Interpreting 705.12(B)(2)

[MOCrocker]

The scenario: the service to a residence is a 200A meter combo (200A main breaker, lugs and 2 blades of each phase for breaker) there is an existing 2-pole, 40A breaker in the meter combo, the interior main panel is fed off the lugs. The interior main panel does not have a main breaker.
I want to tap the conductors feeding the interior main panel with 50A of solar. The length of tap conductors is less than 5 ft. I have a 60A fusible disconnect with 50A fuses prior to the point of interconnection.
My co-worker suggests derating the main in the meter combo to 175A to accommodate the tap. If that is necessary, why not just breaker back-feed the PV on the bus in the meter combo.
Is it necessary to derate the breaker, whether we're tapping the conductors or breaker back-feeding?

 

[ggunn]

The scenario: the service to a residence is a 200A meter combo (200A main breaker, lugs and 2 blades of each phase for breaker) there is an existing 2-pole, 40A breaker in the meter combo, the interior main panel is fed off the lugs. The interior main panel does not have a main breaker.
I want to tap the conductors feeding the interior main panel with 50A of solar. The length of tap conductors is less than 5 ft. I have a 60A fusible disconnect with 50A fuses prior to the point of interconnection.
My co-worker suggests derating the main in the meter combo to 175A to accommodate the tap. If that is necessary, why not just breaker back-feed the PV on the bus in the meter combo.
Is it necessary to derate the breaker, whether we're tapping the conductors or breaker back-feeding?

If you backfeed a breaker on the meter main bus you will have to either verify that the MLO panel is rated for 100% of the service disconnect plus 125% of your inverter max rated power, or you will have to install OCP ahead of it.

 

[MOCrocker]

If you backfeed a breaker on the meter main bus you will have to either verify that the MLO panel is rated for 100% of the service disconnect plus 125% of your inverter max rated power, or you will have to install OCP ahead of it.

Thank you for the response. The interior panel doesn't appear to have the capability of having a main breaker installed. How does your statement change if we were to tap the conductors feeding the MLO?

 

[jaggedben]

The description is a little unclear or incomplete. Sounds like there is a distribution busbar in the meter/main with feed-through lugs to the subpanel.
Are the feed through lugs at the opposite end from the main? Did you mention the 40A breaker because it's the only other circuit in the meter main?

Are you on the 2017 NEC?

First, I believe ggunn is correct, and it makes no difference if you tap the 200A conductors or put a breaker in the meter/main. You have to protect the sub from being able to draw 240amps of power. This means either putting a new 200A overcurrent device to protect it, or downsizing the main service breaker to 150A if load calc allows.

As far as using (B)(2), i.e. the 120% rule, you'd have to downsize the main to 175A and have the PV feed the opposite end. If the feed-through lugs are at the opposite end like usually, then backfeeding those conductors is the only legit way to do that. So tap. Or get a 225A sub in between the main and existing sub.

There is also the 'sum of all breakers' rule, which could avoid downsizing the main (but not avoid protecting the sub) but that's a bit of a loophole in the 2017 vode and the AHJ might have an issue with it.

 

[ggunn]

Thank you for the response. The interior panel doesn't appear to have the capability of having a main breaker installed. How does your statement change if we were to tap the conductors feeding the MLO?

It doesn't. Anywhere between the meter main and the MLO panel exposes the panel to the rating of the service disco plus 125% of your max inverter current.

 

[MOCrocker]

The description is a little unclear or incomplete. Sounds like there is a distribution busbar in the meter/main with feed-through lugs to the subpanel.
Are the feed through lugs at the opposite end from the main? Did you mention the 40A breaker because it's the only other circuit in the meter main?

Are you on the 2017 NEC?

First, I believe ggunn is correct, and it makes no difference if you tap the 200A conductors or put a breaker in the meter/main. You have to protect the sub from being able to draw 240amps of power. This means either putting a new 200A overcurrent device to protect it, or downsizing the main service breaker to 150A if load calc allows.

