Disconnect Question

[GerryB]

I was talking with another electrician about this scenario. Single family 200 amp meter-main outside. Solar taps in main breaker part of enclosure below the 200 amp main, so load side. From there to a fused disco. I was saying I think you need a main breaker in the panel also with this setup, not just a main lug. He didn't think so because of the fused disco on the taps.
I thought the main inside was to keep the combined total solar and utility from exceeding 200 amps. Is that right?

 

[Carultch]

Main panelboards are routinely populated with breakers that add up to far greater than the busbar rating. Statistical models of user behavior reflected in NEC load calculations allow us to do this, such that it is unlikely to trip the main breaker with the loads that will realistically operate simultaneously. That being said, it is only *unlikely*, not impossible, to draw more than the busbar rating among the branch breakers. In the unlikely event that the user overloads the main panel, we want a breaker or fuse to trip before a busbar or wire overheats.

Let's assume your system is 80A (i.e. 64A of total inverter current), just to have a number to make this point. Your main panel was originally depending on the 200A breaker in the meter/main to limit the total load to 200A or less. Inserting an 80A source between the meter/main and the main-lug-only panelboard, means you now supply that panelboard with the potential to draw 280A. It is no longer protected at its busbar rating.

In order to restore code-compliance when adding the 80A source, you'll need a 200A OCPD between the main panel and the point of interconnection. This can either be a main breaker of the panelboard, or you can keep it as an MLO, and place the functionality of the main breaker in a separate device. I.e. enclosed breaker or fused disconnect.

 

[jaggedben]

I was talking with another electrician about this scenario. Single family 200 amp meter-main outside. Solar taps in main breaker part of enclosure below the 200 amp main, so load side. From there to a fused disco. I was saying I think you need a main breaker in the panel also with this setup, not just a main lug. He didn't think so because of the fused disco on the taps.
I thought the main inside was to keep the combined total solar and utility from exceeding 200 amps. Is that right?

You are essentially correct. The feeder and panel downstream of the solar tap are required to be protected from the combined current from both sources.

It's possible, but unlikely, that a breaker to protect the feeder and subpanel is not needed, if the feeder and panel are already rated for the combined current. But I'd wager they're rated 200A, and therefore a main breaker or some OCPD is required as you say.

2020 code reference for this is 705.12(B)(1). It's been in the code under different numbers since 2014.

 

[shortcircuit2]

In this connection arrangement by the OP, with a load side feeder tap in a 200amp meter-main ahead of a distribution panel with the 200amp main at the distribution panel protecting the busbar...does the busbar still need to comply with 705.12(B)(3)(1)? The way 705.12 (2020 NEC) is written, it could be construed that 705.12(B)(3)(1) is still applicable although not necessary.

 

[jaggedben]

In this connection arrangement by the OP, with a load side feeder tap in a 200amp meter-main ahead of a distribution panel with the 200amp main at the distribution panel protecting the busbar...does the busbar still need to comply with 705.12(B)(3)(1)? The way 705.12 (2020 NEC) is written, it could be construed that 705.12(B)(3)(1) is still applicable although not necessary.

No it does not apply. The distribution panel is not "interconnecting equipment".

 

[shortcircuit2]

What is the "interconnecting equipment" if the feeder tap is made in the distribution panel ahead of the 200-amp main?

 

[jaggedben]

What is the "interconnecting equipment" if the feeder tap is made in the distribution panel ahead of the 200-amp main?

It would be any equipment that is placed in series between the interactive source and the primary source. In this case it would be the meter main, but since there are presumably no load side busbars in the meter main there are no relevant provisions of 705.12(B) that apply.

I realize that I'm stating my understanding of the intent, and I've been following the changes and discussions about these specific code sections for over a decade now. I think your raising some valid questions concerning how the language van be unclear or inconsistent. For example some of the provisions of 705.12(B) refer to conductors and not equipment.

 

[shortcircuit2]

Agreed...I understand the intent. But that charging paragraph can run me in circles. It needs work.