panelboard is rated not less than the sum of the ampere ratings of all overcurrent devices supplying it, a connection in a panelboard shall be positioned at the opposite (load) end from the input feeder location or main circuit location. The bus or conductor rating shall be sized for the loads connected in accordance with Article 220. In systems with panelboards connected in series, the rating of the first overcurrent device directly connected to the output of a utility-interactive inverter(s) shall be used in the calculations for all busbars and conductors. A permanent warning label shall be applied to the distribution equipment with the following or equivalent wording:
 
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