120% rule for two Inverter outputs vs sup-panel

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I was asked if you can use two 20a breakers in the 200 amp service instead of adding a sub-panel to combine both inverter outputs in to one feeder to the service with a 40a main and still use the 120% rule on the bus rating. This would be at opposite end of the main and the service has room for 2-2 pole breakers. My first taught was no that would go to 100% of the bus rating and looking at the nec 705 I can't find anything that says I have to do 100% so that leads me to say yes you can use the 120% of bus rating rule if on opposite end of the main breaker. What are your taught on this. Thank You :happyno::happyyes:
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
I was asked if you can use two 20a breakers in the 200 amp service instead of adding a sub-panel to combine both inverter outputs in to one feeder to the service with a 40a main and still use the 120% rule on the bus rating. This would be at opposite end of the main and the service has room for 2-2 pole breakers. My first taught was no that would go to 100% of the bus rating and looking at the nec 705 I can't find anything that says I have to do 100% so that leads me to say yes you can use the 120% of bus rating rule if on opposite end of the main breaker. What are your taught on this. Thank You :happyno::happyyes:
Physically it would be OK but check with your AHJ to see if they will allow two points of interconnection in the MDP.
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
I concur with the other comments. What might trip you up is if an AHJ thinks they are one 'source' that requires a dedicated breaker according to 705.12(D)(1). But I have not had this problem myself. Just say they are two sources. :thumbsup:
 

Smart $

Esteemed Member
Location
Ohio
What makes the most difference here is that the PV breakers are on opposing sides of the bus. For example, occupying #39-41 and #40-42 in a 42 space panel.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
What makes the most difference here is that the PV breakers are on opposing sides of the bus. For example, occupying #39-41 and #40-42 in a 42 space panel.
I don't see an issue with that since there won't be any loads between the two interconnection points.
 

Carultch

Senior Member
Location
Massachusetts
I was asked if you can use two 20a breakers in the 200 amp service instead of adding a sub-panel to combine both inverter outputs in to one feeder to the service with a 40a main and still use the 120% rule on the bus rating. This would be at opposite end of the main and the service has room for 2-2 pole breakers. My first taught was no that would go to 100% of the bus rating and looking at the nec 705 I can't find anything that says I have to do 100% so that leads me to say yes you can use the 120% of bus rating rule if on opposite end of the main breaker. What are your taught on this. Thank You :happyno::happyyes:

NEC2014 revised the language of this, so that it is "125% of the total connected inverter output current", instead of the breaker rating (or sum of the breaker rating). This now prevents "round up errors" when sizing that interconnection breaker from being a showstopper, and it also justifies any mix of breakers for the solar interconnection at the opposite end of the busbar from the main supply, provided that (125% of all inverter current + Main Supply OCPD) <= 120% of busbar rating.
 
120% off bus bar rating

120% off bus bar rating

NEC2014 revised the language of this, so that it is "125% of the total connected inverter output current", instead of the breaker rating (or sum of the breaker rating). This now prevents "round up errors" when sizing that interconnection breaker from being a showstopper, and it also justifies any mix of breakers for the solar interconnection at the opposite end of the busbar from the main supply, provided that (125% of all inverter current + Main Supply OCPD) <= 120% of busbar rating.

All this came from one of my ex-students that was installing 30 micro-inverters on two different roof levels 15 on one roof and 15 on the other roof same house same service and wanted to run the 2 raceway to the service instead of installing a sub-panel. The utility does not require a separate disconnect at this house. My local utility requires a utility disconnect so I have used a sup-panel to meet this requirement.
 

Smart $

Esteemed Member
Location
Ohio
What do I need code for? What's the difference between a 2-pole 40A PV breaker and qty (2) 2-pole 20A breakers plugged onto the same stabs with identical contacts?
Are you talking about a quad that takes up 4 spaces, 4-poles, 2 on each side, or a quad that takes up 4 spaces, 4 poles, all on the same side, i.e. vertically in normal panel orientation? I'd reject the latter.

Regarding 40A, I thought we are doing two inverters, 4 conductors...???
 

jaggedben

Senior Member
Location
Northern California
Occupation
Solar and Energy Storage Installer
Are you talking about a quad that takes up 4 spaces, 4-poles, 2 on each side, or a quad that takes up 4 spaces, 4 poles, all on the same side, i.e. vertically in normal panel orientation? I'd reject the latter.

Regarding 40A, I thought we are doing two inverters, 4 conductors...???

I'm talking about a quad that takes up 2 spaces, 4 poles, like this one. Get it now? The point about 40A is that the panelboard would never 'know' the difference between this breaker assembly and a 40A 2-pole one.
 

Smart $

Esteemed Member
Location
Ohio
I'm talking about a quad that takes up 2 spaces, 4 poles, like this one. Get it now? The point about 40A is that the panelboard would never 'know' the difference between this breaker assembly and a 40A 2-pole one.
Okay... got it. A 2-pole tandem... not what I call a quad though it does have 4 effective poles (2x2). The 2P tandem I'd pass as long as the panel listing designates the last two spaces on either side as useable for tandem breakers.
 
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