As far as using (B)(2), i.e. the 120% rule, you'd have to downsize the main to 175A and have the PV feed the opposite end. If the feed-through lugs are at the opposite end like usually, then backfeeding those conductors is the only legit way to do that. So tap. Or get a 225A sub in between the main and existing sub.

There is also the 'sum of all breakers' rule, which could avoid downsizing the main (but not avoid protecting the sub) but that's a bit of a loophole in the 2017 vode and the AHJ might have an issue with it.

This AHJ is actually on the 2014 NEC.

 

[wwhitney]

On the picture, the central gold colored bolted connections are the main breaker to bus connections, and the conductors at either far side are the feed thru conductors?

If so, looks like each busbar is center fed, and the feed thru lugs are at one end of the busbar. But the left side and right side differ as far as the location of the bus stabs; looks like on the left one bus stab is between the main breaker and the feed thru lug, while on the right side both stabs are on the opposite side of the main breaker connection as the feed thru lug.

Then again, the left hand busbar looks like it's more of a T-shape than a bent bar, so a linear ordering doesn't really apply to the connections to it.

Cheers, Wayne

 

[jaggedben]

Yeah that's a new arrangement for me, at least for a modern Eaton 200A meter main.

Since the feed thrus aren't at the opposite end you can't use the 120% rule. At least not with that 40A in there.

You might be able to convince the AHJ to let you use 705.12(D)(2)(3)(c). Your case is better if you can remove the 40A breaker.

 

[MOCrocker]

On the picture, the central gold colored bolted connections are the main breaker to bus connections, and the conductors at either far side are the feed thru conductors?

If so, looks like each busbar is center fed, and the feed thru lugs are at one end of the busbar. But the left side and right side differ as far as the location of the bus stabs; looks like on the left one bus stab is between the main breaker and the feed thru lug, while on the right side both stabs are on the opposite side of the main breaker connection as the feed thru lug.

Then again, the left hand busbar looks like it's more of a T-shape than a bent bar, so a linear ordering doesn't really apply to the connections to it.

Cheers, Wayne

Correct, the feed through conductors (feeding the interior MLO panel) are on the lugs on either side of the load-side bus.

 

[reethepink]

The description is a little unclear or incomplete. Sounds like there is a distribution busbar in the meter/main with feed-through lugs to the subpanel.
Are the feed through lugs at the opposite end from the main? Did you mention the 40A breaker because it's the only other circuit in the meter main?

Are you on the 2017 NEC?

First, I believe ggunn is correct, and it makes no difference if you tap the 200A conductors or put a breaker in the meter/main. You have to protect the sub from being able to draw 240amps of power. This means either putting a new 200A overcurrent device to protect it, or downsizing the main service breaker to 150A if load calc allows.

As far as using (B)(2), i.e. the 120% rule, you'd have to downsize the main to 175A and have the PV feed the opposite end. If the feed-through lugs are at the opposite end like usually, then backfeeding those conductors is the only legit way to do that. So tap. Or get a 225A sub in between the main and existing sub.

There is also the 'sum of all breakers' rule, which could avoid downsizing the main (but not avoid protecting the sub) but that's a bit of a loophole in the 2017 vode and the AHJ might have an issue with it.

Could you please explain the "225A sub" in between the main and existing sub option? I do not understand this one.

 

[jaggedben]

Could you please explain the "225A sub" in between the main and existing sub option? I do not understand this one.

Actually in the scenario I laid out, where he downsizes the main breaker to 175A, it could be a 200A or 225A panel.

If you have a panel full of breakers you are usually trying to meet the '120% rule' (705.12(B)(2)(3)(b) if we're using 2017 references). That allows the main breaker plus 125% of solar output to be 240A on a 200A busbar panel. If the main breaker is 200A, the solar output is limited to 32A. (125% of 32A is 40A. 200A+40A=240A). He wants to install a 50A solar breaker, which must mean his solar output is larger. So if the busbar is 225A then 120% of 225A is 270A and he's okay. But also if the main breaker is downsized, then 175A + 50A is 225A which is less than 240A, also okay.

Just to be clear, in the actual situation, with the feed through lugs in the photo, there are further issues.